Animal Digestive System Parts and Function
Digestive Tract Parts and Function
The digestive tract consists of:
The Mouth: Initial Digestion
The mouth is the entry zone for food. It performs two types of digestion:
- Mechanical Digestion: This is the chewing done by teeth. Depending on the animal, the dentition is different and adapts to the type of feeding. For example, birds have no teeth but use their beak, tongue, and hard palate. Food is crushed in the gizzard. Carnivores have very developed canine teeth, while herbivores do not have them but have large premolars and molars.
- Chemical Digestion: This occurs due to the salivary glands, which secrete saliva. This liquid contains digestive enzymes that break down polysaccharides into disaccharides and oligosaccharides. The tongue mixes saliva with the shredded food, forming the bolus, which aids swallowing and dissolves particles, allowing the food to be tasted.
The Esophagus: Transport Tube
The esophagus is a tube connecting the mouth to the stomach. In birds, it has a swollen area, the crop, where they store food swallowed without crushing. The function of the esophagus is to propel the bolus into the stomach through peristaltic movements.
The Stomach: Storage and Digestion
The stomach is a vast digestive area. Its development depends on the type of animal (primitive fish have none). In birds, it has two parts: the glandular stomach for chemical digestion and the gizzard for grinding. In ruminants, there are up to four cavities adapted to their type of feeding. The function of the stomach is, on one hand, to store food, and on the other hand, to perform both physical and chemical digestion. Its energetic movements convert the food into a semi-liquid slurry called chyme. Chemical digestion is carried out by gastric juice produced by cells in the wall. This juice contains:
- Pepsin: An enzyme that breaks down proteins into their amino acids.
- Renin: Coagulates milk proteins.
- Hydrochloric Acid: Maintains a very acidic pH in the stomach so that enzymes can function and destroys bacteria or pathogenic microorganisms that may have entered with the food.
The Small Intestine: Nutrient Absorption
The small intestine is divided into the duodenum, jejunum, and ileum. It is the longest tube in the tract. Its length varies with the feeding rate. In herbivores, it is very long. In carnivores, it is short. Its function is to complete the chemical digestion of lipids and finish the chemical digestion of proteins and carbohydrates. To aid this, juices from the liver and pancreas are poured into the duodenum, as well as juice produced by the intestine itself. All these liquids are mixed with the food and form the chyle.
The Large Intestine: Water Absorption
The large intestine is the end of the tube. It has three parts:
- Cecum: The initial part, forming a pouch. Some animals have an appendix prolongation.
- Colon.
- Rectum: The muscular final part that governs the expulsion of feces through the anus.
The main function of the large intestine is to absorb water and ions and form feces from undigested food. This undigested food is broken down by thousands of bacteria living in symbiosis.
Intestinal Absorption Process
Intestinal absorption consists of the passage of molecules obtained from digestion from the intestine into the circulatory system. To increase the absorption surface area, many adaptations appear, such as:
- Having a longer gut.
- Having folds along the tube: intestinal villi and microvilli.
- Cartilaginous fish also present the spiral valve, which favors absorption. This valve is a structure that slows down the passage of food through the intestine, allowing more time for uptake.