KEY POINTS

1. Digestion and absorption are separate, but related, processes.

2. The small-intestinal mucosa has a large surface area and epithelial cells with "leaky" junctions between them.

3. The intestinal surface microenvironment consists of glycocalyx, mucus, and an unstirred water layer.

Digestion

1. Breaking down food particle size by physical action is an important part of the digestive process.

2. Chemical digestion results in the reduction of complex nutrients into simpler molecules.

3. Luminal-phase carbohydrate digestion results in the production of short-chain polysaccharides.

4. Luminal-phase digestion of carbohydrates applies only to starches, because sugars are digested in the membranous phase.

5. Proteins are digested by a variety of luminal-phase enzymes.

6. Membranous-phase digestive enzymes are a structural part of the intestinal surface membrane.

7. Membranous-phase digestion occurs within the microenvironment of the unstirred water layer, intestinal mucus, and glycocalyx.

8. A specific membranous-phase enzyme exists for the digestion of each type of polysaccharide.

9. Complete digestion of peptides to free amino acids takes place both on the enterocyte surface and within the cells.

Intestinal Absorption

1. Specialized nutrient transport systems exist in the apical and basolateral membranes.

2. Secondary and tertiary active transport mechanisms utilize the transcellular sodium ion electrochemical gradient as their source of energy.

3. Passive transport occurs either through ion channels in cell membranes or directly through the tight junctions.

4. The products of membranous-phase digestion are absorbed by sodium co-transport.

Absorption of Water and Electrolytes

1. There are at least three distinct mechanisms of sodium absorption.

2. There are three major mechanisms of chloride absorption.

3. Bicarbonate ion is secreted by several digestive glands and must be recovered from the gut if body acid-base balance is to be maintained.

4. Potassium is absorbed primarily by passive diffusion through the paracellular route.

5. The major mechanisms of electrolyte absorption are selectively distributed along the gut.

6. All intestinal water absorption is passive, occurring because of the absorption of osmotically active solutes.

Intestinal Secretion of Water and Electrolytes

1. Passive increases in luminal osmotic pressure occur during hydrolytic digestion and result in water secretion.

2. Active secretion of electrolytes from the crypt epithelium leads to intestinal water secretion.

Gastrointestinal Blood Flow

1. Water and solute movement between the lateral spaces and villous capillaries is subject to the same forces that govern water and solute movement between the extracellular and vascular fluids in other tissues.

2. Absorbed nutrients enter the capillaries by diffusion from the lateral spaces.

3. A countercurrent, osmotic-multiplier system may increase the osmolality of blood at the tips of the villi, further promoting absorption of water into the blood.

4. Disturbances in the venous drainage from the intestine can greatly affect the mechanisms of capillary absorption in the villi.

Digestion and Absorption of Fats

1. Detergent action as well as enzymatic action is necessary for the digestion and absorption of lipids.

2. Lipids are absorbed through the apical membrane by carrier proteins and simple diffusion.

3. Bile acids are reabsorbed from the ileum by a sodium co-transport system.

4. Absorbed lipids are packaged into chylomicrons before leaving the enterocytes.

Growth and Development of the Intestinal Epithelium

1. The length of intestinal villi is determined by the relative rates of cell loss at the tips and cell replenishment at the base.

Digestion in the Neonate

1. During the first few hours of life, proteins are not digested but are absorbed intact.

2. The major intestinal disaccharidase switches from lactase to maltase with maturity.

Pathophysiology of Diarrhea

1. Diarrhea occurs when there is a mismatch between secretion and absorption.