Digestive system

primary dentition, or deciduous teeth

(sometimes called baby teeth) 20 deciduous teeth, with 4 incisors, 2 canines, and 4 molars in both the mandible and maxilla -The first deciduous teeth to erupt are generally the lower central incisors at about 6 months of age. Deciduous teeth continue to erupt at a rate of about one pair of teeth per month until the age of 24 months, at which time all 20 teeth are usually present.

Deglutition

(swallowing) A specialized type of propulsion that pushes a bolus of food from the oral cavity through the pharynx and esophagus to the stomach. - relies on the coordinated action of the upper alimentary canal, including the soft palate, pharynx, and esophagus - tongue also plays a role in this process, and is the only accessory organ to directly participate in motility

defecation reflex steps

1. Stretch receptors transmit the sensation of rectal distention to the spinal cord. The walls of the rectum house stretch receptors that are activated by the presence of fecal material stretching the wall. Sensory neurons transmit these impulses to the spinal cord. 2. Parasympathetic neurons cause smooth muscle in the sigmoid colon and rectum to contract, and the internal anal sphincter to relax. In an infant or young child, relaxation of the internal anal sphincter results in release of fecal matter from the anal canal. In older children and adults, the defecation reflex ends here and ​there ​is conscious contraction of the external anal sphincter if it is not an appropriate time to defecate. The reflex is initiated again with the next mass movement, and steps 1 and 2 repeat. However, if the time is appropriate for defecation to occur, then we proceed to the next step: 3. Impulses from the cerebral cortex trigger relaxation of the external anal sphincter and contraction of the levator ani muscle, allowing feces to pass out of the body. The cerebral cortex stimulates voluntary relaxation of the external anal sphincter. The levator ani muscle also contracts, which elevates the anus. These three movements combined allow the feces to pass. Note that this activity is assisted by contraction of the abdominal muscles against a closed glottis, a procedure called the Valsalva maneuver, which increases intra-abdominal pressure.

Cholecystokinin (CCK)

A hormone produced by duodenal enteroendocrine cells; triggers the secretion of digestive enzymes and other proteins from the pancreas. - produced by duodenal DNES cells in response to the presence of lipids and partially digested proteins in the duodenum. It acts on acinar cells to trigger the secretion of digestive enzymes and other proteins.

Peritoneum

A set of double serous membranes around several abdominal organs; consists of the inner visceral peritoneum and the outer parietal peritoneum.

Salivary glands

A set of three pairs of glands around the oral cavity that secrete saliva into it.

distal large intestine

A small amount of absorption takes place here, mostly of water - However, its main role is to store fecal material until it is ready to be expelled during defecation. For this reason, the distal large intestine is much less motile than the proximal large intestine. When mass movements force fecal material into the normally empty rectum, it initiates the parasympathetic- mediated defecation reflex

bolus

A term used to describe food after it has been chewed and mixed with saliva

Tooth Structure

A tooth consists of two components: the crown, which is the visible portion above the gum line, and the root, which is embedded within the alveolus. Notice that the tooth consists of outer layers of hard mineralized substances—the enamel—covering a soft, inner gelatinous substance called the pulp. Enamel is composed almost fully of secreted calcium hydroxyapatite crystals with only a small amount of organic material. This makes enamel the hardest substance in the body, allowing it to endure the forces that accompany chewing. The cells that secrete enamel deteriorate after the tooth erupts, so the body cannot repair damaged enamel.

Carbohydrate digestion begins in the mouth with the help of salivary amylase from the salivary glands, which catalyzes the reactions that break long polysaccharides into shorter oligosaccharides. Salivary amylase generally accomplishes little actual chemical digestion because the food simply is not in the mouth long enough for it to have much effect. Some minor digestion by salivary amylase continues in the stomach, but stomach acid generally inactivates this enzyme before it can complete the task. Chemical digestion of carbohydrates resumes in the small intestine. Here polysaccharides and oligosaccharides encounter a pancreatic enzyme called pancreatic amylase. This enzyme is very similar in structure and function to salivary amylase, and ❶ catalyzes the reactions that break the remaining polysaccharides into oligosaccharides. Oligosaccharide digestion is completed by reactions catalyzed by enzymes that are products of enterocytes on the brush border of the small intestine (brush border enzymes), including the enzymes lactase, maltase, and sucrase. Lactase catalyzes only one reaction: the digestion of the sugar lactose (found in milk and milk-based products) into glucose and galactose. ❷ Both maltase and sucrase catalyze reactions that break oligosaccharides into disaccharides and monosaccharides. The end result of all this activity is the production of the monosaccharides glucose, fructose, and galactose. After carbohydrates have been digested into monosaccharides, they are ready to be absorbed. Notice in Figure 22.29 that both glucose and galactose are transported across the enterocyte's apical membrane by the same mechanism. Note also that the concentrations of glucose and galactose are higher in the enterocyte's cytosol than they are in the lumen of the small intestine. This means that absorption of glucose and galactose requires these molecules to move against their concentration gradients. In the cell chapter, we discussed the fact that anytime we move a substance against its concentration gradient, energy must be expended (see Chapter 3). The energy in this case comes via a secondary active transport mechanism known as the Na+/glucose cotransporter. Recall that secondary active transport is a form of membrane transport in which ATP is used indirectly to move a chemical across the membrane. In this case, the Na+/K+ pump consumes ATP to drive sodium ions out of the enterocyte and into the lumen of the small intestine. This creates a concentration gradient that favors the movement of sodium ions back into the enterocyte, another example of the Gradients Core Principle (Module 1.5.5) and of the importance of gradients in anatomy and physiology. Remember that a concentration gradient is actually a form of potential energy. The cell harnesses this energy by ❹❹ allowing sodium ions to diffuse back into the enterocyte, and this energy drives the transport of glucose and galactose into the enterocyte. Fructose is unable to bind the Na+/glucose cotransporter, so it crosses the apical enterocyte membrane by a separate mechanism (see Figure 22.29). ❺❺ Fructose binds a channel that mediates its facilitated diffusion across the membrane. Facilitated diffusion is a passive process and requires no net input of energy, and so a concentration gradient must be present or fructose will not be absorbed. All three monosaccharides ❻❻ cross the basal enterocyte membrane by the same facilitated diffusion mechanism. This process involves a membrane protein that is very similar to the one that helps fructose cross the apical enterocyte membrane. After the monosaccharides cross the basal membrane, they diffuse through the extracellular fluid and into the capillaries in the villus. Once in the blood, they are delivered to the liver via the hepatic portal vein for processing.

Digestion and Absorption of Carbohydrates

small intestine

Digestive organ where most chemical digestion and absorption of food takes place - 6-meter-long (almost 20-foot-long) - also called small bowel -the longest portion of the alimentary canal (it is considerably shorter in a living person—about 3 meters or 10 feet—than in a cadaver because of smooth muscle tone). Four main processes occur in the small intestine: secretion, digestion, absorption, and propulsion.

Mesentery

Double folds of visceral peritoneum located around certain abdominal organs, such as the small and large intestines. - support and bind these organs together and keep the small intestine in a particular shape that fits within the abdominopelvic cavity. This function is key, as we don't want loops of intestine wandering around our abdominopelvic cavities. - also house blood vessels, nerves, and lymphatic vessels, anchoring them in place

migrating motor complex

During fasting, the small intestine exhibits slow, rhythmic contractions along its length in a pattern called the.... - These contractions clear any remaining material from the small intestine, including leftover food and secretions. - requires about 2 hours to push digesting food from the duodenum to the ileocecal valve. This movement is controlled by both the ENS and a hormone called motilin, which is produced by cells in the duodenal mucosa.

Absorption of Electrolytes

Electrolytes are both taken in from the diet and present in secretions from digestive organs. Most of these electrolytes are absorbed in the small intestine, although significant amounts are absorbed in the large intestine as well. You have already seen mechanisms for absorption of sodium ions (cotransport with monosaccharides and amino acids). This sodium ion absorption is also key in the absorption of anions—as sodium ions enter the enterocytes, an electrical gradient is created that drives the absorption of anions such as chloride and bicarbonate ions. Other electrolytes have specialized mechanisms for absorption. For example, as you've discovered, calcium ions are absorbed with the help of vitamin D (see Chapter 16). Certain electrolytes that are present in smaller amounts, such as iron and magnesium ions, are absorbed by active transport mechanisms.

VIP (vasoactive intestinal peptide)

Enteric interneurons that are inhibitory to smooth muscle use which of the following neurotransmitters?

gastric pacemaker

The peristaltic waves are initiated and controlled by a group of specialized cells collectively called the..... - controls the rate of the waves, which remains relatively constant at about three per minute. Their strength, however, is subject to regulation by several factors, including stimulation from the vagus nerve and secretion of certain hormones. For example, certain DNES cells of the stomach produce the hormone serotonin, which stimulates gastric motility. Another hormone with the same effect is intestinal gastrin, which is produced by duodenal DNES cells.

