Chapter 23 - The Digestive System

What are the types of cells lining the small intestine?

- Absorptive cells - Goblet Cells - Enteroendocrine cells - Paneth cell

Chylomicron transport

- Chylomicrons are exocytosed from basolateral side and enter lymphatic lacteal. - Chylomicrons are eventually emptied into venous blood at thoracic duct

Describe how gastrin controls digestion

- Enterendocrine G cells secrete gastrin when the stomach is distended. - Partially digested proteins and caffeine are in the stomach, or if the pH of the stomach chyme is high. - Gastrin promotes secretion of gastric juice, increases gastric motility and promotes growth of gastric mucosa

Describe how secretin controls digestion

- Enterendocrine S cells in the mucosa of duodenum secrete secretin when acidic chyme enters the small intestine. - Secretin stimulates secretion of pancreatic juice and bile that are rich in bicarbonate ions

Absorption in the Small Intestine

- Fats are absorbed as small short-chain fatty acids. They diffuse across the absorptive cells of the villus on the small intestine through simple diffusion. - Amino acids are absorbed through active trnasport or secondary active transport with sodium across the absorptive cells of the villus. - Monosaccharides are absorbed through secondary active transport with sodium or facilitated diffusion. The enzymes on the brush border break up these glucose and fructose molecules into monosaccharides. - In the absorptive cells of the villus, monosacchardies get across through facilitated diffusion. After diffusing across, the absorptive cells of the membrane, they enter the blood capillary of a villus and are then transported to the hepatic portal vein and then the liver

Mechanical and Chemical Digestion in the Colon

- Haustral Churning - Peristalsis - Mass peristalsis - The last stages of digestion occur through bacterial action. Substances are further broken down by bacterial action. Some vitamins are synthesized by bacterial action

What are the functions of the colon?

- Haustral churning, peristalsis, and mass peristalisis drive contents of the colon into the rectum - Bacteria in colon convert proteins into amino acids, break down amino acids, and produce some B vitamins - Absorption of some water, ions, and vitamins - Formation of feces - Defecation

Chemical digestion

- Involves breaking down the food into simpler nutrients that can be used by the cells. - Chemical digestion begins in the mouth when salivary amylase converts polysaccharides to disaccharides

Mechanical digestion

- Involves physically breaking the food into smaller pieces. - It begins in the mouth as the food is chewed. - The food is mixed with saliva to form a bolus that can be easily swallowed

Segmentation

- Mixes chime with digestive juices and brings food into contact with mucosa for absorption - Nonadjacent segments of the alimentary canal organs contract and relax. - Food is moved forward then backward - Localized contraction that serves to mix chyme with intestinal secretion

Gastric Phase

- Neural and hormonal mechanisms: Distension activates stretch receptors, initiating both long and short reflexes - Chemical stimuli, such as partially digested proteins, caffeine, and low acidity, activate enteroendocrine G cells to secrete gastrin

Intestinal Phase

- Partially digested food enters small intestine, causing a brief release of intestinal (enteric) gastrin. Gastrin encourages gastric glands of stomach to continue secretory activities. - The intestinal phase begins as chyme enters the duodenum

Peristalsis

- Propels chime through small intestine - Adjacent segments of the alimentary canal organs alternatively contract and relax - Food is moved distally along the tract - Propulsive contraction that serves to move content forward

Describe the composition of pancreatic juice

- Proteases are secreted in an inactive form and are activated when they reach the duodenum - Enteropeptidase is an enzyme bound to plasma membrane of duodenal epithelial cells. It activates pancreatic protease trypsinogen to trypsin - Once trypsin is activated, it can then activate: more trypsinogen; procarboxypeptidase to active carboxypeptidase; chymotrypinsogen to active chymotrypsin

Describe the structure of salivary glands

- Salivary glands are composed of two types of secretory cells, serous cells and mucous cells. - Serous cells produce watery secretions, enzymes, ions, and a bit of mucin. - Mucous cells produce mucous. - There are 3 paris of salivary glands - parotid, submandibular, and sublingual glands. - Parotid and submandibular glands contain msotly serous cellls, but sublingual gland consists mostly of mucous cells.

