The Digestive System

Mastication

"Chewing" The first step in the digestive process

Why must pepsin be stored as pepsinogen?

-Pepsin can digest protein -It must be stored and secreted in an inactive form so it doesn't digest the proteins of the cells in which it is formed -Therefore, pepsin is maintained in the inactive form of pepsinogen until it reaches the gastric lumen, where it is activated by HCl secreted into the lumen by a different cell type

Intrinsic factor

-another secretory product of the parietal cells in addition to HCl -essential for absorption of vitamin B12 -this vitamin can be absorbed only when in combination with intrinsic factor -vitamin B12 is essential for the normal formation of red blood cells -decrease or absence of intrinsic factor leads to pernicious anemia -pernicious anemia is typically caused by an autoimmune attack against the parietal cells

Effects of vomiting

-large losses of secreted fluids and acids that normally would be reabsorbed -resulting reduction in plasma volume can lead to dehydration and circulatory problems, and the loss of acid from the stomach can lead to metabolic alkalosis

Pepsinogen

-major digestive constituent of gastric secretion -an inactive enzymatic molecule produced by the chief cells -stored in the chief cell's cytoplasm within secretory vesicles known as zymogen granules, from which it is released by exocytosis on appropriate stimulate -when pepsinogen is secreted into the gastric lumen, HCl cleaves off a small fragment of the molecule, converting it into the active form of the enzyme pepsin -pepsin initiates protein digestion by splitting certain amino acid linkages in proteins to yield peptide fragments; it works most effectively in the acid environment provided by HCl

Vomiting

-the forceful expulsion of gastric contents out through the moth -the stomach doesn't actively participate in vomiting -the major force for expulsion comes from contraction of the respiratory muscles (diaphragm and the abdominal muscles) -act of vomiting is coordinated by a vomiting center in the medulla of the brain stem

Function of Mucus

-the surface of the gastric mucosa is covered by a layer of mucus derived from the surface epithelial cells and mucous cells -this mucus serves as a protective barrier against several forms of potential injury to the gastric mucosa -mucus-secreting cells secrete HCO3- that is trapped in the mucus and neutralizes acid in the vicinity 1. protects the gastric mucosa against mechanical injury because of its lubricating properties 2. helps protect the stomach wall from self-digestion because pepsin is inhibited when it comes in contact with the layer of mucus coating the stomach lining 3. being alkaline, mucus helps protect against acid injury by neutralizing HCl in the vicinity of the gastric lining, but it doesn't interfere with the function of HCl in the lumen

What makes up the digestive system?

1. Accessory digestive organs: -salivary glands -exocrine pancreas -biliary system (liver and gallbladder) 2. Digestive tract: continuous from mouth to anus -mouth -pharynx (throat) -esophagus -stomach -small intestine (consisting of the duodenum, jejunum, and ileum) -large intestine (the cecum, appendix, colon, and rectum) -anus

Three segments of small intestine

1. Duodenum 2. Jejunum 3. Ileum

Four main aspects of gastric motility

1. Filling: -receptive relaxation is the reflex relaxation of the stomach as it is receiving food -the interior of the stomach is thrown into deep folds that get smaller and nearly flatten out as the stomach relaxes -enhances the stomach's ability to accommodate the extra volume of food with little rise is stomach pressure 2. Storage: -because only feeble mixing movements occur in the body and fundus, food delivered to the stomach from the esophagus is stored in the relatively quiet body without being mixed -the fundic area usually doesn't store food but contains only a pocket of gas -food is gradually fed from the body into the antrum, where mixing does take place 3. Mixing & Emptying: -the strong antral peristaltic contractions mix the food with gastric secretions to produce chyme -contraction propels the chyme forward -a small portion of chyme is pushed through the partially open sphincter into the duodenum -sphincter closes when contraction reaches the pyloric sphincter and no further emptying takes place -chyme is tossed back into the antrum

Four Basic Digestive Processes

1. Motility 2. Secretion 3. Digestion 4. Absorption

Four Major Tissue Layers of Digestive Tract

1. Mucosa 2. Submucosa 3. Muscularis externa 4. Serosa

Three types of gastric exocrine secretory cells found in the walls of the pits and glands in the oxyntic mucosa