Absorption of Vitamins

Vitamins are chemicals generally provided by the diet that are involved in a host of metabolic reactions. There are two types of vitamins: water-soluble vitamins, which are polar molecules; and fat-soluble vitamins, which are lipid-based and mostly nonpolar. Most of the water-soluble vitamins are absorbed in the small intestine by diffusing through the enterocytes' plasma membranes. One important exception is vitamin B12, which must bind to intrinsic factor—a chemical produced by the parietal cells of the stomach—to be absorbed in the ileum. A shortage of intrinsic factor can impair vitamin B12 absorption, as discussed in A&P in the Real World: Intrinsic Factor and Vitamin B12 Deficiency. Fat-soluble vitamins are packaged into micelles with fats and other lipids and are absorbed with them. The fat-soluble vitamins include A, D, E, and K.

1. Mucous neck cells. The cells located near the top, or "neck," of the gland are called mucous neck cells. As their name implies, these cells secrete mucus much like the surface epithelial cells. However, these surface cells secrete alkaline mucus, whereas mucous neck cells secrete acidic mucus. This prevents their mucus from neutralizing the acid produced by other cells known as parietal cells. 2. Parietal cells. The next cells are the parietal cells, which secrete the hydrochloric acid (HCl) that is responsible for the acidic pH of gastric juice. Acid is an important component of gastric juice because it is required to activate a precursor enzyme called pepsinogen and also because it destroys most disease-causing organisms we ingest. In addition, parietal cells produce the chemical intrinsic factor, which is required for intestinal absorption of vitamin B12, found in various foods. 3. Chief cells. Next down are the chief cells, which secrete the inactive precursor enzyme pepsinogen. When pepsinogen encounters an acidic pH, it becomes the active enzyme pepsin, which begins protein digestion in the stomach. 4. Diffuse neuroendocrine system cells. Located at the very bottom of the gland are the diffuse neuroendocrine system cells (or DNES cells; also known as enteroendocrine cells). As implied by their names, these are endocrine cells found in many places through the body—hence the term "diffuse"— that secrete many of the same products as neurons. DNES cells are closest to the blood vessels in the underlying submucosa, which gives the hormones they produce ready access to the blood. DNES cells are found throughout the digestive system, and several different types are located within gastric glands. The DNES cells are called G cells, which secrete the hormone Gastrin. This hormone, along with the hormone histamine produced by a different type of DNES cell, stimulates acid secretion from parietal cells.

We find four main types of cells in or near gastric glands, each of which secretes a different product

Much of the churning of the stomach is under the control of hormones and enteric nervous system (ENS) neurons so this poison would have little effect on these control mechanisms.

What effect would a poison that stimulates the parasympathetic nervous system, by activating the vagus nerve; have on the motility of the stomach?

Supplemental digestive enzymes are proteins and will be broken down in the stomach by the acids and pepsin, rendering them useless.

What will happen to supplemental digestive enzymes in the stomach after they are ingested?

secretin (secretin (from S cells in the duodenum) causes both the liver and pancreas to secrete bicarbonate into the small intestine.)

Which of the following GI hormones promotes a pancreatic juice rich in bicarbonate ions?

CCK (cholecystokinin)

Which of the following intestinal hormones stimulates the release of bile from the gall bladder?

The parotid glands have only serous cells, the submandibular glands have mostly serous cells and a small number of mucous cells, and the sublingual glands contain mostly mucous cells. For this reason, the parotid glands secrete mainly water and enzymes, the submandibular glands secrete enzymes mixed with some mucus, and the sublingual glands secrete mainly mucus with a small amount of enzymes.

The three main types of salivary glands differ in the proportion of mucous and serous cells they contain.

filiform, fungiform, circumvallate, and foliate papillae. All papillae except filiform contain epithelium with sensory receptors called taste buds, which detect chemicals associated with different taste sensations. Filiform papillae play no role in taste, and are instead covered with stratified squamous keratinized epithelium. The keratinized cells make the surface of the tongue rough, which assists in mechanical digestion. Human tongues have a limited number of keratinized cells, so our tongues do not feel rough to the touch. Cats' tongues, however, have a large number of keratinized cells, which are responsible for their sandpaper-like texture.

There are four kinds of papillae

alimentary canal/ gi tract

a continuous passage through which food moves. It consists of the oral cavity (mouth), pharynx, esophagus, stomach, small intestine, and large intestine.

saliva

a fluid containing water, enzymes, mucus, and other solutes

Esophagus

a muscular tube about 25 cm (10 in.) long found posterior to the trachea. It transmits the bolus from the pharynx to the stomach. Like the pharynx, the esophageal mucosa is lined with stratified squamous nonkeratinized epithelium. The mucosa and submucosa contain esophageal glands that secrete mucus to lubricate the bolus as it passes through the esophagus. The muscularis externa of the esophagus consists of two layers of muscle, but it differs from the remainder of the alimentary canal. Rather than being only smooth muscle, its superior third is composed of skeletal muscle, its middle third is a mixture of skeletal and smooth muscle (see Figure 22.8a), and its inferior third is composed of smooth muscle.

lesser omentum

a smaller mesentery that extends from the medial surface of the stomach to the liver.

serous cells, which secrete a water-based fluid with enzymes and other solutes; and mucous cells, which secrete mucus. The secretions from serous cells are involved in digestive processes, and are generally released just before or during eating. However, secretions from mucous cells are primarily involved in keeping the oral mucosa moist, and so are released continually.

There are two main types of acinar cells in salivary glands:

rectum

This portion of the large intestine runs anterior to the sacrum and is retroperitoneal. The walls of the rectum feature horizontal folds called rectal valves, which allow the passage of flatus (gas) without risking the simultaneous passage of feces.

extrinsic and intrinsic

Two groups of skeletal muscles control tongue movement:

In this process, ❶❶ the stomach's smooth muscle produces peristaltic contractions that propel a small amount of chyme through the pyloric sphincter into the duodenum. The rest of the chyme is pushed backward into the stomach. ❷❷ The peristaltic waves then churn and mix the remaining chyme, and ❸❸ the process repeats

churning steps

greater omentum

is unique among the mesenteries in that it consists of four layers of folded visceral peritoneum, is named for the fact that it covers the abdominal organs like an apron - is the first structure visible when the abdominal cavity is opened - extends from the base of the stomach down into the pelvis

retroperitoneal

located behind the peritoneum or partly/ completely outside the peritoneal cavity

The digestive organs are drained by a set of veins that drain into the hepatic portal vein. The hepatic portal vein then delivers the blood to the liver for processing. Blood drains from the liver by a set of hepatic veins, which in turn deliver blood to the inferior vena cava.

returning blood to heart

papillae

rough, bumpy elevations on dorsal surface of tongue

Large intestine, or large bowel

runs along the border of the abdominal cavity, surrounding the small intestine and other abdominal organs like a frame - About 1.5 meters (5 feet) long, it is so named because it has a larger diameter than the small intestine. The large intestine receives material from the small intestine that was not digested or absorbed. - a passageway for the leftovers, known as Feces, or fecal matter - The tasks of the large intestine also include secretion (primarily in the form of mucus), propulsion, and defecation. In addition, it houses numerous bacteria that perform important functions such as synthesizing vitamins.

enteric nervous system

the nervous system of the digestive tract - supplies the alimentary canal from the esophagus to the anus (the terminal portion of the large intestine). Nerve plexuses of the ENS work with the sympathetic and parasympathetic nervous systems to control secretion from and motility of the alimentary canal.

parietal peritoneum

the outer layer of the peritoneum that lines the interior of the abdominal wall

proximal large intestine

the primary site of water and electrolyte absorption and bacterial activity, and exhibits two main types of motility. The first is a type of segmentation, or churning, similar to what we saw in the small intestine, in which the circular muscle of each haustrum contracts repeatedly. This swirls the material around in the haustrum, which aids in water and electrolyte absorption. These contractions are controlled primarily by local neurons of the ENS and are triggered by stretch. The other type of motility in the proximal large intestine is a propulsive motion known as a mass movement, or mass peristalsis. During a mass movement, multiple haustra undergo peristalsis, which propels their contents toward the distal large intestine. Mass movements occur three to four times per day, and appear to be triggered by food consumption, which initiates reflexes controlled by the ENS.

