BIOLOGY

Digestive System

The digestive system or digestive system is the set of organs responsible for the processing of food in animals . It is responsible for transforming food so that it can be used by the body.

Animals need external sources to maintain vital functions, unlike plants. It is then from the food that energy and matter are obtained in the form of nutrients for the different cellular activities.

In order for the nutrients to enter the cell, the food is processed, cut and divided into its molecular components, which then pass to the blood, which carries them to the rest of the body. This is possible thanks to the digestive system.

Animals can be herbivores, carnivores or omnivores, depending on whether they feed on plants, animals, or both. Humans are omnivores , although there are people who have a predilection for vegetables (vegetarians) or meat (Eskimos).

Parts of the digestive system

The digestive system is made up of the digestive tract and the accessory or accessory glands . The digestive tract goes from the mouth to the anus and, certainly, it is a very long tube through which food travels by gravity and by peristalsis.

The peristalsis is the effect produced by the muscles of the gastrointestinal tract to contract; imagine that you have a flexible hose and you want to pass a marble through it. Closing your fist around the hose, just behind the marble, you can push it to the other end. The same happens along the digestive tract.

Digestive tube and its parts

Oral cavity

The mouth or oral cavity extends from the lips to the pharynx. It contains the tongue, gums, teeth and orifices of the salivary ducts. It is in this part where the first phase of digestion takes place: chewing and swallowing.

Pharynx

The pharynx is a tube through which air, food and liquids enter. The tonsils and adenoids are found in the pharynx. The epiglottis is also obtained, a lamina behind the tongue that closes the larynx when we swallow.

Esophagus

The esophagus is a tube of fibers and muscle that measures approximately 25 cm. It connects the pharynx with the stomach and is behind the trachea in its upper part. It enters the abdomen through the diaphragm. Its function is to allow the arrival of food to the stomach.

Stomach

The stomach is a muscular organ, similar to a bag or fanny pack, which mixes the food with the gastric juice to form the chyme . It is located on the left side of the abdomen, between the esophagus and the first part of the small intestine.

You can store up to three liters of food.

The cardia is responsible for letting the food from the esophagus to the stomach.

At the entrance and exit of the stomach are two muscular rings, called sphincters, which open and close regulating the transit of food. The cardiac sphincter opens to let in the food bolus, and closes to prevent the reflux of the chyme.

The pylorus is the sphincter at the end of the stomach, which opens to let the chyme pass into the small intestine.

Small intestine

The small intestine is a tube of approximately 6 to 7 m, which runs from the stomach to the large intestine. It has three parts: the duodenum, the jejunum and the ileum.

The duodenum is just after the stomach and measures 25 cm. In it pancreatic and biliary juices are spilled. The jejunum continues to the duodenum and represents two fifths of the small intestine. It continues with ileus, which is the longest part and connects to the large intestine.

The inner part of the small intestine is especially rich in surface. As a consequence, it increases the contact area with the intestinal content, and, therefore, the absorption. This is due to the formation of large-scale folds and villi, and the presence of cellular microvilli.

Large intestine

The large intestine is the last part of the digestive tract, approximately 1.5 m in length. It can be divided into eight segments:

  1. the blind,
  2. the appendix,
  3. the ascending colon,
  4. the transverse colon,
  5. the descending colon,
  6. the sigmoid colon,
  7. the rectum,
  8. the anal channel.

Glands attached to the digestive system

Attached or accessory glands are structures that connect to the digestive tract and secrete their products there. There are three: the salivary glands, the liver and the pancreas.

Salivary glands

The main product of the salivary glands is saliva. There are three pairs of salivary glands: the parotid glands, the sublingual glands and the submandibular glands.

Functions of saliva and salivary glands

  • Cleaning and humidification of the mouth.
  • Secretion of digestive enzymes such as amylase, and antibacterial enzymes, such as lysozyme.
  • Dissolution of compounds and contribution to the sense of taste.

The liver is the largest glandular organ in the body, weighing approximately 1.5 kg. It is located on the right side of the abdomen, below the diaphragm.

Bile produced by the liver is stored in a pear-shaped sac, called a gall bladder . A thin tube or duct comes out of the gallbladder to release the bile in the duodenum.

Liver functions
Function Mechanism
Protein metabolism

Synthesis of plasma proteins.

Synthesis of coagulation factors.

