Human Digestive System: Anatomy, Physiology, and Metabolism

The Human Digestive System

Definition: The digestive system is a network of organs and tissues that work together to perform digestion.

Digestion: Digestion is a complex process that involves the breakdown and absorption of nutrients from ingested food.

Composition: This system consists of the Gastrointestinal (GI) tract, accessory organs, and various digestive processes.

The Gastrointestinal (GI) Tract

The GI tract is a long, hollow tube extending from the mouth to the anus, including the esophagus, stomach, small intestine, and large intestine.

Accessory Organs

The accessory organs include the liver, gallbladder, pancreas, and salivary glands.

• Functions: Produce digestive enzymes, produce saliva, and produce bile.

Main Functions of the Digestive System

1. Ingestion: Food enters the mouth.
2. Digestion: Involves mechanical digestion (chewing) and chemical digestion (enzymes breaking down nutrients).
3. Absorption: Nutrients are absorbed into the bloodstream.
4. Elimination: Waste is eliminated through feces.

Anatomical Diagram Labels

The system includes the mouth, salivary glands, epiglottis, esophagus, liver, gallbladder, stomach, pancreas, small intestine, large intestine, appendix, rectum, and anus.

Primary Organs of the GI Tract

The digestive system includes organs from the mouth to the anus, consisting of:

1. Mouth / Oral cavity
2. Pharynx
3. Esophagus (food pipe)
4. Stomach
5. Small intestine
6. Large intestine
7. Rectum and Anus

Supporting Accessory Organs

1. Salivary glands
2. Liver
3. Gallbladder
4. Pancreas

Layers of the GI Tract

The GI tract is composed of four distinct layers:

• Mucosa: The innermost layer, composed of epithelial cells, lamina propria, and muscularis mucosae. It produces mucus for protection, contacts luminal contents, and handles absorption and secretion (HCl, gastric juice).
• Submucosa: Made up of loose connective tissue; contains nerves, lymphatic vessels, and glands. It provides support, blood supply, and nerve supply.
• Muscularis: A layer of smooth muscles responsible for motility, movement, and peristalsis.
• Serosa / Adventitia: The outermost layer that covers the GI tract, providing protection and support.

The Oral Cavity (Mouth)

Also known as the buccal cavity, it is the beginning of the alimentary canal. It consists of:

• Teeth: Break down food into smaller pieces (mechanical digestion/chewing).
• Tongue: Mixes food with saliva, assists in swallowing, and facilitates taste.
• Salivary Glands: Produce saliva containing enzymes (amylase and lipase) that help break down carbohydrates and fats.

Main functions of the mouth:

• Ingestion of food.
• Mechanical breakdown of food by teeth.
• Formation of a bolus (food mixed with saliva) and moistening of food.
• Mixing and swallowing food with the help of the tongue.

The Stomach

The stomach is a hollow, J-shaped, bag-like structure located between the esophagus and the small intestine. It is approximately 15–20 cm long and 10–15 cm wide, though size varies by individual.

Physiology and Functions of the Stomach

• Mechanical breakdown and temporary storage of food (Capacity: 1–1.5 L).
• Chemical digestion by enzymes (pepsinogen, gastric amylase, and gastric lipase) and hydrochloric acid (HCl), which activates pepsin.
• Formation of chyme (food mixed with acid and enzymes).
• Secretion of gastric juice: mucus, HCl, pepsinogen, gastric amylase, and lipase.
• Peristaltic movement, denaturation of proteins, and breakdown of fats and carbohydrates.

The Small Intestine

A long, thin tube (about 6–7 meters) that performs the final digestion of chyme using pancreatic juice and bile. It is primarily responsible for nutrient absorption and consists of three parts: duodenum, jejunum, and ileum. The walls contain finger-like projections called villi, which increase the surface area for absorption.

The Large Intestine

Also known as the colon, this wider tube (1.5 m long, 6–7 cm diameter) absorbs water and electrolytes. It consists of the cecum, appendix, and the ascending, transverse, descending, and sigmoid colons. It facilitates the elimination of waste as feces.

Rectum and Anus

The rectum is the terminal 12 cm of the large intestine, storing feces until stretch receptors activate the nervous system to trigger a bowel movement. The anus is the external opening, featuring an involuntary internal sphincter (smooth muscle) and a voluntary external sphincter.

