Unit Twelve: Gastrointestinal Physiology

Chapter 65: Digestion and Absorption

In the Gastrointestinal Tract

Guyton and Hall, Textbook of Medical Physiology, 12th edition

Digestion of the Various Foods by Hydrolysis

• Hydrolysis of Carbohydrates

a. When cbhs are digested, the water molecule thatwas removed during the synthesis reaction, is added back to split the polysaccharide

Digestion of the Various Foods by Hydrolysis

• Hydrolysis of Fats

a. The fat digesting enzymes return 3 water moleculesto the triglyceride and break the fatty acids away from the glycerol

• Hydrolysis of Proteins

a. Reverse of the condensation synthesis reaction also;the hydrogen from water goes to one of amino acidin a peptide bond and the hydroxyl ion goes to theother amino acid

Digestion of the Various Foods by Hydrolysis

• Digestion of Carbohydrates

a. Cbh foods of the diet- consists of sucrose, lactose, and starches

b. Digestion in the mouth and stomach-ptyalin (alpha-amylase)-eventually blocked by the acid of the stomach

d. Small intestine-pancreatic amylase; usually converted to maltose or other small glucose polymers

Digestion of the Various Foods by Hydrolysis

• Digestion of Carbohydrates

e. Hydrolysis by intestinal epithelial enzymes-enterocytes containing lactase, sucrase, maltase,and alpha-dextrinase

Fig. 65.1 Digestion of carbohydrates

Digestion of the Various Foods by Hydrolysis

• Digestion of Proteins

a. In the stomach- pepsin is the most important pepticenzyme of the stomach; active at pH 2-3 but inactive at pH5

Fig. 65.2 Digestion of proteins

Digestion of the Various Foods by Hydrolysis

• Digestion of Proteins

b. Most protein digestion results from actions of pancreatic proteolytic enzymes: trypsin,chymotrypsin, carboxypeptidase and proelastase

c. Digestion of peptides by peptidases in the entero-cytes that line the small intestinal villi; cells have abrush border containing hundreds of microvilli

d. More than 99% of the absorbed products are individual amino acids

Digestion of the Various Foods by Hydrolysis

• Digestion of Fats- the most abundant fats in the dietare triglycerides

Fig. 65.3 Hydrolysis of neutral fat catalyzed by lipase

Digestion of the Various Foods by Hydrolysis

• Digestion of Fats

a. Triglycerides or neutral fats are more commonlyfound in foods of animal origin

b. Digestion of fats in the stomach- small amount(<10%) by a lingual lipase (lingual glands of thetongue)

c. First step of fat digestion is emulsification bybile acids and lecithin

Digestion of the Various Foods by Hydrolysis

• Digestion of Fats

d. Triglycerides are digested by pancreatic lipase

e. End products of fat digestion are free fatty acids

Fig. 65.4 Digestion of fats

Digestion of the Various Foods by Hydrolysis

• Digestion of Fats

f. Bile salts from micelles that accelerate fat digestion

g. Digestion of cholesterol esters and phospholipids-two other lipases in the pancreatic juice; cholesterol ester hydrolase and phospholipase A

Basic Principles of GI Absorption

• Anatomical Basis of Absorption- total quantity offluid absorbed each day by the intestines is equal to the ingested fluid plus that secretedby the various gastrointestinal secretions (8-9liters); about 7 is reabsorbed in the small intestine.

Only about 1.5 passes through the ileocecal valve into the colon each day; stomach is a poor absorptive area (except alcohol and aspirin for example)

Basic Principles of GI Absorption

• Anatomical Basis of Absorption

a. Folds of Kerckring, villi, and microvilli increase theintestinal mucosal absorptive area by 1000 fold

Fig. 65.5 Longitudinal section of the small intestine, showing the valvulae conniventes covered by villi

Basic Principles of GI Absorption

• Anatomical Basis of Absorption

b. The folds are called valvulae conniventes and extend most of the way around the intestine

c. Also, there are millions of villi all the way down tothe ileocecal valve

Basic Principles of GI Absorption

• Anatomical Basis of Absorption

Fig. 65.6 Functional organization of the villus; longitudinal (A) and cross-sections (B)

Basic Principles of GI Absorption

• Absorption in the Small Intestine

a. Absorption of water by osmosis

b. Absorption of ions- active transport of sodium;sodium is also co-transported by several specificcarrier proteins:

1) Sodium-glucose co-transporter2) Sodium-amino acid co-transporters3) Sodium-hydrogen exchanger

Basic Principles of GI Absorption

Fig. 65.8 Absorption of sodium, chloride, glucose, and amino acids through the intestinal epithelium; note the osmotic absorption of water

Basic Principles of GI Absorption

c. Aldosterone greatly enhances the absorption ofsodium.

d. Absorption of choride ions in the small intestine-mainly by diffusion following an electrochemicalgradient caused by sodium flow

e. Absorption of bicarbonate ions in the duodenumand jejunum-when sodium is absorbed, small amounts of hydrogen are secreted into the lumen of the gut in exchange for some of the sodium.

Basic Principles of GI Absorption

Hydrogens combine with bicarbonate to form carbonicacid which then dissociates to form water and carbondioxide. Water remains as part of the chyme, but thecarbon dioxide is absorbed into the blood, and expiredthrough the lungs—this is called the “active absorptionof bicarbonate ions” and is the same mechanism thatoccurs in the tubules of the kidney.

f. Secretion of bicarbonate ions in the ileum and largeintestine—simultaneous absorption of chloride ions

Basic Principles of GI Absorption

g. Active absorption of calcium, iron, potassium, magnesium, and phosphate

• Absorption of Nutrients

a. Cbh are mainly absorbed as monosaccharidesb. Proteins are absorbed as dipeptides, tripeptides, or

amino acidsc. Fats are absorbed as micelles into lacteals; small

amounts are absorbed as fatty acids into the portalblood

Absorption in the Large Intestine-Formation of Feces

• Absorption and Secretion of Electrolytes and Water

a. High capacity for the active absorption of sodiumand chloride (due to electrochemical gradient of sodium)

b. Creates an osmotic gradient which in turn causesabsorption of water

c. Mucosa secretes bicarbonate to counter the acidityof bacterial end products

Absorption in the Large Intestine-Formation of Feces

• Maximum Absorption Capacity of the Large Intestine

a. 5-8 liters of fluids and electrolytes each day

b. Bacterial action in the colon-vitamin K, vitamin B-12,thiamine, riboflavin, and gases (carbon dioxide, hydrogen gas, and methane)

c. Composition of the feces- 75% water and 25% solidmatter; solid matter is 30% dead bacteria, 19-20% fat,10-20% inorganic matter, 2-3% protein, 30% undigestedroughage; color is due to bilirubin by-products; odordue to bacterial actions