Human Digestive System: Processes and Functions
Human Digestive System: Processes and Functions
- The human digestive system is composed of:
- A digestive tract that extends from the mouth to the anus, through which food passes.
- A set of glands that pour their secretions into the digestive tract. These include salivary glands, the pancreas, the liver, and gastric glands.
Structure of the Digestive Tract
It consists of four layers:
- Mucosa: The inner layer, formed by lining epithelium. It contains cells that secrete mucus, which lubricates and protects the surface.
- Muscle: Consists of smooth muscle. It produces rhythmic waves of contraction and relaxation, called peristaltic waves. In some places, there are bottlenecks called sphincters that prevent the backward movement of materials. There is a sphincter between the esophagus and the stomach (cardia), and another between the stomach and the small intestine (pylorus).
- Submucosa: Composed of tissue rich in blood vessels, lymph vessels, and nerves.
- Serosa: The outer layer is thin and composed of connective tissue.
Types of Digestive Processes
It comprises two types of processes:
- Mechanical: Chewing and grinding of food in the stomach.
- Chemical: Enzymes produced by the digestive juices act on food crushed in a liquid medium.
Digestion in the Mouth
In the mouth, food is crushed by the teeth and mixed with saliva until it becomes a kind of pulp.
Saliva moistens food to aid swallowing and passage through the esophagus.
Saliva consists of water, salts, enzymes, and mucus with lubricant and antiseptic action. It contains immunoglobulin A, the first defense against bacteria and viruses, and lysozyme, which attacks bacterial walls.
Saliva is produced by three pairs of salivary glands.
Saliva contains two enzymes:
- Salivary amylase or ptyalin: Starts to digest starch and glycogen.
- Lingual lipase: Hydrolyzes milk fat droplets upon contact with the stomach’s acidic environment.
During swallowing, the communication between the pharynx and nasal cavity closes, and the epiglottis prevents the passage of food into the airways.
Stomach Digestion
Glands located in the walls of the stomach secrete gastric juice and pancreatic juice, which mix with food and create a juice called chyme.
- Hydrochloric acid: Provides the acidity required for the activation of pepsin and destroys most of the microorganisms that accompany food.
- Pepsin: A protein enzyme that transforms proteins into peptides. The inactive precursor secreted by the gastric mucosa is pepsinogen.
- Intrinsic factor: A glycoprotein secreted by cells producing hydrochloric acid in the gastric mucosa. It is necessary for the absorption of vitamin B12 in the small intestine. This is the only gastric secretion essential for life. The digestive function of the other gastric secretions can be replaced by the activity of pancreatic enzymes.
- Mucous secretions: Cover the walls of the stomach and help protect them from the action of gastric juice.
Chyme passes through the pylorus into the duodenum.
In the stomach, absorption does not occur, except for some fat-soluble substances.
Alcohol is absorbed more quickly if accompanied by CO2.
Serious injury can occur in children and young people, whose livers are not yet ready to metabolize all the alcohol absorbed.
Intestinal Digestion
The intestine absorbs about 8.5 liters of water, where dissolved nutrients are found.
To absorb this volume of fluid, the intestinal mucosa forms folds or villi.
In addition to the villi, the intestinal lining is formed by cells with microvilli.
It is estimated that the total extent of intestinal absorption is 300 m2.
The chemical transformations of food take place primarily in the small intestine, through the action of pancreatic juice, bile, and enzymes in the epithelial cells of the mucosa.