Serous glands secrete serous fluid. They contain serous acini, a grouping of serous cells that secrete serous fluid, isotonic with blood plasma, that contains enzymes such as alpha-amylase.
Serous glands are most common in the parotid gland and lacrimal gland but are also present in the submandibular gland and, to a far lesser extent, the sublingual gland.
The human digestive system consists of the gastrointestinal tract plus the accessory organs of digestion (the tongue, salivary glands, pancreas, liver, and gallbladder). Digestion involves the breakdown of food into smaller and smaller components, until they can be absorbed and assimilated into the body. The process of digestion has three stages: the cephalic phase, the gastric phase, and the intestinal phase.
The first stage, the cephalic phase of digestion, begins with secretions from gastric glands in response to the sight and smell of food, and continues in the mouth with the mechanical breakdown of food by chewing, and the chemical breakdown by digestive enzymes in the saliva. Saliva contains amylase, and lingual lipase, secreted by the salivary glands, and serous glands on the tongue. Chewing mixes the food with saliva to produce a bolus to be swallowed down the esophagus to enter the stomach. The second stage, the gastric phase, takes place in the stomach, where the food is further broken down by mixing with gastric juice until it passes into the duodenum, the first part of the small intestine. The intestinal phase where the partially digested food is mixed with pancreatic digestive enzymes completes the process of digestion.
Lingual lipase is a member of a family of digestive enzymes called triacylglycerol lipases, EC 3.1.1.3, that use the catalytic triad of aspartate, histidine, and serine to hydrolyze medium and long-chain triglycerides into partial glycerides and free fatty acids. The enzyme, released into the mouth along with the saliva, catalyzes the first reaction in the digestion of dietary lipid, with diglycerides being the primary reaction product. However, due to the unique characteristics of lingual lipase, including a pH optimum 4.5–5.4 and its ability to catalyze reactions without bile salts, the lipolytic activity continues through to the stomach. Enzyme release is signaled by the autonomic nervous system after ingestion, at which time the serous glands under the circumvallate and foliate papillae on the surface of the tongue secrete lingual lipase into the grooves of the papillae, co-localized with fat taste receptors. The hydrolysis of the dietary fats is essential for fat absorption by the small intestine, as long chain triacylglycerides cannot be absorbed, and as much as 30% of fat is hydrolyzed within 1 to 20 minutes of ingestion by lingual lipase alone.
Lingual lipase, together with gastric lipase, comprise the two acidic lipases.