Cardia. The region where the esophagus empties into the stomach is the cardia. The cardia receives the bolus when the gastroesophageal sphincter relaxes. Fundus. The dome-shaped top of the stomach is its fundus. Body. The largest portion of the stomach is its body. Pyloric antrum. The inferior portion of the stomach is the pyloric antrum. Pylorus. The terminal portion of the stomach is the pylorus, which abuts the first portion of the small intestine, the duodenum. Like the cardia, the pylorus contains a sphincter that controls the flow of ingested food. In the pylorus, a thick ring of smooth muscle called the pyloric sphincter regulates the flow of materials between the stomach and the small intestine.

the stomach has five anatomical regions

lingual frenulum

thin band that anchors tongue to floor of mouth

pancreas

An endocrine and exocrine organ located inferior and posterior to the stomach; secretes enzymes and other products for digestion and the hormones insulin, glucagon, and somatostatin. - mostly located in the left upper quadrant of the abdomen, where it extends from the duodenum to the spleen; most of it is retroperitoneal. - It has three portions: (1) a wide head that contacts the duodenum, (2) a middle body, and (3) a thinner tail that tapers off toward the spleen. - Running down the middle of the pancreas is the main pancreatic duct, which receives secretions from acinar cells. Near the duodenum, the main pancreatic duct merges with the duct from the liver and gallbladder, after which it empties into the duodenum.

gastroesophageal sphincter

At the inferior end of the esophagus is another sphincter, also known as the lower esophageal sphincter, that regulates the passage of the bolus into the stomach. This sphincter also prevents the contents of the stomach from re-entering the esophagus.

upper esophageal sphincter

At the junction of the pharynx and the esophagus, the muscularis externa is modified into a sphincter called the ________________________________ - which controls the passage of the bolus into the esophagus

Digestion and Absorption of Proteins

Chemical digestion of proteins does not begin until they reach the stomach, where they encounter the enzyme pepsin. Recall that the chief cells of the gastric glands produce the inactive precursor pepsinogen. Pepsinogen requires a pH of about 2 to become pepsin, and pepsin is inactivated completely at a pH of 7. Activated pepsin catalyzes reactions that digest proteins into smaller polypeptides, oligopeptides, and some free amino acids. However, the reactions catalyzed by pepsin digest only about 10-15% of the proteins in ingested food. Individuals who do not produce pepsin do not appear to have any less ability to digest proteins. The remainder of protein digestion takes place in the small intestine with the help of pancreatic enzymes and brush border enzymes (step ❶❶ of Figure 22.30). There are five pancreatic enzymes that digest proteins, all of which are released as inactive precursors. This protects the pancreas from autodigestion, or digestion of its cells by its own enzymes. The first pancreatic enzyme to become activated is the precursor trypsinogen (trip-SIN-oh-jen), which becomes the active enzyme trypsin (TRIP-sin) when it encounters enzymes on the intestinal brush border. Trypsin in turn catalyzes the reactions that convert the other pancreatic enzymes to their active forms, and activates additional trypsinogen as it is secreted by the pancreas. These enzymes catalyze reactions that digest proteins and polypeptides into oligopeptides and some free amino acids. The final enzymes to act on proteins are associated with the enterocytes. There are multiple brush border enzymes that catalyze the digestion of oligopeptides into free amino acids. These enzymes are limited in the reactions they are able to catalyze, so some small oligopeptides (two to three amino acids long) remain undigested. These small oligopeptides are taken up by enterocytes, and in the enterocyte cytoplasm they encounter more enzymes that catalyze their breakdown into free amino acids. To be absorbed in the small intestine, proteins consumed for nutrition must generally be broken down into small oligopeptides and free amino acids. However, humans do have the ability to absorb small amounts of whole, undigested proteins involved in immunity by the process of endocytosis. Specialized cells overlying the Peyer's patches in the small intestine ingest these proteins and deliver them to the lymphatic tissue in the patches. This is particularly important in the development of the immune system in newborn infants. Oligopeptides and free amino acids cross the enterocyte apical membrane primarily by secondary active transport membrane proteins that use a sodium ion gradient established by the Na+/K+ pump steps ❷❷ and ❸❸ of Figure 22.30. Within the enterocyte, oligopeptides are broken down into free amino acids. The free amino acids then exit the basal enterocyte membrane by facilitated diffusion (step ❹❹), after which they enter the capillaries in the villus. Like carbohydrates, the amino acids are then delivered to the liver for processing via the hepatic portal vein.

Receptive Function

In its resting state, the stomach has a volume of only about 50 ml (0.2 cup), but it can expand to about 1500 ml (6.3 cups) when filled with food and liquid. When food or liquid is swallowed, the gastroesophageal sphincter and smooth muscle of the fundus and body of the stomach relax to allow the stomach to fill. This is known as receptive relaxation, and is mediated by both the medulla (as part of the swallowing reflex) and the vagus nerve. Another factor that allows the stomach to fill is the inherent ability of smooth muscle to relax when it is stretched. These two factors, along with the rugae, allow the stomach to receive food and fluid without raising its internal pressure. This is important, as increases in gastric pressure can trigger vomiting, expelling of the stomach contents through the mouth

acinar cells, Yes, in response to a high fat and protein meal, CCK would be stimulated and in turn would stimulate an enzyme-rich secretion from the pancreas.

In response to a steak dinner, certain secretions are needed to aid digestion. What cells in the pancreas would provide these secretions?

note

Like gastric secretion, pancreatic secretion occurs at a basal rate between meals. During eating, pancreatic secretion rises due to parasympathetic and hormonal stimulation.

pancreas, liver, and gallbladder

Like the salivary glands, these accessory organs are exocrine glands that secrete a product through a duct to the outside of the body.

❶❶ Micelles escort lipids to the enterocyte plasma membrane. Digested triglycerides and other lipids such as cholesterol and fat-soluble vitamins (such as vitamin D) remain associated with bile salts in micelles. Notice that these bile salts have their polar portions on the outside, facing the water-based environment, and their nonpolar portions on the inside, facing the digested lipids. The outer polar portion of the micelle allows it to approach the polar mucus layer and phosphate heads of the plasma membrane. ❷❷ Lipids diffuse through the phospholipid bilayer and enter the cytosol. Once the lipids in the micelles are at the plasma membrane, the attraction of the fatty acid tails of the phospholipid bilayer draws the lipids into the cytosol. This generally occurs by simple diffusion, although some lipids use facilitated diffusion by carrier-mediated transport. Note that the leftover bile salts in the micelles remain in the lumen of the small intestine and are reabsorbed by active transport mechanisms in the ileum. ❸❸ Lipids are reassembled into triglycerides and packaged into chylomicrons. Within the enterocyte, enzymes catalyze reactions that turn the free fatty acids and monoglycerides back into triglycerides. The triglycerides are packaged with cholesterol, other dietary lipids, phospholipids, and lipid-binding proteins known as apoproteins into structures called chylomicrons (ky-loh-MY-krahnz). Chylomicrons are similar to micelles in that their nonpolar lipids face the inside and their polar portions face the outside. This allows chylomicrons to travel with the polar water molecules in blood. ❹❹ Chylomicrons are released into the interstitial fluid by exocytosis and then enter a lacteal. The newly formed chylomicrons are packaged into vesicles by the Golgi apparatus and released by exocytosis from the basal enterocyte membrane into the interstitial fluid in the core of the villus. Chylomicrons vary in size but are generally large—in fact, too large to enter the capillaries in the villus. However, lacteals, which are also found in the core of the villus, have valves in their walls that allow large substances to enter and exit. Chylomicrons therefore enter a lacteal, where they join the lymph.

Lipid Absorption Just as lipid digestion is more complicated than that of carbohydrates and proteins, lipid absorption is also more complex. Again, this is due to lipids' nonpolar nature. Lipids face several water-based barriers that deter them from passing into the cytosol of the enterocytes, including the mucus lining of the small intestine and the polar phosphate heads of the enterocytes' plasma membranes. For this reason, lipids require assistance to approach the enterocyte membranes and eventually cross them. Let's walk through the steps of lipid absorption

Bile Production The liver's main digestive function is to produce bile, a liquid that contains multiple components, including water, electrolytes, and organic compounds. Bile serves two critical functions: (1) It is required for the digestion and absorption of lipids; and (2) it is the mechanism by which the liver excretes wastes and other substances that the kidneys cannot excrete. One of its main organic compounds is bile salts, which are derived from cholesterol. Bile salts are amphiphilic, meaning they have both polar and nonpolar parts. This allows them to interact with both lipids and the watery environment of the small intestine. When bile is released into the duodenum, the bile salts coat the lipids and physically break them apart into smaller pieces, a process known as emulsification. Although emulsification is mechanical digestion, it is necessary for the chemical digestion and absorption of lipids. Bile also contains varying amounts of materials that the liver excretes, including cholesterol, waste products, and harmful chemicals (toxins) such as heavy metals. Many of these products are not reabsorbed by the small or large intestine and so pass into the feces. An important example of the liver's excretory function involves bilirubin, a waste product that results from the breakdown of hemoglobin by the spleen. Hepatocytes secrete bilirubin into bile, and the normal flora in the large intestine convert it to urobilinogen. Bacteria convert most of the urobilinogen to stercobilin, which is brown and is responsible for the brown color of feces. A small portion of the urobilinogen is reabsorbed by the large intestine and ends up back in the hepatic portal system. The majority of this urobilinogen is simply re-secreted into the bile, although a small portion remains in the blood and is excreted by the kidneys (note that yellow urobilinogen is largely responsible for the color of urine).

Main function of the Liver

sympathetic postganglionic fibers

Norepinephrine is the neurotransmitter released by which fibers?

