3. 2. Intestinal tenue
• Structural modifications of the small intestine wall increase
surface area
– Plicae circulares/Folds of Kercking: deep circular folds of the mucosa
and submucosa
– Villi – fingerlike extensions of the mucosa
– Microvilli – tiny projections of absorptive mucosal cells’ plasma
membranes
5. Area of simple cylinder
Folds of Kercking
Villi
Microvilli
(brush border)
Degree by which different anatomical features increase surface area in the small intestine:
Increase in Surface Area
(Relative to cylinder)
Surface Area (cm2)Structure
1
600
30
3
4 cm Dia. x 260 cm L
~3,300
~10,000
~100,000
~2,000,000
6. • Carbohidrat
via cotransport with Na+, and facilitated diffusion
– Enter the capillary bed in the villi
– Transported to the liver via the hepatic portal vein
• Protein
Absorption: similar to carbohydrates
• Lipid
Diffusion into intestinal cells . The intestinal villi are coated by an
unstirred water layer which reduces the absorption of the poorly
water soluble lipids.
• Nucleat acid
Absorption: active transport via membrane carriers
Absorbed in villi and transported to liver via hepatic portal vein
7. • Vitamins
– Water soluble vitamins are absorbed by diffusion
– Fat soluble vitamins are absorbed as part of micelles
• Vitamin B12 requires intrinsic factor
• water
Absorption of water is passive and is determined
by differences in osmolarity of luminal
content and blood, therefore net transport of
water can occur in both direction.
8. • Electrolyte
Most ions are actively absorbed along the length of small
intestine
– Na+ is coupled with absorption of glucose and amino acids
– Ionic iron is transported into mucosal cells where it binds to ferritin
Anions passively follow the electrical potential established by
Na+
K+ diffuses across the intestinal mucosa in response to osmotic
gradients
Ca2+ absorption:
– Is related to blood levels of ionic calcium
– Is regulated by vitamin D and parathyroid hormone (PTH)