1. Mucous cells: -line the gastric pits and the entrance of the glands -they secrete a thin, watery mucus 2. Chief cells: -line the deeper parts of the gastric lands -secrete the enzyme precursor pepsinogen 2. Parietal (or oxyntic) cells -secrete HCl and intrinsic factor *These exocrine secretions are all released into the gastric lumen and make up the gastric digestive juice

Two components of pancreatic juice secreted by the exocrine pancreas

1. Pancreatic enzymes actively secreted by the acinar cells that form the acini 2. An aqueous alkaline solution actively secreted by the duct cells that line the pancreatic ducts. The aqueous alkaline component is rich in sodium bicarbonate (NaHCO3)

Two basic types of phasic digestive motility

1. Propulsive Movements-propel or push the contents forward through the digestive tract (transit of food through the esophagus is rapid) 2. Mixing movements-promote digestion by mixing food with digestive juices and facilitate absorption by exposing all parts of the intestinal contents to the absorbing surfaces of the digestive tract

Changes during the oropharyngeal stage of swallowing to prevent food from entering the wrong passageways

1. Swallowing center inhibits respiratory center in brain stem. 2. Elevation of uvula prevents food from entering nasal passages 3. Position of tongue prevents food from reentering mouth. 4. Epiglottis is pressed down over closed glottis as auxiliary mechanism to prevent food from entering airways 5. Tight apposition of vocal folds across glottis prevents food from entering respiratory airways

Three main functions of the stomach

1. To store ingested food until it can be emptied into the small intestine at a rate appropriate for optimal digestion and absorption. 2. To secrete HCl and enzymes that begin protein digestion. 3. Through it's mixing movements, the ingested food is pulverized and mixed with gastric secretions to produce a thick liquid mixture known as chyme. The stomach contents must be converted to chyme before they can be emptied into the duodenum.

Four factors involved in regulating digestive system function

1. autonomous smooth muscle function 2. intrinsic nerve plexuses 3. extrinsic nerves 4. gastrointestinal hormones

3 Phases of Gastric Secretion

1. cephalic 2. gastric 3. intestinal

Submucosa

A thick layer of connective tissue that provides the digestive tract with its distensibility and elasticity

Lamina propria

A thin middle layer of connective tissue on which the epithelium rests Houses the gut-associated lymphoid tissue, which is important in the defense against disease-causing intestinal bacteria

The exocrine pancreas

Acini Secretes into the duodenal lumen a digestive juice composed of digestive enzymes secreted by the acinar cells and an aqueous NaHCO3 solution secreted by the duct cells.

Absorption in small intestine

All products of carbohydrate, protein, and fat digestion, as well as most of the ingested electrolytes, vitamins, and water, are absorbed by the small intestine indiscriminately (EXCEPT FOR CALCIUM AND IRON) Most absorption occurs in the duodenum and jejunum

Smooth Muscle Activity in Digestion

Although the smooth muscle in the walls of the digestive tract is phasic smooth muscle that displays action-potential-induced bursts of contraction, it also maintains a constant low level of contraction known as tone. Contraction of the smooth muscle within the walls of the digestive organs accomplishes movement of material through most of the digestive tract (except for ends of tract).

The most important salivary proteins and their contributions to the functions of saliva

Amylase, mucus, and lysozyme 1. Saliva begins digestion of carbs in the mouth through action of salivary amylase 2. Saliva facilitates swallowing by moistening food particles, thereby holding them together, and by providing lubrication through the presence of mucus, which is thick and slippery. 3. Saliva exerts some antibacterial action (involving lysozyme) 4. Serves as a solvent for molecules that stimulate the taste buds. Only molecules in solution can react with taste bud receptors. 5. Saliva aids speech by facilitating movements of the lips and tongue. 6. Saliva keeps the mouth and teeth clean. THe constant flow of saliva helps flush away food residues, foreign particles, and old epithelial cells that have shed from the oral mucosa. 7. Saliva neutralizes acids in foods and produced by bacteria in mouth.

