Chapter 3Part 2

Digestion is the process of breaking down foods into nutrients to prepare for absorption

Visual, olfactory, and auditory senses Taste (smell)

◦ Sweet sour, salty, bitter, umami (savoriness) Presence of food in the mouth

◦ Activate secretory responses from Salivary glands Stomach Pancreas

◦ Contraction of gall bladder Bile

Anatomy – Gastrointestinal (GI) tract

Flexible muscular tube from mouth to anus

Lumen - inner space of the tract.

Anatomy –GI tract◦ Mouth - beginning of digestive system and

digestion Mastication (chewing) -stimulates taste buds, Swallowing - epiglottis closes to prevent food from

entering the pharynx.

Anatomy - GI tract◦ Esophagus - tube carrying bolus to stomach. ◦ Sphincters – control release, keeps food moving

forward Peristalsis Upper and lower esophageal sphincters (cardiac

sphincter - LES)

Anatomy - GI tract◦ Stomach – HCl and enzymes turn food into a

fluidy paste Holds 2 to 4 cups of food (6 L) Grinds the bolus to a semiliquid mass (chyme) Holding “tank”

Emptying closely regulated 5 ml/min (teaspoon) 1-4 h to empty Pyloric sphincter

Anatomy - GI tract◦ Stomach

Prevention of autodigestion Mucus lining HCO3

-

Prevents HCl in the stomach from burning through the stomach lining

Stomach Acid (HCl)◦ Destroys biological activity of protein◦ Activates digestive enzymes◦ Partially digests dietary protein◦ Assists in vitamin B12 absorption

B12 requires gastric acid to be released from food◦ Improves absorption of minerals

Gastrin stimulates secretion of gastric hydrochloric acid, performing many functions.◦ Activates pepsin◦ Stimulates the release of hydrochloric acid◦ Kills pathogenic organisms◦ Improves absorption of iron and calcium◦ Inactivates hormones of plant and animal origin◦ Denatures food proteins, making them more vulnerable

to enzyme action

Anatomy - GI tract◦ Small intestine - digestive juices

Gallbladder (Bile) and pancreas (enzymes that break down CHO, fats and proteins)

Three parts of small intestine Duodenum Ileum Cecum

◦ Large intestine (colon) Ileocecal valve to rectum and anus.

Anatomy – GI tract◦ Approximately 10 feet long

Narrow◦ Duodenum (10 in)

Where enzymes and bile are added Regulatory center that senses:

Nutrient content (particularly fat) pH (HCO3

- from pancreas raises pH of chyme from stomach)

◦ Jejunum (4 ft)◦ Ileum (5 ft)

Anatomy – GI tract◦ Physical structure – intestinal wall

Folded walls Villi – made up of absorptive cells Increase surface area greatly Rapid turnover absorptive cells (daily)

The Muscular Action of Digestion – under autonomic control◦ Peristalsis - pushes the digestive contents along.

Muscles are circular, longitudinal, and diagonal muscles.

◦ Sphincter contractions open and close passageways Prevents reflux, controls the passage of contents.

Fig. 3-2, p. 75

Longitudinal

Circular

Diagonal

Salivary glands Pancreas Liver (gall bladder) Stomach Small intestine

Begins in the mouth Saliva

◦ Electrolytes (Na, Cl)◦ Solvent◦ Enzymes (salivary amylase)◦ Mucus

Saliva moistens food◦ Begins breakdown of CHO

Small Intestine ◦ Pancreatic amylase (duodenum)

Smaller CHO molecules◦ Brush border enzymes

Maltase, sucrase, lactase Disaccharides to monosaccharides

Maximum rate of glucose absorption◦ 50 – 80 g/h (70 kg)◦ 4 g CHO/min or 240 g/h

Absorbed into circulation◦ Hepatic portal vein

Liver

Large intestine◦ Water reabsorption◦ Some digestion

Mostly bacterial Vitamin absorption

Vitamins K, B12, thiamin and riboflavin

Compacts and prepares feces for defecation

Mouth ◦ Lingual lipase

Stomach◦ Gastric lipase

Pancreas ◦ Pancreatic lipase◦ Bile

Cholecystokinin (CCK)◦ Released from wall of duodenum◦ In response to fat and protein in the chyme◦ Causes the release of bile and pancreatic

enzymes Secretin

◦ Stimulates bile secretion by liver and release of bicarbonate into duodenum

Micelles ◦ Monoglyceride, FFA + bile salts

Micelles are formed after bile acts on large fat droplets

Pancreatic and brush border enzymes breakdown micelles to fatty acids

These fatty acids are then repackaged as chylomicrons, which are largely absorbed into the lymphatic system

They then travel to the heart and enter the circulation

Pepsinogen - precursor Pepsin

◦Released from stomach wall Gastrin

◦Release of HCL ◦Controls pepsin release

Trypsinogen Secreted by pancreas

Trypsin Tripeptides, dipeptides,

and single amino acids. Brush border enzymes Small intestine into

portal vein Active absorption

Vitamin absorption Passive diffusion In the jejunum and ileum.

Fat-soluble vitamins With dietary lipids Transport

Chylomicrons, lipoproteins

Water-soluble vitamins Diffuse into the blood Excess (exceeds renal capacity)

Pass into the urine Vitamin B12.

Intrinsic factor Intestine absorbs by endocytosis.

Dietary factors Type of food source impacts absorption

Cellular factors The body does not absorb minerals very well. Availability depends on its chemical form

Heme vs nonheme

Passive process of osmosis in small intestine

9 L of water each day.◦ 72% absorbed in the proximal small intestine ◦ 20% absorbed from the distal segment of the

small intestine ◦ 8% absorbed from the large intestine