Digestive (digestive) system: how digestion and anatomy happens

The digestive system, also called digestive or gastro-intestinal (SGI) is one of the main systems of the human body and is responsible for the processing of food and absorption of nutrients, allowing the proper functioning of the body. This system is made up of several bodies, which act together in order to perform the following main functions:

Promote the digestion of proteins, carbohydrates and lipids from the foods and drinks consumed; Absorb fluids and micronutrients; Provide a physical and immunological barrier to microorganisms, foreign bodies and antigens consumed with the food.

Thus, the SGI is responsible for regulating the metabolism and the immune system, in order to maintain the proper functioning of the body.

Organs of the digestive system

The digestive system is made up of organs that allow the conduction of the ingested food or drink and, along the way, absorption of essential nutrients for the proper functioning of the organism. This system extends from the mouth to the anus, with its constituent organs:

Mouth: responsible for receiving the food and reducing the size of the particles so that it can be digested and absorbed more easily, in addition to mixing it with saliva; Esophagus: responsible for transporting food and liquids from the oral cavity to the stomach; Stomach: plays a fundamental role in the temporary storage and digestion of food eaten; Small intestine: responsible for most of the digestion and absorption of food and receives secretions from the pancreas and liver, which assist this process; Large intestine: this is where water and electrolyte absorption occurs. This organ is also responsible for temporarily storing end products of digestion that serve as a means for bacterial synthesis of some vitamins; Rectum and anus: are responsible for controlling defecation.

In addition to the organs, the digestive system consists of several enzymes that ensure the correct digestion of food, the main ones being:

Salivary amylase, or ptyline, which is present in the mouth and is responsible for the initial digestion of starch; Pepsin, which is the main enzyme in the stomach and is responsible for the breakdown of proteins; Lipase, which is also present in the stomach and promotes the initial digestion of lipids. This enzyme is also secreted by the pancreas and performs the same function; Trypsin, which is found in the small intestine and leads to the breakdown of fatty acids and glycerol.

Much of the nutrients cannot be absorbed in their natural form due to their size or the fact that they are not soluble. Thus, the digestive system is responsible for transforming these large particles into smaller, soluble particles capable of being absorbed quickly, which is mainly due to the production of several digestive enzymes.

How digestion happens

The digestive process begins with the ingestion of food or drink and ends with the release of feces. The digestion of carbohydrates starts in the mouth, although digestion is minimal, while the digestion of proteins and lipids starts in the stomach. Most of the digestion of carbohydrates, proteins and fats takes place in the initial portion of the small intestine.

The digestion time of food varies according to the total volume and characteristics of the food consumed, and can last up to 12 hours for each meal, for example.

1. Digestion in the oropharyngeal cavity

In the mouth, the teeth grind and crush the food eaten into smaller particles and the formed food cake is moistened by saliva. In addition, there is release of a digestive enzyme, salivary amylase or ptialin, which initiates the digestion of the starch that constitutes carbohydrates. The digestion of starch in the mouth by the action of amylase is minimal and its activity is inhibited in the stomach due to the presence of acidic substances.

The bolus passes through the pharynx, under voluntary control, and the esophagus, under involuntary control, reaching the stomach, where it is mixed with gastric secretions.

2. Digestion in the stomach

In the stomach, the secretions produced are rich in hydrochloric acid and enzymes and are mixed with food. In the presence of food in the stomach, pepsin, which is one of the enzymes present in the stomach, is secreted in its inactive form (pepsinogen) and converted to pepsin by the action of hydrochloric acid. This enzyme plays a fundamental role in the protein digestion process, changing its shape and size. In addition to the production of pepsin, there is also the production, to a lesser extent, of lipase, which is an enzyme responsible for the initial degradation of lipids.

Gastric secretions are also important to increase intestinal availability and absorption of vitamin B12, calcium, iron and zinc.

After processing the food through the stomach, the bolus is released in small amounts into the small intestine according to the contractions of the stomach. In the case of liquid meals, gastric emptying lasts around 1 to 2 hours, while for solid meals it lasts about 2 to 3 hours and varies according to the total volume and characteristics of the food ingested.

3. Digestion in the small intestine

The small intestine is the main organ of digestion and absorption of food and nutrients and is divided into three parts: duodenum, jejunum and ileum. In the initial part of the small intestine, most of the food eaten is digested and absorbed due to the stimulation of enzyme production by the small intestine, pancreas and gallbladder.

Bile is secreted by the liver and gallbladder and facilitates the digestion and absorption of lipids, cholesterol and fat-soluble vitamins. The pancreas is responsible for secreting enzymes that are able to digest all major nutrients. The enzymes produced by the small intestine reduce carbohydrates of lower molecular weight and peptides of medium and large size, in addition to triglycerides that are degraded into free fatty acids and monoglycerols.

Most of the digestive process is completed in the duodenum and the upper part of the jejunum, and the absorption of most nutrients is almost complete by the time the material reaches the middle of the jejunum. The entry of partially digested foods stimulates the release of several hormones and, consequently, of enzymes and liquids that interfere with gastrointestinal motility and satiety.

Throughout the small intestine almost all macronutrients, vitamins, minerals, trace elements and liquids are absorbed before reaching the colon. The colon and rectum absorb most of the remaining fluid from the small intestine. The colon absorbs electrolytes and a small amount of remaining nutrients.

The remaining fibers, resistant starches, sugar and amino acids are fermented by the brush border of the colon, resulting in short chain fatty acids and gas. Short-chain fatty acids help maintain normal mucosal function, release a small amount of energy from some of the residual carbohydrates and amino acids, and facilitate the absorption of salt and water.

The intestinal contents take 3 to 8 hours to reach the ileocecal valve, which serves to limit the amount of intestinal material that passes from the small intestine to the colon and prevents its return.

What can interfere with digestion

There are several factors that can prevent digestion from being performed correctly, resulting in consequences for the person's health. Some of the factors that can affect digestion are:

Quantity and composition of the food eaten, this is because depending on the characteristic of the food the digestion process can be faster or slower, which can influence the feeling of satiety, for example. Psychological factors, such as appearance, smell and taste of the food. This is because these sensations increase the production of saliva and secretions from the stomach, in addition to favoring the muscular activity of the SGI, causing the food to be little digested and absorbed. In the case of negative emotions, such as fear and sadness, for example, the reverse occurs: there is a decrease in the release of gastric secretions as well as a reduction in peristaltic bowel movements; Digestive microbiota, which can suffer interference due to the use of medications such as antibiotics, inducing bacterial resistance, or to situations that lead to decreased production of hydrochloric acid by the stomach, which can result in gastritis. Food processing, since the way food is consumed can interfere with the speed of digestion. Cooked foods are normally digested more quickly than those eaten raw, for example.

If you notice any symptoms related to the gastrointestinal system, such as excessive gas, heartburn, bloated abdominal feeling, constipation or diarrhea, for example, it is important to go to the gastroenterologist for tests to identify the cause of the symptoms and start the best treatment.