Absorption of Water

On average, over 9 liters of water enter the small intestine each day. About 2 liters of water are ingested, and the remaining liters are secreted into the alimentary canal by the alimentary canal itself and accessory organs. Of these 9 liters, about 8 liters are absorbed into the enterocytes of the small intestine, and most of the remaining water is absorbed into the enterocytes of the large intestine. This leaves only about 0.1 liter of water to be excreted in feces. Water absorption occurs exclusively by osmosis. This requires both the cytosol of the enterocytes and the extracellular fluid in the villi to be more concentrated than the fluid in the lumen of the intestine to drive water movement. For this reason, anything that causes excess solutes to remain in the lumen of the intestines will hold onto water and decrease its reabsorption. This fact can be exploited in the use of drugs known as osmotic laxatives, which soften the feces. These drugs consist of solutes that either are not absorbed or are given in amounts in excess of what can be absorbed. As a result, water is retained in the feces, softening them and making them easier to eliminate.

porta hepatis

On the liver's posterior side we find an indentation called the porta hepatis. Numerous blood vessels enter and exit the liver at the porta hepatis, including the hepatic artery, which brings oxygen-rich blood to the liver; and the hepatic portal vein, which brings nutrient-rich, deoxygenated blood to the liver from multiple abdominal organs. In addition to blood vessels, we find nerves, lymphatic vessels, and the common hepatic duct entering and exiting the porta hepatis. Note that the hepatic veins, which drain blood from the sinusoids, empty into the inferior vena cava on the liver's superior surface.

cementum

On the outer portion of the root of the tooth, we find a different kind of mineralized bone like tissue known as _________________________ - composed of about half calcium hydroxyapatite crystals and half organic compounds such as collagen fibers. This is approximately the same composition that bone has, and for this reason cementum is about as hard as bone. The periodontal ligament extends collagen fibers into the cementum, which helps this substance "cement" the tooth in place.

Nutrient metabolism. The liver processes nutrients obtained from the diet. Carbohydrates and proteins absorbed from the alimentary canal are delivered to the liver by the hepatic portal vein, and lipids by the hepatic arteries. Within the liver, some of these nutrients are stored for later use, modified into another form, or used to synthesize other compounds. For example, some of the glucose taken in by the liver is stored in the form of glycogen, and many of the dietary amino acids are used to synthesize plasma proteins such as albumin and clotting proteins. We discuss the liver's role in multiple metabolic processes in the metabolism chapter (see ​Chapter 23​). Detoxification. The liver detoxifies substances produced by the body, such as the previously mentioned bilirubin. In addition, the liver processes substances that we eat or drink, some of which are toxins harmful to the body (such as alcohol). These substances are generally delivered to the liver first via the hepatic portal vein, where the liver converts them into less harmful materials that can be excreted in bile or in urine. The liver also metabolizes many drugs, such as antibiotics. Individuals with impaired liver function metabolize drugs more slowly than those with normal liver function, and the drugs remain in their systems much longer. Excretion. Recall that the liver directly excretes bilirubin in bile. Several other substances the liver processes are excreted in bile, particularly drugs such as certain antibiotics. The liver also modifies substances so that they can be excreted by the kidneys.

Other Functions of the Liver

dentin

Primary material found in teeth. It is covered by the enamel in the crown and a protective layer of cementum in the root. - inner layer of mineralized tissue in both the root and crown - composed of about 70% calcium hydroxyapatite crystals - second hardest material in the body. However, dentin also has some degree of elasticity, which is critical for preventing the overlying enamel from fracturing when chewing hard substances. Unlike enamel, dentin is formed throughout life by cells called odontoblasts that line its inner surface.

Acid Secretion from the Stomach

Recall that the gastric glands of the stomach secrete multiple exocrine products, including hydrochloric acid (HCl) released by parietal cells. The hydrogen ions are secreted by an ATP-consuming pump in the plasma membrane, called the H+/K+ pump, or proton pump. The H+/K+ pump drives a hydrogen ion out of the cell and into the lumen of the gland while taking a potassium ion into the cytosol. The chloride ion follows the hydrogen ion via passive diffusion through a membrane channel. Acid secretion occurs in this manner continuously throughout the day between meals, at what is known as the basal rate. During eating, gastric acid secretion changes from the basal rate under the influence of the parasympathetic nervous system and multiple hormones.

Stomach Muscularis Externa

Recall that the muscularis externa of the alimentary canal usually has two layers of smooth muscle: an inner circular layer and an outer longitudinal layer. in the stomach there is an additional inner layer of smooth muscle in the stomach's body with its fibers oriented obliquely. This oblique layer of smooth muscle allows the stomach to perform churning, a motion that pummels the food into a liquid called chyme.

- Salivary amylase. The first digestive enzyme that ingested food encounters is salivary amylase (AM-uh-layz). It catalyzes the beginning of carbohydrate digestion, breaking down large polysaccharides into smaller polysaccharides. - Lysozyme. The chemical lysozyme (LY-soh-zy'm) catalyzes the perforation of bacterial plasma membranes. This allows bacteria-killing substances in the saliva to enter and kill the bacteria. - Secretory IgA. Recall that the antibody immunoglobulin A, or IgA, is found in the body's secretions, including saliva (see Chapter 20). IgA binds specific antigens on pathogens and mediates their destruction. - Bicarbonate ions. During eating, when flow rates of saliva are high, basic bicarbonate ions (HCO3−) are added to saliva. Their primary function is to neutralize any acid from the stomach that regurgitates into the esophagus.

Saliva consists primarily of water; electrolytes such as sodium, chloride, and potassium ions; and variable amounts of mucus, depending on the type of salivary gland. It also contains the following components:

the cephalic phase, gastric phase, and intestinal phase

Secretion can be divided into three phases based on the primary source of regulation:

the peritoneal membrane, or peritoneum

The abdominopelvic cavity houses the largest serous membrane in the body

abdominal aorta, including the celiac trunk, superior mesenteric artery, inferior mesenteric artery, and branches from each of these arteries.

The arterial supply of the digestive organs consists of branches from the:

acinar cells

The basic secretory cell of the salivary glands

1. Voluntary phase: The tongue pushes the bolus posteriorly toward the oropharynx. During the voluntary phase, the tongue pushes the bolus superiorly against the hard palate and posteriorly toward the oropharynx. The voluntary phase is the only stage of swallowing under conscious control. 2. Pharyngeal phase: The bolus enters the oropharynx; the soft palate and epiglottis seal off the nasopharynx and larynx, respectively. The bolus passes from the pharynx to the esophagus during the pharyngeal phase. This phase involves the involuntary contraction of skeletal muscles such as the pharyngeal constrictor muscles, and is controlled by the swallowing reflex, a reflex initiated by the medulla oblongata. The reflex arc is initiated when the bolus contacts sensory receptors in the oropharynx. This triggers the elevation of the uvula and soft palate, which prevents the bolus from going into the nasopharynx. It also triggers the elevation of the larynx. The bolus pushes the epiglottis down. This, together with the elevation of the larynx, ​prevents ​aspiration, or the entry of food into the larynx. 3. Esophageal phase: Peristaltic waves move the bolus down the esophagus to the stomach. During the final phase of swallowing, the esophageal phase, the bolus passes through the esophagus to the stomach. This phase begins as the upper esophageal sphincter relaxes. Neurons of the enteric nervous system then stimulate the inner circular and outer longitudinal layers of the muscularis externa to undergo peristalsis and "massage" the bolus inferiorly toward the stomach. Notice that control of swallowing is almost entirely neural. The voluntary phase is under control of the cerebral cortex, and the remaining two phases are regulated by the medulla and the enteric nervous system. The ANS does not directly control any phase of swallowing, but it does influence the esophageal phase—the parasympathetic division stimulates peristalsis, and the sympathetic division inhibits it.

Swallowing consists of three phases: voluntary, pharyngeal, and esophageal.

The gallbladder, a small sac that sits on the posterior liver, receives most of the bile from the common hepatic duct. The gallbladder stores bile, concentrates it (removing water), and releases it when stimulated. Bile release is stimulated by the hormone CCK, which triggers contraction of the smooth muscle in the wall of the gallbladder. This causes the gallbladder to release bile into the cystic duct. the cystic duct joins the common hepatic duct to form the common bile duct. The common bile duct joins the main pancreatic duct near the duodenum to form the hepatopancreatic ampulla. The ampulla is surrounded by a ring of smooth muscle called the hepatopancreatic sphincter, which controls the emptying of bile and pancreatic fluids into the duodenum. Recall that the contents of the hepatopancreatic ampulla then empty into the duodenum at the major duodenal papilla. Bile contains calcium salts and cholesterol, and both can precipitate and form hard lumps called gallstones.Gallstones are generally asymptomatic, but occasionally they become lodged in the cystic duct or the common bile duct and block the outflow of bile. This results in abdominal pain, particularly when ​eating​ a fatty meal, and the presence of undigested fats in the feces. The feces also take on a clay color due to the lack of bilirubin excretion. Painful gallstones are generally treated by surgically removing the gallbladder, a procedure known as a cholecystectomy. The absence of a gallbladder may temporarily reduce the ability of the small intestine to digest fats. However, over time the common hepatic duct enlarges and takes over some of the gallbladder's functions.

The Gallbladder and Its Relationship to the Liver

stomach

The J-shaped organ in the abdominopelvic cavity that is responsible for chemical and mechanical digestion and propulsion of ingested food. - sits primarily in the left upper quadrant just inferior to the diaphragm muscle. The stomach's convex left side is known as its greater curvature, and its concave right side is its lesser curvature.

