Digestive Glands: Types, Functions, and Digestive Process
Digestive glands are a set of glands responsible for producing digestive juices needed for the chemical digestion of food in the digestive tract. These glands include the salivary glands, gastric glands, intestinal glands, liver, and pancreas.
Gastric and intestinal glands are numerous, very tiny, and located in the inner wall of the stomach and intestines, respectively. The salivary glands, liver, and pancreas are located outside the digestive tract and pour their secretions into it through ducts.
Salivary Glands
The salivary glands produce saliva, the first digestive juice that acts on food. Saliva consists mainly of:
- Water
- Mucin (a substance that makes saliva thick and viscous)
- Salivary amylase (a digestive enzyme)
- Bactericidal substances (that inhibit the growth of microorganisms)
Gastric Glands
Gastric glands produce gastric juice, composed of:
- Hydrochloric acid (HCl)
- Pepsin (a digestive enzyme that breaks down proteins)
Liver
The liver produces bile, which is stored in the gallbladder, a small sac attached to the liver. From the gallbladder, bile passes into the duodenum, where it acts on fats. Bile is a greenish liquid that contains no digestive enzymes. It consists mainly of water and bile salts, which promote the dissolution of fats by acting as a detergent.
Pancreas
The pancreas secretes pancreatic juice into the duodenum. Pancreatic juice contains different digestive enzymes:
- Lipase, which acts on lipids
- Amylase, which breaks down starch
- Trypsin, which breaks down protein molecules
Intestinal Glands
Intestinal glands produce intestinal juice, which contains various digestive enzymes that complete the digestion of starch and proteins.
Mucous Glands
Mucous glands, located in the inner wall of the digestive tract, secrete substances that lubricate the walls and protect them from the action of digestive juices.
Digestive Juices and Enzymes
| Gland | Juice | Enzymes |
|---|---|---|
| Salivary | Saliva | Salivary amylase |
| Gastric | Gastric juice | Pepsin |
| Intestinal | Intestinal juice | Various intestinal enzymes |
| Liver | Bile | Bile salts |
| Pancreas | Pancreatic juice | Lipase, amylase, trypsin |
The Digestive Process
Digestion is a process of chemical transformation of food into soluble substances that occurs in the digestive tract. The act of swallowing the food bolus is called deglutition. During swallowing, the tongue pushes the bolus into the pharynx, while the epiglottis closes the larynx and the soft palate rises to close the nasal passages. This ensures that the food bolus enters the esophagus. The movements of contraction and relaxation of the esophagus muscles are called peristaltic movements and occur throughout the digestive tract to move food.
Digestion in the Stomach
In the stomach, pepsin breaks down long chains of proteins into fragments called amino peptides. The food bolus, under the action of gastric juice, becomes chyme, which exits the stomach through the pylorus in small quantities.
Digestion in the Small Intestine
Most chemical transformations of digestion are carried out in the duodenum. The chyme is mixed with pancreatic juice, bile, and intestinal juice, which act as follows:
- Bile salts act on fats as a detergent, facilitating the action of pancreatic digestive enzymes.
- Pancreatic juice enzymes: lipase digests fats, amylase degrades starch, and trypsin digests proteins.
- Intestinal juice enzymes convert disaccharides into glucose and complete the digestion of proteins.
The chyme acquires a milky appearance and becomes a liquid called chyle.