Surface mucous cell

- Secrete mucus to form a protective barrier that prevents digestion of the stomach wall. - The cells also absorb small quantities of water, ions, short-chain fatty acids, and some drugs enter the bloodstream

Parietal cell

- Secretes hydrochloric acid and intrinsic factor. - Hydrochloric acid kills microbes in food, denature proteins, and convert pepsinogen into pepsin - Intrinsic factor is needed for the absorption of vitamin B12 that is used in red blood cell formation (erythropoiesis)

Mucous neck cell

- Secretes mucus to form a protective barrier that prevents digestion of the stomach wall. - The cells also absorb small quantities of water, ions, short-chain fatty acids, and some drugs enter the bloodstream

Chief cell

- Secretes pepsinogen and gastric lipase - Pepsinogen when activated becomes pepsin to break down protein into peptides. - Gastric lipase splits triglycerides into fatty acids and monoglycerides

G cell

- Secretes the hormone gastrin to stimulate parietal cells to secrete HCl and chief cells to secrete pepsinogen. - It also causes the lower esophageal sphincter to contract and increase the motility of the stomach and relaxes the pyloric sphincter

Muscularis externa

- Stomach has three layers of muscle (circular, longitudinal, and oblique) - The muscularis externa is responsible for segmental contractions and peristaltic movement in the GI tract. - Tiniae coli: there are three, separate longitudinal ribbons of smooth muscle on the outside of the ascending, transverse, descending, and sigmoid colons

GI Tract

- The GI tract is a long tube that is open at both ends for transporting food during processing - Includes mouth, pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anus

Esophageal Stage

- The circular muscles behind the bolus contracts to shorten, creating propulsive movement - The longitudinal muscles contract - The bolus moves pass the relaxed lower esophageal sphincter and into the stomach

Describe the function of the gallbladder

- The gallbladder stores bile until it is needed.

Describe the structure of the liver

- The liver has two lobes, the right and left lobe. Each lobe has its own hepatic duct. - The hepatic portal vein delivers nutrient-rich blood to the lvier from the digestive tract. - All blood from the digestive organs as well as the spleen is delivered to the liver in the hepatic portal vein before being returned to the general circulation. - The capillary walls in the lobules of the liver are sinusoidal and allow large proteins and even blood cells to pass, permitting the liver to perform its function. - The liver is composed of hepatocytes, bile canaliculi, and hepatic sinusoids.

Describe the function of the liver

- The liver makes bile, which is important in the emulsification of fats - Bile functions to carry bilirubin formed from breakdown of work-out RBCs, emulsify fats, and is an excretory and a digestive secretion - The liver also metabolizes carbohydrates, lipids, and proteins. - It processes drugs and hormones. - It excretes bilirubin. It synthesizes bile salts for the breakdown of fat and serves as storage for glycogen and iron. - It is also involved in phagocytosis and vitamin D activation

Describe the function of the small intestine

- The majority of digestion and absorption occur in the small intestine (jejunum and ileum) - Segmentation and peristalsis - The small intestine completes digestion of carbohydrates, proteins, lipids. It begins and completes digestion of nucleic acids - The small intestine absorbs about 90% of nutrients and water that pass through the digestive system

Mouth

- The mouth is formed by the cheeks, hard and soft palates, and tongue - The voluntary phase of swallowing takes place within the mouth

Mucosa

- The mucosa is full of gastric glands and pits. The gastric pits secrete mucus, acid, and digestive enzymes. The gastric glands secrete gastric juice and contain hydrochloric acid, mucus, and pepsin - The mucosa also has rugae that are a structural modification that allow expansion of the stomach wall. - The mucosa is made up of 3 layers (epithelium, lamina propria, and muscularis mucosae). - Epithelium are responsible for most digestive, absorptive, and secretory processes. - The lamina propria is a layer of connective tissue.

Describe the mucosa

- The mucosa is the innermost layer, and surrounds the lumen, the open space within the digestive tube. - The mucosa is made of three layers (epithelium, lamina propria, and muscularis mucosae) - It comes in direct contact with digested food (chyme) and functions in absorption and secretion

Describe the muscularis

- The muscularis is responsible for the segmental contractions and peristaltic movements in the GI tract. - The muscularis consists of an inner circular muscular layer and a longitudinal outer muscular layer. - The coordinated contractions of these layers is called peristalsis, which propels food through the GI tract. - In the colon, the muscularis externa is much thicker because the feces are large and heavy, requiring more force to push along. - The stomach has a third layer of muscularis externa: the inner oblique layer to help churn the chyme in the stomach

Describe the pharynx

- The pharynx is a funnel shaped tube that extends from the internal nares to the esophagus posteriorly and to the larynx anteriorly - The pharynx is composed of skeletal muscle and lined with mucous membrane. These skeletal muscle helps to swallow food - The involuntary portion of swallowing takes place in the pharynx

Describe the serosa

- The serosa consists of a secretory epithelial layer and a thin connective tissue layer that reduces the friction from muscle movements - Cells oft he serous layer secrete a serous fluid that provides lubrication to reduce friction - The serous layer provides a partition between the internal organs and the abdominal cavity. The connective tissue layer provides blood vessels and nerves.

