Saliva: Classification and Composition

Introduction

• Salivary glands are a group of exocrine, and tubulo-acinar glands that secrete saliva. The glands are formed of parenchymal components constituted by secretory acini and excretory ducts as well as extra-parenchymal components constituted by connective tissue formed of collagen fibres, ground matrix, and neurovascular bundle.
• The secretory acini are responsible for secretion of a watery fluid that forms a coating over oral mucosa, which is known as saliva.
• The term ‘saliva’ is a Greek derived word, which is a complex fluid and helps in maintenance of oral well-being.
• General properties of saliva (Sembulingum 2012)
  • An average of 1-1.5 litres of saliva is produced by secretory acini per day.
  • Saliva has a pH of 6.35-6.85 and is hypotonic to plasma. It has a turbid appearance.

Classification of Salivary Glands

1. Based on the size of the gland

2. Based on histology

Composition

(Jenkins 1966; Sembulingum K 2012)

Factors affecting the Composition of Saliva

(de Almeida 2008)

1. Non-dietary factors:

   • Composition of saliva differs between the major salivary glands. The parotid gland contributes 30% of salivary secretion and has salivary secretion low in calcium and high in phosphate secretion. On the other hand, submandibular gland salivary secretion contributes 60% of the total volume of saliva produced and is rich in calcium.

2. Flow rate:

   • Normal flow rate of ranges between 0.1-0.2 ml/minute. Variation in flow of saliva is affected by external factors such as sight of food, mastication, sleep, intake of medications, etc. the rate of flow of saliva affects the concentration of ions in saliva such as
     • Slight increase in flow rate shows an increase in the concentration of sodium and bicarbonate ions and decrease in the concentration of potassium, and phosphate ions.
     • On further increase in flow rate, concentration of all ions increase except phosphate ions.
   • Variation in flow rate depends on the posture of person i.e., the salivary flow rate is higher in individuals standing than lying down.
   • Also, flow of saliva is higher during day or light than dark.

3. Duration of stimulation:

   • When the duration of stimulation of salivary secretion is longer, bicarbonate and calcium ions, as well as protein concentrations increase.

4. Nature of the stimulus:

   • The salivary protein concentration is affected differently to electric, pharmacologic and gustatory stimuli.

5. Plasma concentration:

   • Salivary concentrations of calcium, urea, and potassium are affected by their plasma concentrations.

6. Time of the day:

   • Similar to hormones, circadian variation in salivary composition is seen.

7. Dietary factors:

   • Fibrous foods or highly-flavoured food increase salivary flow rate as well as its composition.

Summary

• Saliva is vital for many functions, including mastication, swallowing and taste.
• Salivary glands can be broadly divided in to major and minor glands, the major glands being parotid, submandibular and sublingual glands.
• They can also be divided based on the saliva they produce – serous or mucous.
• Saliva is composed of many factors including digestive and immune factors.
• The composition of saliva is affected by non-dietary factors, flow rates, duration and nature of stimulus as well as other factors.

Recommended Reading

• • Essentials of Medical Physiology
  • Master Dentistry Volume 3 Oral Biology: Oral Anatomy, Histology, Physiology and Biochemistry, 1e