Human Physiology: Digestion and Absorption

Digestion and Absorption are key processes in human physiology that allow the body to break down food and absorb essential nutrients for energy, growth, and repair. Here’s an overview of both processes:

1. Digestion:

•Digestion is the process of breaking down food into smaller, absorbable components. It involves both mechanical digestion (physical breakdown of food) and chemical digestion (enzymatic breakdown of macromolecules).

■ Stages of Digestion:

●Mouth:

1.Mechanical digestion: Chewing breaks down food into smaller pieces

2. Chemical digestion: Saliva contains the enzyme amylase, which begins the digestion of starches into simpler sugars.

●Stomach:

1. Mechanical digestion: The stomach churns the food, mixing it with gastric juices to form chyme.

2. Chemical digestion: The stomach secretes pepsin, an enzyme that begins the digestion of proteins. The stomach also secretes hydrochloric acid (HCl), which creates an acidic environment to aid in protein breakdown.

● Small Intestine:

Chemical digestion: The small intestine is the major site for digestion. Enzymes from the pancreas (e.g., amylase, lipase, and proteases) and the intestinal lining further digest carbohydrates, proteins, and fats. Bile from the liver (stored in the gallbladder) emulsifies fats, making them easier to break down by enzymes like lipase.

2. Absorption:

Absorption is the process of taking nutrients from the digestive tract into the bloodstream or lymphatic system.

■ Sites of Absorption:

Small Intestine (Primary Site):

1.The walls of the small intestine are lined with villi and microvilli that increase surface area for absorption.

2.Carbohydrates: Absorbed as monosaccharides (glucose, fructose, and galactose) through active transport and facilitated diffusion into the bloodstream.

3.Proteins: Absorbed as amino acids and small peptides via active transport into the bloodstream.

4.Fats (Lipids): Absorbed as fatty acids and monoglycerides into the lymphatic system via the lacteals (specialized lymph vessels) after being packaged into chylomicrons.

5.Vitamins and Minerals: Fat-soluble vitamins (A, D, E, K) are absorbed with fats, while water-soluble vitamins (B and C) and minerals are absorbed through various mechanisms.

Large Intestine:

1. The large intestine absorbs water and electrolytes (sodium, potassium). It also compacts undigested food material into feces.

2.Some vitamins, like vitamin K, are produced by gut bacteria in the large intestine and absorbed.

Key Enzymes and Substances in Digestion:

1.Amylase: Breaks down carbohydrates.

2.Pepsin: Breaks down proteins in the stomach.

3.Lipase: Breaks down fats.

5.Bile: Emulsifies fats for easier digestion.

Summary of Nutrient Absorption:

Carbohydrates: Broken down into simple sugars and absorbed into the bloodstream.

Proteins: Broken down into amino acids and absorbed into the bloodstream.

Fats: Broken down into fatty acids and absorbed into the lymphatic system.

Understanding digestion and absorption is fundamental to knowing how the body derives nutrients from food for various functions, including energy production and cellular repair.

Digestion is the process by which food is broken down into smaller components that can be absorbed into the bloodstream, while absorption is the process by which these digested nutrients pass through the lining of the small intestine into the blood or lymphatic system for distribution to the body.

1. Digestive Organs and Their Roles:

● Mouth:

Digestion begins in the mouth where food is mechanically broken down by chewing (mastication) and mixed with saliva. Saliva contains the enzyme salivary amylase, which begins the chemical breakdown of carbohydrates into simpler sugars like maltose.

●Esophagus:

A muscular tube that moves the food bolus from the mouth to the stomach via peristalsis (rhythmic muscular contractions).

● Stomach:

The stomach is involved in both mechanical and chemical digestion. Gastric juice, which contains hydrochloric acid (HCl) and pepsinogen (activated to pepsin in acidic conditions), helps break down proteins into peptides. The stomach churns food into a semi-liquid mixture called chyme.

●Small Intestine:

The small intestine is the primary site of digestion and absorption. It is divided into three parts: the duodenum, jejunum, and ileum.

•Duodenum: Here, chyme is mixed with bile from the liver and gallbladder (to emulsify fats) and pancreatic juice from the pancreas (containing enzymes like lipase, amylase, and proteases). This allows the digestion of fats, carbohydrates, and proteins, respectively.

•Jejunum and Ileum: These are where most nutrients are absorbed through the intestinal walls into the blood or lymph.

● Large Intestine (Colon):

The large intestine absorbs water, electrolytes, and vitamins produced by gut bacteria. It also compacts the remaining undigested material into feces for excretion.

● Rectum and Anus:

The rectum stores feces, which are eventually expelled through the anus during defecation.

2.. Enzymes and Their Functions:

Carbohydrates:

Salivary amylase in the mouth begins carbohydrate digestion.Pancreatic amylase in the small intestine further breaks down complex carbohydrates into disaccharides.Disaccharidases (like lactase, sucrase, and maltase) in the brush border of the small intestine break disaccharides into monosaccharides like glucose, fructose, and galactose, which are absorbed into the bloodstream.

Proteins:

Pepsin in the stomach breaks proteins into smaller polypeptides.Trypsin and chymotrypsin, released from the pancreas into the small intestine, continue to break polypeptides into shorter chains.Peptidases in the brush border of the small intestine break peptides into amino acids for absorption.

Fats:

Bile emulsifies fats into micelles, increasing the surface area for the enzyme lipase to act.Pancreatic lipase breaks down triglycerides into monoglycerides and free fatty acids, which are absorbed into the lymphatic system via lacteals.