Salivary Glands and Liver

Salivary Glands: Located in the mouth and throat, they produce saliva (99% water, enzymes, mucus, electrolytes, and antibodies) to aid digestion and oral health.

Liver: The largest internal organ (approx. 1.4 kg, 18 cm long), located in the upper right abdomen. It is divided into four lobes (left, right, caudate, and quadrate) and has a reddish-brown color.

The Gallbladder

A pear-shaped structure under the liver that stores and concentrates bile. It releases bile into the small intestine via the cystic duct to aid in the digestion and absorption of fats.

Physiology of the Digestive Process

• Ingestion: Mechanical breakdown begins in the mouth.
• Swallowing: The bolus passes through the esophagus via peristalsis.
• Gastric Digestion: Gastric juices break down proteins and fats in the stomach.
• Pancreatic Digestion: Pancreatic juice and bile emulsify fats and break down nutrients in the small intestine.
• Absorption: Nutrients enter the bloodstream through the small intestine and are processed by the liver.
• Elimination: The large intestine absorbs water, and waste is expelled as feces.

Hormonal Regulation

• Gastrin: Stimulates gastric juice and acid production.
• Secretin: Stimulates pancreatic juice and bile secretion.
• Cholecystokinin (CCK): Stimulates bile release and pancreatic juice.
• Insulin: Lowers blood sugar by regulating glucose absorption.
• Glucagon: Raises blood sugar by regulating glucose release.

Acid Production in the Stomach

Parietal cells produce HCl to maintain a low pH (1–3), which is essential for protein digestion and killing bacteria. The process involves:

1. CO₂ reacts with H₂O to form carbonic acid (H₂CO₃) via carbonic anhydrase.
2. Carbonic acid dissociates into bicarbonate (HCO₃^-) and hydrogen ions (H⁺).
3. H⁺ is pumped into the stomach lumen via the H⁺/K⁺ ATPase proton pump.
4. Chloride ions (Cl^-) diffuse into the lumen to form HCl.

Regulation of Acid Production by the PNS

The Parasympathetic Nervous System (PNS) regulates acid through the cephalic phase, vagus nerve stimulation (releasing acetylcholine), and the stimulation of Gastrin and Histamine release. Conversely, somatostatin, secretin, and prostaglandins inhibit acid production. Dysregulation can lead to GERD or peptic ulcers.

Role of Pepsin in Digestion

Pepsin is an enzyme that breaks proteins into peptides and amino acids. Chief cells secrete inactive pepsinogen, which HCl activates into pepsin. HCl also denatures proteins to make them accessible for proteolysis.

ATP: Adenosine Triphosphate

ATP is the primary energy currency of the cell, composed of adenine, ribose, and three phosphate groups. It is formed via cellular respiration:

1. Glycolysis: Occurs in the cytosol; converts glucose to pyruvate (Net: 8 ATP).
2. Krebs Cycle: Pyruvate becomes Acetyl-CoA, producing NADH and FADH₂ (Net: 22 ATP from 2 mol of Acetyl-CoA).
3. Oxidative Phosphorylation: Occurs in the mitochondria, yielding approximately 32–34 ATP.

Roles of ATP: Muscle contraction, protein synthesis, membrane transport, cell signaling, and DNA replication.

Creatine Phosphate

Also known as phosphocreatine (PCr), this high-energy compound is stored in muscle cells. It acts as an energy buffer, donating a phosphate group to ADP to rapidly regenerate ATP during intense activity.

Basal Metabolic Rate (BMR)

BMR is the minimum rate of energy expenditure required to maintain basic body functions at rest, such as heart function, nerve conduction, and ion transport.

Factors Affecting BMR

• Age and Sex: BMR decreases with age; men typically have a higher BMR.
• Body Composition: BMR increases with weight and height.
• Genetics and Activity: Genetics influence BMR, while regular exercise increases it.
• Starvation: Can decrease BMR by up to 50%.
• Environment: Climate and altitude play a role.

Significance of BMR

BMR helps calculate caloric requirements for diet planning. Abnormal BMR levels can indicate health issues: it is lower in cases of undernutrition and higher during fever, diabetes, leukemia, or polycythemia.