Physio Chp. 14 Digestive System

Identify the hormones of the GI tract and adipose tissue that influence hunger and satiety, and explain their actions.

Ghrelin - secreted by the stomach, which increases between meals when the stomach is empty, and stimulates hunger. CCK (cholecystonkinin)- secreted by the small intestine, which reduces appetite. PYY (polypeptide Y-Y) - recently found! secreted in the small intestine, which suppresses hunger on a more intermediate term, helping to space meals. Leptin - secreted by adipose tissue, which increases as adipose cells store fat, which then acts on the hypothalamus to suppress appetite and increase the metabolic rate.

Explain the actions of intrinsic factor, ghrelin, gastrin, and histamine in stomach function

Intrinsic factor - absorbs vitamin B12 Ghrelin - promotes hunger when stomach is empty Gastrin - directly stimulates the parietal cells to secret HCl and ECL cells to secrete histamine Histamine - stimulates the secretion of HCl by binding to H2 histamine receptors.

List and describe the three components of the body's caloric expenditures.

1) Basal Metabolic Rate (BMR) - 60% of total calorie expenditure; energy expenditure of a relaxed, resting person at a comfortable temperature. 2) Adaptive thermogenesis - heat energy expended to maintain homeostasis of body temperature and digest and absorb food. 3) Physical activity increases metabolic rate of skeletal muscles, which can significantly raise the body's BMR.

List and describe the functions of the digestive system:

1) Motility - movement of food through the espophagus into the stomach, the churning of the stomachs contents, and then the movement of food through the small and large intestines. 2) Secretion - digestive juices and hormones are exocrine/endocrine secretions regulate digestive functions. 3) Digestion - the breakdown of food molecules into their smaller subunits; proteins à amino acids; starch/disaccharides à monosaccharides; tryglycerides à fatty oils/glycerol 4) Absorption - the transport of the subunits above from the lumen of the GI tract into the blood or lymph 5) Excretion - wastes that are not absorbed are temporarily stored in the large intestine and then excreted

Pancreatic juice contains bicarbonate and digestive enzymes:

1) Pancreatic amylase - starch 2) Trysin - proteins 3) Lipase - tryglycerides 4) Phospholipase - phospholipids 5) Ribonuclease/Deoxyribonuclease - nucleic acids

Describe some of the different functions of the liver

1) Production and secretion of bile 2) Detoxication of the blood 3) Regulation of blood glucose 4) Metabolism of lipids 5) Synthesis of plasma proteins

Identify the composition and functions of bile and pancreatic juice

Bile is produced by the liver when it converts cholesterol into bile acid, which bind to the amino acids glycine and taurine form bile salts. Bile emulsifies fat, which means it breaks down larger fats into smaller fats, creating a finer suspension of fat in water, which aids with digestion.

Identify the enzymes involved in carbohydrate digestion and their locations. Describe how monosaccharides are absorbed.

Carbohydrates (commonly in the form of starch) is digested with the help of salivary amylase (in the mouth) and pancreatic amylase (in the pancreatic juice). Monosaccharides are absorbed by the brush border enzymes in the microvilli in the small intestine.

Describe the regulation of gastric activity during cephalic, gastric, and intestinal phases.

Cephalic - regulation of the stomach by the brain. Vagus nerve stimulates G cells to secrete gastrin and ECL cells to secrete histamines. Gastrin also stimulates ECL cells to secrete histamines, which stimulates the parietal cells to secrete HCl Gastric - begins with the arrival of partially digested food into the stomach. Shorter polypeptides and amino acids stimulate pepsinogen and gastrin secretion. Gastrin then stimulates increased pepsinogen secretion from the chief cells and histamine secretion from the ECL cells. Histamine then stimulates the parietal cells to secret HCl Intestinal phase - inhibition of the stomach when chyme reaches the small intestine. Chyme activates a neural reflex that inhibits gastric motility and secretion. The fat in the chyme stimulates the secretion of inhibitory hormones called enterogastrone, which keeps the chyme longer, allowing the intestine more time to process the fatty chyme.

Describe how food is transported along the esophagus and how gastric juice is normally prevented from entering the esophagus.

Food is transported along the esophagus in the form of bolus and through contraction of smooth muscle behind the bolus and relaxation of smooth muscle ahead of the bolus (peristalsis, wave like contraction). Gastric juice is prevented from entering the esophagus by the lower esophageal sphincter, which closes after the food passes into the stomach to prevent stomach juices from coming up into the esophagus.

List the cells of the gastric glands and the substances they secrete

Goblet cells - mucus Parietal cells - hydrochloric acid and intrinsic factor Chief cells - pepsinogen (pre-cursor to pepsin) Enterochromoaffin-like (ECL) cells - histamine G-Cells - Gastrin Ghrelin secreting cells - promotes hunger

Identify the enzymes involved in protein digestion and their locations. Describe how amino acids are absorbed.

Pepsin, trysin and other protein-digestion enzymes in pancreatic juice break down protein. Most of it occurs in the duodenum and jejunum. Free amino acids are transported through the intestinal epithelial cells and released into the blood capillaries of the villi. The di/tripeptides are transported into the epithelial cell cytoplasm and then digested into separate amino acids within the cells.

Identify the brush border, and the significance of the brush border enzymes

The brush border is made up of microvilli on the apical surface of the villi. The brush border enzymes are digestive enzymes imbedded within the microvilli, and complete the digestion of food molecules that other enzymes. i.e. - sucrase and lactase.

Explain the nature and significance of the intestinal microbiota.

The colon harbors a lot of bacteria (ten times more than total human body cells) and provide the host (us) with several benefits: 1) They produce significant amounts of vitamin K and folic acid 2) The produce short-chain fatty acids, which are used for energy and aid the absorption of Na, H2CO3, magnesium, and Fe 3) They reduce the ability of pathogenic bacteria to cause disease, promoting anti-inflammatory processes that help protect the intestine from injury.

Describe the structure of the large intestine, and how it differs from the small intestine.

The large intestine is made up of the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal. The large intestine doesn't have villi and microvilli.

Describe the structure of the small intestine, identifying the parts that increase the surface area exposed to the chyme

The small intestine in divided into three parts - the duodenum, jejunum, and ileum. The villi and micro villi increase the surface area of the small intestine..

Explain how the secretions of bile and pancreatic juice are regulated

When acidic chyme enters the duodenum and the pH falls below 4.5, secretin is released, which secretes Bicarbonate and neutralizes the acidity of the chyme. Fat and protein content of chyme stimulate the small intestine to release Cholecystokinin (CCK), which stimulates the pancreas to produce and secrete digestive enzymes. Theses two hormones make up the two components of pancreatic juice (bicarbonate and digestive enzymes). Secretin and CCK also stimulate the liver to produce bile. CCK stimulates the contraction of the gallbladder, causing the bile to be ejected into the duodenum.