Describe the sphincters of the stomach

- The stomach has 2 sphincters, the cardiac sphincter and the pyloric sphincter. - The cardiac sphincter at the upper portion of the stomach prevents the acidic contents of the stomach from moving upward into the esophagus - The pyloric sphincter controls what will or the amount of stomach content leaving

Describe the layers of the stomach

- The stomach has the same four layers as the rest of the GI tract - mucosa, submucosa, muscularis externa, and serosa

Describe the structure of the stomach

- The stomach is a J-shaped enlargement of the GI tract. - The cardia is where the esophagus connects to the stomach and where food passes through the stomach. - Above and to the left of the cardia is the fundus. - Below the fundus is the body of the stomach, the main part - The funnel-shaped pylorus connects the stomach to the duodenum. - The wider end of the funnel, the pyloric antrum connects to the body of the stomach

Describe the functions of the stomach.

- The stomach mixes saliva, food, and gastric juice to form chyme. The bolus is liquefied to form chyme - the stomach serves as a reservoir for food before release into the small intestine - The stomach secretes gastric juice, which contains HCl, pepsin, intrinsic factor, and gastric lipase - The stomach secretes gastrin into the blood

Describe the submucosa

- The submucosa is a dense, irregular layer of connective tissue with large blood vessels, lymphatics, and nerves that support the mucosa. - The absorbed elements that pass through the mucosa are picked up from the blood vessels of the submucosa - Tiny parasympathetic ganglia are scattered around to form the submucosal plexus where preganglionic parasympathetic neurons create synapses with postganglionic nerve fibers that supply the muscularis mucosae

Describe the function of teeth

- The teeth project into the mouth and are adapted for mechanical digestion. - Food we consume is broken down into smaller parts to mix with digestive enzymes.

Describe the function of the tongue

- The tongue forms the floor of the oral cavity, manipulates food for chewing and swallowing, shapes food, and senses taste. - The tongue participates in chewing, swallowing, and speech

Pharyngeal Stage

- The tongue rises against the palate and closes the nasopharynx - The uvula and palate seal off the nasal cavity - The epiglottis covers the larynx - Breathing is temporarily interrupted

Describe the structure of the tongue

- The tongue, together with associated muscle, forms the floor of the oral cavity - The tongue is composed of skeletal muscles covered with mucous membrane. - The upper and lateral surfaces of the tongue are covered with papillae, some of which contain taste buds

Describe the structure of teeth

- There are two main parts of the teeth, the crown and the root. - The crown has a covering, the enamel that protects the teeth from wear and tear. - Below the enamel is the dentin which makes up the majority of the tooth. - The root of the tooth is a pulp cavity that contains connective tissue containing nerves and blood vessels - Inside the root is the root canal, which is an extension of the pulp cavity that contains nerves and blood vessels.

Emulsification

- Triglycerides and their breakdown products are insoluble in water. - Bile salts break large fat globules into smaller ones

Mass peristalsis

- a strong peristaltic wave that begins in the transverse colon and quickly drives the contents of the colon into the rectum - The defecation reflex occurs when the rectal wall stretches as mass movements force feces into it

Describe the function of the pancreas.

- produce enzymes that digest carbohydrates, proteins, fats, and nucleic acids - Produces sodium bicarbonate which buffers stomach acid - The pancreas empties its contents into the duodenum

Digestive Process

6 basic processes: - ingestion - secretion - motility - digestion - absorption - defecation

Absorptive Cell

Absorbs water

What are the types of cells in the colon?

Absorptive Cell and Goblet Cell

Phases of Digestion

Cephalic Phase Gastric Phase Intestinal Phase

Innervation of the gut

Controlled by the enteric nervous system and the autonomic nervous system

Haustral Churning

Distension reaches a certain point and the walls of the haustra contract to squeeze contents onward

What are the divisions of the small intestine?

Duodeunum, jejunum, ileum

Describe lipid digestion in the small intestine

Emulsification Digestion Micelle Formation Diffusion Chylomicron formation Chylomicron Transport

Describe the neural innervation of the gut.

Enteric nervous system Autonomic nervous system

Describe how CCK controls digestion

Enteroendocrine CCK cells in the mucosa of small intestine secrete CCK when partially digested proteins (amino acids), triglycerides, and fatty acids enter the small intestine. CCK stimulates secretion of pancreatic juice rich in digestive enzymes, causes bile ejection from the gallbladder and opening of the sphincter of the hepatopancreatic ampulla, and induces satiety.

