SCC A&P 2 Ch. 29 (Bourgeade)

What is the primary function of the digestive system?

The primary function of the digestive system is to bring essential nutrients into the internal environment so that they are available to each cell of the body.

What is absorption?

Passage of substances through the intestinal mucosa into blood or lymph - most absorption occurs in the small intestine

What is chemical digestion?

changes in chemical composition of food during digestion

What is the function of mastication?

chewing, reduces the size of food particles, mixes food with saliva to prepare for swallowing

What organ secretes bile? What organ concentrates bile?

secreted by liver and concentrated in gall bladder

Name and describe the three stages of deglutition.

1. Oral stage: mouth to oropharynx 2. Pharyngeal stage: oropharynx to esophagus 3. Esophageal stage - esophagus to stomach

What does gastric juice contain?

1. Pepsin (secreted by chief cells) - begins digestion of proteins 2. HCl (secreted by parietal cells) - decreases the pH of chyme for activation and optimum function of pepsin 3. Intrinsic factor (secreted by parietal cells) - protects vitamin B12 and later facilitates its absorption 4. Mucus and water lubricate, protect and facilitate the mixing of chyme

What is pancreatic juice composed of?

1. Proteases - trypsin, chymotrypsin 2. Lipases - digest emulsified fats 3. Nucleases - enzymes that digest nucleic acids like DNA, RNA 4. Amylase 5. Sodium bicarbonate - increase the pH for optimum enzyme function, restore blood pH

What mechanisms are used to accomplish this primary function?

A. Ingestion - food taken in B. Digestion - breakdown of complex nutrients into simple ones C. Motility of the GI tract - physically breaks down large chunks of food material and moves food along tract. D. Secretion - digestive enzyme secretion facilitates chemical digestion. E. Absorption - movement of nutrients through GI mucosa into intestine. F. Elimination - excretion of material that is not absorbed G. Regulation - coordination of various functions of digestive system

Describe what occurs during intestinal motility.

- Segmentation in duodenum and upper jejunum mixes chyme with digestive juices from pancreas, liver and intestinal mucosa - Peristalsis rate picks up as chyme approaches end of jejunum, moving it through small intestine into large intestine - 5 hours for chyme to pass through small intestine - Peristalsis regulated in part by intrinsic stretch reflexes, stimulated by cholecystokinin (CCK)

What is the structure of carbohydrates?

- carbohydrates are saccharide compounds, their molecules contain 1 or more saccharide groups : C6 H10O5

What is constipation? Diarrhea?

- contents of lower part of colon and rectum move at a slower than normal rate, extra water is absorbed from feces and results in hardened stool. - increased motility of small intestine, causing decreased absorption of water and electrolytes and watery stool.

How are lipids digested?

- fats must be emulsified by bile in small intestine before being digested - pancreatic lipase is the main fat-digesting enzyme

What substances are present in saliva?

- mucus lubricates food and with water, facilitates mixing - amylase: enzymes that begins digestion of starches - sodium bicarbonate increases the pH for optimum amylase function

What enzyme breaks down polysaccharides?

- polysaccharides (starches and glycogens) contain several saccharide groups

What is the structure of proteins?

- proteins made of amino acids chains - proteases catalyze hydrolysis of proteins into intermediate compounds and then finally into amino acids

What are the functions of secretin? Of CCK?

- secretin begins the production of pancreatic fluid low in enzyme content but high in bicarbonate CCK - causes increase exocrine secretion from pancreas - opposes gastrin, thus inhibiting gastric HCl secretion - stimulates contraction of the gall bladder so that bile is ejected into duodenum

What are the 3 phases of gastric secretion?

1. Cephalic phase - "psychic phase" 2. Gastric phase 3. Intestinal phase

Describe the mechanisms of absorption.

1. For water - absorbed by osmosis 2. Other substances - secondary active transport Ex: Na - epithelial cells that form the outer wall of the villus constantly pump Na from the GI lumen into the intestinal environment 3. Glucose - cannot pass freely through mucosal cell, must be transported by a carrier Process is called sodium contrasport (coupled transport) - carriers that bind both Na ions and glucose molecules passively transport these molecules together out of GI lumen 4. Fatty acids, monoglycerides and cholesterol are transported with the aid of bile salts from the lumen to absorbing cells of the villi - After food is absorbed, it travels to the liver via the portal system.

What are the main proteases and where are they found?

1. pepsin in gastric juice 2. trypsin in pancreatic juice 3. peptidases in intestinal brush border

What are the characteristics of digestive enzymes?

1. specific in their action 2. function optimally at a specific pH 3. most enzymes catalyze a chemical reaction in both directions 4. enzymes are constantly synthesized (destroyed or eliminated)

What is elimination?

Expulsion of feces from digestive tract - defecation

How is gastric emptying under hormonal control? Under nervous control?

Hormonal - fats in duodenum stimulate release of gastric inhibitory peptide, which acts to decrease peristalsis of gastric muscle and slows passage of chyme into duodenum Nervous - enterogastric reflex, receptors in duodenal mucosa are sensitive to the presence of acid and to distension, impulses over sensory and motor fibers in the vagus nerve cause a reflex inhibition of gastric peristalsis

What is segmentation?

a mixing, forward and backward movement, helps break down food particles, mixes food and digestive juices and brings it in contact with intestinal mucosa to facilitate absorption

What structure secretes gastric juice?

gastric glands

How are fats emulsified?

lecithin and bile salts emulsify fats by encasing them in shells to form tiny spheres called micelles

What is the function of intestinal juice?

mucus and water lubricate and aid in continued mixing of chyme

What is defecation?

results from a reflex brought about by stimulation of receptors in rectal mucosa that is produced when the rectum is distended

Describe peristalsis.

wavelike ripple of the muscle layer, causes forward movement of matter