The circulatory (cardiovascular) system.

The cardiovascular^yst^m is the system of blood circulation. Its function is mainly that of transportation system: The nutrients, oxygen, special substances which are requir ed by cells are carried by the blood stream; and cellular wastes and sometimes other materials produced by the cells are carried away by the blood stream. The cardiovascular system includes the heart, the vessels — arteries, veins and capillaries and the blood.

The center of the system is the heart. The human heart is a cone-shaped organ, about 5 inches long and 3,5 inches broad. It weighs about 10 ounces in the adult man and 6 ounces in the female. It lies in the thoracic cavity, just behind the breastbone and between the lungs. The heart is a powerful muscular organ, hollow in the center. It is covered by a transparent membrane called p ericardium. The inner surface of the heart, including the valves, is also covered by a thin transparent membrane called the endocardium,

The heart is divided into four c hambers: a right auricle or a trium and ventricl e, and a left auricle or atrium and ventricle. The two sides are completely separated from each other by a muscular septum. In the "right heart" between the atrium and the ventricle there is a one-way valve called the tricuspid valve. In the "left heart" between the atrium and the ventricle there is the mitral valve. The left side of the heart is stronger, thicker and more powerful than the right one.

The vessels of the system are the arteries, the veins and the capillaries. The arteries carry blood from the heart to all parts of the body except the lungs. The largest artery of the body is the aorta. It is situated partly in the thorax, partly in the abdomen, and it is divided into three parts - the arch, thoracic and abdominal aorta.

The veins are the vessels which carry blood from the organs (except lungs) to the heart.

The capillaries are the finest vessels. The wails of the capillaries are so thin that nourishment, oxygen can pass through them into tissues, and impurities are taken away from them into the blood.

The blood is a red fluid, which coagulates when it escapes from a blood vessel. It consists of a colourless fluid called plasma or serum and many millions of minute bodies, the corpu s cles. They are too small to be seen by the naked eye and have given the blood its colour and substance.

Speaking about circulation we should keep in mind that there are two types of blood circulation - the greater and

the lesser. The function of the greater circulation is to supply the organs with nourishment and oxygen and to take

away waste products. The greater circulation begins in the left ventricle and finishes in the right atrium. Blood is pumped through the ao rtic valve to the aorta and then to the arteries, arterioles and capillaries all over the body. In the capillaries the exchange of substances takes place. Gradually joining together and getting larger they become veins. The veins carry the blood to the right atrium. From here the blood passes to the right ventricle where the lesser or pulmonary circulation begins. The function of the lesser circulation is to supply the blood with oxygen and to depriv e it of carbon dioxide. The pulmonary arteries carry the blood to the lungs, where the exchange of gases occurs, and then

the pulmonary veins carry well-oxygenated blood to the left atrium.

Дата добавления: 2015-10-23