Digestive Glands

Introduction

The gastrointestinal tract characterises the digestive system. It is a portal by which various nutrients enter our bodies. It plays an important role in maintaining the body. It starts in the mouth and ends in the anus.

 It is composed of various organs and structures such as the oesophagus, stomach, small intestine, and large intestine. In addition to these, there are auxiliary organs that aid digestion, such as teeth, tongue, salivary glands, liver, and pancreas.

The digestive process includes the functions of the gastrointestinal tract (GI tract), appendages, and several organ systems. The nature of the single human stomach makes the process much easier compared to some other animals.

Salivary Glands

There are three pairs of salivary glands. They are the parotid, mandibular and sublingual glands.

Parotid gland

The largest salivary gland is the parotid gland. They are on both sides of the face, just below and in front of the ears. The parotid glands secrete saliva from the parotid duct into the mouth, helping to chew and swallow.

The secretions of each parotid gland flow through the Stensen duct leading to the mouth, opposite to the tip of the second molar. Mumps is a disease caused by a viral infection of the parotid glands.It is characterised by swelling, inflammation and pain.

Submandibular gland

These are embedded in the mucous membrane of the oral cavity floor under the tongue. The tubes of these glands lead to the sublingual part of the mouth below the tongue. It secretes a mixture of serous fluid and mucus. This secretion reaches the oral cavity through the submandibular duct or Wharton’s duct. It is much smaller than the parotid gland, and is responsible for producing 65 – 70% of saliva in the oral cavity.

Sublingual gland

It is the smallest of all salivary glands. It also includes the parotid and submandibular glands. It is located between the muscles of the oral cavity. There is no tube with striated muscle, unlike other salivary glands, so saliva is secreted directly from the Rivinus ducts. It is the only large salivary gland that is not encapsulated. First and foremost, it smoothens the oral cavity, aids swallowing, buffers pH, stimulates digestion and secretes thick mucus that promotes dental hygiene. 

Saliva and its Function

The salivary glands produce saliva, a viscous liquid. Human saliva is a viscous, colourless, turbid, milky white liquid. The optimum pH is 6.8, the range is 5.6 to 7.6, and the specific gravity is 1.002 to 1.08 g/ml. It is secreted evenly in small amounts and moisturises the oral cavity. In the presence of food, saliva moistens the food and helps smooth its subsequent passage through the gastrointestinal tract, thus increasing the secretion rate. It also stimulates digestion. Saliva contains 98.5-99% water and 1-1.5% high density residues.

Gastric Glands

There are various gastric glands on the stomach wall. They are simple or branched tubular glands. About 2-3 litres of gastric juice is excreted daily from these gastric glands in adults. There are at least three types of gastric glands on the inner wall of the stomach. These are parietal cells (oxygen cells), chief cells, and mucous cells. Parietal cells supply hydrochloric acid to gastric juice. The main cells supply pepsin and other enzymes such as rennin, gastric lipase, and mucin cells secrete mucin. The intrinsic factor in the castle is secreted by parietal cells and aids in the absorption of vitamin B₁₂. These secretions together form a low acidic gastric juice with a pH of 2 and a specific gravity of 1.002 to 1.006 g/ml. Gastric fluid is composed of sodium chloride containing trace amounts of potassium chloride and phosphate, mucin and the enzyme pepsin, rennin and gastric lipase, and approximately 0.5% solids containing hydrochloric acid and mucin. Gastric secretion is under the control of both nerves and hormones. Gastric secretions are caused by thinking about food, smelling it, chewing it, and bringing it into contact with the stomach wall. There are also two unusual types of glands in the gastric glands.

Normally, the Argentaffin cells at the base of the gland secrete the powerful vasoconstrictor serotonin. Endocrine cells are present in the pyloric antrum and produce gastrin. Gastrin causes the secretion of enzymes and HCl.

Pancreas

The pancreas is an elongated yellowish gland. It is behind the stomach and horizontally in the curve of the duodenum. It is about 12-15 cm long, 2.5 cm wide, and weighs about 60 g. It consists of a head, body and tail. The head is on the curve of the duodenum, the body is behind the stomach, and the tail connects the spleen. The small tubes in the pancreas make up the main tube of the pancreas. The main tube of the pancreas leads to the liver-pancreatic ampulla. It also has an accessory pancreatic duct (Ductus Santorini) that leads directly to the duodenum. The pancreas essentially has both exocrine and endocrine glands. The acinus, exocrine tissue, is home to numerous branched tubules or lobules. The acinus is embedded in connective tissue, including blood vessels, lymph vessels, nerves, and pancreatic ducts. The acinus is composed of cuboidal epithelial cells and secretes alkaline pancreatic juice at pH 8.8.

About 500 – 800 ml of pancreatic juice is excreted daily. The main pancreas carries pancreatic juice to the duodenum through the hepatopancreatic ampulla. Pancreatic juice is poured directly into the duodenum via the accessory pancreatic duct. Pancreatic juice contains three enzymes: sodium bicarbonate, trypsinogen, chymotrypsinogen, procarboxypeptidase, and several enzymes from nucleases such as pancreatic amylase pancreatic lipase, DNAse and RNAse. Pancreatic juice helps digest starch, proteins, fats and nucleic acids. The activity of chyme caused by HCl is neutralised by baking soda. The endocrine tissue of the pancreas is composed of several groups of cells located between the sinuses, the so-called islets of langerhans. Each islet of langerhans is composed of the following cell types. These secrete hormones that are poured into the circulating blood; 

i) Alpha cells (Azel) are more towards the perimeter of the island, occupying about 25% of the islets of langerhans. These cells produce glucagon, which converts glycogen to glucose in the liver. 

ii) Beta cells (b cells) are more towards the centre of the islet, occupying about 60% of the islet and about 60% of the islets of langerhans. These cells supply the hormone insulin, converting glucose into glycogen in the liver and muscles. Insulin deficiency causes diabetes. 

iii) Delta cells (d cells) are located around the islet and occupy about 10% of the islets of langerhans. These cells produce the hormone somatostatin, which stops the secretion of glucagon from alpha cells and, to some extent, insulin from beta cells. 

iv) Pancreatic polypeptide (PP) cells are also found in the pancreas. Pancreatic polypeptide (PP) is secreted by these cells and inhibits the release of pancreatic juice. Therefore, the pancreas serves two main functions;

i) With the secretion of pancreatic juice containing digestive enzymes 

ii) Hormone production 

Conclusion

Digestion is both important and complex. Turning food into useful ingredients requires multiple organs and systems, different chemicals, and impressive adjustments. The road from the hamburger to the poop is long and winding.

In this article, you learned about the human digestive glands. We have come to know the detailed function of the glands. The digestive glands are exocrine glands that deliver their secretions through channels to target organs. Examples include salivary glands, gastric glands, liver, pancreas, and intestinal glands. The salivary glands produce saliva. The gastric glands secrete HCl, pepsin, rennin, other enzymes and mucus. The pancreas secretes pancreatic juice into the duodenum. The intestinal glands located in the small intestine secrete intestinal juice or intestinal juice. All of these secretions help digest complex foods into simpler forms.