Absorption in small intestine (villi)

After food has been mechanically and chemically digested in the mouth, stomach, and small intestine, the resulting small, soluble molecules are ready to be absorbed. Absorption is the process by which these digested food molecules pass from the lumen of the small intestine into the blood or lymph. The small intestine is specifically adapted for this vital role.

Key functions of the small intestine:

• Digestion: Completes the digestion of carbohydrates, proteins, and fats using enzymes produced by the intestinal wall and pancreas.
• Absorption: Absorbs most of the digested nutrients, as well as water, vitamins, and minerals.

The small intestine is remarkably adapted to maximise the efficiency of nutrient absorption. Its structure incorporates several features that significantly increase the surface area and facilitate rapid transport of molecules.

Key adaptations:

1. Long length: The small intestine is approximately 6-7 meters long in adults, providing ample time and surface area for absorption.
2. Folded inner surface: The inner lining (mucosa) is folded into numerous circular folds, or plicae circulares, which increase the surface area.
3. Villi: The most prominent adaptation are the millions of tiny, finger-like projections called villi (singular: villus) that cover the entire inner surface of the small intestine.
   • Each villus is about 0.5-1.5 mm long.
   • They give the inner surface a velvety appearance.
4. Microvilli: The epithelial cells that line each villus have their own microscopic folds on their surface, called microvilli. These collectively form the 'brush border'.

Combined effect: These adaptations (length, folds, villi, and microvilli) increase the effective surface area for absorption by an estimated 600 times, making it equivalent to the size of a tennis court.

Each villus is a highly specialised structure designed to efficiently absorb nutrients. Understanding its internal components is crucial.

Components of a villus:

• Epithelial cells: A single layer of columnar epithelial cells covers the surface of each villus. These cells are crucial for absorption.
  • Microvilli: As mentioned, these are tiny folds on the apical (lumen-facing) surface of the epithelial cells, forming the 'brush border'. They further increase surface area and contain digestive enzymes.
  • Mitochondria: Epithelial cells have numerous mitochondria to provide ATP for active transport processes.
• Blood capillaries: Each villus contains a dense network of blood capillaries. These are tiny blood vessels that run very close to the surface, maintaining a steep concentration gradient for absorbed nutrients to diffuse into the bloodstream.
  • Transport: Glucose, amino acids, water, mineral salts, and vitamins are absorbed into the blood capillaries.
• Lacteal: A central lymphatic vessel called a lacteal is present in each villus. This vessel is responsible for absorbing digested fats.
  • Transport: Fatty acids and glycerol (recombined into triglycerides) are absorbed into the lacteal.
• Goblet cells: Scattered among the epithelial cells, these cells secrete mucus to lubricate and protect the intestinal lining.
• Thin wall: The wall of the villus is only one cell thick (the epithelial layer), allowing for a short diffusion distance for nutrients.

Nutrients are absorbed across the villi by various mechanisms, depending on the type of nutrient and its concentration gradient.

1. Diffusion:

• Definition: Movement of molecules from a region of higher concentration to a region of lower concentration, down a concentration gradient.
• Examples: Small, lipid-soluble molecules (e.g., fatty acids and glycerol) can diffuse directly across the cell membrane. Some water-soluble molecules (e.g., glucose, amino acids) can diffuse if their concentration in the lumen is significantly higher than in the blood.

2. Facilitated Diffusion:

• Definition: Movement of molecules down a concentration gradient with the help of specific carrier proteins in the cell membrane. No energy is required.
• Examples: Fructose and some amino acids are absorbed this way.

3. Active Transport:

• Definition: Movement of molecules from a region of lower concentration to a region of higher concentration, against a concentration gradient. This process requires energy (ATP) and specific carrier proteins.
• Examples: Glucose and amino acids are often absorbed by active transport, especially when their concentration in the lumen is low, ensuring maximum uptake.

4. Osmosis:

• Definition: The movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.
• Examples: Water is absorbed by osmosis, following the movement of dissolved solutes (like glucose and mineral salts) into the blood.

Once absorbed into the villi, nutrients follow different pathways to reach the rest of the body.

1. To the Bloodstream (via capillaries):

• Nutrients: Glucose, amino acids, water, mineral salts, water-soluble vitamins (B and C).
• Pathway: Absorbed into the capillaries within the villi -> Capillaries merge to form venules -> Venules merge to form the hepatic portal vein -> Hepatic portal vein carries nutrient-rich blood directly to the liver.
• Role of the liver: The liver processes these absorbed nutrients, converting excess glucose to glycogen for storage, detoxifying harmful substances, and synthesising plasma proteins, before releasing them into the general circulation.

2. To the Lymphatic System (via lacteal):

• Nutrients: Fatty acids and glycerol (re-esterified into triglycerides within the epithelial cells), fat-soluble vitamins (A, D, E, K).
• Pathway: Absorbed into the lacteal within the villi -> Lacteals drain into larger lymphatic vessels -> Lymphatic vessels eventually merge to form the thoracic duct -> Thoracic duct empties into the subclavian vein, returning the absorbed fats to the bloodstream.
• Reason for lymphatic pathway: Fats are too large to directly enter the blood capillaries and are transported via the lymphatic system, which eventually connects to the circulatory system.