A&P Ch. 40

Describe the primary mechanisms of the digestive system

Table 40-1

How the body digests carbohydrates

Carbohydrate digestion: Carbohydrates are saccharide compounds. Polysaccharides are hydrolyzed by amylases to form disaccharides. Final steps of carbohydrate digestion are catalyzed by sucrase, lactase, and maltase, found in the cell membrane of epithelial cells covering the villi that line the intestinal lumen.

Explain the processes involved in chemical digestion and the locations of these processes

Chemical digestion—all changes in chemical composition of food as it travels through the digestive tract. Chemical changes result from hydrolysis—process in which compound unites with water and breaks down further. Numerous enzymes in the various digestive juices catalyze the hydrolysis of foods.

The mechanisms that are involved in controlling secretion

Control of salivary secretion: Only reflex mechanisms control the secretion of saliva. Chemical and mechanical stimuli come from the presence of food in the mouth. Olfactory and visual stimuli come from the smell and sight of food. Control of gastric secretion—three phases 1. Cephalic phase—"psychic phase," because mental factors activate the mechanism; parasympathetic fibers in branches of the vagus nerve conduct stimulating efferent impulses to the glands; stimulate production of gastrin (by G cells in the stomach) 2. Gastric phase—when products of protein digestion reach the pyloric portion of the stomach, they stimulate release of gastrin; gastrin accelerates secretion of gastric juice, ensuring enough enzymes are present to digest food 3. Intestinal phase—various mechanisms seem to adjust gastric secretion as chyme passes to and through the intestinal tract; endocrine reflexes involving gastric inhibitory peptide, secretin, and CCK inhibit gastric secretions Control of pancreatic secretion—stimulated by several hormones released by intestinal mucosa. Secretin evokes production of pancreatic fluid low in enzyme content but high in bicarbonate. CCK—several functions: Causes increased exocrine secretion from the pancreas. Opposes gastrin, thus inhibiting gastric HCl secretion. Stimulates contraction of the gallbladder so that bile is ejected into the duodenum. Control of bile secretion—bile secreted continually by the liver; secretin and CCK stimulate ejection of bile from the gallbladder. Control of intestinal secretion—little known about how intestinal secretion is regulated; suggested that the intestinal mucosa is stimulated to release hormones that increase the production of intestinal juice.

Pepsin

Enzyme that breaks down proteins in the stomach

Describe how digestive enzymes work

Extracellular, organic (protein) catalysts which accelerates chemical reactions. Operate in lumen of digestive tract, outside any cells of the body. Properties of digestive enzymes: Specific in their action which means they act only on specific substrate. Function optimally at a specific pH. Most enzymes catalyze a chemical reaction in both directions. Enzymes are continually being destroyed or eliminated from the body and must continually be synthesized. Most digestive enzymes are synthesized as inactive proenzymes. All digestive enzymes are classified chemically as hydrolases because they catalyze the hydrolysis of food molecules—the breakdown of a molecule using water.

How the body digests fats

Fat digestion: Fats must be emulsified by bile in small intestine before being digested. Lipases are the main lipid-digesting enzymes.

List the locations where absorption of different substances takes place

Figure 40-22

Explain how the body regulates motility

Food in the stomach is churned (propulsion and retropulsion) and mixed with gastric juices to form chyme. Chyme is ejected about every 20 seconds into the duodenum; emptying the stomach takes approximately 2 to 6 hours. Gastric motility: Mixing actions in the stomach include both propulsion (forward movement) and retropulsion (backward movement). As peristaltic contractions become stronger, some of the liquid chyme squirts past the pyloric sphincter (which has decreased its muscle tone) and into the duodenum. The stomach continues to mix the chyme as it is gradually released into the duodenum. Gastric emptying controlled by hormonal and nervous mechanisms: Hormonal mechanism—fats in duodenum stimulate the release of gastric inhibitory peptide, which acts to decrease peristalsis of gastric muscle and slows passage of chyme into duodenum. Nervous mechanism—enterogastric reflex; receptors in the duodenal mucosa are sensitive to presence of acid and to distention; impulses over sensory and motor fibers in the vagus nerve cause a reflex inhibition of gastric peristalsis

Gastric inhibitory peptide (GIP)

Hormone produced by the intestinal mucosa that inhibits gastric secretion and motility; because it also enhances pancreatic insulin secretion in the presence of high plasma glucose, it has been more recently called glucose-dependent insulinotropic polypeptide (GIP).

Processes involved in mechanical digestion and the locations of these processes

Mechanical digestion—movements of the digestive tract. Change ingested food from large particles into minute particles, facilitating chemical digestion. Churn contents of the GI lumen to mix with digestive juices and ensure contact with the surface of the intestinal mucosa, facilitating absorption. Propel food along the alimentary tract, eliminating digestive waste from the body. Table 40-2

emulsification

Mechanical process that breaks fat drops into smaller droplets to facilitate digestions of fats

Segmentation

Mixing movement; digestive reflexes cause a forward-and-backward movement with a single segment of the GI tract; helps break down food particles, mixes food and digestive juices, and brings digested food in contact with intestinal mucosa to facilitate absorption. Steps: Ring like regions of contraction occur at interval along the GI tract. Previously contracted regions relax and adjacent regions now contract, effectively "chopping" the contents of each segment into smaller chunks.

