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Ch:- 3

Pg:- 35,36

Structure of Villi and Microvilli:

• Villi: Each villus (singular of villi) is a tiny projection about 0.5-1.5 millimeters long, covering the inner surface of the small intestine. Villi are densely packed, covering a large area and increasing the surface area significantly.

• Microvilli: On the surface of the epithelial cells lining each villus, there are even smaller projections called microvilli. Together, these form the “brush border,” which increases surface area even further, maximizing the intestine’s ability to absorb nutrients.

Process of Nutrient Absorption in the Villi:

• Digestion in the Small Intestine: By the time food reaches the villi in the small intestine, it has been broken down into small, absorbable molecules: simple sugars from carbohydrates, amino acids from proteins, fatty acids and glycerol from fats, as well as vitamins and minerals.

• Absorption of Nutrients:

  • Carbohydrates: Simple sugars like glucose are absorbed through the cell membranes of the epithelial cells in the villi via a process called active transport. Glucose is then transported into the capillaries within each villus.

  • Proteins: Amino acids are similarly absorbed through active transport and enter the bloodstream through capillaries in the villi.

  • Fats: Fatty acids and glycerol, due to their nonpolar nature, diffuse passively into the epithelial cells. Within these cells, they are reassembled into triglycerides, packaged into lipoprotein particles called chylomicrons, and absorbed into lymphatic capillaries (lacteals) within the villi. These chylomicrons eventually enter the bloodstream after being transported through the lymphatic system.

  • Water and Electrolytes: Water is absorbed passively through osmosis, while electrolytes like sodium and potassium are absorbed via active and passive transport mechanisms.

• Transport into Circulation: The capillaries in each villus carry absorbed nutrients directly to the hepatic portal vein, which transports blood to the liver for further processing before nutrients are distributed to the rest of the body.

Adaptations of Villi for Effective Absorption:

• Large Surface Area: The vast number of villi and microvilli increases the surface area, enhancing absorption efficiency.

• Thin Epithelial Layer: The single layer of epithelial cells allows for easy diffusion or transport of nutrients into the blood.

• Rich Blood Supply: Each villus contains a network of capillaries, ensuring that absorbed nutrients are quickly transported into the bloodstream.

• Lacteals: The lymphatic vessels (lacteals) within villi facilitate the absorption of large lipid molecules and fat-soluble vitamins, which can’t directly enter the blood.

Importance of Villi in Nutrient Absorption:

The villi and microvilli play an indispensable role in nutrient absorption by creating a massive surface area within the confined space of the small intestine, allowing almost all nutrients from digested food to be efficiently absorbed into the bloodstream. If the villi are damaged or reduced in number (as in conditions like celiac disease), nutrient absorption is significantly impaired, leading to malnutrition and other health complications.