1. Figure 65-6; Guyton & Hall

2. 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.

3. 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

4. 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

5. 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 ml
Pancreas (1) 2
S.I. (2)
Colon (1.4)

6. 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.

7. 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

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

9. 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

10. 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

11. “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

12. 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)

13. Figure 62-4; Guyton & Hall