Figure 65-6; Guyton & Hall

Life Cycle of S.I. Enterocytes Villi house self-renewing population of epithelial cells with a 5-day turnover. Cell types in villus include: secretory cells, endocrine cells, goblet cells, and mature absorptive epithelial cells. Cells in enterocyte lineage divide and differentiate as they migrate up crypts, becoming mature absorptive cells. Enterocytes are shed into lumen to become part of ingesta to be digested and absorbed.

Mechanisms of Absorption Four mechanisms are important in transport of substances across intestinal cell membrane Active Transport - - primary - secondary (co-transport, counter-transport) Passive Diffusion Facilitated Diffusion - carrier mediated Endocytosis

Absorptive Pathway of Nutrients A nutrient must cross 8 barriers to be absorbed by blood or lymph 1. Mucous layer 2. Glycocalyx 3. Apical cell membrane 4. Cytoplasm of enterocyte 5. Basolateral cell membrane 6. Intercellular space 7. Basement membrane 8. Wall of capillary or lymph vessel

Fluid Entering and Exiting the Gut Volume entering Volume absorbed 10 Diet (2) Duodenum and Jejunum (4) 8 Saliva (1) Volume (L/day) 6 Stomach (2) Ileum (3.5) 4 Bile (1) Volume Excreted 100-200 ml Pancreas (1) 2 S.I. (2) Colon (1.4)

Sodium Absorption in Small Intestine Na+ Gl Cl- H+ P K+ 1 2 3 4 Aldosterone increases Na+ reabsorption and K+ secretion in S.I. and colon.

Protein Digestion and Absorption Pepsin Pancreatic proteases Di- and Tri- peptides Large peptides Free amino acids Carriers Peptidases Carriers Small amounts Dipeptides and tripeptides Amino acids Cytoplasmic peptidases Amino acids

Assimilation of Lipids – Overall Scheme Triglyceride Duodenum Fatty acids & Monoglyceride Enterocyte Triglyceride

Basic Steps of Lipid Assimilation Most dietary lipid is neutral fat or triglyceride. Three main processes must occur for triglyceride to be absorbed into blood: Emulsification - large aggregates of dietary triglyceride are broken down. Enzymatic digestion - to yield monoglyceride and fatty acids. Both can diffuse into enterocyte. Reconstitution of triglyceride and chylomicron formation

Assimilation of Lipids lecithin Emulsified fat lipase-colipase 2-MG FFA FOOD bile salts micelles bile salts (enterocyte) apoprotein + TG 2-MG FFA (micelles) 2-MG FFA TG chylomicrons lymph vessel

“Malabsorption” Malabsorption as a general phenomenon is defined clinically in terms of fat malabsorption because fat can be measured easily in stool, unlike carbohydrates and proteins. Motility disorders - moving through too rapidly Digestion disorder - pancreatitis / cystic fibrosis - (not enough lipase) Absorption disorder - tropical and nontropical sprue - resection of small intestine

Neural Control of GI Tract Intrinsic Control - Enteric nervous system Myenteric (Auerbach’s) plexus Submucosal (Meissner’s) plexus Extrinsic Control - Autonomic nervous system Parasympathetic - mainly stimulates (Ach) Sympathetic - mainly inhibits (NE)

Figure 62-4; Guyton & Hall