Histology of the Liver

The basic unit of the liver is the liver lobule. Liver lobules are separated from one another by septa that branch in from the connective tissue capsule of the liver. A liver lobule is composed of flattened plates of cells, called hepatocytes, arranged in the shape of a hexagon and stacked on each other. In the center of the lobule is a small central vein. At each of the six corners of the lobule, we find three structures collectively referred to as a portal triad: (1) a branch of the hepatic artery called a hepatic arteriole, (2) a branch of the portal vein called a portal venule, and (3) a small bile duct that carries bile.

motility

The capability of the GI tract to move material along its length is called - In the oral cavity, the pharynx, the superior portion of the esophagus, and the last portion of the large intestine, motility is due to skeletal muscle. In the rest of the alimentary canal, motility is the work of smooth muscle. Motility takes several forms, including swallowing, churning, peristalsis, and defecation. -Each type of motility is regulated by the nervous system and/or the endocrine system. Nervous system regulation of motility is accomplished by the nerves of the ANS. The sympathetic and parasympathetic branches of the ANS generally have opposite effects on gastrointestinal motility—sympathetic activity inhibits digestive processes, and parasympathetic activity stimulates them. - In addition, motility is regulated by the ENS. The functions stimulated by the ENS are often called short reflexes because the reflex pathways are confined to local neurons. In contrast, functions stimulated by the ANS are known as long reflexes because they must travel outside the local digestive neurons to the CNS to function.

Splanchnic circulation

The collection of blood vessels that supplies and drains the digestive organs in the abdominopelvic cavity.

Pancreatic Juice

The collective secretions of the pancreatic acinar and duct cells - consists of water and multiple digestive enzymes and other proteins. In addition, the duct cells secrete bicarbonate ions, a base, which make pancreatic juice alkaline. This helps to neutralize the acidic chyme that enters the duodenum from the stomach and protects the duodenum from damage by the acid. The digestive enzymes, secreted by acinar cells, are crucial in chemical digestion and catalyze the reactions that digest carbohydrates, proteins, lipids, and nucleic acids.

the organs of the alimentary canal, also known as the gastrointestinal (GI) tract​, or digestive tract; and the accessory organs

The digestive system consists of two types of organs:

- Ingestion. Food and water are brought into the digestive system by​ ingestion,​ which occurs via the mouth under normal conditions. - Secretion. Digestive organs contain both endocrine and exocrine glands that secrete a variety of substances—such as mucus, enzymes, acid, and hormones—to aid other digestive processes. - Propulsion. Ingested food and liquids pass from one digestive organ to the next by the process of propulsion. Propulsion is accomplished largely by rhythmic contractions of the smooth muscle of the alimentary canal called peristalsis and is aided by mucus secreted by multiple organs. - Digestion. Food breakdown occurs by the process of digestion. There are two kinds of digestion. The first is mechanical digestion, in which digestive organs physically break food down into smaller pieces via processes such as chewing and mixing food by movements by the muscles of the alimentary canal. In the second, chemical digestion, enzymes secreted by digestive organs catalyze reactions that break the chemical bonds within food particles until only small compounds remain. - Absorption. Once food particles are mechanically and chemically digested, nutrients move through the wall of the alimentary canal into blood or lymphatic vessels by a process called absorption. Water, electrolytes, and vitamins are also absorbed into the blood in the same manner. - Defecation. Certain ingested materials are not digestible or usable by the body. Such materials continue their transit through the alimentary canal until they exit the body as feces through defecation. Defecation also provides the body with a way to eliminate certain metabolic wastes. Note that defecation is simply a specialized form of propulsion.

The digestive system must perform the following six basic processes to carry out these functions:

Duodenum

The initial 25-cm-long segment of the small intestine into which bile and pancreatic juice are secreted. - begins at the pylorus - shortest of the three divisions, it is only about 25 cm (10 in.) long. - arches into a "C" shape as it curves around the pancreas. Only the proximal portion of the duodenum is within the peritoneum; the remainder sits posterior to the peritoneal cavity and so is retroperitoneal. Internally, the duodenum houses the major duodenal papilla, which is where secretions from the gallbladder and pancreas enter the small intestine. The duodenal submucosa contains specialized glands called duodenal (Brunner's) glands, which produce an alkaline mucus to protect the duodenum from the acidic chyme.

pulp

The final component of a tooth is the inner pulp, which is composed of loose connective tissue and housed within the central pulp cavity. Pulp contains blood vessels and nerves that supply the other tissues of the tooth with nutrients and innervation. The pulp cavity extends into the root via the thin root canal. Occasionally, the pulp becomes infected, which results in inflammation and generally a great deal of pain. When the infection risks the health of the tooth, a root canal procedure may be performed to remove the pulp from the pulp cavity and root canal, and fill the newly hollow space with an inert material.

Emptying Function

The final function of gastric motility is to control the movement of chyme into the duodenum. Different materials pass through the pyloric sphincter at different rates. Liquids such as water and saline move rapidly from the stomach to the duodenum with essentially no delay. Solids must be converted to a nearly liquid state before they are able to enter the small intestine. This occurs relatively easily with carbohydrates and proteins, which mix with gastric juice with few problems. It may take much longer with lipids, because they require extensive churning due to their nonpolar structure. A key factor that determines the rate of gastric emptying is the amount and composition of chyme in the duodenum. When sensory receptors in the wall of the duodenum detect a high degree of stretch, a low pH, a high lipid composition, and/or a high solute concentration in the chyme, they trigger a negative feedback loop that delays gastric emptying. This feedback loop is mediated by both the vagus nerve and hormones secreted by the duodenum, including secretin, cholecystokinin, and gastric inhibitory peptide. Control of gastric emptying is critical because the duodenum must mix the incoming chyme thoroughly before it moves to the rest of the small intestine. There are two reasons for this: (1) Chyme is acidic, and the duodenum must mix it with bicarbonate ions to avoid damaging the intestinal mucosa; and (2) chyme is generally very concentrated and must be diluted with water from pancreatic juice to prevent the chyme from drawing water into the intestinal lumen by osmosis. The duodenum can process incoming ​chyme ​only so quickly, and therefore must receive small amounts at a time.

Intestinal Phase

The final phase of gastric acid secretion, the intestinal phase, is responsible for the remaining approximately 10% of acid secretion, after which further acid secretion is inhibited. The intestinal phase is triggered by the presence of partially digested proteins in the fluid entering the duodenum. As in the stomach, these partially digested proteins trigger duodenal DNES cells to release intestinal gastrin. This hormone has the same effect as gastrin produced by the stomach, and stimulates hydrogen ion secretion from parietal cells. The stimulatory effect of the intestinal phase is brief. As chyme enters the duodenum, the declining pH and presence of lipids trigger the enterogastric reflex, which decreases vagal activity and acid secretion. The low pH in the duodenum also triggers the production of hormones by the cells of the duodenal mucosa, including secretin and gastric inhibitory peptide (GIP). Both hormones reduce acid secretion.

Gastric Phase

The gastric phase begins when food enters the stomach, and continues the stimulation provided during the cephalic phase. There are two stimuli that trigger acid secretion during the gastric phase. The first is the simple presence of food in the stomach. Distention of the stomach wall stimulates neurons of the ENS and sensory receptors involved with vagus nerve reflexes. Both the vagus nerve and ENS neurons release ACh, which has the same four effects we discussed with the cephalic phase. The net effect is to increase acid secretion. The second stimulus is the presence of partially digested proteins in gastric juice. These protein fragments stimulate G cells to produce and release gastrin, which in turn stimulates acid secretion. The acid activates pepsin, which catalyzes protein digestion. Digested proteins stimulate more gastrin release, which triggers further acid secretion and more protein digestion.Other stimuli trigger gastrin secretion as well, including caffeine and alcohol. Together, gastrin and the neural stimulation of parietal cells during the gastric phase account for about 50-60% of total acid secretion.

Ileum

The ileum, the small intestine's final segment, is also intraperitoneal. About 3.6 meters (10.8 feet) in length, it terminates at the portion of the large intestine called the cecum. A sphincter known as the ileocecal valve controls the movement of materials from the ileum into the cecum. This sphincter also prevents materials in the large intestine from flowing backward into the ileum. This function is key because, as we discuss shortly, the large intestine houses a great number of bacteria that could cause illness if they entered the ileum.

Cephalic Phase

The initial cephalic phase is mediated by the sight, smell, taste, or even thought of food. This phase, which is directed by the CNS, prepares the stomach to receive food by increasing the release of hydrogen ions into it. These stimuli trigger output from the vagus nerve, the main nerve of the parasympathetic nervous system, resulting in four physiological effects: - Direct stimulation of hydrogen ion release. The vagus nerves release ACh onto parietal cells, which directly stimulates them to release hydrogen ions. - Stimulation of gastrin secretion. Vagal nerve stimulation triggers the release of a peptide that stimulates G cells to release the hormone gastrin. Gastrin triggers hydrogen ion secretion. - Stimulation of histamine secretion. The vagus nerve stimulates a specific type of DNES cell to release the hormone histamine. Like gastrin, histamine triggers hydrogen ion secretion. - Inhibition of somatostatin secretion. The hormone somatostatin​ is produced by a type of DNES cell located in the antrum of the stomach. Somatostatin inhibits acid secretion. Vagal nerve stimulation inhibits somatostatin release, which has the effect of increasing hydrogen ion secretion. Together, the four changes during the cephalic phase are responsible for about 30-40% of total hydrogen ion secretion from parietal cells.

