Digestive System: Food Digestion and Nutrient Absorption
The digestive system performs the processes of food digestion and absorption of nutrients. These processes require two types of actions:
- Mechanical: This involves the fragmentation of food and the movement of the bolus through the digestive tract via muscle contractions called peristaltic contractions.
- Chemical: This involves the hydrolysis of food by enzymes found in digestive juices and bile. These processes break down food into smaller molecules that can cross the gastrointestinal wall and enter the internal environment to be distributed to different tissues via the bloodstream.
Mouth
The mouth fragments food and initiates the chemical digestion of carbohydrates, thanks to enzymes present in saliva. The mouth also performs other important functions, such as lubricating and swallowing food.
Chemical Digestion and Lubrication of Food
Saliva mixes with food and initiates the chemical digestion of carbohydrates. Saliva is a watery solution composed of water and proteins. Once shredded and lubricated, food becomes a bolus. Besides its digestive function, saliva lubricates the bolus, promoting swallowing, protecting the oral mucosa, and dissolving particles, stimulating the taste buds to produce taste sensations.
Swallowing
Swallowing is the stage where the bolus passes from the mouth to the esophagus. First, the bolus is propelled into the pharynx by the tongue voluntarily. Then, the bolus enters the esophagus, and peristaltic waves move it along.
Esophagus
The esophagus is a tube connecting the mouth to the stomach. In birds, it has a widened section called the crop, where they store food that they swallow without crushing. Its function is to continue the mechanical treatment of food and chemical digestion. It also secretes a mucoid substance that lubricates the bolus and prevents abrasion of the walls.
Stomach
The stomach is located between the esophagus and the small intestine. In birds, it undergoes a major transformation and is divided into a muscular part, especially for the trituration of food, called the gizzard. In the case of ruminants, it has four chambers, as an adaptation for rumination. Its function is to store food, regulate its passage to the bowel, and perform part of the digestion. It also allows for a small amount of water absorption of some ions and some chemicals, such as aspirin or ethanol.
Chyme and Gastric Secretions
- Chyme: When ingested food reaches the stomach, it mixes with digestive juices, acquiring a semi-liquid consistency. Food at this stage is called chyme.
- Gastric Secretions: These are secreted by the glands of the stomach lining. They are composed of water, mucin, which has a protective function; pepsinogen, the precursor of pepsin; rennin, which acts on milk protein so that it can be coagulated and treated by enzymes; and hydrochloric acid, which maintains an acidic pH for enzyme function and also has a bacterial action.
Regulation of Gastric Secretions
Regulation has both nervous and hormonal components. When food reaches the stomach, gastrin is secreted into the blood, reaching the stomach cells that produce hydrochloric acid.
Small Intestine
The small intestine is divided into three parts: the duodenum, jejunum, and ileum. It represents the longest portion of the digestive tract and performs most of the chemical digestion and nutrient absorption. For this, secretions from the pancreas, liver, and intestinal glands are discharged and mixed with the chyme, thanks to back-and-forth peristaltic waves. This mixture forms chyle, containing water, salts, minerals, etc., which are absorbed by the intestinal mucosa to be incorporated into the circulatory system.