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Digestive System and Digestion of Food

All organs and tissues involved in ingesting and digesting food and removing waste from the body constitute the digestive system. To better understand the digestive process, it is necessary to know the digestive system.

Digestive system

The digestive system consists of the digestive tract, the route of food in the body and organs that help digestion. The schematic representation of the digestive system is given below;

digestive system
digestive system

Digestive tract organs;

1. Mouth

Both chemical and mechanical digestion takes place in the mouth. While the teeth break down the food, allowing the food to break down and shrink, the tongue turns the food into a bite. The oral cavity is lined with a mucous membrane.

The saliva secreted from 3 pairs of salivary glands located around the mouth (under the chin, under the tongue and under the ear) is given to the oral cavity. Saliva in the mouth keeps food wet. However, the amylase enzyme in saliva initiates the chemical digestion of starch, a carbohydrate, in the mouth.

2. Pharynx

It is the space between the mouth and the esophagus. Mechanical or chemical digestion does not occur in the pharynx. It has an important task to prevent the transition of consumed foods to the trachea. The lid of the larynx (epiglottis) in the pharynx closes the trachea during the transition of the bite, preventing the bite from going to the trachea and the bite goes to the esophagus.

3. Esophagus

It is the channel where the food transition takes place between the pharynx and the stomach. The smooth muscles in their structure contract and relax during digestion (peristaltic movement) and allow the food to move toward the stomach.

While the peristaltic movement is considered mechanical digestion in some sources, it is not accepted in some sources. We prefer to regard peristaltic motion as mechanical digestion.

4. Stomach

It can be said that it is the main organ of digestion. It can be thought of as a bag where digestion is carried out and the consumed food is stored for a while. At the stomach entrance, there is a valve called “cardia” that prevents the stomach contents (chyme or chymus) from passing into the esophagus.

In cases where the cardia cannot fully perform its function, the highly acidic chyme may pass into the esophagus and a burning sensation (reflux) may occur in the esophagus.

Both chemical and mechanical digestion takes place in the stomach. When the bite reaches the stomach and touches the stomach surface, the stomach secretes gastric juice. Gastric juice contains water, mucus, HCl and pepsinogen. Pepsinogen turns into pepsin with the effect of HCl in the stomach and provides the chemical digestion of proteins.

In addition to digestion in the stomach, absorption of water, some vitamins and minerals also occur.

5. Small intestine

The length of the small intestine in an adult is approximately 6-7 meters. It consists of three parts. The first 25 cm part is called the “duodenum”, the middle part is called the “jejunum” and the last part is called the “ileum”. The duodenum is very important for digestion. The absorption occurs in the last two parts of the small intestine (jejunum and ileum).

The small intestine folds in the abdominal cavity. It consists of connective tissue, smooth muscle and epithelial tissue. In the inner surface of the small intestine, there are folds called “villi” and “microvilli” that increase the surface area.

Thanks to the villi and microvilli, the small intestine of an adult has a very large surface area of ​​120 m2. Considering that 120 m2 is the surface area of ​​an average house, it is pretty interesting that a human small intestine has such a large surface area.

Dextrinase, sucrase, lactase, maltase, dipeptidase, tripeptidase, aminopeptidase and enterokinase enzymes are secreted from the small intestine. The digestion of carbohydrates, fats and proteins is completed in the small intestine and the absorption of the building blocks (monosaccharides, amino acids, fatty acids and glycerol) is carried out.

6. Large intestine

The canal from the small intestine to the anus is called the large intestine. It is about 2 meters long and is folded in the abdominal cavity. The cecum is located at the junction of the large intestine and the small intestine.

At the end of the caecum is a protrusion called the appendix. The part where it joins the anus is called the “rectum”. Protrusions such as villi and microvilli are not found in the large intestine.

No digestion takes place in the large intestine. However, the absorption of water, vitamins, and minerals takes place.

7. Anus

It is the part where wastes formed after digestion are removed from the body.