Mucosa

The innermost mucosa consists of three components: First is a layer of epithelium facing the lumen, followed by a thin layer of loose connective tissue called the lamina propria, and finally two thin layers of smooth muscle together known as the muscularis mucosae. The epithelium from the stomach to the end of the large intestine is simple columnar epithelium with copious mucus-secreting cells such as goblet cells. The mucus coats the epithelium, protecting it and the underlying tissues from ingested food and chemicals secreted by digestive organs. The mucosa also houses regenerative epithelial cells that have a high rate of mitosis. These cells allow the mucosa to replace epithelial cells as they are damaged or sloughed off in the alimentary canal. The lamina propria houses blood and lymphatic vessels, glands, and mucosa-associated lymphatic tissue. The two layers of the muscularis mucosae are arranged in different directions—the inner layer is circular and the outer layer is longitudinal. As discussed later, this arrangement allows propulsion as the two layers contract alternately.

The largest type of folds, which are visible with the naked eye as ridges in the wall, are called circular folds (or plicae circulares). Circular folds involve both the mucosa and submucosa of the small intestine. These folds not only increase surface area but also slow down the transit of chyme through the small intestine, which gives the nutrients more time to be digested, and the small intestine cells, called enterocytes, more time to absorb nutrients. The smaller two types of folds are not visible to the naked eye. The mucosa folds into projections called villi. Notice that each villus consists of a layer of enterocytes and occasional mucus-secreting goblet cells surrounding a central core of blood capillaries and a lymphatic vessel called a lacteal. Between villi, the mucosa indents to form intestinal crypts, which house glands that also contain hormone-secreting DNES cells. The smallest folds, the microvilli​, are found in the plasma membrane of the enterocytes. Each enterocyte has as many as 3000 microvilli, which gives the cell the appearance of a bristle brush, or brush border, on microscopic examination. Associated with the brush border are numerous digestive enzymes produced and secreted by enterocytes, such as sucrase, maltase, and lactase, which catalyze reactions that break down disaccharides, and peptidases, which catalyze reactions that break down peptides.

The internal surface of the small intestine contains three progressively smaller types of folds.

Jejunum

The jejunum, the middle portion of the small intestine, measures about 2.5 meters (7.5 feet) in length. It begins at the duodenojejunal flexure and sits within the peritoneal cavity. It is the most active site for chemical digestion and absorption.

proximal and distal. The proximal large intestine consists of the ascending and transverse colon, and the distal large intestine consists of the descending and sigmoid colon, rectum, and anal canal.

The large intestine has two functional segments

the cecum, the colon, and the rectum

The large intestine is made up of three segments:

liver

The large organ in the right upper quadrant of the abdominopelvic cavity; functions include diverse metabolic activities, filtering blood from most abdominal organs, and bile production.

oral cavity structure

The lateral walls of the oral cavity are formed by the cheeks, which are composed largely of the buccinator muscles and lined internally by stratified squamous nonkeratinized epithelium. The cheeks terminate anteriorly as the lips, which are formed by the orbicularis oris muscle and covered with stratified squamous keratinized epithelium. The integument covers most of the lips, but the portion near the mouth, called the red margin, contains less keratin than the surrounding skin. As a result, the red margin is fairly translucent and the blood in the vessels of the dermis is more visible, giving the lips a slight reddish tint. The mouth's inferior wall is composed of muscles of the tongue and muscles that attach to the hyoid bone. Posterior to the lips and cheeks we find the gums, or gingivae. The gums are covered with stratified squamous nonkeratinized epithelium overlying connective tissue and the maxilla and mandible, where the teeth are housed. A narrow band of mucosa called the labial frenulum attaches the internal surfaces of the upper and lower lips to the gums on the midline. The narrow space between the teeth and gums and the internal surfaces of the lips and cheeks is the vestibule. The space posterior to the teeth and gums is the oral cavity proper. The superior boundary, or "roof," of the mouth is the palate (PAL-it), which consists of two portions: the anterior two-thirds is the hard palate, and the posterior one-third is the soft palate. The hard palate consists of stratified squamous epithelium and connective tissue covering the palatine processes of the maxillary bones and the palatine bones. The surface of the hard palate is slightly rough, which assists in mechanical digestion. The arch-shaped soft palate consists of stratified squamous epithelium overlying skeletal muscle. Extending inferiorly from the soft palate is a projection called the uvula (YOO-vyoo-luh). When we swallow, the soft palate and uvula move posteriorly to prevent food from entering the nasal cavity.

Anatomy of the Liver

The liver is wrapped in a thin connective tissue capsule, and most of it is covered by the visceral peritoneum. It is composed of four lobes: the large right lobe and left lobe —located on the right and left sides of the liver, respectively—and the smaller caudate lobe and quadrate lobe, located on the posterior side of the liver's right lobe. The right and left lobes are separated by a fold of visceral peritoneum called the falciform ligament, which also anchors the liver to the anterior abdominal wall. On its inferior surface we find the round ligament, a remnant of the umbilical vein that was present in the fetus.

Bile Secretion

The liver produces bile continually but generally does not secrete it at a basal rate, the way pancreatic and gastric juices are released. Instead, bile secretion does not occur until the gallbladder contracts and the sphincter at the hepatopancreatic ampulla relaxes. This activity is mostly accomplished by cholecystokinin (CCK) and to a small extent by the vagus nerve. Other factors influence bile secretion, including the hormone secretin, which stimulates bile production and release by hepatocytes. However, the most potent stimulus for bile production and release is bile itself, specifically bile salts. Recall from our earlier discussion of the liver that bile salts are reabsorbed in the last section of the ileum and transported back to the liver through the hepatic portal vein. As bile salts re-enter the liver, bile secretion rises dramatically in a positive feedback loop. Bile secretion continues into the duodenum until the duodenum empties, at which point CCK and secretin levels decline.

mesocolon

The mesentery attached to much of the large intestine

Histology of the Large Intestine

The mucosa of the large intestine lacks villi and its cells lack microvilli. These structural adaptations reflect the fact that nutrient absorption is not the large intestine's primary function. Like much of the alimentary canal, its mucosa is rich with goblet cells that secrete protective and lubricating mucus. The muscularis externa of the large intestine is unique in that its longitudinal layer is not continuous throughout most of its length. Instead, this layer is gathered into three bands or ribbons of muscle called taeniae coli. Their constant tension bunches the colon into pockets referred to as haustra. The serosa, or visceral peritoneum, of the large intestine contains fat-filled pouches known as epiploic appendages.

Stomach Mucosa: Gastric Glands

The mucosa of the stomach is heavily indented to form deep structures called gastric pits. Between the gastric pits we find columnar epithelial cells that secrete a thick mucus layer that lines and protects the cells of the stomach from its own secretions. Conditions that decrease the amount of mucus secreted by these cells can lead to a gastric ulcer (also called a peptic ulcer), in which acid eats away at the mucosa and exposes the underlying tissues. Ulcers are often associated with the presence of certain bacteria and/or excessive production of acid.

Muscularis externa

The muscularis externa is a thick muscular layer composed of smooth muscle in most of the alimentary canal. We generally find two layers of smooth muscle that are arranged in the same manner as in the muscularis mucosae, with inner circular and outer longitudinal layers. The motility of the muscularis externa is regulated by groups of nerves of the enteric nervous system, called the myenteric plexus.

Peritoneal cavity

The narrow space between the visceral and parietal peritoneal membranes that is filled with serous fluid. This fluid lubricates organs as they slide past one another.

the celiac plexus, the superior mesenteric plexus, and the inferior mesenteric plexus.

The nerves of the sympathetic and parasympathetic divisions that serve the abdominal digestive organs are located in three main clusters:

Digestion and Absorption of Nucleic Acids

The nucleic acids in the food we eat begin chemical digestion in the small intestine with the help of pancreatic enzymes called nucleases. These enzymes catalyze the reactions that break nucleic acids into individual nucleotides. Further digestion occurs via brush border enzymes, which remove the phosphate group and the sugar from the nucleotide to leave a phosphate ion, ribose or deoxyribose, and a nitrogenous base. These three substances are absorbed via primary and secondary active transport mechanisms into capillaries in the villi. The hepatic portal system then takes them via the blood to the liver to be metabolized.

Histology of the Alimentary Canal

The organs of the alimentary canal follow the same general tissue pattern of other hollow organs we have studied: concentric layers of tissue surround a space called the lumen. Like other hollow organs, those of the alimentary canal contain an inner epithelium, a layer of connective tissue, a layer of smooth muscle, and an outer layer of connective tissue. Most regions of the alimentary canal have four named tissue layers: mucosa, submucosa, muscularis externa, and either the serosa or adventitia.

