Transcript Introduction to Physiology: The Cell and General Physiology

Figure 65-6; Guyton & Hall
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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.
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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
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Absorptive Pathway of Nutrients
• A nutrient must cross 8 barriers to be
absorbed by blood or lymph
1.
2.
3.
4.
5.
6.
7.
Mucous layer
Glycocalyx
Apical cell membrane
Cytoplasm of enterocyte
Basolateral cell membrane
Intercellular space
Basement membrane
8. Wall of capillary or lymph vessel
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Fluid Entering and Exiting the Gut
Volume
absorbed
8
Diet (2)
6
Saliva (1)
Stomach
(2)
Duodenum
and
Jejunum (4)
Volume (L/day)
10
Volume
entering
4
2
Bile (1)
Ileum
(3.5)
Pancreas (1)
0
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S.I. (2)
Colon (1.4)
Volume
Excreted
100-200 ml
Sodium Absorption in Small Intestine
1
2
3
4
Aldosterone
increases
Na+ reabsorption
and K+ secretion in
S.I. and colon.
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Na+
Na+
Na+
Gl
Gl
Na+
Na+
Cl-
ClNa+
H+
Na+
Cl-
Na+
K+
Na+
K+
P
Na+
H+
Cl-
Protein Digestion and Absorption
Protein
Pepsin
Pancreatic proteases
Di- and Tripeptides
Large
peptides
Free
amino acids
Carriers
Peptidases
Carriers
Dipeptides and tripeptides
Cytoplasmic peptidases
Amino acids
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Small
amounts
Amino acids
Assimilation of Lipids –
Overall Scheme
Triglyceride
Duodenum
Fatty acids &
Monoglyceride
Enterocyte
Triglyceride
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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
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Assimilation of Lipids
lecithin Emulsified
FOOD
fat
bile salts
apoprotein + TG
(enterocyte)
TG
chylomicrons
lymph vessel
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2-MG
FFA
lipase-colipase
2-MG
FFA
micelles
bile salts
2-MG
FFA
(micelles)
“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
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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)
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Figure 62-4; Guyton & Hall
Copyright © 2006 by Elsevier, Inc.