Digestive Process in Vertebrates: Stages and Functions
Digestive Process – Stages
- Ingestion: Food enters the organism. In most animals, food enters through the mouth.
- Digestion: Transforms food into the simplest molecules that can be used by cells. In most animals, food undergoes a double transformation: mechanical and chemical.
- Absorption: The molecules obtained during digestion pass through the walls of the digestive tract to be incorporated into all cells of the organism.
- Egestion: The process of eliminating all food waste that could not be digested and is expelled to the outside of the body.
Systems Involved in Nutrition
- Digestive system: Processes food into nutrients.
- Circulatory system: Transports nutrients to cells and waste material to the excretory and respiratory systems.
- Urinary system: Eliminates the waste obtained from cellular metabolism.
In the relationship of the systems involved in nutrition, nutrients in the cells can be used for the manufacture of more complex molecules or be degraded for the energy they contain. The CO2 and waste generated by metabolism are returned to the circulatory system.
Digestive Process in Vertebrates
Digestion in the Mouth
- Chewing: Carnivores have highly developed canines. Herbivores have highly developed molars and premolars. Omnivores have a similar degree of development of all types of teeth.
- Insalivation: When the saliva secreted by salivary glands mixes with food. Saliva is composed of water, mucin, and ptyalin. After chewing and insalivation, the food becomes the bolus.
- Swallowing: The food is pushed by the tongue toward the pharynx, passes through the esophagus, and then to the stomach. Progress through the esophagus is produced by peristaltic wave contraction movements. In birds, the widening of the esophagus is called the crop.
Gastric Digestion
The stomach, which can have one or more compartments with a muscular wall, carries out gastric digestion, which consists of two phases: mechanical and chemical. The mechanical phase is caused by the contraction of the stomach wall, while the chemical phase is done through the gastric juice secreted by cells in the wall of the stomach. This juice consists of hydrochloric acid, pepsin, and mucin.
Intestinal Digestion
Digestion is completed in the intestine, a tube of variable length based on the type of diet. The intestine is composed of two parts:
- Small intestine: Has three parts: duodenum, jejunum, and ileum.
- Large intestine: Is divided into the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.
Digestion of chyme is completed in the small intestine, which becomes a whitish substance called chyle, which is made up of water, minerals, and nutrients.
Function of Glands Annexed to the Intestine
- Liver: Secretes bile, which is stored in the gallbladder. Bile causes fat emulsion, facilitating the action of lipases, both pancreatic and intestinal.
- Intestinal glands: Two types: Brunner’s and Lieberkühn’s. They are found in the small intestinal mucosa and secrete intestinal juice, which is made up of water, mucus, and numerous enzymes such as maltase, sucrase, lactase, peptidase, and lipase.
- Pancreas: Secretes pancreatic juice and hormones involved in regulating blood glucose levels. Pancreatic juice is made up of hydrolytic enzymes: amylase, lipase, trypsin, and chymotrypsin.