Physiology of Digestion and Absorption

The process of digestion and absorption involves the breakdown of food into simpler nutrients that can be absorbed into the bloodstream and utilized by the body. This complex process occurs throughout the gastrointestinal (GI) tract, which consists of the mouth, esophagus, stomach, small intestine, and large intestine. The organs involved in digestion and absorption work together to ensure that nutrients are efficiently extracted and waste products are eliminated.

1. Digestion: The Breakdown of Food

Digestion involves the mechanical and chemical breakdown of food into smaller, absorbable components. This process occurs through a combination of mechanical actions (e.g., chewing and churning) and the action of digestive enzymes.

Mouth (Oral Cavity)

• Mechanical Digestion: The teeth chew the food, breaking it into smaller pieces. The tongue mixes food with saliva.
• Chemical Digestion: Saliva, produced by the salivary glands, contains the enzyme salivary amylase, which begins the digestion of starches into simple sugars. The enzyme lingual lipase also starts the digestion of lipids, though this process is less significant in the mouth.

Esophagus

• Peristalsis: The food (now a bolus) is pushed through the esophagus via peristalsis (a series of wave-like muscular contractions), leading it from the mouth to the stomach.

Stomach

• Mechanical Digestion: The stomach churns food to mix it with gastric juices, producing a semi-liquid substance called chyme.
• Chemical Digestion:
  • Gastric juice, which includes hydrochloric acid (HCl), creates an acidic environment that activates the enzyme pepsinogen into pepsin, which begins the breakdown of proteins into smaller peptides.
  • Lipase in the stomach further breaks down fats, although this process is minimal compared to the small intestine.

Small Intestine

The majority of digestion occurs in the small intestine, where digestive enzymes and bile further break down food into absorbable components.

• Duodenum (First part of the small intestine):
  • Bile produced by the liver and stored in the gallbladder emulsifies fats, breaking them into smaller droplets to facilitate digestion by lipases.
  • Pancreatic enzymes are released from the pancreas into the duodenum. These include:
    • Pancreatic amylase: Breaks down carbohydrates (starches) into simple sugars (e.g., maltose).
    • Pancreatic lipase: Breaks down fats into fatty acids and glycerol.
    • Proteases (e.g., trypsin, chymotrypsin): Break down proteins into smaller peptides and amino acids.
  • Bicarbonate: The pancreas also releases bicarbonate ions to neutralize the acidic chyme from the stomach, creating a more alkaline environment optimal for enzyme activity.
• Jejunum (Middle section of the small intestine):
  • Absorption begins here. The walls of the jejunum contain villi, finger-like projections that increase the surface area for absorption. Each villus is lined with microvilli, creating the brush border.
  • Digestive enzymes embedded in the microvilli, such as lactase (for breaking down lactose) and sucrase (for breaking down sucrose), finalize the breakdown of sugars.
• Ileum (Last section of the small intestine):
  • The ileum absorbs remaining nutrients, particularly vitamins, bile salts, and water-soluble vitamins (like B12).
  • The nutrients are absorbed into the blood through the villi and transported to the liver via the hepatic portal vein.

Large Intestine

• Absorption of Water and Electrolytes: The large intestine absorbs water, sodium, and other electrolytes from the chyme, converting it into solid waste (feces).
• Fermentation of Undigested Food: Some undigested carbohydrates and fibers are fermented by gut bacteria in the colon, producing gases and short-chain fatty acids (SCFAs) like butyrate, which can be absorbed for energy.

2. Absorption: Movement of Nutrients into the Bloodstream

Absorption refers to the process by which digested food is taken up by the cells lining the gastrointestinal tract and transported into the blood or lymphatic system.

Mechanisms of Absorption

1. Passive Diffusion: Nutrients move from areas of high concentration to low concentration without energy expenditure. This mechanism is used by small molecules like water, fatty acids, and small lipids.
2. Facilitated Diffusion: Nutrients are absorbed through specialized transport proteins in the cell membrane. This mechanism is used for nutrients like glucose and amino acids.
3. Active Transport: This process requires energy (ATP) to move nutrients against their concentration gradient. For example, glucose and amino acids are absorbed into the bloodstream in the small intestine using sodium-dependent transporters.
4. Endocytosis: This is the process by which large molecules, such as vitamin B12, are absorbed through cell membranes by engulfing the molecule into the cell.

Key Nutrient Absorption in the Small Intestine

1. Carbohydrates:
   • Digested into monosaccharides (glucose, fructose, galactose) by enzymes like amylase (from saliva and pancreas) and sucrase, lactase, and maltase (from the brush border of the small intestine).
   • Glucose and galactose are absorbed by active transport mechanisms (sodium-glucose transporters), while fructose uses facilitated diffusion.
2. Proteins:
   • Proteins are broken down into amino acids and small peptides by pepsin (in the stomach) and proteases like trypsin (from the pancreas).
   • Amino acids are absorbed by active transport across the intestinal cells and into the bloodstream.
3. Fats:
   • Bile salts emulsify fats into smaller droplets in the duodenum.
   • Pancreatic lipase breaks down fats into fatty acids and monoglycerides.
   • These components are absorbed by the intestinal cells, where they are reassembled into triglycerides and packaged into chylomicrons (lipoproteins) that enter the lymphatic system before eventually reaching the bloodstream.
4. Vitamins:
   • Fat-soluble vitamins (A, D, E, K) are absorbed along with dietary fats through the lymphatic system.
   • Water-soluble vitamins (B-complex, C) are absorbed through active or passive transport into the bloodstream.
5. Minerals:
   • Sodium, potassium, calcium, magnesium, and iron are absorbed through active transport mechanisms.
   • Iron is absorbed in its ferrous (Fe2+) form and requires vitamin C for better absorption.
6. Water:
   • About 7-9 liters of water are secreted into the digestive tract daily, and most is absorbed in the small intestine (90%) and the large intestine (remaining 10%).

3. Transport of Absorbed Nutrients

• Bloodstream: Nutrients absorbed into the bloodstream from the small intestine are transported to the liver via the hepatic portal vein. In the liver, nutrients are metabolized or stored as needed. For example, glucose is stored as glycogen, and fats are converted into lipoproteins.
• Lymphatic System: Fat-soluble nutrients and lipids are absorbed into the lymphatic system in the form of chylomicrons, which are eventually emptied into the bloodstream near the heart.

Conclusion

The processes of digestion and absorption are vital for breaking down complex food molecules into simple nutrients that the body can use for energy, growth, and repair. Mechanical digestion (chewing, churning) and chemical digestion (enzyme action, bile) occur throughout the gastrointestinal tract, while absorption takes place primarily in the small intestine. Nutrients like carbohydrates, proteins, fats, vitamins, minerals, and water are absorbed using various mechanisms, ensuring the body receives the nutrients it needs to function optimally.