copyright © 2006 by elsevier, inc. figure 65-6; guyton & hall

Copyright © 2006 by Elsevier, Inc.

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

1. Mucous layer2. Glycocalyx3. Apical cell membrane4. Cytoplasm of enterocyte5. Basolateral cell membrane6. Intercellular space7. Basement membrane

8. Wall of capillary or lymph vessel

• A nutrient must cross 8 barriers to be absorbed by blood or lymph


Stomach(2)

Fluid Entering and Exiting the Gut

Bile (1)

0

2

4

6

8

10

Vol

ume

(L/d

ay)

Diet (2)

Pancreas (1)

Saliva (1)

S.I. (2)

Duodenum and

Jejunum (4)

Colon (1.4)

Ileum(3.5)

Volumeentering

Volume absorbed

Volume Excreted

100-200 ml


Sodium Absorption in Small Intestine

Na+Na+

Na+

Gl

Gl

Na+

Na+Cl-Na+ Cl-

Na+

H+

Na+

H+

PNa+ Na+

K+ K+

Cl-Cl-

1

2

3

4

Aldosterone increasesNa+ reabsorption and K+ secretion in S.I. and colon.


Protein Digestion and Absorption

Dipeptides and tripeptides

Cytoplasmic peptidases

Amino acids

Di- and Tri-peptides

Largepeptides

Freeamino acids

CarriersPeptidases

Smallamounts Amino acids

Protein

PepsinPancreatic proteases

Carriers


Assimilation of Lipids –Overall Scheme

Triglyceride

Fatty acids &Monoglyceride

Triglyceride

Duodenum

Enterocyte


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

Emulsified fat

2-MGFFAFOOD

lecithinbile salts

lipase-colipase

2-MGFFA

(micelles)

bile saltsmicelles

2-MGFFATG

(enterocyte)apoprotein + 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

copyright © 2006 by elsevier, inc. figure 65-6; guyton & hall

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