Pancreas

An elongated gland that lies behind and below the stomach, above the first loop of the duodenum. Contains both exocrine and endocrine tissue. The exocrine part consists of grapelike clusters of secretory cells that form sacs known as acini, which connect to ducts that eventually empty into the duodenum. The endocrine part consists of isolated islands of endocrine tissue, the islets of Langerhans, which are dispersed throughout the pancreas.

Mucous membrane

An inner epithelial layer that serves as a protective surface Contains exocrine gland cells for secretion of digestive juices, endocrine gland cells for secretion of blood-borne gastrointestinal hormones, and epithelial cells specialized for absorbing digested nutrients.

Why would the removal of the terminal ileum be detrimental?

Because vitamin B12 and bile salts are reabsorbed here. The specialized transport mechanisms for these two substances are located only in this region. (All other substances can be absorbed throughout the small intestine's length)

Bile Salts

Bile contains no digestive enzymes, but is important for the digestion and absorption of fats, primarily through the activity of bile salts. Derivatives of cholesterol. Aid in fat digestion and absorption Recycled between the small intestine and liver through enterohepatic circulation.

Absorption of carbohydrate and protein

Both are absorbed by secondary active transport and enter the blood Accomplished by Na+-dependent symport, and both categories of end products are absorbed into the blood

Gastroileal reflex

Both the duodenum and the ileum start to segment simultaneously when a meal first enters the small intestine The duodenum starts primarily in response to local distension caused by the presence of chyme Segmentation of the empty ileum is brought about by gastrin secreted in response to the presence of chyme in the stomach, a mechanism known as the gastroileal reflex.

Types of Muscle that contain pacemaker cells

Cardiac & some smooth muscle cells

Role of CCK in pancreatic secretion

Cholecystokinin is important in regulating pancreatic digestive enzyme secretion. The main stimulus for release of CCK from the duodenal mucosa is the presence of fat and, to a lesser extent, protein products. The circulatory system transports CCK to the pancreas where it stimulates the pancreatic acinar cells to increase digestive enzyme secretion. Among these enzymes are lipase and the proteolytic enzymes, which appropriately further digest the fat and protein that initiated the response and also help digest carbohydrate. **Even though the total amount of enzymes released varies depending on the type of meal consumes, the proportion of enzymes released does not vary on a meal-to-meal basis.

Large Intestine

Consists of the colon (makes up most of the large intestine), cecum (ileocecal valve), appendix (lymphoid tissue housing lymphocytes), and rectum (straight part of colon). Extracts H2O and salt from the contents delivered to the colon (indigestible food residues, unabsorbed biliary components, and the remaining fluid). The primary function of the large intestine is to store feces before defacation.

Gastric Mucosa

Contains the cells that secrete gastric juices Divided into two distinct areas: 1. The oxyntic mucosa-lines the body and fundus 2. The pyloric gland area-lines the antrum

Pharyngoesophageal sphincter

Contraction of circular skeletal muscle During swallowing, tonic contraction of this upper esophageal sphincter keeps the entrance to the esophagus closed to prevent large volumes of air from entering the esophagus and stomach during breathing. Instead, air is directed only into the respiratory airways.

Chymotrypsinogen & Procarboxypeptidase

Converted by trypsin to their active forms, chymotrypsin and carboxypeptidase, within the duodenal lumen

Enamel

Covers the exposed part of a tooth The hardest structure of the body Enamel can't be regenerated after the tooth has erupted

Lactose intolerance

Deficiency of lactase, the disaccharidase specific for the digestion of lactose (milk sugar) Undigested lactose remains in the lumen and its accumulation creases an osmotic gradient that draws H2O into the intestinal lumen Bacteria living in the large intestine have lactose-splitting ability and thereby produce large quantities of CO2 and methane gas in the process

Carbohydrate digestion and absorption

Dietary carbohydrate is presented to the small intestine for absorption mainly in the forms of the disaccharides maltose, sucrose and lactose. The monosaccharides flucose and galactose are absorbed into the epithelial cells by Na+- and energy-dependent secondary active transport (via the symporter SGLT) located at the luminal membrane. The monosaccharide fructose enters the cell by passive facilitated diffusion via GLUT-5 Glucose, galactose, and fructose exit the cell at the basal membrane by passive facilitated diffusion via GLUT-2 These monosaccharides enter the blood by simple diffusion

