Absorption

• Uptake of digested nutrients and water from the lumen of the digestive tract into the bloodstream and lymphatic vessels.

Small intestine (specifically the duodenum)

• Major site of nutrient absorption.
• Three folded mucosal structures maximize surface area for absorption
  1 – Plicae circulares: wavy, folds on the inner walls of small intestine → form circular folds → increase surface area 3-fold.
  2 – Villi: finger-like projections that protrude from the plicae circulares → surface area by 10-fold. Arterioles, venules, and lymphatic vessels pass through the villi and uptake absorbed nutrients.
  3 – Microvilli (brush border): hair-like projections on columnar small intestine epithelial cells (face the lumen of the small intestine) → increase surface are 20-fold. Together, all folded layers = 600-fold surface area increase

Nutrients are absorbed in the small intestine

• Monosaccharides (digested carbohydrate products)
• Amino acids, di-peptides and tri-peptides (digested protein products)
• Intact proteins
• Short-chain fatty acids, long-chain fatty acids, and glycerol (which are digested lipid products)
• Vitamins
• Water and electrolytes

Nutrients cross the apical and basolateral surfaces of the intestinal epithelium for absorption into circulation or the lymphatic system.

• Apical surface: interfaces the intestinal lumen and epithelium.
• Basolateral surface: opposite to the apical surface, lines the inside of the villi.
• Capillaries and lacteals inside villi
  – Most nutrients cross the basolateral surface and pass directly into circulation.
• Fats, however, pass directly into lacteals (lymphatic system).

Key Transport Mechanisms

• Apical Surface
  – Secondary active transport: transporter moves an ion movement down its concentration gradient, which generates energy for it to move another ion (or molecule) against its concentration gradient.
  – Facilitated diffusion: Transporter passively moves an ion or molecule across the plasma membrane, down its concentration gradient.
  – Simple diffusion: in which non-charged, lipid, and hydrophobic molecules passively cross through the plasma membrane (without a transmembrane protein) down their concentration gradient.
  – Endocytosis: form of active, energy-requiring cellular ingestion, which transports large substances into the cell.

• Basolateral Surface
  – Facilitated diffusion.
  – Simple diffusion.
  – Exocytosis: the opposite of endocytosis – a vacuole actively fuses with the plasma membrane to release its contents into the extracellular environment.

• Water Absorption

Most relevant to the function of the large intestine but also occurs in the small intestine.

– Large intestine stores and concentrates fecal material before elimination.
– Mainly absorbs water and electrolytes to do so
– Also absorbs bacterial byproducts.

• Three General Steps of Water Absorption:

Step 1:

• Sodium-potassium pump on the basolateral surface
  – Pumps K+ into cell and Na+ out of cell (pump utilizes ATP to move sodium and potassium against their concentration gradients).
• Na+ passively enters via general sodium ion transporter on the apical surface (possible because of the active transport of sodium out of the cell)
• Net positive charge in the cell

Step 2:

• Chloride enters the cell through a general chloride ion transporter on the apical surfacen via facilitated diffusion down the electrical gradient.
• Higher solute concentration inside the cell relative to the lumen.

Step 3:

• Water crosses the apical surface via osmosis.