Serosa or adventitia.

The outer connective tissue layer is the serosa in the organs within the peritoneal cavity and the adventitia in organs outside the cavity. The serosa, also called the visceral peritoneum, is composed of simple squamous epithelial tissue and loose connective tissue, whereas the adventitia is composed of dense irregular connective tissue. Both structures support digestive organs and anchor them to surrounding structures.

Digestion and Absorption of Lipids

The process of lipid digestion is more complex than either carbohydrate or protein digestion because lipids are nonpolar molecules. Their nonpolar nature causes lipids to stick to one another instead of interacting with water, and as a result they form large globules in a water-based solution. If this happened in the alimentary canal, digestive enzymes would have very little surface area on which to work to catalyze reactions that break the bonds in the lipids. For this reason, the first part of lipid digestion must involve physically breaking up large lipid globules into smaller globules by mechanical digestion to give digestive enzymes more surface area on which to work. This is accomplished first by physical processes such as mastication in the mouth, churning in the stomach, and segmentation in the small intestine. However, even after these processes, the lipid globules would still be too large for efficient chemical digestion to take place. To mechanically break lipids into even smaller globules, we need the help of bile salts. When bile salts mix with lipids, their nonpolar parts interact with the lipids, while their polar parts interact with the surrounding watery fluid. This physically breaks up the lipid globules into smaller pieces by the process of emulsification. The end result is multiple tiny lipid droplets each coated with bile salts, a mixture called an emulsion (ee-MUL-shun). This gives digestive enzymes enough surface area on which to work to efficiently digest the lipids.

❶❶ Lipids are broken apart by stomach churning and broken down in reactions catalyzed by gastric lipase. Lipid digestion begins in the stomach with the help of the enzyme gastric lipase. Gastric lipase catalyzes the reactions that remove one fatty acid from triglycerides, leaving some free fatty acids and diglycerides. About 15% of total dietary fats are digested this way. ❷❷ Lipids enter the small intestine and are emulsified by bile salts. The undigested and partially digested triglycerides enter the small intestine, where they interact with bile salts. Bile salts coat the lipids and physically break them apart into smaller pieces. This is purely a mechanical process—no bonds are broken during emulsification. ❸❸ Pancreatic lipase catalyzes reactions that digest the lipids into free fatty acids and monoglycerides. The pancreas releases an enzyme, pancreatic lipase, that catalyzes lipid breakdown. In this process, triglycerides are digested into monoglycerides and free fatty acids. ❹❹ Bile salts remain associated with the digested lipids to form micelles. After chemical digestion by lipase is complete, the bile salts and digested lipids stay together in structures known as micelles (my-SELZ). If the digested lipids did not remain associated with bile salts, they would re-form into large globules and absorption would not be possible.

The process of lipid digestion proceeds as follows in these steps

Pharynx

The region of the respiratory tract that is located posterior to the nasal cavity, oral cavity, and larynx. It consists of three divisions: the nasopharynx, oropharynx, and laryngopharynx. Of these three divisions, only the oropharynx and laryngopharynx are part of the alimentary canal. Like the oral cavity, both are lined with stratified squamous epithelium to protect them from abrasion by food. The oropharynx houses two sets of tonsils—the palatine tonsils and the lingual tonsils. The tonsils perform defensive functions and help protect the remainder of the alimentary canal from any pathogens that enter the body via the oral and nasal cavities. The primary function of the pharynx is propulsion in the form of swallowing, during which the bolus passes through the pharynx and into the esophagus. Recall that the pharynx is surrounded by three pairs of skeletal muscles: the upper, middle, and lower pharyngeal constrictor muscles (see Chapter 9). These muscles contract sequentially during swallowing and propel the bolus inferiorly.

the duodenum, jejunum, and ileum

The small intestine consists of three divisions

peristalsis and segmentation As with other regions of the alimentary canal, peristalsis in the small intestine is accomplished by alternating contractions of the longitudinal and circular layers of smooth muscle in the muscularis externa. Its primary function is to propel chyme toward the ileum and ultimately through the ileocecal valve to the cecum. However, segmentation, also known as intestinal churning, involves contractions of only the circular layer of smooth muscle, which produces a squeezing motion. The ​primary ​functions of segmentation are mechanical digestion and mixing the chyme with intestinal and pancreatic enzymes as well as bile. The vagus nerve appears to regulate both peristalsis and segmentation.

The small intestine undergoes two types of movement during eating:

gallbladder

The small, hollow organ on the posterior side of the liver that stores and releases bile.

secretion, propulsion, and digestion

The stomach performs three primary functions

(1) receive food from the esophagus, (2) churn the incoming bolus into chyme, and (3) control the rate at which chyme empties into the small intestine.

The stomach's motility enables it to perform three actions:

Submucosa

The submucosa is composed of dense irregular connective tissue with blood and lymphatic vessels and submucosal glands. Here we find nerve clusters of the enteric nervous system, referred to as the submucosal plexus. Each plexus regulates secretion from and blood flow to its area of the alimentary canal.

The primary functions of the esophagus

are propulsion and a small amount of secretion of mostly mucus. During swallowing, the skeletal muscle and smooth muscle of the muscularis undergo peristalsis, which pushes the bolus inferiorly. Although the esophagus' thick epithelium prevents any significant absorption from taking place, it does protect the esophagus from abrasion by food.

Which of the following is a source of dietary fiber and promotes timely movement through the colon?

cellulose

plexus

cluster of nerves

normal flora or gut flora

consist of about 500 different bacterial species that coexist with humans in a symbiotic (mutually beneficial) relationship. Humans provide the bacteria with the environment they need to survive, and the bacteria perform a number of useful functions for humans, including the following: - Produce vitamins. Bacteria produce vitamins such as vitamin K, which is necessary for blood clotting. - Metabolize undigested materials. Bacteria metabolize carbohydrates such as soluble fibers that the small intestine is unable to digest, converting them into fatty acids and other compounds the body can absorb and use. This also aids in the absorption of certain vitamins and electrolytes. A somewhat unfortunate byproduct of this metabolism is the production of gas within the intestine that is released as flatus. - Deter the growth of harmful bacteria. The normal flora prevent the growth of pathogenic, or disease-causing, microorganisms by competing for nutrients and producing chemicals that kill certain harmful bacterial species. - Stimulate the immune system. During infancy, the normal flora induce immune tolerance to their own antigens. At the same time, they stimulate the development of mucosa-associated lymphatic tissue (MALT) and the production of antibodies to pathogens. This creates a favorable environment for the normal flora while also protecting the host from pathogenic bacteria.

extrinsic muscles

control the position of the tongue. involved during the ingestion phase of digestion, move the tongue during chewing and help turn the food into a bolus

intrinsic muscles

control the shape and size of the tongue. Intrinsic muscles push the food against the hard palate during chewing, which assists in mechanical digestion, and also push the bolus posteriorly during swallowing.

cecum

first part of the large intestine; a blind pouch that is intraperitoneal and located in the right lower quadrant of the abdomen. - features a smaller blind-ended pouch extended from its posteroinferior end, called the Vermiform appendix, which is generally shortened to simply appendix

In the small intestine, which of the following enzymes breaks down oligosaccharides?

glucoamylase

appendix

houses multiple lymphatic nodules and plays a role in the immune system

colon

longest portion of the large intestine; divided into four portions: - Ascending colon. The retroperitoneal ascending colon travels superiorly along the right side of the abdomen from the right lower quadrant to the right upper quadrant. When it reaches the liver, it makes a sharp left-hand turn at a junction called the hepatic flexure, also known as the right colic flexure. - Transverse colon. At the hepatic flexure, the ascending colon becomes the intraperitoneal transverse colon, so named because it passes transversely across the superior abdominal cavity. At the spleen, it takes a sharp turn inferiorly at a junction called the splenic flexure, also known as the left colic flexure. - Descending colon. The splenic flexure gives rise to the retroperitoneal descending colon, which passes along the left side of the abdominal cavity. - Sigmoid colon. In the left lower quadrant, the descending colon becomes the S-shaped sigmoid colon (sigmoid = "S-shaped"), which is intraperitoneal and passes toward the sacrum.

tongue

manipulates food for chewing and swallowing; a taste organ - consists of skeletal muscle covered with a layer of stratified squamous epithelium

accessory organs

not part of the alimentary canal but assist in digestion in some way. They are located around the alimentary canal and include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.

Acinar cells are modified simple cuboidal epithelial cells. They are found in clusters known as acini. Each acinus surrounds a small duct into which its cells secrete their products. The duct cells also secrete products, discussed shortly. Most of these small ducts merge and drain into the main pancreatic duct, although some secrete into the smaller accessory pancreatic duct.

note

hydrolysis reactions

reactions that use a water molecule to break a chemical bond This is one reason why so much water is secreted with fluids like gastric and pancreatic juices—the water molecules are key components of the reactions that chemically break down food. These hydrolysis reactions would occur without digestive enzymes, but they would occur far too slowly and we would be unable to extract nutrients from the food we eat. Our digestive enzymes speed up the reactions, a process known as enzymatic hydrolysis.

