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Figure 65-6; Guyton & Hall

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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.

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life cycle of s i enterocytes
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
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
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
Fluid Entering and Exiting the Gut

Volume

entering

Volume

absorbed

10

Diet (2)

Duodenum

and

Jejunum (4)

8

Saliva (1)

6

Volume (L/day)

Stomach

(2)

Ileum

(3.5)

4

Bile (1)

Volume

Excreted

100-200 ml

Pancreas (1)

2

S.I. (2)

Colon (1.4)

0

sodium absorption in small intestine
3

4

2

1

Na+

Na+

Na+

Gl

Gl

Na+

Na+

Na+

P

K+

K+

Na+

Cl-

Cl-

Na+

Na+

H+

Na+

H+

Cl-

Cl-

Sodium Absorption in Small Intestine

Aldosterone increases

Na+ reabsorption and K+ secretion in S.I. and colon.

protein digestion and absorption
Protein Digestion and Absorption

Protein

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
Assimilation of Lipids –Overall Scheme

Triglyceride

Duodenum

Fatty acids &

Monoglyceride

Enterocyte

Triglyceride

basic steps of lipid assimilation
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
Assimilation of Lipids

lipase-colipase

Emulsified

fat

lecithin

2-MG

FFA

FOOD

bile salts

bile salts

micelles

(enterocyte)

apoprotein + TG

2-MG

FFA

(micelles)

2-MG

FFA

TG

chylomicrons

lymph vessel

malabsorption
“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
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)
ad