The digestive system's primary function is to break down food into smaller molecules that can be absorbed and utilized by the body. This process involves both mechanical digestion, such as chewing and churning, and chemical digestion, where enzymes break down complex molecules into simpler ones. Nutrients from food are absorbed through the walls of the small intestine into the bloodstream, where they are transported to cells for energy, growth, and repair. The system also eliminates undigested waste, maintaining the body's overall health and homeostasis.

Define mechanical/ chemical digestion

1. Mouth:

   • Function: Begins mechanical digestion through chewing and chemical digestion with saliva.
   • Key Structures: Teeth, tongue, salivary glands.

2. Pharynx:

   • Function: Passageway for food and air.
   • Key Structures: Epiglottis (prevents food from entering the trachea).

3. Esophagus:

   • Function: Transports food from the pharynx to the stomach.
   • Key Mechanism: Peristalsis (wave-like muscle contractions).

4. Stomach:

   • Function: Mechanical digestion through churning and chemical digestion with gastric juices.
   • Key Structures: Gastric glands (secrete hydrochloric acid and pepsinogen).

   gastric pits are the entry points to the deeper gastric glands where the actual secretion of digestive substances occurs.

Key cells of Gastric Glands

1. Parietal Cells:
   • Location: Gastric glands in the stomach lining.
   • Function: Secrete hydrochloric acid (HCl), which lowers the pH of the stomach, aiding in protein digestion and creating an acidic environment that kills pathogens.
2. Chief Cells:
   • Location: Gastric glands in the stomach lining.
   • Function: Produce and secrete pepsinogen, an inactive enzyme that is converted to pepsin in the presence of HCl. Pepsin breaks down proteins into smaller peptides.
3. Mucous Cells:
   • Location: Surface and neck of the gastric glands.
   • Function: Secrete mucus, which forms a protective layer on the stomach lining to prevent damage from the acidic environment and digestive enzymes.
4. Enteroendocrine Cells:
   • Location: Gastric glands in the stomach lining.
   • Function: Release hormones such as gastrin, which stimulates the secretion of HCl and pepsinogen, enhancing the digestive process.

These cells work together to create an optimal environment for digestion within the stomach, ensuring efficient breakdown of food and protection of the stomach lining.

1. Small Intestine:
   • Function: Primary site for nutrient absorption and chemical digestion.
   • Key Sections:
     • Duodenum: Receives chyme from the stomach and digestive juices from the pancreas and liver.
     • Jejunum: Absorbs nutrients.
     • Ileum: Absorbs remaining nutrients and bile acids.
   • Key Structures: Villi and microvilli (increase surface area for absorption).
2. Pancreas:
   • Function: Produces digestive enzymes and bicarbonate to neutralize stomach acid.
   • Key Enzymes: Amylase, lipase, proteases.

Liver

• Function:
  • Bile Production: The liver produces bile, a digestive fluid that emulsifies fats, breaking them into smaller droplets to enhance their digestion and absorption in the small intestine.
  • Detoxification: The liver detoxifies various metabolites, drugs, and toxins, converting them into less harmful substances or facilitating their excretion.
  • Metabolism Regulation: It plays a central role in carbohydrate, protein, and lipid metabolism, regulating blood glucose levels, synthesizing proteins, and managing cholesterol levels.
  • Storage: The liver stores essential nutrients, including glycogen (a form of glucose), vitamins (A, D, E, K, and B12), and minerals (iron and copper).
  • Synthesis of Plasma Proteins: It synthesizes various important plasma proteins, including albumin (maintains blood's osmotic pressure) and clotting factors (essential for blood coagulation).
• Key Structures:
  • Hepatocytes:
    • Description: The primary functional cells of the liver, making up about 70-85% of the liver's mass.
    • Function: Hepatocytes perform most of the liver's metabolic, detoxifying, and synthetic functions. They produce bile, process nutrients absorbed from the digestive tract, and detoxify harmful substances.
  • Bile Canaliculi:
    • Description: Small ducts between hepatocytes that collect bile produced by these cells.
    • Function: Transport bile to the bile ducts, eventually leading to the gallbladder and small intestine.
  • Sinusoids:
    • Description: Specialized capillary networks between rows of hepatocytes.
    • Function: Allow blood from the hepatic artery and portal vein to mix and bathe hepatocytes, facilitating nutrient and waste exchange.
  • Kupffer Cells:
    • Description: Specialized macrophages located within the liver sinusoids.
    • Function: Engulf and break down old red blood cells, pathogens, and cellular debris, playing a crucial role in the liver's immune function.
  • Stellate (Ito) Cells:
    • Description: Star-shaped cells found in the space of Disse (between hepatocytes and sinusoids).
    • Function: Store vitamin A and fat, and play a role in liver fibrosis when activated.

The liver's multifaceted functions and complex structure are essential for maintaining the body's metabolic balance, detoxifying harmful substances, and producing critical substances for digestion and overall health.

1. Gallbladder:

   Function: Stores and concentrates bile.

   • Key Process: Releases bile into the duodenum.