How is saliva secreted?

Saliva comes from special parts of our body called salivary glands. Everyone has 3 major pairs of glands: there are 2 under your tongue called sublingual glands, two under your chin called submandibular glands, and two under your ears called parotid glands.

These little blobs-like glands are made from clusters of myoepithelial cells and these clusters contain several basic units such as serous acini, mucous acini, Striated ducts, and intercalated ducts. Parotid glands are the largest salivary gland and they are responsible for the secretion of about 25% of saliva. The parotid gland contains predominantly serous acini and it secretes a protein-rich liquid including enzymes that can help to digest food. The submandibular gland is the second-largest salivary gland that produces 70% of saliva. It is a mixture of serous and mucous acini and is released through submandibular ducts. The smallest glands are sublingual glands, approximately 5% of the saliva comes from these glands.

The formation of saliva occurs in 2 stages. In stage 1, the acinar cells whether serous or mucous cells secrete a NaCl-rich fluid called primary saliva. In the second stage, the ductal cells modify the primary saliva composition by reabsorbing part of NaCl and secreting a large quantity of bicarbonate and K+. Bicarbonate secretion is important in the second stage because it can interact with phosphate, and provides a buffer capacity to neutralize the acid produced in the oral cavity, the final PH of saliva is around 6.6.

The unstimulated saliva flow rate has been shown to fluctuate in a 24-hr period, the period is also known as the circadian rhythm. The maximum saliva output is usually observed in the late afternoon and declines to nearly zero during sleep.