In addition to the numerous small digestive glands embedded in the mucous membrane of the gastrointestinal tract, the major digestive glands – the liver and pancreas, mentioned above – play an important role in the chemical processing of food substances. These glands in the period of embryonic development are formed in the wall of the duodenal bud, but as they grow, they are moved out of its bounds and turn into large organs that are connected to it for life using their ducts, through which their secrets are secreted into the intestinal canal. However, they differ from the rest of the intestinal digestive glands not only by their size, but also by the essential features of the structure and functions.
The pancreas is located on the back of the abdominal cavity in its upper part behind the stomach (see Fig. 6). Its right end (head) is surrounded by a horseshoe-shaped curved duodenum, and the left end is in contact with the spleen. Iron consists of two sections: one of them produces a juice that is very important for digestion, which can be said to be universal, since it contains enzymes for all the main organic nutrients. Another section of the gland is represented by groups of cells (so-called islets) that are not connected with its ducts. Their secret – the hormone insulin – is secreted directly into the blood and has the most significant effect on carbohydrate metabolism (Fig. 14).
Fig. 14. Pancreas and ways of removing bile. 1,2,3, 4, 6 – ducts for removing bile; 5 – gallbladder; 7 – a section of the duodenum into which the bile duct and the pancreatic duct flow; 8 – pancreas and its duct.
Fig. 15. Liver (front view): 1 – the right lobe of the liver; 2 – the left lobe of the liver; 3 – gallbladder
The liver is the largest gland in our body (Fig. 15). It weighs 1.5 kg, which is approximately 1/50 of the weight of an adult. In a newborn child, the relative size of the liver is even larger – it is 1/20 of the body weight. The liver is located in the upper abdominal cavity right below the diaphragm. Its immediate neighbors are the gallbladder, stomach, duodenum, right kidney. The bile duct enters the duodenum with the pancreatic duct. Everyone knows what animal liver, such as cows, looks like. The human liver in appearance and form is very similar to this organ in animals. It has a brown color and a semi-liquid consistency. The structure of the liver is very difficult. It is formed by cells of the liver epithelium, a variety of blood vessels and nerves. If you look at a piece of the liver under a microscope, it is easy to make sure that its substance is divided into many small segments that have the same structure and are separated from each other by thin layers of connective tissue that stretch to the capsule of the liver that covers it outside. Everyone knows that the liver secretes bile, which is a digestive juice that is important for the digestion of fats. During the day, from 600 ml to 1 l of bile is formed, but it enters the intestines as needed when fatty food enters. In between meals, bile is collected in the gallbladder. Despite the fact that its volume is small (only about 40-60 ml), a significant amount of dense substances of bile (acids and pigments) can be stored in it. In the gall bladder there is a rapid absorption of water, so the concentration of its dense substances increases 10 times. Thus, 50 ml of the concentrated contents of the gallbladder corresponds to approximately 500 ml of hepatic bile.
For a long time, scientists thought that the formation of bile is the main function of the liver. But this opinion turned out to be erroneous. The role of the liver in the functioning of the body, as it turned out, is much more significant. Experiments on animals and the study of human liver diseases have shown that it is a vital organ. Switching off the bile-forming function does not cause disturbances in the activity of the organism incompatible with life. Removal of the liver or the complete shutdown of its functions inevitably leads to death within a short time (several days). The fact is that the liver is, figuratively speaking, the central chemical laboratory of the body, participating in all types of metabolism (carbohydrate, fat, protein, vitamin, water), in the process of blood formation and, most importantly, in the implementation of protective functions.