How the body digests proteins

Protein digestion: Protein compounds are made up of twisted chains of amino acids; stomach acid helps enzymes reach more parts of a protein by loosening its folds. Proteases catalyze hydrolysis of proteins into intermediate compounds and, finally, into amino acids. Main proteases: pepsin in gastric juice, trypsin in pancreatic juice, peptidases in intestinal brush border.

The processes involved in digestive secretion and the locations of those processes

Saliva—secreted by salivary glands. Mucus lubricates food and, with water, facilitates mixing. Amylase—an enzyme that begins digestion of starches. Lingual lipase works at low pH, so can digest fats in stomach and upper duodenum. Sodium bicarbonate increases the pH for optimum amylase function Gastric juice—secreted by gastric glands. Pepsin (secreted as inactive pepsinogen by chief cells)—a protease that begins the digestion of proteins. Hydrochloric acid (HCl, secreted by parietal cells). HCl decreases the pH of chyme for activation and optimum function of pepsin, Released actively into the gastric juice by H-K pumps (proton pumps), Vesicles in the resting parietal cell move to the apical surface when the cell becomes active—thus increasing the surface area for the process of secretion. Intrinsic factor (secreted by parietal cells) protects vitamin B12 and later facilitates its absorption. Mucus and water lubricate, protect, and facilitate mixing of chyme. Pancreatic juice—secreted by acinar and duct cells of the pancreas Proteases (e.g., trypsin and chymotrypsin)—enzymes that digest proteins and polypeptides Lipases—enzymes that digest emulsified fats Nucleases—enzymes that digest nucleic acids such as DNA and RNA Amylase—an enzyme that digests starches Sodium bicarbonate increases the pH for optimum enzyme function; its manufacture also helps restore normal pH of blood. Bile—secreted by the liver; stored and concentrated in the gallbladder Lecithin and bile salts emulsify fats by encasing them in shells to form tiny spheres called micelles Sodium bicarbonate increases pH for optimum enzyme function Cholesterol, products of detoxification, and bile pigments (e.g., bilirubin) are waste products excreted by the liver and eventually eliminated in the feces Intestinal juice—secreted by intestinal exocrine cells. Mucus and water lubricate and aid in continued mixing of chyme. Sodium bicarbonate increases pH for optimum enzyme function.

Explain the processes involved with defecation

The act of expelling feces is called defecation. Defecation is a reflex brought about by stimulation of receptors in the rectal mucosa. Normally, the rectum is empty until mass peristalsis moves fecal matter out of the colon into the rectum. This distends the rectum and produces the desire to defecate. Also, it stimulates colonic peristalsis and initiates reflex relaxation of the internal sphincter of the anus. Voluntary straining efforts and relaxation of the external anal sphincter may then follow as a result of the desire to defecate. Together, these several responses bring about defecation.

Hydrolysis

a chemical process in which a compound unites with water and then splits into simpler compounds.

Motility

ability to move spontaneously.

Trypsin

an enzyme from the pancreas that digests proteins in the small intestine.

Intrinsic factor

binds to molecules of B12, protecting them from the acids and enzymes of the stomach; secreted by parietal cells.

Chief cells (Zymogenic cells)

cells lining the gastric glands of the stomach that secrete pepsinogen and intrinsic factor.

Pancreatic juice

digestive secretion; secreted by the exocrine acinar cells of the pancreas.

Protease

enzyme that catalyzes the hydrolysis of proteins into intermediate compounds (proteoses and peptides).

Amylase

enzyme that digests carbohydrates

Hydrolase

hydrolyzing enzyme.

Proenzyme

inactive form in which many enzymes are synthesized

Enterogastric reflex

nervous reflex causing inhibition of gastric peristalsis in response to the presence of acid and distention of duodenal mucosa; also may inhibit gastric secretion.

Proton pump

or H-K pump. Ion pump for hydrogen ions (H+) and potassium ions (K+) in the plasma membrane of gastric parietal cells.

Lipase

pancreatic enzyme necessary to digest fats

Chymotrypsin

pancreatic enzyme that digests proteins in the digestive tract.

Chyme

partially digested food mixture leaving the stomach.

Peptidase

protease found in the intestinal brush border; hydrolyzes peptides to amino acids

Saliva

secretion of the salivary glands that is made up of water, mucus, amylase, sodium bicarbonate, and lipase.

Gastric juice

stomach secretion containing acid and enzymes; aids in the digestion of food.

Lecithin

substance in bile that emulsifies dietary oils and fats in the lumen of the small intestine.

Kinase

substance that converts proenzymes to active enzymes

Peristalsis

wavelike ripple of the muscle layer of a hollow organ; progressive motility that produces forward movement of matter along the GI tract. Steps: A bolus stretches the GI wall, triggering a reflex contraction of circular muscle that pushes the bolus forward. This, in turn, triggers a reflex contraction in that location, pushing the bolus even farther. This process continues as long as the stretch reflex is activated by the presence of food. Peristalsis is a progressive kind of motility—that is, a type of motion that produces forward movement of ingested material along the GI tract. (See Fig. 40-3).

Bilirubin

yellowish pigment formed when the heme group is removed from the hemoglobin molecule and stripped of its iron atom; a product of the breakdown of red blood cells. (pigment released by the liver in bile).