Digestive Glands: Salivary Glands, Sjögren's Syndrome, Liver, Pancreas, Gastric Glands, Intestinal Glands, Practice Problems and FAQs

The moment you hear of, see or even think of noodles, manchurian, cakes, chocolates, or anything delicious, your mouth starts watering. Have you ever wondered where this water comes from and what it actually is? This watery substance in our mouth is known as saliva. This comes from the salivary glands that are situated in different locations in the mouth. Similarly, there are various types of glands present in the digestive system that aid in the process of digestion. Digestion is the process by which large food molecules are broken down into smaller ones. These smaller molecules are absorbed by the various tissues. The digestive system of humans is composed of two parts: alimentary canal and digestive glands. In this topic, we will discuss various types of digestive glands.

Table of Contents:

• Digestive Glands
• Salivary Glands
• Liver
• Pancreas
• Ducts of the Liver and Pancreas
• Digestive Glands within Alimentary Gland
• Practice Problems of Digestive Glands
• FAQs of Digestive Glands

Digestive Glands

Digestive glands are the specialised structures that secrete digestive juices that help in the process of digestion.

There are two types of digestive glands:

1. Accessory digestive glands
2. Digestive glands within alimentary canal

The accessory glands are associated with the alimentary canal but are not a part of the alimentary canal. They are exocrine glands and have ducts to carry out their secretions. There are three types of accessory digestive glands:

1. Salivary glands
2. Liver
3. Pancreas

Salivary Glands

Salivary glands secrete saliva into the buccal cavity. Three pairs of salivary glands are present at different locations.These are:

1. Parotid glands
2. Submandibular glands/submaxillary glands
3. Sublingual glands

Parotid Glands

‘Par’ means near and ‘-otid’ means ear. They are located just in front of each ear. These are the largest salivary glands and contribute around 10% of the saliva. The duct of the parotid gland is known as the Stenson’s duct.

Submandibular/submaxillary Glands

These glands are located at the angles of the lower jaw. These glands are considered as the second largest salivary gland and contribute 70% to the saliva. The duct of the submandibular gland is known as the Wharton’s duct.

Sublingual Glands

‘Sub’ means below and ‘lingual’ means tongue. These are located below the tongue and are considered as the smallest salivary glands. These glands contribute 5% to the saliva. The ducts of sublingual glands are known as the ducts of rivinus.

Saliva

Saliva is a watery substance produced in the mouth. Around 1-1.5 L of saliva is secreted per day. It is slightly acidic (6.8 pH). Components of saliva are as follows:

• Water: Water prevents drying of the mouth.
• Electrolytes: They act as buffering agents.
• Salivary amylase: It is an enzyme that helps in the digestion of starch.
• Lysozyme: It is a naturally occurring enzyme that has antimicrobial properties.

Sjögren's Syndrome

There is a disease known as Sjögren's Syndrome that affects some people. The major symptoms of Sjögren's syndrome include dryness of mouth and eyes. People affected by this also suffer from a decreased sense of taste and sometimes they cannot sense any taste. This is probably due to the dryness of the mouth. Dryness of the mouth reduces the amount of saliva that is very important for the sensation of taste.

Liver

Liver is considered as the largest gland of the human body (weight: 1.2-1.5 kg in adults). It plays a crucial role in digestion. It is located in the upper right side of the abdominal cavity. It has two lobes- right and left. These lobes are separated by a falciform ligament.

Each lobe of the liver is divided into lobules. Lobules are the structural and functional units of the liver. Lobules are covered with a thin connective tissue called Glisson’s capsule. Glisson’s capsule is a characteristic feature of the mammalian liver. Cells of the liver are known as hepatocytes.

Hepatocytes are arranged in the form of cords known as hepatic cords. The space present between the adjacent hepatic lobules is known as hepatic sinusoid. Blood flows through these spaces. Sinusoids are lined by endothelial cells. In some places, macrophages are present called Kupffer cells.

