000367

53 The musculature is usually divided into three types: striated (or skeletal), cardiac, and smooth muscles. Skeletal muscles are responsible for move‑ ments of the body such as running and jumping. Smooth muscles are found within the organs and blood vessels. They are responsible for many body functions such as movement of the organs of the digestive tract, blood ves‑ sels, secretory ducts. Cardiac muscle is found only in the heart. Smooth and cardiac muscles are under the control of the involuntary, or autonomic, nerv‑ ous system. Striated muscle, on the other hand, is mainly under the control of the voluntary, or central, nervous system. The cardiovascular (or circulatory) system is the system of blood circula‑ tion. It is composed of the heart, blood, and blood vessels. The heart is the main organ of the cardiovascular system. It consists of two separate chambers divided by the septum. Each of the chambers has two connected parts: the atrium and the ventricle. There are three types of blood vessels: arteries, veins and capillaries. Arteries carry blood away from the heart to other organs and tissues of the body. Arteries divide into smaller and smaller vessels which fially divide into capillaries. Capillaries are microscopic vessels; their walls are only one epithelial cell in thickness. The exchange between the blood and body cells occurs through the walls of the capillaries: the cells are supplied with oxygen and nutrients, waste products are carried away. Then the blood is collected from the capillaries and carried back to the heart by veins. The respiratory system obtains from the air the oxygen necessary for cellular metabolism; it also returns to the air the carbon dioxide, a waste product of such metabolism. The system consists of the breathing passages and two lungs. The breathing passages are the nasal cavity, the pharynx, the larynx, the trachea and two bronchi. They conduct air to the lungs. The lungs are located in the lateral cavities of the chest. The left lung is smaller than the right one; it is divided into two lobes. The right lung has three lobes. Each lung is covered with the pleura (or pleural membrane). The digestive system breaks down food into nutrients, which are then absorbed from the blood or lymph; this system also eliminates the waste materials which cannot be absorbed. The system consists of the digestive tract and digestive glands. The digestive tract is 8–10 meters long. It is di‑ vided into the mouth, pharynx, esophagus, stomach, small intestine and large intestine. The digestive glands secrete juices which take part in the chemical process of digestion. The largest glands are the liver with gallblad‑ der and pancreas.