Digestive System in Vertebrates: From Oral Cavity to Midgut
Oral Cavity
The oral cavity, or mouth, is surrounded by folds of skin called lips, except in turtles and birds. Inside the mouth, there are teeth. In some species, all teeth are the same, while in others, the upper teeth have different shapes. Their function is to cut, grind, and crush food (mechanical digestion). The tongue is also found in the mouth. It contains a large number of taste buds, and its function is to mix food and facilitate its transit into the pharynx. In fish, the tongue is not mobile. Salivary glands discharge their secretions into the oral cavity. These glands appear in all vertebrate groups except fish. Saliva is an alkaline liquid formed by 98% water, mineral salts, amylase, lysozyme, etc. Saliva has several functions: it acts as a lubricant for food, facilitating swallowing; it destroys some bacteria ingested with food; and it begins chemical digestion with amylase.
Pharynx
The pharynx is a muscular tube involved in the swallowing process. It belongs to both the digestive and respiratory tracts and serves as a transit point for food and water to lower portions of the digestive tract. The pharynx has a fold (epiglottis) that prevents food from entering the respiratory system.
Esophagus
The esophagus is a tube whose straight muscular contractions propel the bolus into the stomach. The combined action of these contractions produces peristaltic movements, which resemble the movement of an earthworm.
Stomach
The stomach is a dilation of the digestive tract where food is stored for a while before moving to the intestine in an advanced state of disintegration.
- Fish, amphibians, and reptiles: Spindle-shaped stomach.
- Birds: Divided into a muscular part, specializing in food trituration called the gizzard, and a glandular part, or stomach.
- Ruminant mammals: The stomach is very complex and is divided into four chambers: rumen, reticulum, omasum, and abomasum. Grass goes to the reticulum, from which small portions are regurgitated and chewed (rumination). This mass is swallowed a second time, returns to the stomach, and ferments. Later, it is driven to the omasum, where there is a partial absorption of water. From there, it passes to the abomasum, which secretes gastric juice and carries out authentic digestion.
In the vertebrate stomach, chemical digestion begins through the action of gastric juice secreted by glands lining its walls. Gastric juice consists of mucin, a substance that protects the gastric mucosa; pepsinogen, the inactive precursor of pepsin, a hydrolytic enzyme that converts proteins into polypeptides; hydrochloric acid, which maintains the pH to allow the action of enzymes and kill bacteria; and renin, which appears only in mammals during lactation and acts on proteins in coagulated milk. After remaining in the stomach, food forms a slurry called chyme, which passes into the intestine.
Midgut
The midgut performs two different and very important functions: total chemical digestion and absorption of food. In all vertebrates, the midgut receives secretions from:
- Bile juice: Produced by the liver. It is stored in the gallbladder until it is excreted into the duodenum through the bile duct. It emulsifies fatty substances.
- Pancreatic juice: Produced by the pancreas.
On the walls of the intestinal mucosa, there are two types of glands: one secretes mucus, and the other secretes intestinal juice. The enzymes contained in these three juices are not the same in all animals. However, the result of their action is common to all. At the end of digestion, chyme has been transformed into chyle, a milky liquid. The resulting products of digestion must penetrate the intestinal wall to enter the blood and be transported to every cell in the body. This process is called absorption. Most organic nutrients, such as glucose and amino acids, are absorbed by active transport, which requires a lot of energy. Other substances pass into the blood capillaries by simple diffusion or facilitated diffusion.