Bile

Bile is secreted by the liver cells or hepatocytes. It is stored in the gallbladder. Gallbladder is a pear-shaped organ attached to the posterior surface of the liver by the connective tissues. It acts as a concentrating reservoir for bile.

Functions of Liver

The liver performs various functions that are enlisted below:

• Detoxification: Detoxification is the primary function of the liver. In this process, the liver converts toxic substances into harmless substances. These harmless substances are absorbed easily.
• Haemopoiesis: Haemopoiesis is characterised as the process of blood cells formation. Liver is responsible for the production of RBCs in an embryo.
• Deamination: Deamination is the process through which amino groups are removed from the excess amino acids and urea is formed in the process.
• Phagocytosis: Phagocytosis is the process of engulfing pathogens. The Kupffer cells present in the liver engulf disease causing microbes and dead cells.
• Carbohydrate metabolism: The liver metabolises carbohydrates through glycogenesis, glycogenolysis, gluconeogenesis and glyconeogenesis.
• Storage: The liver stores excess fats in the form of glycogen. 

Pancreas

Pancreas secretes enzymes as well as hormones. Externally, the pancreas is divided into four parts: Head, Neck, Body and Tail. Head is present in the curve of duodenum.

The exocrine part of the pancreas which is the internal part consists of rounded lobules known as acini. The acinar cells secrete an alkaline pancreatic juice which helps in digestion. The pancreatic juice has a pH: 8.4. Around 500-800 mL of pancreatic juice is secreted per day.

The pancreatic juice is composed of sodium bicarbonate and enzymes such as trypsinogen, chymotrypsinogen, procarboxypeptidase, pancreatic lipase, DNase and RNase.

Ducts of the Liver and Pancreas

The pancreatic juice and the bile are transported to the duodenum of the small intestine through various ducts:

1. Common bile duct
2. Hepato-pancreatic duct
3. Cystic duct

The bile is stored in the gallbladder which is then transported into the common bile duct. Common bile duct is formed by the joining of the hepatic duct (duct of the liver) and the cystic duct (duct of the gallbladder). The bile is reached to the hepato-pancreatic duct from the common bile duct. Hepato-pancreatic duct is formed by joining the common bile duct and the pancreatic duct. The bile flows through the hepato-pancreatic duct and reaches the duodenum.

Digestive Glands Within the Alimentary Canal

There are two types of digestive glands that are present within the alimentary glands:

1. Gastric glands
2. Intestinal glands

Gastric Glands

Ever had a burning sensation in the chest after gobbling down lots of spicy food? Do you know why spicy food causes such heartburn? It is caused due to the acids released by one of the major organs of the digestive system, the stomach. The gastric glands of the stomach are responsible for this.

The gastric glands are tubular glands formed by the epithelium of the stomach. These glands secrete enzymes necessary for digestion. 

There are several cells that make up the gastric glands:

Chief cells/Peptic Cells

These cells secrete enzymes in their inactive form. Inactive precursor of an enzyme is known as zymogen. Secretions of chief cells help in the breakdown of proteins. The opening of this gastric gland is known as the gastric pit.

Oxyntic/Parietal Cells

These cells secrete hydrochloric acid. HCl provides an acidic medium for the activation of pepsinogen. These cells also secrete the castle's intrinsic factor. The intrinsic factor helps in the absorption of vitamin B12.

Mucus Cells/ Goblet Cells

These cells are present throughout the medium and secrete mucus. Mucus protects the lining of the stomach from strong acids.

Enteroendocrine Cells

These cells are situated at the basal part. They are of two types:

1. Argentaffin cells: Secrete serotonin which is a vasoconstrictor.
2. G cells: Secrete the hormone gastrin which stimulates the release of HCl from the oxyntic cells.

Intestinal Glands

In the small intestine, epithelial cells are modified into glands. There are two types of glands present in the small intestine:

1. Brunner’s gland
2. Crypts of Lieberkuhn

Brunner’s Gland

Brunner’s glands are located in the submucosal layer of the duodenum. They synthesise and secrete non enzymatic and alkaline secretion of intestinal juice. They also secrete mucus which protects the duodenal wall from getting digested. They open into the crypts of Lieberkuhn.