Lipid metabolism

Formation of lipoproteins and fatty acids.

Synthesis of cholesterol.

Transformation of cholesterol into bile salts.

Carbohydrate metabolism

Synthesis and degradation of glycogen.

Glucose production

Secretion of bile

Production of bile salts.

Elimination of bilirubin.

Storage

Glycogen

Vitamins A and B12

Transformation and detoxification Processing of drugs and exogenous substances.
Protection

Filtration of blood from the intestine.

Removal of bacteria by phagocytosis.

Hematopoiesis The liver of the fetus produces red blood cells.

Pancreas

The pancreas is an endocrine and exocrine gland. The endocrine cells of the pancreas (islets of Langerhans) produce the hormones insulin, somatostatin and glucagon that pours into the blood. The exocrine cells produce digestive enzymes that end up in the duodenum.

Main pancreatic enzymes
Enzyme Act on
Tripsin Proteins and polypeptides
Pancreatic lipase Triglycerides
Pancreatic alpha-amylase Starch
Phospholipase-A2 Phospholipids

Functions of the digestive system

Digestion of food

The main function of the digestive system is the digestion of food, that is, the processing of food to be assimilated by the body. In order to take advantage of the nutritional content of food, digestion consists of several stages: chewing, swallowing, chemical and enzymatic digestion and absorption. Next, we explain each of these steps.

1. Chewing

When we introduce food into the mouth and chew it, the teeth and tongue perform a mechanical function by breaking and mixing the food with saliva and enzymes. The large pieces of food are chopped and crushed, facilitating the digestion process.

The molecules dissolve in the saliva and stimulate the sense of taste. The food bolus is the combination of crushed and chewed foods with saliva.

2. Swallowing

In the process of swallowing, the epiglottis prevents the passage of food to the respiratory tract.

After chewing and forming the food bolus, the next step is to swallow it. The tongue pushes the bolus through the palate into the pharynx. Here, the epiglottis plays a critical role, since it prevents the food from being diverted to the trachea and thus reaching the lungs, which would cause serious inconveniences.

Then, the esophagus is responsible for transporting the food bolus to the stomach.

3. Chemical and enzymatic digestion

Hydrochloric acid and enzymes in the stomach degrade the food bolus to its molecular components.

It is carried out in the stomach and in the small intestine. In the stomach, gastric acid juice activates enzymes that begin to break down proteins and fats. Then a liquid mass called chyme is formed .

In the duodenum, pancreatic juice, bile and intestinal juice are secreted, with enzymes that help break down the proteins, fats, carbohydrates and nucleic acids of the chyme.

This is how proteins are transformed into amino acids, carbohydrates into monosaccharides, fats into fatty acids and glycerol, and nucleic acids into nucleotides. The result is now an aqueous substance with a milky appearance called chyle .

The chyme takes between 2 and 3 hours to pass through the small intestine.

The process by which food molecules pass into the blood is called absorption. This takes place in the stomach, the small intestine and the large intestine. Through the wall of the stomach, you can absorb alcohol, water, salts and some medications.

Most of the absorption of the products of digestion takes place in the enterocytes (intestinal cells) in the small intestine. This thanks to the large amount of contact surface, due to the folds, villi and microvilli.

The villi look like glove fingers that project into the intestine. Microvilli are extensions of the plasma membrane of intestinal cells.

The main function of the colon is the absorption, especially of water.

Excretion

The digestive system is also responsible for the elimination of what could not be processed during digestion. The defecation is carried out by the rectum and the anus. The stools are what remains of food that were not absorbed in the digestive tract.

Exocrine and endocrine secretion

The organs and glands of the digestive system also have an exocrine and endocrine function. Substances that are discharged directly into the digestive tract, such as, for example, hydrochloric acid, saliva and bile salts and bile, are exocrine secretions.

Endocrine secretions are the hormones that are poured into the bloodstream that produce their effect on separate organs. Insulin and glucagon are hormones that secrete the pancreas, important in the regulation of glucose in the body.

Defense against microorganisms

The digestive tract goes through the body, connecting the outside with the inside of the body. That is why it also fulfills the function of defending ourselves from microorganisms or foreign agents, by means of:

  • the secretion of gastric acids.
  • The reflection of vomit.
  • The intestinal microbiota: beneficial bacteria that live inside the tube.
  • The immune response and antibody secretion.

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