Organs that help digestion;

1. Salivary glands

There are three pairs of salivary glands around the mouth, under the chin, under the tongue, and under the ear. The mission of the salivary glands is to help digest food by secreting saliva. The saliva contains water, mucus, electrolytes, amylase and lysozyme enzymes.

2. Liver and gallbladder

The liver can be called the body’s chemistry laboratory. It weighs approximately 1.5 kg and consists of 2 lobes, right and left. It is the heaviest organ in the body after the skin. It synthesizes many enzymes and hormones necessary for the body and converts many molecules.

The liver’s function in digestion is to secrete bile. Bile fluid performs the mechanical digestion of fats. Bile produced in the liver is sent to the gallbladder and stored there. The bile liquid in the gallbladder is sent to the duodenum during digestion. There are no enzymes in the bile fluid; It ensures that the oils are reduced to tiny oil droplets.

3. Pancreas

The pancreas is an organ where enzymes necessary for digestion are produced. The pancreatic juice is secreted during digestion and this fluid is sent to the duodenum. Pancreatic juice contains water, bicarbonate ions, amylase, lipase, trypsinogen, chymotrypsinogen, carboxypeptidase and nuclease enzymes.

The high acidity (pH 2.0) of chyme reaching the duodenum from the stomach is neutralized by bicarbonates in pancreatic juice. This prevents high acidity from damaging the intestines.

Digestion of Food

Digestion is the process of breaking down food physically and chemically and making it suitable for use by the body. A person gets the energy he needs for his life activities and the molecules he needs for cell construction, enzyme and hormone production from food. In order to benefit from nutrients, that food must be broken down into its building blocks.

The breakdown of nutrients into building blocks; In other words, digestion has two aspects, mechanical and chemical. Mechanical digestion has only physical effects such as shredding, grinding and size reduction and no enzymes are used. Therefore, there is no change in the chemical structures of nutrients.

1. Chewing and grinding performed with the help of teeth in the mouth,

2. Peristaltic movements along the esophagus,

3. The reduction of fat globules by bile fluid is mechanical digestion.

Mechanical digestion aims to break down the food into smaller sizes and increase the surface area on which the enzymes can act. Increasing the surface area facilitates chemical digestion. An example of this is that a bite that is swallowed without chewing is more difficult to digest.

In chemical digestion, the enzyme effect breaks the bonds in large molecules and the building blocks become free.

It has been stated in previous issues that food consists of different nutrients such as water, carbohydrates, fat, protein, vitamins and minerals. There is no single standard digestive procedure in which every nutrient is digested. The digestion and absorption procedures of carbohydrates, fats and proteins are different from each other.

For example, while the chemical digestion of carbohydrates takes place in the mouth and small intestine; the chemical digestion of proteins takes place in the stomach and small intestine. Naturally, the enzymes used in the chemical digestion of these items are also different from each other.

Water, vitamins and minerals are not digested; that is, they move through the digestive system without any disintegration and are absorbed. Carbohydrates, fats and proteins are broken down into their building blocks, monosaccharides, fatty acids, glycerol and amino acids. These building blocks are then absorbed and presented to the body for use.

When food is eaten, the digestive system immediately begins the procedure to break down that food and obtain its building blocks. Even seeing the food or even the thought of that food is often enough to start the digestive activity. For example, when we see a food that we think is delicious, the salivary glands begin to work and they secrete saliva, causing our mouths to water.

The digestive system focuses on what nutrients the food contains rather than what it is. Let’s take bread, for example. Bread is food containing water (37%), carbohydrates (50%), protein (8%), fat (1%), minerals and vitamins.

If the events that took place from the moment the bread is eaten are listed;

1. Saliva is secreted from the salivary glands. It is ensured that the food is wetted with saliva; In this way, the grinding and chewing of the bread by the teeth and the transition of the bite through the pharynx and esophagus become easier.

In addition, the enzyme amylase begins to break down the starch in the bread chemically. Of course, there is not only starch as a carbohydrate in bread. Other carbohydrates are also present. However, only starch is chemically digested from carbohydrates in the mouth.