Fat digestion and absorption

Different from carbohydrate and protein absorption because of insolubility of fat in water Bile salts cause lipid emulsion and lipase hydrolyzes the triglycerides into monoglycerides and free fatty acids. These water-insoluble products are carried within water-soluble micelles, which are formed by bile salts and other bile constituents. Micelles and hydrolyzed fat products passively diffuse through the lipid bilayer of luminal membranes. Reformed triglycerides form water-soluble chylomicrons which are unable cross the basement membrane of capillaries, so instead they enter the lymphatic vessels.

Secretion

Digestive juices are secreted into the digestive tract lumen by exocrine glads along the route, each with its own specific secretory product. Each digestive secretion consists of water, electrolytes, and specific organic constituents important in the digestive process, such as enzymes, bile salts, or mucus.

Functions of HCl in digestive system

Doesn't actually digest anything (doesn't break apart nutrient chemical bonds) 1. Activates the enzyme precursor pepsinogen to an active enzyme, pepsin, and provides an acid medium that is optimal for pepsin activity. 2. Aids in the breakdown of connective tissue and muscle fibers, reducing large food particles into smaller particles. 3. Denatures protein. 4. Along with salivary lysozyme, HCl kills most of the microorganisms ingested with food, although some do escape and continue to grow and multiply in the large intestine.

Pancreatic Lipase

Extremely important because it's the only enzyme secreted throughout the entire digestive system that can digest fat. Secreted in its active form because there is no risk of pancreatic self-digestion by lipase. Pancreatic lipase hydrolyzes dietary triglycerides into monoglycerides and free fatty acids, and triglycerides aren't a structural component of pancreatic cells.

Control of secretion of gastric digestive juices

G cells secrete gastrin Enterochromaffin-like (ECL) cells secrete histamine D cells secrete somatostatin **Parietal cells have separate receptors for each of these chemical messengers **ACh, gastrin, and histamine are stimulatory **Somatostatin inhibits HCl secretion ***ACh and gastrin also increase pepsinogen secretion through their stimulatory effect on the chief cells

What controls pancreatic exocrine secretion?

Hormonal mechanisms The release of the two major enterogastrones, secretin and cholecystokinin (CKK), in response to chyme in the duodenum plays the central role in controlling pancreatic secretion.

Importance of Pancreatic Enzymes

Important because they can almost completely digest food in the absence of all other digestive secretions. The acinar cells secrete three different types of pancreatic enzymes capable of digesting all three categories of foodstuffs: 1. Proteolytic enzymes for protein digestion 2. Pancreatic amylase for carbohydrate digestion 3. Pancreatic lipase for fat digestion

Hepcidin

Iron absorption is largely controlled by hepcidin which is released from the liver when iron levels in the body become too high. Hepcidin prevents further iron export from the small-intestine epithelial cell into the blood.

Iron Absorption

Iron is essential for hemoglobin production Two main steps are involved in absorption of iron into blood: 1. absorption of iron from the lumen into small intestine epithelial cells 2. absorption of iron from the epithelial cells into the blood Two possible fates of iron after absorption into the small-intestine epithelial cells: 1. Iron needed immediately for production of red blood cells is absorbed into the blood for delivery to the bone marrow, the site of red blood cell production. 2. Iron not immediately needed is irreversibly stored within the small-intestine epithelial cells in a granular form called ferritin, which can't be absorbed into the blood.

Gastroesophageal sphincter

Is smooth muscle in contrast to the upper esophageal sphincter Except during swallowing, stays tonically contracted by beans of myogenic activity to maintain a barrier between the stomach and esophagus, reducing the change of reflux of acidic gastric contents into the esophagus As the peristaltic wave sweeps down the esophagus, the gastroesophageal sphincter relaxes so that the bolus can pass into the stomach. After the bolus has entered the stomach, the swallow is complete and this lower esophageal sphincter again contracts.

