What starch partially digests

Gastrointestinal Diseases - Digestion

  1. Nutrition tips
  2. Gastrointestinal diseases
  3. digestion


The food supplied by mouth cannot be used by the body in its original, non-unlocked form. In the course of digestion, the food is broken down into smaller components that can be absorbed in the blood or lymph and introduced into the metabolism. This is achieved through a complex interaction of physical, chemical and enzymatic processes.

The digestive tract extends from the mouth to the anus and consists of several hollow organs that form a kind of long, intertwined muscle tube. This includes the mouth, gullet (esophagus), stomach, small intestine, large intestine, rectum, and anus. Food travels through the digestive tract through peristaltic (forward-facing) muscle movements. Various glands flow into these, causing the food pulp to be mixed with various digestive secretions. The following describes what happens to food in the individual organs.


The digestion of the food begins in the mouth. With the help of the teeth, the food is mechanically crushed, whereby the chewing movement stimulates the flow of saliva. The salivary glands produce about 1-1.5 liters of secretion daily and release it into the oral cavity. The saliva makes the food slippery and prepares it for further transport through the esophagus. By chopping and loosening the food, its taste can be better perceived. This in turn increases the secretion of saliva and initiates gastric juice secretion.

Food comes into contact with an enzyme for the first time through saliva, because the enzyme α-amylase is released with the saliva. This splits complex carbohydrates (oligo-, polysaccharides) into smaller, sweet-tasting building blocks. This is also the reason why bread takes on a sweet taste after prolonged chewing! Various spices such as pepper, chilli, curry, paprika and mustard increase saliva production and the activity of α-amylase.

Another important task of saliva is the cleaning of the teeth and the neutralization of acids created in the mouth or supplied with food (e.g. from fruit juice). These properties of saliva are of particular importance in the prevention of caries.


The chyme is passed through the esophagus into the stomach with the help of peristaltic movements, where it is mixed with the gastric juice, of which 1.5-3 liters are formed daily. The low pH value of the acidic gastric juice has a bacteria-killing effect and leads to a flocculation (denaturation) of protein, which makes it more "vulnerable" to enzymes.

Beer and white wine are powerful stimulators of gastric acid secretion. Among the nutrients, protein is the best acid loosener, while fat tends to inhibit acid production. The mucus produced in the so-called "side cells" protects the stomach wall from the attack of the aggressive stomach acid.

A small amount of fat-digesting enzymes, but mainly protein-splitting enzymes, are released through the stomach wall into the stomach. This is the enzyme pepsin, which is formed from its precursor (pepsinogen). The digestion of the carbohydrates, which already begins in the mouth with the saliva, rests in the stomach because the pH value drop in the chyme inhibits the α-amylase due to the action of the gastric acid.

The so-called intrinsic factor, a glycoprotein that is necessary for the absorption of vitamin B12 into the body, is formed in the parietal cells of the gastric mucosa. A deficiency in this substance leads to an undersupply of vitamin B12, which can lead to pernicious anemia, a special form of anemia.

The length of time food stays in the stomach depends on various factors. So delayed z. B. a high percentage of fat gastric emptying. The consistency and temperature of the pulp also have an influence. Through the stomach gate (pylorus) the food is passed on to the duodenum (duodenum).

Small intestine

Splitting of nutrients

The small intestine is an essential digestive organ. With the help of enzymes, the food components that have already been pre-digested in the mouth and stomach are broken down further.

In the small intestine, the carbohydrates are broken down into their smallest components by special enzymes (disaccharidases). H. the enzymes split, for example, the household sugar into grape sugar and fructose or the lactose (milk sugar) into grape sugar and mucus sugar. A deficiency in these enzymes leads to intolerances such. B. lactose intolerance.

Most of the fat digestion takes place in the upper parts of the small intestine. The bile produced by the liver is stored in the gallbladder and released into the duodenum. The bile is important to emulsify the fats, which makes them easier for the digestive enzymes (lipases) to attack. The bile also facilitates the absorption of the fats, which because of their size are initially transported onwards via the lymph, which in turn feeds them into the blood vessel system. The fat can then be transported via the blood to different parts of the body and used or stored there.

The digestion of proteins that started in the stomach continues in the intestine. The larger protein fragments are broken down into small molecules (peptides, amino acids) by the attack of the enzymes released with the secretion of the pancreas (e.g. trypsin) and absorbed into the bloodstream. Bicarbonate also reaches the small intestine via the pancreatic secretion, which neutralizes the acidic stomach contents.

Vitamins are also absorbed in the small intestine. While the fat-soluble vitamins can be well stored in the liver and adipose tissue, excess, unneeded amounts of water-soluble vitamins are soon excreted through the kidneys.

Nutrient absorption

Another task of the small intestine is the absorption of the split food components into the bloodstream. Most of the nutrients are absorbed in the small intestine. In order to be able to fulfill this function optimally, the surface of this organ is very much enlarged. This is achieved through folds of the mucous membrane (Kerckring folds), on which finger-shaped protuberances protrude into the intestine. The so-called brush border (microvilli) is located on these small intestinal villi, which increases the absorption area of ​​the intestine - in contrast to a "tube" with a smooth surface - by a factor of 600, resulting in a total surface of approx. 200 m2 is achieved. The intestinal cells bring the nutrients into the blood, which they carry to other parts of the body. What happens next depends on the respective nutrient - part goes into metabolic processes, another part is stored.


In the large intestine, water is withdrawn from the previously very liquid chyme, which is then absorbed together with minerals. The food pulp is thickened by the dehydration. Remaining z. B. Dietary fiber. Dietary fiber is largely inaccessible to human enzymes and therefore reaches the large intestine undigested. When it comes to dietary fiber, a distinction is made between soluble and insoluble. Soluble fiber dissolves easily in water and forms a gel-like substance, which promotes the transport of the food pulp. Some of the dietary fiber is broken down in the large intestine by bacteria or excreted with the faeces. The bacterial degradation results in short-chain fatty acids from the dietary fibers. T. can be absorbed and used for energy. Vitamins (e.g. vitamin B12, niacin) formed by bacteria in the large intestine can only be absorbed and used in small quantities.

Regulation of digestion

Many hormones are involved in the regulation of digestion. The most important ones - gastrin, secretin and cholecystokinin (CCK) - are produced in the lining of the stomach and small intestine. From there they first get into the blood vessel system and are then transported back to the digestive tract in order to stimulate the release of digestive secretions there.

In addition, the digestive tract is innervated by the extrinsic and intrinsic nervous systems. The extrinsic nervous system promotes or slows down the peristaltic movements of the digestive tract and can stimulate the stomach and pancreas to produce digestive juices. While the extrinsic nervous system is in external contact with the digestive tract, the intrinsic nervous system is located as a dense network in the wall of the digestive tract. The intrinsic nervous system reacts to stretching stimuli that arise from the transport of food. This releases various substances that increase or decrease the speed of transport and the release of digestive secretions.

Gut and immune system

The digestive tract has a surface area of ​​300-500 m2. This makes it the largest contact area between the human body and the outside world. The digestive tract not only comes into contact with food components, but also with pathogens and allergens. It is therefore protected accordingly by the body. A large part of the immune system is located in the intestine.

This as GALT (Gut associated lymphoid tissue) - intestinal-associated immune system - designated tissue has important functions in the protection against pathogenic substances. It is divided into three different functional areas: Peyer's plaques, lymph follicles and mucosal lymphocytes, which form an immunological barrier against pathological substances.

Read in the following chapters which diseases of the digestive tract can occur and what their causes are.