Anything that increases or decreases the motility of the large intestine affects the amount of water present in feces. When motility increases, the large intestine does not have enough time to absorb water from fecal material. This produces watery feces, a condition known as diarrhea (dy-ah-REE-ah). Factors that may increase motility include irritation of the colon due to bacterial or viral infections or drugs that stimulate the parasympathetic nervous system. Conversely, when motility decreases, the large intestine absorbs too much water and the fecal material becomes hard, a condition called constipation. The large intestine's motility may be slowed by drugs such as opiate narcotics or those that block the effects of acetylcholine (ACh).

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GIP (from cells in the duodenum and proximal jejunum) stimulates secretion of insulin by the pancreas.

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Once nutrients are digested to their component monomers, they must enter the body. Remember that the mucosa of the alimentary canal is actually an external membrane, so even though it might seem that the food is "inside" the body, it technically is not until it is absorbed. Nutrients and other substances must be absorbed across the epithelial lining of the alimentary canal and enter the bloodstream before they can be delivered to body cells.

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Saliva performs several functions in the oral cavity, including moistening, lubricating, and cleansing the oral mucosa. In addition, its lysozyme and IgA deter the growth of pathogenic bacteria in the oral cavity. Saliva also functions in (1) mechanical digestion, by moistening and helping to mix ingested food into a bolus so it can be swallowed, and (2) chemical digestion, by the actions of salivary amylase. Finally, many food polymers dissolve in the water of saliva, and the resulting monomers then stimulate taste receptors on the tongue.

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The hepatic arteriole and portal venule both drain into large, leaky capillaries, the hepatic sinusoids, that pass between rows of hepatocytes. Sinusoids have walls that are not continuous, which allows large substances to enter and exit. Blood flows slowly through the sinusoids as materials are exchanged between the blood and hepatocytes, eventually draining into the central vein. The central veins merge and drain into the hepatic veins, which in turn feed into the inferior vena cava. Notice that bile flows through the liver lobule in the opposite direction, from the hepatocytes into tiny ducts called bile canaliculi, which eventually drain into a bile duct.

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The stomach has the same four tissue layers as the rest of the alimentary canal, with a mucosa, submucosa, muscularis externa, and serosa. However, the stomach's muscularis externa and mucosa are modified to better suit its functions.

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The teeth are key organs of mechanical digestion. They are located in their bony sockets, called alveoli, within the mandible and maxilla and are held in place by bands of connective tissue known collectively as the periodontal ligament (pehr′-ee-oh-DAHN-tuhl; peri- = "around," odont- = "tooth"). With the assistance of the masseter and temporalis muscles, the teeth masticate, or chew, ingested food, grinding it into smaller pieces. Mastication aids digestion by increasing the overall surface area of the food, giving digestive enzymes more places to catalyze the reactions of chemical digestion.

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Unlike proteins and carbohydrates, lipids are not delivered directly to the liver via the hepatic portal vein after absorption. However, the hepatic portal vein delivers the leftover bile salts to the liver, where they are used to make new bile. The lipids travel within chylomicrons through the lymphatic vessels and eventually to the thoracic duct, where they join the blood with the rest of the lymph as the thoracic duct meets the junction of the left internal jugular and left subclavian veins. As chylomicrons travel through blood capillaries, lipids are progressively removed and enter cells. Lipids removed from chylomicrons in the liver are processed by a number of metabolic processes. Chylomicrons that have been depleted of all their lipids are taken in by hepatocytes and dismantled.

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chief cells produce pepsinogen, the inactive form of pepsin.

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secretin

which is released by duodenal cells in response to acid and lipids in the duodenum. Secretin primarily triggers duct cells to secrete bicarbonate ions.

Salivation

primarily controlled by the parasympathetic nervous system in a reflex arc. The arc begins with sensory stimuli such as the smell or taste of food, which communicate this information to the salivatory nucleus in the brainstem. Parasympathetic fibers from the brainstem exit via the facial nerve (cranial nerve VII) to innervate the submandibular and sublingual glands, and via the glossopharyngeal nerve (cranial nerve IX) to innervate the parotid glands. Neurons from these nerves release acetylcholine (ACh) onto the acinar cells, which triggers the acinar cells to secrete saliva. The importance of ACh in salivation is evident any time ACh receptors are blocked. A number of medications such as antihistamines for allergies have anticholinergic properties that block the effects of ACh around the body. Anticholinergic drugs bind to the ACh receptors on the salivary glands and decrease saliva secretion, leading to the common side effect of a dry mouth. The sympathetic nervous system also innervates the salivary glands. Interestingly, this is one of the only instances in the body in which the sympathetic and parasympathetic divisions have similar effects on an organ. Sympathetic nerves innervate the salivary glands and their ducts, triggering increased saliva production and facilitating saliva transport through ducts. The primary difference is that sympathetic nerves stimulate mostly mucous cells, whereas parasympathetic nerves stimulate mostly serous cells.

intraperitoneal

within the peritoneal cavity

enterocytes

specialized absorptive cells in the villi of the small intestine - produce multiple digestive enzymes, hormones, and mucus. These enzymes, along with those released by the pancreas, are responsible for the bulk of chemical digestion

lumen

the hollow inside portion of an organ

visceral peritoneum (serosa)

the inner layer of the peritoneum that surrounds the organs of the abdominal cavity

anal canal

the last portion of the large intestine where the rectum Recall from the muscular system chapter that the anal canal passes through the levator ani muscle in the floor of the pelvic cavity. The walls of the anal canal feature longitudinal grooves called anal columns. Between the anal columns are anal sinuses, which secrete mucus when feces pass through the anal canal during defecation. The terminal portion of the anal canal has two sphincters. The first is the involuntary internal anal sphincter, which is simply the thickened circular layer of the muscularis externa. The internal anal sphincter is supplied by parasympathetic motor neurons. The second is the voluntary external anal sphincter, which is composed of skeletal muscle. As this sphincter is voluntary, it is innervated by somatic motor neurons controlled by the cerebral cortex.

- Parotid glands. The parotid glands are large glands located over the masseter muscle just anterior to the ear. These glands secrete saliva through the parotid duct, which passes over the masseter muscle and pierces the buccinator muscle to open into the oral cavity near the second molar. The parotid glands secrete 25-30% of total saliva. - Submandibular glands. The smaller submandibular glands are located just medial to the inferior portion of the body of the mandible. They secrete saliva through the submandibular duct, which empties into the floor of the oral cavity. In spite of their smaller size, the submandibular glands are very active, and secrete 65-70% of total saliva. - Sublingual glands. As implied by their name, the Sublingual glands (sub-LING-gwuhl) are situated inferior to the tongue. The sublingual glands secrete saliva through several small sublingual ducts that empty into the oral cavity just under the tongue. These are the smallest salivary glands and secrete only about 5% of total saliva.

three pairs of salivary glands:

- Incisors. The incisors are the central teeth that are broad and flat with a narrow crown. They are specialized for cutting off pieces of food. The middle two incisors are the central incisors, and those to either side are the lateral incisors. - Canines. The canines (KAY-nynz), also known as cuspids, are on either side of the incisors. Their pointed crowns are specialized for ripping and tearing. - Molars. The teeth posterior and lateral to the canines are the premolars and the molars. Both types of molars have broad crowns with rounded projections called cusps that are specialized for grinding.

three types of teeth, which are classified according to their shape

the greater omentum and lesser omentum

two mesenteries

gastric glands

unusual in that they contain both endocrine cells that secrete hormones into the bloodstream and exocrine cells that secrete an acidic, enzyme-containing fluid called gastric juice into the lumen of the stomach - at base of gastric pits

oral cavity, or mouth

where the digestive system begins - is the area posterior to the teeth and bounded by the palate and tongue - Four digestive processes take place here: ingestion, secretion, chemical and mechanical digestion, and propulsion - although the oral cavity is technically part of the alimentary canal, it houses two accessory organs: the teeth and the tongue. In addition, three pairs of accessory organs, the salivary glands, are located in and around the oral cavity.

esophageal hiatus

where the esophagus passes through the diaphragm

secondary dentition is the set of permanent teeth

which are situated above the primary dentition in the maxilla and below it in the mandible When the child is about 6 years of age, these teeth enlarge and begin to press on the deciduous teeth. This causes the root to gradually dissolve and the deciduous tooth falls out of the bone. The permanent tooth then erupts and takes its place. There are 32 permanent teeth, with 4 incisors, 2 canines, 4 premolars, and 6 molars in both the mandible and maxilla. Generally, by age 12 all the deciduous teeth have fallen out and all but the third set of secondary molars have erupted. The third set of molars, known as the wisdom teeth, erupt somewhat later, between ages 17 and 21. Sometimes wisdom teeth remain embedded in the bone, a condition called impaction. Any tooth has the potential to become impacted, but wisdom teeth become impacted more often than any other teeth because of their position at the back of the jaw. Impacted teeth can be removed surgically.