Anatomy and Function of the Human Digestive System

The Human Digestive System

1. The Mouth

• Digestion Initiation: Digestion begins here, involving both mechanical action (chewing) and chemical breakdown (via enzymes).
• Saliva Production: The mouth produces saliva, which is composed of water, mucus, and the enzyme amylase.

2. The Esophagus

The esophagus is a muscular tube connecting the pharynx to the stomach.

• Length and Movement: It is approximately 25 cm long and facilitates the movement of food through peristalsis—a series of rhythmic muscular contractions and relaxations.
• Associated Structures: The passage of food involves the pharynx and the epiglottis, which prevents food from entering the trachea (windpipe).

3. The Stomach

This sac-like organ is involved in both mechanical and chemical digestion.

Stomach Structure and Secretions

• Wall Composition: The stomach walls consist of three layers of smooth muscle. The inner lining is composed of epithelial cells.
• Gastric Pits: These structures release enzymes and mucus into the stomach, forming the highly acidic gastric fluid.

The Digestion Process in the Stomach

1. Mechanical Digestion: Food enters, and the cardiac sphincter closes to prevent reflux into the esophagus. Muscle contractions churn the contents, breaking them up and mixing them with gastric fluid to form chyme.
2. Chemical Digestion: This is primarily carried out by the Gastric Fluid.
   • Acidic Environment: Strong acids (hydrochloric acid) break down food. The stomach is protected from this corrosive environment by a thick layer of mucus.
   • Ulcers: An ulcer occurs when the acid erodes the protective mucus lining and the underlying stomach tissue.

4. Accessory Organs: Liver, Gallbladder, and Pancreas

The Liver

The liver performs vital functions, including storing nutrients, detoxification, and secreting bile. Bile helps dissolve fats, making them easier to digest.

The Gallbladder

The gallbladder serves primarily to store and concentrate bile produced by the liver.

The Pancreas

The pancreas functions as both an endocrine and exocrine gland:

• Endocrine Function: Secretes hormones directly into the bloodstream, such as insulin.
• Exocrine Function: Produces digestive juices, including:
  • Sodium Bicarbonate: Neutralizes the acidic chyme entering the small intestine, protecting the intestinal lining.
  • Amylase: An enzyme that breaks down carbohydrates.

5. The Small Intestine

This long, coiled organ is approximately 7 meters in length and is the primary site for chemical digestion and nutrient absorption.

Three Sections of the Small Intestine

1. Duodenum (First Section): Receives chyme from the stomach and secretions from the accessory organs. It has extra mucus production to protect against acid and bile.
2. Jejunum (Middle Section): Major site for the absorption of nutrients and water, facilitated by fingerlike projections called villi.
3. Ileum (Last Section): Continues the absorption of nutrients and water, also utilizing villi.

6. The Large Intestine

The large intestine processes undigested material and absorbs water.

Major Parts of the Large Intestine

It consists of four main sections of the colon, leading to the rectum and anal canal:

• Ascending Colon
• Transverse Colon
• Descending Colon
• Sigmoid Colon (leading to the rectum and anal canal)

Function and Waste Formation

• Absorption: Only water and nutrients produced by symbiotic bacteria (primarily E. coli) are absorbed here.
• Movement: Slow contractions move the material through the colon toward the rectum.
• Feces Formation: The absorption of water solidifies the remaining mass, which is then referred to as feces.