Human Digestive System: Processes and Anatomy Explained

1. Digestion: Mechanical and Chemical Processes

Digestion is the set of mechanical and chemical processes involved in food processing. It transforms ingested substances into nutrients—substances contained in food that can be absorbed and utilized by the body.

2. The Digestive System

2.1. The Mouth

• 2.1.1. Lips: Feature the orbicularis oris muscle, a characteristic of mammals that allows infants to suckle without injury.
• 2.1.2. Cheeks: The internal side walls of the mouth.
• 2.1.3. Palate: The roof of the mouth, which separates the oral cavity from the nasal cavity.
• 2.1.4. Teeth: Enable the mechanical processes of chewing, cutting, and grinding food.
• 2.1.5. Tongue: A muscular organ that acts as the primary engine for swallowing and contains gustatory papillae for taste.
• 2.1.6. Salivary Glands: Three pairs of glands—the parotid (behind the mandibular ramus), submaxillary (on the floor of the mouth), and sublingual (located anteriorly)—secrete approximately 1.4 liters of saliva per day.

3. Functions of the Mouth

• Convert food into a soft mass through mechanical breakdown and salivary dissolution.
• Initiate chemical digestion to form the food bolus.
• Facilitate taste sensitivity, swallowing, and speech.

4. The Digestive Canal

The digestive tract consists of the pharynx, esophagus, stomach, small intestine, large intestine, rectum, and anal canal.

5. Pharynx

• Located behind the larynx, it facilitates swallowing.
• Swallowing is voluntary until the food bolus reaches the pharynx, which then triggers the involuntary swallowing reflex.

6. Esophagus

• Approximately 30 cm in length.
• Connects the pharynx to the stomach, transporting the food bolus.
• Utilizes involuntary contractions known as peristaltic movements.

7. Stomach

• A muscular section of the digestive tract.
• Sphincters: The entrance is controlled by the cardiac sphincter, and the exit is controlled by the pyloric sphincter.
• Secretions: Glands secrete hydrochloric acid (HCl), pepsin (which degrades proteins into amino acids), and mucus (for protection against acid).
• Chyme: The final mixture of the food bolus and gastric secretions is called chyme.
• Absorption: The stomach absorbs water, alcohol, and certain medications.

8. Small Intestine

• A long portion (approximately 6 meters) specialized for cellular absorption via villi.
• Cells in the walls secrete enzymes, mucus, and bicarbonate.
• Converts chyme into chyle.
• Divisions: The small intestine is divided into the duodenum (25 cm), jejunum (2.5 m), and ileum (3.25 m).
• The duodenum receives secretions from the liver and pancreas.