GI Reflex Pathways

GI reflex pathways regulate GI secretion and motility in response to stimuli within the GI tract

Intestinal juice

Intestinal juice provides a vehicle for absorption of substances from chyme as they come in contact with the villi

Movement of Nutrients

Lacteal in villus, lymphatic capillary, lymphatic vessel, lymph flow that drains into blood

Chylomicron formation

Lipid products are converted back into triglycerides and packaged with lecithin and lipoproteins, forming chylomicron

Diffusion

Lipids products leave micelles and cross epithelial membrane via diffusion

Digestion

Pancreatic lipases break down fat into monoglyceride plus two free fatty acids

Why are protease enzymes secreted in an inactive form?

Protease enzymes are secreted in an inactive form because the cells producing inactive enzymes are protected from the enzymes until they are safely within the lumen of the GI tract.

Trace the flow of the bile from the liver to the duodenum

Right/left hepatic duct, common hepatic duct, cystic duct, common bile duct, pancreatic duct, hepatopancreatic ampulla, duodenum

Describe the function of salivary glands

Salivary glands lie outside the mouth and empty their contents that they secrete from serous or mucous cells into ducts which deliver saliva into the oral cavity

Goblet Cells

Secrete mucus

Enteroendocrine cells

Secrete the hormones secretin, cholecystokinin, or GIP

Goblet Cell

Secretes mucus

Cephalic Phase

Stimulates gastric secretion and motility It is triggered by sight, smell, and thought of food.

Outline the digestive sequence of fats

Stomach (lingual lipase and gastric lipase), small intestine (pancreatic lipase), duodenum (chyme enters, bile is released --> emulsification)

What are the types of cells in the stomach glands?

Surface mucous cell Mucous neck cell Parietal cell Chief Cell G Cell

Describe the esophagus

The esophagus is a collapsible, muscular tube that lies posterior to the trachea and connects the pharynx to the stomach The esophagus serves as a passageway for the bolus to go from the pharynx to the stomach

Describe the structure of the gallbladder

The gallbladder contains the cystic duct. When the cystic duct joins the common hepatic duct from the liver, it forms the common bile duct

Anatomy of the Large Intestine (Colon)

The large intestine is divided into the cecum, the colon, the rectum, and the anus. - The ileocecal valve, located at the opening between the ileum and the large intestine, controls the flow of chyme from the small intestine to the large intestine. The colon is made of the ascending colon, transverse colon, descending colon, and sigmoid colon

Describe the structure of the pancreas

The pancreas primarily consists of exocrine tissue that produces pancreatic enzymes for digestion The remaining tissue are endocrine tissue called Isle of Langerhands

Serosa

The serosa consists of a secretory epithelial layer and a thin connective tissue layer that reduce the friction from muscle movements

Describe the specific organization of the wall of the small intestine.

The small intestine has structural modification, circular folds, villi, and microvilli that amplify its absorptive surface. Circular folds increases the surface area for digestion and absorption

Submucosa

The submucosa is a dense, irregular layer of connective tissue with large blood vessels, lymphatics, and nerves that supports the mucosa

Voluntary Swallowing

The tongue shapes the chewed, lubricated food (bolus) and moves it to the back of the mouth cavity

Deglutition

There are three stages of swallowing: voluntary, pharyngeal, and esophageal

Peristalsis

Waves of muscular contractions that propel contents from one point to another

Brush border enzymes

are found on the surfaces of the microvilli of absorptive cells. They are responbible for breaking down food products

Accessory structures

are not part of the FI tract but they do contribute to food processing - include teeth, tongue, salivary glands, liver, gall bladder, and pancreas

What is the function of the digestive system?

breaks down food that is then absorbed to supply body with energy

Absorptive cells

digest and absorb nutrients.

Mesentery

fan-shaped fold that separates the coils of the small intestine

Enteric nervous system

is made of submucosal plexus and myenteric plexus. Innervation of submucosal plexus stimualtes the production of enzymes from submucosal gland. - myenteric plexus is responsible for contraction of smooth muscles (circular and longitudinal) in muscularis to provide peristalsis.

Outline the digestive sequence of carbohydrates

mouth (salivary amylase), duodenum (amylase from pancreatic juices), enzymes on brush border of small intestinal wall (maltase, sucrases, and lactases)

What are the layers of the GI tract?

mucosa, submucosa, muscularis, serosa

Autonomic Nervous System

parasympathetic sympathetic

Micelle formation

products from digestion become coated with bile salts

Paneth cells

secrete lysozyme and is capable of phagocytosis

Outline the digestive sequence of proteins

stomach (pepsin breaks protein into peptides), duodenum (trypsin, elastase, and chymotrypsin reduce them into smaller peptides), small intestine (carboxypeptidase, dipeptidase, and aminopeptidase break them down more)

Greater omentum

the fatty apron overlying the intestines

Peritoneum

the peritoneum is the largest serous membrane in the body The parietal peritoneum lines the abdominal wall while the visceral peritoneum forms the outermost layer along most of the alimentary canal