The endocrine pancreas

Islets of Langerhans Secretes the hormones insulin and glucagon into the blood

Ileocecal Juncture

Juncture between the small and large intestine The juncture between the ileum and large intestine is the ileocecal valve, which is surrounded by thickened smooth muscle, the ileocecal sphincter. Pressure on the cecal side pushes the valve closed and contracts the sphincter, preventing the bacteria-laden colonic contents from contaminating the nutrient-rich small intestine. The valve/sphincter opens and allows ileal contents to enter the large intestine in response to pressure on the ileal side of the valve and to the hormone gastrin secreted as a new meal enters the stomach.

Liver

Largest and most important metabolic organ in the body (the body's biochemical factory) Secretes bile salts, which aid in fat digestion and absorption Also performs a wide variety of functions not related to digestion (See P. 615-616)

Pancreatic Amylase

Like salivary amylase, pancreatic amylase contributes to carbohydrate digestion by converting polysaccharides into the disaccharide maltose. Amylase is secreted in the pancreatic juice in an active form because active amylase doesn't endanger the secretory cells bc they don't contain any polysaccharides.

Mucosa

Lines the luminal surface of the digestive tract Divided into 3 layers: 1. Mucous membrane 2. Lamina propria 3. Muscularis mucosa Surface is generally highly folded, with many ridges and valleys that greatly increase the surface area available for absorption.

What secretes bile?

Liver

Hepatocytes

Liver cells Even though the liver is involved in many different functions, cells have little specialization. Specialization is due to the organelles within each hepatocyte Phagocytic activity (not performed by hepatocytes) carried out by Kupffer cells (macrophages in the liver)

Biliary system

Liver, gallbladder, and associated ducts

The brush-border

Microvilli-special hairlike projections on the luminal surface of the small-intestine epithelial cells that form the brush border The brush-border plasma membrane contains three categories of integral proteins that function as enzymes: 1. Enterokinase: activates trypsinogen 2. Disacccharides: complete carb digestion 3. Aminopeptidases: completes protein digestion

Motility

Motility refers to the muscular contractions that mix and move forward the contents of the digestive tract In addition to action-potential-induced bursts of contraction, smooth muscle maintains a constant low level of contraction known as tone. Tone is important in maintaining a steady pressure on the contents of the digestive tract as well as in preventing its walls from remaining permanently stretched following distension.

Is saliva essential for digesting and absorbing foods?

No. Enzymes produced by the pancreas and small intestine can complete food digestion even in the absence of salivary and gastric secretion.

Does absorption of foodstuff occur in the mouth?

No. However, some drugs can be absorbed by the oral mucosa.

Slow waves

Not action potentials and don't directly induce muscle contraction; they are rhythmic, wavelike fluctuations in membrane potential that cyclically bring membrane closer to or farther from threshold potential If these waves reach threshold at the peaks of depolarization, a volley of action potentials is triggered at each peak, resulting in rhythmic cycles of muscle contraction

Pancreatic Aqueous Alkaline Secretion

Pancreatic enzymes function best in a neutral or slightly alkaline environment, so the the acidic chyme emptied into the duodenal lumen must be quickly neutralized. Pancreatic duct cells secrete an alkaline (NaHCO3-rich) fluid into the duodenal lumen to neutralize the chyme.

Two secretory products released into duodenal lumen

Pancreatic juice and bile

Protein digestion and absorption

Protein needed to be digested comes from both food and endogenous protein Dietary and endogenous proteins are hydrolyzed into their constituent amino acids and a few small peptide fragments by gastric pepsin and the pancreatic proteolytic enzymes Amino acids are absorbed across the intestinal cells by symporters, similar to glucose and galactose absorption. Also, small peptides gain entry by means of another Na+-dependent carrier in a process known as tertiary active transport. In this case, the symporter simultaneously transports both H+ down its concentration gradient and the peptide against its concentration gradient.

Peristalsis

Ringlike contractions of the circular smooth muscle of a tubular organ that move progressively forward with a stripping motion, pushing the contents of the organ ahead of the contraction.

