Saliva and Gastric Juice: Composition and Functions

Composition and Function of Saliva and Gastric Juice

Major Salivary Glands

The salivary glands in mammals are exocrine glands associated with the oral cavity. In humans and animals, there are three major paired (extramural) salivary glands:

  • Gl. Parotis - Also known as the parotid gland, it primarily produces serous saliva rich in enzymes.
  • Gl. Mandibularis - Known as the submandibular gland, it produces mixed saliva (both mucous and serous).
  • Gl. Sublingualis - The sublingual gland predominantly secretes mucous saliva.

Small Salivary Glands

In addition to the major glands, there are small salivary glands deeply embedded in the mucosa of the oral cavity, which can be classified based on their location:

  • Labial Glands - Located in the lips.
  • Buccal Glands - Found in the cheeks.
  • Pharyngeal Glands - Situated in the throat region.

Salivary Gland Locations in Various Species

The anatomical arrangement of salivary glands differs among species:

  • Cattle and Horses: Key locations include the parotid, mandibular, and sublingual glands, including variations such as polystomatic and monostomatic sublingual glands.
  • Pigs and Dogs: Feature similar gland structures with distinct parotid and zygomatic glands, and variations in the sublingual glands.

Obtaining Saliva

Mixed saliva can be collected directly from the oral cavity, comprising secretions from all salivary glands along with desquamated epithelial cells, microorganisms, lymphocytes, and metabolic by-products.
The process of saliva collection for research began with Ivan Pavlov, who created a fistula in dogs in 1899 to study salivary gland function.

Composition and Function of Saliva

The composition of mixed saliva includes:

  • 50-52% from the parotid gland
  • 8-10% from the submandibular gland
  • 2-3% from the sublingual gland
  • 35-38% from minor salivary glands

Saliva appears transparent to slightly opalescent and is a foamy liquid that serves various functions in the digestive system:

  • Lubrication: Saliva helps bind and lubricate masticated food, creating a bolus for swallowing.
  • Dissolution: It solubilizes food, aiding in taste and initiating digestive reactions.
  • Homeostasis: It maintains pH balance and temperature regulation (especially in ruminants).
  • Waste Removal: Saliva removes metabolic waste like urea.
  • Digestive Role: Contains enzymes like amylase that initiate starch digestion.
  • Protection: Provides a protective coating in the oral cavity and esophagus, with antibacterial functions due to enzymes such as lysozyme and immunoglobulin A (IgA).

Salivary Secretion Mechanism

Salivary glands are structured as acinar glands, with collecting ducts leading to cellular acini. Initial saliva secretion includes water and electrolytes filtered from blood plasma. Cytological components and specific saliva components are actively secreted by glandular cells and modified as saliva moves through the ducts. The result is a hypotonic saliva with lower sodium concentration compared to extracellular fluid, except for ruminant saliva, which is hypertonic with higher bicarbonate and phosphate levels.

Regulation of Salivary Secretion

Salivary secretion is primarily regulated by the nervous system. Parasympathetic nerve fibers stimulate saliva production via cholinergic receptors, while sympathetic nerves can trigger secretion during stress or feeding circumstances through β-adrenergic receptors. Reflexive salivation can occur when food or non-edible substances enter the mouth, and it may also be a conditioned response triggered by the thought or sight of food.

Composition and Properties of Gastric Juice

Gastric juice, produced in the stomach, is characterized by its transparency and acidity (with pH values varying significantly among species). For example:

  • Cattle: pH = 2.06 – 4.14
  • Sheep: pH = 1.90 – 6.70
  • Dogs: pH = 0.80 – 0.98
  • Pigs: pH = 1.70 – 2.00
  • Humans: pH = 0.9 – 1.5

Composition of Gastric Juice

Gastric juice is predominantly composed of:

  • 98.5 – 99% water
  • 1 – 1.5% dry substances, including:
    • Non-organic: Hydrochloric acid (0.2 – 0.6%), electrolytes such as sodium and potassium, and some organic acids.
    • Organic: Mucin, proteolytic enzymes (like pepsin), lipase, gastric intrinsic factor, and other enzymes (e.g., lysozyme).

Understanding these components and functions is crucial for grasping the digestive processes in both mammals and other animal species.

Conclusion

The intricate structures and diverse functions of salivary and gastric secretions highlight the significance of these fluids in digestion and overall animal physiology. Accurate knowledge of gland locations, compositions, and functions is key to understanding animal health and nutrition.