Crypts of Lieberkuhn

These are simple and tubular structures which are located throughout the small intestine between the villi. Several types of cells are found in the lining:

1. Enzyme secreting cells: secrete the intestinal juice.
2. Goblet cells: secrete mucus
3. Paneth cells: secrete antimicrobial proteins
4. Enteroendocrine cells: secrete secretin and serotonin hormones. Secretin inhibits the activity of gastric glands and stimulates salt secretion in bile and pancreatic juice. Serotonin promotes smooth muscle contraction for peristalsis.

Practice Problems of Digestive Glands

Q1. Which of these glands is located in front of the ear?

• Parotid glands
• Sublingual glands
• Submandibular glands
• Gastric glands

Solution: ‘Par’ means near and ‘-otid’ means ear. The parotid glands are salivary glands located just in front of the ear. It is the largest salivary gland. It contributes 10% of the saliva produced. Hence, the correct option is a.

Q2. Match the following:

• A - 2, B - 1, C - 3
• A - 3, B - 1, C - 2
• A - 1, B - 3, C - 2
• A - 3, B - 2, C -1

Solution: Parietal cells secrete hydrochloric acid which provides an acidic medium for the activation of pepsinogen. These cells also secrete the castle's intrinsic factor which helps in the absorption of vitamin B12. 

The argentaffin cells in the stomach secrete the vasoconstrictor serotonin.

The goblet cells secrete mucus that protects the stomach lining from the HCl.

Hence, the correct option is b.

Q3. Common bile duct and pancreatic duct are joined to form which of the following ducts?

• Stenson’s duct
• Wharton’s duct
• Hepato-pancreatic duct
• Cystic duct

Solution: Hepato-pancreatic duct is formed by joining the common bile duct and the pancreatic duct. The bile flows through the hepato-pancreatic duct and reaches the duodenum. Hence, the correct option is c.

Q4. Which of the following is not one of the functions of the liver?

• Deamination of amino acids
• Haemopoiesis
• Secretion of gastrin
• Storage of extra glucose as glycogen

Solution: The G cells of the stomach release the hormone gastrin which promotes the release of HCl from the oxyntic or parietal cells of the stomach.

Thus, the correct option is c.

Frequently Asked Questions of Digestive Glands

Question 1. How many types of accessory digestive glands are there?

Answer. There are three types of accessory digestive glands present in the human body. These are enlisted below:

• Salivary glands: These glands secrete saliva in the buccal cavity.
• Liver: Liver is considered as the largest gland of the human body and it secretes bile.
• Pancreas: Pancreas is a heterocrine gland, i.e. it secrete enzymes as well as hormones.

Question 2. How many types of salivary glands are present in the human body?

Answer. There are three types of salivary glands present in the human body. These are present in different locations that are enlisted below;

• Parotid gland: Parotid gland is present in front of the ear. It contributes to 10% of the saliva.
• Submandibular gland: These glands are located at the angles of the lower jaw. They contribute to 70% of the saliva.
• Sublingual gland: These glands are present below the tongue and contribute 5% of the saliva.

Question 3. How many types of cells make up the gastric glands?

Answer. There are four types of cells that collectively make up the gastric glands. These are:

• Peptic cells: Peptic cells secrete enzymes in their inactive form (zymogens).
• Parietal cells: Parietal cells secrete hydrochloric acid that is required for the activation of pepsinogen.
• Goblet cells: Goblet cells secrete mucus that prevents the stomach inner lining from strong acids.
• Enteroendocrine cells: These cells secrete serotonin, somatostatin, histamine and gastrin.

Question 4. What is the composition of pancreatic juice?

Answer. The composition of pancreatic juice is:

• Sodium bicarbonate
• Trypsinogen
• Chymotrypsinogen
• Procarboxypeptidase
• Pancreatic lipase
• DNase
• RNase

Other Related Topics

Other Related Topic Of Biology

• Disorders of the digestive system
• Stomach
• Nutrition
• Buccal cavity
• Balanced diet