Uvula

Seals off the nasal passages during swallowing

Gastric parietal cells

Secrete H+ and absorb HCO3-, so the acid base balance of the body is NOT altered by digestive secretion

Pancreatic duct cells

Secrete HCO3- and absorb H+

D cells

Secrete the paracrine somatostatin in response to acid acts locally as a paracrine in negative-feedback fashion to inhibit secretion by the parietal cells, G cells, and ECL cells, thus turning off the HCl-secreting cells and their most potent stimulatory pathway

Trypsinogen

Secreted into the duodenal lumen and activated to its active enzyme form trypsin, by enterokinase (enteropeptidase) Trypsin then autocatalytically activates more trypsinogen Trypsinogen must remain inactive within the pancreas to prevent this proteolytic enzyme from digesting the proteins of the cells in which it is formed.

Enterochromaffin-like (ECL) cells

Secretes the paracrine histamine in response to ACh and gastrin histamine acts locally on nearby parietal cells to speed up HCl secretion and potentiates the actions of ACh and gastrin

Role of secretin in pancreatic secretion

Secretin release is stimulated by acid in the duodenum. Secretin is then carried by the blood to the pancreas, where it stimulates the duct cells to markedly increase their secretion of a NaHCO3- rich aqueous fluid into the duodenum.

Segmentation

Segmentation mixes (allows for digestive juices to come in contact with more chyme) and slowly propels (allows for maximum absorption) the chyme ***The mixing accomplished by segmentation serves as the dual functions of mixing the chyme with the digestive juices secreted into the small-intestine lumen and exposing all the chyme to the absorptive surfaces of the small-intestine mucosa. Consists of oscillating, ring-like contractions of circular smooth muscles Within a matter of seconds, the contracted segments relax and the previously relaxed areas contract. These oscillating contractions thoroughly mix the chyme within the small-intestine lumen. Segmentation is slight or absent between meals but becomes very vigorous immediately after a meal. The frequency of segmentation declines along the length of the small intestine

External anal sphincter

Skeletal muscle; voluntary movement

Haustral Contractions

Slowly shuffle the colonic contents back and forth The outer longitudinal smooth muscle layer doesn't completely surround the large intestine, but consists instead of only three separate, conspicuous, longitudinal bands of muscle, the taeniae coli, which run the length of the large intestine. Most of the time, movements of the large intestine are slow and nonpropulsive, as is appropriate for its absorptive and storage functions. The colon's main motility is haustral contractions initiated by the autonomous rhythmicity of colonic smooth muscle cells.

Digestion in the small intestine

Small intestine doesn't secrete digestive enzymes (digestive enzymes are secreted by the pancreas) Small intestine enzymes complete digestion within the brush-border region Fat digestion is completed in the small intestine lumen, but carbohydrate and proteins still need further digestion Membrane-bound enzymes on microvilli in the brush-border further digest these materials

Internal anal sphincter

Smooth muscle, involuntary movement

Acetylcholine

Stimulates the parietal, chief, G, and ECL cells

Hepatic artery

Supply hepatocytes with fresh blood supply for O2 and nutrient needs.

Digestion

The biochemical breakdown of the structurally complex foodstuffs of the diet into smaller, absorbable units by the enzymes produced within the digestive system. 1. Starch, glycogen, and disaccharides are converted into their constituent monosaccharides, principally glucose with small amounts of fructose and galactose. 2. Proteins are degraded primarily into their constituent amino acids as well as a few small polypeptides. 3. Fats, consisting of a glycerol with three fatty acids, are degraded into a monoglyceride, a glycerol molecule with one fatty acid, and free fatty acids.

Pharynx

The cavity at the rear of the throat that acts as a common passageway for both the digestive system (by serving as the link between the mouth and esophagus, for food) and the respiratory system (by providing access between the nasal passages and trachea, for air).

Liver Blood Flow

The liver receives blood from two sources: 1a. Arterial blood, which provides the liver's O2 supply and carries blood-borne metabolites for hepatic processing, is delivered by the hepatic artery. 1b. Venous blood draining the digestive tract is carried by the hepatic portal vein to the liver for processing and storage of newly absorbed nutrients. 2. Blood leaves the liver via the hepatic vein.

