Blood vessels are classified into three groups depending upon their functions, accordingly they have peculiar structures.
Arteries are thick walled elastic vessels that convey blood from heart to different organs of the body. All arteries carry oxygenated blood except pulmonary arteries which carry deoxygenated blood from right ventricle to lungs. Arteries are also called distributing vessels.
Veins are thin walled, flexible vessels which collect blood from various peripheral organs and return it to right atrium of heart. All veins carries deoxygenated blood except pulmonary veins which carries oxygenated blood from lungs to left atrium of heart. Veins are also called collecting vessels or reservoir vessels.
Capillaries are very fine microscopic vessels present in organs which connects arteries and veins. Capillaries are involved in exchange of nutrients and waste products between blood and tissue cells. Capillaries are also known as exchange vessels.
Entire circulatory system is further subdivided into three segments based on the organs they supply. First is systemic circulation which is composed aorta and its brunches, systemic capillaries in venacavae. Through this segment oxygenated blood is carried from left ventricle to all body organs and deoxygenated blood from these organs is returned back to heart (Fig32a). Second is pulmonary circulation which consists of pulmonary arteries, capillaries and pulmonary veins. Through this segment deoxygenated blood is carried to lungs from right ventricle and oxygenated blood is returned to heart. Third is coronary circulation which consists of coronary arteries, capillaries and coronary veins. Through this segment, oxygenated blood is carried from aorta to heart wall (myocardium) and deoxygenated blood is returned to right atrium.
Arterioles are specialized vessels present in all organs and arise out of arteries. Special feature of arterioles is that they act as ‘stop cock’ to regulate amount of blood entering particular organ. The wall of arterioles is composed of smooth muscles that can regulate their diameter, so blood flow. The functioning of arterioles also contributes to blood pressure through peripheral resistance.
Sinusoids are wider vascular spaces present in some specialized organs such as liver, adrenal gland, spleen, bone marrow etc. They can be regarded as wider capillaries through which much greater exchange of nutrients is possible between blood and tissue cells.