Human Digestive System: From Mouth to Anus
The Human Digestive System
Oral Cavity
Teeth are structures responsible for crushing food. They are harder than bone and contain pieces composed of calcium and fluoride.
- Incisors: Cut food.
- Canines: Tear food.
- Premolars and molars: Grind and blend food.
Premolars have one or two roots and a crown with two protuberances. Molars have four or more roots and four or five protuberances.
Tongue: Participates in the mastication process by moving food around. It also facilitates the passage of the food bolus into the pharynx and esophagus during deglutition.
Saliva:
- Facilitates the passage of the bolus into the pharynx and esophagus.
- Starts the digestion of starch, transforming it into simpler sugars because it contains amylase, a digestive enzyme.
- Destroys some existing bacteria.
Deglutition and Esophagus
The bolus is formed and, thanks to the tongue, passes through the pharynx and esophagus, ducts that end in the stomach. The advancement of the bolus through the esophagus is produced by muscular contractions of its walls. This movement is called peristaltic movement.
Stomach Digestion
Upon entering the stomach, the bolus causes contractions of the musculature. The bolus remains in the stomach for three or four hours, where it mixes with gastric juice produced by glands that coat its internal wall. The resulting mixture constitutes chyme, a more fluid mass than the bolus. Chyme contains:
- Pepsin: An enzyme that begins the digestion of proteins.
- Hydrochloric acid: Activates pepsin and facilitates the breakdown of food fibers. It also destroys bacteria.
Liver
The liver is a large organ located on the right side of the abdomen, partially covering the stomach. It has a purplish color due to the blood it contains and performs many functions. It produces bile, which is poured into the duodenum through the ampulla of Vater. Bile is not produced directly but is stored in the gallbladder. Bile emulsifies fats but does not contain digestive enzymes. It contains bile salts that emulsify fats into smaller particles so that they can be more easily digested by digestive juices.
Pancreas
The pancreas is located behind and below the stomach. It has two main functions:
- Endocrine function: Produces hormones that regulate the amount of glucose and its accumulation in the liver.
- Exocrine function: Secretes pancreatic juice, which contains sodium bicarbonate.
Absorption of Nutrients
All nutrients are absorbed in the small intestine, while water is absorbed in the large intestine. The inner wall of the small intestine is covered with intestinal villi, which are toured by capillaries that collect nutrients after digestion. To enhance the surface area, the plasma membrane of the cells features microvilli. Digested products, except for fats, are collected by blood capillaries. Fats are collected by the lymphatic system and then passed into the blood.
Large Intestine
The remains of undigested food pass into the large intestine. It is approximately one meter long and has no villi. It is responsible for the absorption of water and the compaction of waste from digestion, forming feces. Fecal waste progresses due to peristaltic movements and is expelled to the outside through the anus.