Bile

The opening of the bile duct into the duodenum is guarded by the sphincter of Oddi, which prevents bile from entering the duodenum except during digestion of meals. Secreted by liver and diverted back up into the gallbladder, a small, saclike structure tucked beneath but no directly connected to the liver. Thus, bile is not transported directly from the liver to the gallbladder. The bile is subsequently stored and concentrated in the gallbladder between meals. After a meal, bile enters the duodenum as a result of the combined effects of the gallbladder emptying and increased bile secretion by the liver.

Autonomous Smooth Muscle Function

The prominent type of self-induced electrical activity in digestive smooth muscle is slow-wave potentials (interstitial cells of Cajal) Electrical activity initiated in a digestive tract pacemaker cell spreads to the adjacent contractile smooth muscle cells

Saliva

The secretion associated with the mouth Produced largely by three major pairs of salivary glands that lie outside the oral cavity and discharge saliva through short ducts into the mouth. 99.5% H2O and 0.5% electrolytes and protein

Small Intestine

The site where most digestion and absorption take place. No further digestion or absorption of ingested nutrients occurs after small intestine. Exception: water and salt in the large intestine

Absorption

The small absorbable units that result from digestion, along with water, vitamins, and electrolytes, are transferred from the digestive tract lumen into the blood or lymph.

Lipid Emulsion and Detergent Action of Bile Salts

The term detergent action refers to bile salts' ability to convert large fat globules into a lipid emulsion consisting of many small fat droplets suspended in the aqueous chyme, thus increasing the surface area available for attack by pancreatic lipase. A bile salt consists of a lipid-soluble part that dissolves in the fat droplet and a negatively charged, water-soluble part that projects from the surface of the droplet. When a large fat droplet is broken up into smaller fat droplets by intestinal contractions, bile salts absorb on the surface of the small droplets, creating shells of negatively charged, water-soluble bile salt components that cause the fat droplets to repel one another. This emulsifying action holds the fat droplets apart and prevents them from recoalescing, increasing the surface area of exposed fat available for digestion by pancreatic lipase.

Intrinsic nerve plexuses

The two major networks of nerve fibers (the submucosal plexus and the myenteric plexus) that lie entirely within the digestive tract wall and run its entire length. Together, these two plexuses are often termed the enteric nervous system. Coordinate local activity within the digestive tract.

Occlusion

The upper and lower teeth normally fit together when the jaws are closed Allows food to be ground and crushed between the tooth surfaces.

Mass Movements

Three to four times a day, generally after meals, a marked increase in motility takes place during which large segments of the ascending and transverse colon contract simultaneously, driving the feces one third to three fourths of the length of the colon in a few seconds.

Primary Function of the Digestive System

Transfer nutrients, water, and electrolytes from the food we eat into the body's internal environment.

Three major pancreatic proteolytic enzymes

Trypsinogen, chymotrypinogen, and procarboxypeptidase (each of which is secreted in an inactive form)

Gastrointestinal hormones

Tucked within the mucosa of certain regions of the digestive tract are endocrine gland cells that, on appropriate stimulation, release hormones into the blood. These gastrointestinal hormones exert either excitatory or inhibitory influences on smooth muscle and exocrine gland cells.

Hepatic portal system

Unique vascular connection between the digestive tract and liver

Pancreatic and Biliary Secretions

When gastric contents are emptied into the small intestine, they are mixed not only with juice secreted by the small-intestine mucosa but also with the secretions of the exocrine pancreas and liver that are released into the duodenal lumen.

Defacation Reflex

When mass movements of the colon move feces into the rectum, the resultant distension of the rectum stimulates stretch receptors in the rectal wall, initiating the defacation reflex. If both the internal and external anal sphincters are relaxed, defacation occurs.

Migrating motility complex

When most of the meal has been absorbed, segmentation contractions cease and are replaced between meals by this complex. Consists of weak, repetitive peristaltic waves that move a short distance down the intestine before dying out. With each contraction, the short peristaltic waves sweep any remnants of the preceding meal plus mucosal debris and bacteria forward toward the colon.

