The lungs
The lungs are the main organs of the respiratory system. There are 2 lungs in the human body located in the lateral cavities of the chest. The lungs are separated from each other by the mediastinum. The lungs are covered with the pleura. They are conical in shape. Each lung has the base, apex, 2 borders and 3 surfaces. The right lung consists of 3 lobes, but the left one has only 2 lobes.
The weight of the lungs varies according to many conditions. In the adult male the weight of the lungs is about 1,350 gr. The vital capacity of the lungs is 3,5-4 liters in the male and it’s 3-3,5 liters in the female. When the depth of respiration increases the vital capacity may be 6 litres and even more.
In infants the lungs are of a pale rose colour, but later they become darker.
The structure of the lung consists of an external serous coat, the visceral layer of the pleura, a subserous elastic tissue and the parenchyma or proper substance of the lungs.
The physiology of the lungs is associated with their structure. There are over 700,000,000 alveoli and many capillaries in the lungs. This particular structure of the lungs enables the exchange of gases between the alveolar air and the blood.
Elastic fibers of connective tissue composing of the alveoli, alveolar passages and the visceral pleura enable the lungs to dilate.
The lungs take part in the production of physiologically active substances, in the regulation of blood coagulation, in the metabolism of proteins, fats and carbohydrates.
The exchange of gases in the lungs
The exchange of gases takes place in the alveoli of the lungs. Oxygen passes into the blood and carbon dioxide passes into the atmospheric air.
The exchange of oxygen and carbon dioxide is due to the difference of partial pressure of these gases in the alveolar air and in the venous blood.
The partial pressure of oxygen in the alveolar air is higher than in the venous blood. The partial pressure of carbon dioxide is higher in the venous blood, so carbon dioxide passes from the blood into alveolar air.
The process of transfer of gases into the environment (or medium) with a lower partial pressure is called diffusion.
Hemoglobin is substance of the blood which transfers oxygen in the blood. The oxygen capacity of the blood averages to 18-20 ml per 100 gr of blood. Carbon dioxide is transferred in combination with hemoglobin and as bicarbonic salts. The combination of oxygen and hemoglobin is called oxyhemoglobin, that of carbon dioxide and hemoglobin – carbohemoglobin.
The respiratory rate and its depth depend on the amount of carbon dioxide in the blood. Normally during one inspiration the man breathes in about 500 ml of air. On deep inspiration one breathes in 1,5-2 litres of air.
The alimantary tract
The alimentary tract is a musculomembraneous canal about 8 ½ metres in length. It extends from oral cavity to the anus. It consists of the mouth, pharynx, esophagus, stomach, small and large intestine, the liver with gallbladder and pancreas.
Important structures of the mouth are teeth, the tongue, soft and hard palates, and salivary glands.
From the mouth food passes through the pharynx to the esophagus and then to the stomach.
The stomach is a dilated portion of the alimentary canal. It’s in the upper part of the abdomen under the diaphragm. It measures about 21-25 cm in lehgth, 8-9 cm in its greatest diameter. It has a capacity of from 2,14 to 4,28 litres.
The small intestine is a thin-walled muscular tube about 6,5 m long. It’s located in the lower and central portions of the abdominal and pelvic cavities. The small intestine is composed of the duodenum, jejunum and ileum.
The large intestine is about 1,5 m long. It’s divided into caecum, colon and rectum.
The liver is the largest gland in the human body. It’s in the right upper part of the abdominal cavity under the diaphragm. The liver is in the right side of the abdomen. The weight of the liver is 1,500 gr.
The gallbladder is a hollow sac lying on the lower surface of the liver.
The pancreas is a long thin gland lying under and behind the stomach.