Other carbohydrates reach the stomach without undergoing chemical digestion in the mouth. In the mouth, the proteins and fats in the bread do not undergo chemical digestion; Therefore, their chemical structure does not change.

2. Teeth perform mechanical digestion by chewing and grinding. Thus, a large food surface is provided for chemical digestion. All food undergoes mechanical digestion in the mouth.

3. Crumbled and shrunken bread passes through the pharynx and esophagus and reaches the stomach. Peristaltic movements provide the transition of the bite from the esophagus. The mucous layer secretes mucus, making it easier to pass. The bite reaches the stomach from the pharynx in about 3 seconds.

4. When the bite touches the stomach wall, the stomach begins to secrete gastric juice. Gastric juice contains water, hydrochloric acid (HCl), pepsinogen enzyme and mucus. In the stomach, no chemical digestion is applied to the carbohydrates and fats in the bread.

There is only chemical digestion of proteins in the stomach and this digestion does not occur until the amino acids are their monomers. Proteins are broken down into polypeptides in the stomach.

It is no longer possible to talk about a bite or bread in the stomach. It would be more accurate to say a mixture containing nutrients. This mixture is called chyme and is highly acidic (pH 2). The chyme goes to the small intestine after the stomach.

5. When the chyme reaches the duodenum, the duodenum stimulates the pancreas, liver, gallbladde and stomach through the hormones it secretes. For the pancreas and liver to secrete enzymes; drain the gallbladder of bile fluid; it stimulates the stomach to slow down.

Nutrients are broken down into their monomers in the small intestine. Carbohydrates have building blocks such as glucose, fructose and galactose; proteins that have been polypeptided before in the stomach are converted to amino acids; fats are broken down into glycerol and fatty acids.

Bile juice is not secreted for chemical digestion but for mechanical digestion. The bile fluid only allows the fat globules to break down into smaller pieces. After mechanical digestion, the chemical digestion of fats is carried out with the enzyme lipase. As the name suggests, indigestible carbohydrates, known as dietary fiber, are not digested.

6. Digestion is now complete in the small intestine. Consumed bread is now in a watery mixture of glucose, amino acids, fatty acids, glycerol, dietary fibers, vitamins and minerals. At this stage, components other than dietary fibers are absorbed.

Glucose, amino acids, vitamin C and B group vitamins are absorbed in the small intestine and delivered to the liver via capillaries. Fatty acids, glycerol, vitamins A, D, E and K are absorbed and transferred to the bloodstream through lymph capillaries.

Generally, 98% of glucose, fructose, galactose and other monosaccharides, 95% of glycerol and fatty acids and 92% of amino acids can be absorbed by the body.

After digestion and absorption in the small intestine, the remainder of the mixture is water, dietary fiber, minerals, some vitamins and non-absorbable building blocks of carbohydrates, proteins and fats. This mixture continues on its way to the large intestine.

7. Digestion is no longer present in the large intestine. Some water, minerals and vitamins in the remaining mixture are absorbed in the large intestine. Bacteria ferment non-absorbable carbohydrate, fat and protein building blocks in the large intestine.

This fermentation is very positive for the body. The activities of probiotic bacteria have very important functions, such as strengthening immunity, preventing weight gain and regulating digestion. After absorption and fermentation of bacteria, what remains is now waste and enters the discharge process.

Bread is given as an example because it contains all the nutrients. No matter how many different foods the individual is fed at dinner, the digestion procedure is carried out through the nutrients contained in the food.

For example, if the digestion of meat is considered; The digestion procedure proceeds through the nutrients, as in the example of bread. There will be no carbohydrate digestion and absorption because there is no carbohydrate in meat only.

But the whole other digestion procedure is exactly the same. As a difference, it can be said that the digestion time will be longer because the protein and fat ratio in meat is high.

The digestion procedure can be schematized as follows;

digestive procedure
digestive procedure

In addition to the above information regarding absorption;

1. In the mouth, nicotine, some poisons and some minerals can be absorbed,

2. In the stomach, alcohol and some poisons can be absorbed.


Here’s an article that might interest you;

Energy Metabolism – I: Energy Production in the Body

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