Malocclusion

When the teeth don't make proper contact with one another, and can't accomplish their normal cutting and grinding action adequately. Can cause abnormal wearing of affected tooth surfaces and dysfunction and pain of the temporomandibular joint (TMJ), where the jawbones articulate with each other.

Esophagus

a fairly straight muscular tube that extends between the pharynx and stomach guarded at both ends by sphincters (pharyngoesophageal and gastroesophageal sphincter)

Muscularis mucosa

a sparse layer of smooth muscle the outermost mucosal layer that lies adjacent to the submucosa

Food isn't absorbed through the stomach, but what is?

alcohol and aspirin alcohol absorption occurs more slowly if gastric emptying is delayed so that the alcohol remains in the more slowly absorbing stomach longer (absorbed faster in intestine due to greater surface area) because fat is the most potent duodenal stimulus for inhibiting gastric motility, consuming fat-rich foods before or during alcohol ingestion delays gastric emptying and prevents the alcohol from producing its effects as rapidly

Protein digestion

begins in antrum

Gastric phase of gastric secretion

begins when the food actually reaches the stomach protein in the stomach stimulates chemoreceptors that activate the intrinsic nerve plexuses, which in turn stimulate the secretory cells alcohol and caffeine also stimulates secretion of a highly acidic gastric juice

Epiglottis

cartilaginous tissue that further protects food from entering the respiratory airways

Carbohydrate digestion

continues in body of stomach

Intestinal phase of gastric secretion

encompasses the factors originating in the small intestine that influence gastric secretion this phase is inhibitory important in helping shut off the flow of gastric juices as chyme begins to be emptied into the small intestine

Increase ACh secretion leads to what?

increased secretion of HCl and pepsinogen by the secretory cells

Interstitial cells of Cajal

musclelike but noncontractile cells that are the pacemaker cells that instigate cyclic slow-wave activity connected with smooth muscle cells by gap junctions through which charge-carrying ions can flow

Glottis

prevents food from entering the trachea primarily by elevation of the larynx and tight closure of the vocal folds across the laryngeal opening

Cephalic phase of gastric secretion

refers to the increased secretion of HCl and pepsinogen that occurs in feedforward fashion in response to stimuli acting in the head even before food reaches the stomach

G cells

secrete the hormone gastrin into the blood in response to protein products in the stomach lumen and in response to ACh gastrin is a major gastrointestinal hormone that stimulates the parietal and chief cells, promoting secretion of a highly acidic gastric juice indirectly promotes HCl secretion by stimulating the ECL cells to release histamine gastrin is the main factor that brings about increased HCl secretion during meal digestion

Effect of the parasympathetic system on the digestive system

tend to increase smooth muscle motility and promote secretion of digestive enzymes and hormones

Effect of the sympathetic system on the digestive system

tends to inhibit or slow down digestive tract contraction and secretion

Muscularis externa

the major smooth muscle coat of the digestive tube that surrounds the submucosa contractile activity of these smooth muscle layers produces the propulsive and mixing movements

Swallowing

the motility associated with the pharynx and esophagus a sequentially programmed all-or-nothing reflex

Extrinsic nerves

the nerve fibers from both branches of the autonomic nervous system that originate outside the digestive tract and innervate the various digestive organs influence digestive tract motility and secretion either by modifying ongoing activity in the intrinsic plexuses, altering the level of gastrointestinal hormone secretion, or, in some instances, acting directly on the smooth muscle and glands

Serosa

the outer connective tissue covering of the digestive tract that secretes a watery, slippery fluid that lubricates and prevents friction between the digestive organs and surrounding viscera

Oropharyngeal stage

the stage of swallowing that consists of moving the bolus (ball of chewed or liquid food) from the mouth through the pharynx and into the esophagus

Esophageal stage

the swallowing center triggers a primary peristaltic wave that sweeps from the beginning to the end of the esophagus, forcing the bolus ahead of it through the esophagus to the stomach the term peristalsis refers to ringlike contractions of the circular smooth muscle that move progressively forward, pushing the bous into a relaxed area ahead of the contraction