The small intestine is specifically designed (adapted) to absorb the maximum amount of digested food (nutrients) into the bloodstream. Its key adaptations include: a very long length (~6–7 m), inner wall lined with millions of finger-like projections called villi (which are further covered with microvilli forming the brush border), and a rich blood and lymph supply. Together, these features provide an enormous surface area for nutrient absorption.
Small intestine length: ~6–7 m — long to maximise absorption time.
Villi: finger-like projections on inner wall — increase surface area to ~2 m².
Microvilli (brush border): cover villi — increase surface area to ~250 m² (tennis court size).
Each villus has blood capillaries (for glucose/amino acids) and lacteals (for fats).
Single-cell thick walls of villi allow rapid nutrient passage into blood.
Glucose and amino acids → blood capillaries → hepatic portal vein → liver.
Fats → lacteals (lymph vessels) → lymph → bloodstream.
Long Length: • The small intestine is about 6–7 metres long in a living adult • This long path provides maximum time for absorption • The partially digested food moves slowly, allowing nutrients to be absorbed
Villi (singular: Villus): • The inner lining (mucosa) of the small intestine has millions of tiny finger-like projections called villi • Each villus is about 0.5–1 mm long • Villi greatly increase the surface area available for absorption • Without villi, the inner surface area ≈ 0.33 m² • With villi, surface area ≈ 2 m²
Microvilli (Brush Border): • Each villus is covered with even smaller projections called microvilli • Microvilli form the 'brush border' of the intestinal cells (enterocytes) • With microvilli, total surface area ≈ 250 m² (as large as a tennis court!) • This massively increased surface area ensures rapid and efficient absorption
Rich Blood Supply: • Each villus contains a network of capillaries (tiny blood vessels) • Absorbed glucose and amino acids enter these capillaries → carried to liver via hepatic portal vein
Lacteals: • Each villus also contains a lacteal — a lymph vessel • Absorbed fats enter the lacteals → transported through the lymph system
Single-cell thick walls: • The wall of each villus is only one cell thick (epithelial cells) • This short distance allows nutrients to pass quickly from the gut into the blood
What is absorbed in the small intestine:
Carbohydrates → Glucose (monosaccharides) • Salivary amylase (starts in mouth) and pancreatic amylase break starch → maltose • Intestinal enzymes (maltase, sucrase, lactase) break di-saccharides → glucose • Glucose is absorbed through villi → blood capillaries
Proteins → Amino acids • Pepsin (stomach) + trypsin, chymotrypsin (pancreas) break proteins → peptides • Peptidases in intestinal wall break peptides → amino acids • Amino acids absorbed through villi → blood capillaries
Fats → Fatty acids + Glycerol • Bile salts (from liver/gallbladder) emulsify fats • Pancreatic lipase breaks fats → fatty acids + glycerol • Absorbed → lacteals (lymph vessels) in villi → lymph system → blood
Water, minerals, vitamins: • Absorbed throughout the small intestine • Remaining water absorbed in large intestine
Summary: • Surface area maximised: length + villi + microvilli = 250 m² • Single-cell walls allow rapid passage • Capillaries carry glucose + amino acids to liver • Lacteals carry fats into lymph system
The small intestine is designed for maximum absorption through: (1) Long length (~6–7 m) for prolonged contact; (2) Villi — finger-like projections on the inner wall increasing surface area; (3) Microvilli (brush border) on villi — increasing surface area to ~250 m²; (4) Single-cell thick walls for rapid passage; (5) Rich capillary supply (for glucose and amino acids) and lacteals (for fat absorption).
Villi are tiny finger-like projections on the inner lining of the small intestine. Each villus is 0.5–1 mm long. They increase the surface area of the small intestine enormously (from 0.33 m² to ~2 m², and with microvilli to ~250 m²). This huge surface area ensures efficient absorption of digested nutrients. Each villus has blood capillaries (for glucose/amino acids) and a lacteal (for fats).
Fats are broken down by pancreatic lipase (after emulsification by bile salts) into fatty acids and glycerol. These are absorbed by the lacteals (lymph vessels inside villi) rather than blood capillaries. The absorbed fats enter the lymph system and eventually reach the bloodstream through the thoracic duct.
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