Digestive Glands — Salivary Glands, Liver and Pancreas

Digestion depends not only on the mechanical breakdown of food but, far more importantly, on a sequence of chemical secretions poured into the alimentary canal at successive points along its length. Some of these secretions come from glands built into the wall of the canal itself, while others are produced by large, separate accessory organs and delivered to the canal through ducts; this section takes up the three principal accessory digestive glands named in the syllabus — the salivary glands, the liver and the pancreas — along with the glands of the intestinal wall itself.

There are three pairs of salivary glands in the human mouth. The parotid glands, the largest of the three, lie just in front of and below each ear and empty their secretion into the buccal cavity opposite the upper second molar. The submandibular (submaxillary) glands lie beneath the floor of the mouth, along the inner side of the lower jaw, and open on either side of the base of the tongue. The sublingual glands, the smallest pair, lie immediately beneath the tongue and open through several small ducts into the floor of the mouth. Together these three pairs secrete saliva, a watery fluid that moistens and lubricates food to ease chewing and swallowing, and contains the enzyme salivary amylase (also called ptyalin), which begins the digestion of cooked or boiled starch, together with a trace of the antibacterial enzyme lysozyme and mucus that binds chewed food into a cohesive bolus. Saliva also keeps the mouth moist, helps dissolve food particles so that they can be tasted, and plays a minor role in maintaining oral hygiene.

The liver is the largest gland in the human body, a reddish-brown organ occupying much of the upper right side of the abdominal cavity, just beneath the diaphragm. Though it performs a great many metabolic functions beyond digestion (regulating blood glucose, synthesising plasma proteins, detoxifying harmful substances, and more), its direct digestive contribution is the continuous secretion of bile by its functional cells, the hepatocytes. Bile drains from the liver through the hepatic duct, and between meals is diverted into the gall bladder, a small pear-shaped sac tucked beneath the liver, where it is stored and considerably concentrated by the reabsorption of water. When fatty food enters the duodenum, the gall bladder contracts (under hormonal control, Section 14.6) and releases this concentrated bile through the cystic and then the common bile duct into the duodenum. Bile itself contains no digestive enzyme; its digestively active components are the bile salts, along with the bile pigments bilirubin and biliverdin (breakdown products of haemoglobin, which give bile and, ultimately, faeces their characteristic colour) and cholesterol. As covered in more detail in Section 14.9, the bile salts act as a detergent-like emulsifying agent, breaking large fat globules into a suspension of much smaller droplets and so vastly increasing the surface area on which pancreatic lipase can act.

The pancreas is an elongated, pale gland lying transversely behind the stomach, with its head fitted into the curve of the duodenum and its tail extending toward the spleen; it is a mixed gland, meaning it has both an endocrine and an exocrine role, though only its exocrine, digestive role concerns this chapter. The bulk of the pancreatic tissue is made of small clusters of secretory cells called acini, which produce pancreatic juice; this alkaline, bicarbonate-rich fluid drains via the pancreatic duct, which typically joins the common bile duct just before both open into the duodenum. Pancreatic juice is the single most important digestive secretion of the body, containing enzyme precursors and active enzymes capable of acting on all three major classes of food — trypsinogen and chymotrypsinogen (inactive precursors of the protein-digesting enzymes trypsin and chymotrypsin) and procarboxypeptidase, pancreatic amylase (acting on starch), pancreatic lipase (acting on fat) and nucleases (acting on nucleic acids) — together with a high concentration of bicarbonate ions that neutralise the strongly acidic chyme arriving from the stomach, raising the pH to a level at which the intestinal and pancreatic enzymes can function. …