The circulatory system is the complex network of blood vessels and lymphatic ducts concerned with the transport of blood and lymph throughout the body. It carries oxygen, glucose, amino acids, carbon dioxide, and hormones to the different body tissues and organs. It consists of the heart, the blood, arteries, veins, and lymph vessels. The circulatory system is divided into the greater (or systemic) circulation, and the lesser (or pulmonary) circulation. The lymphatic system is part of the greater circulation, emptying the body tissues excess fluid into the venous blood. The lung, kidney, and liver play an active role in the blood gaseous exchange, excretion of cellular waste contained in the blood, and metabolism of nutrients brought into the liver by the portal vein.
Function
The function of the circulatory system is to convey nutrients, oxygen, and hormones to the different organs and tissues of the body to keep them alive and functional, and to bring back carbon dioxide and cellular waste to the lungs and kidney to be exhaled and excreted, respectively. Meanwhile, the lymphatic vessels function is to drain excess fluid from the intercellular spaces in the body tissues, carrying it to the subclavia vein. Therefore, by the circulatory system, cells and tissues are supplied with nutrients and oxygen, and the byproducts of cellular metabolism are eliminated.
In human beings and rest of vertebrates, the heart is the main organ of the circulatory system, which makes the blood flow by pumping it at every ventricular contraction of the myocardium. The vessels that carry oxygenated blood from the heart are called arteries, and the vessels that carry blood back to the heart are called veins. In a closed circulatory system blood from the arteries flows into vessels of increasingly smaller caliber and finally into arterioles, from which the blood enters the capillaries. The capillaries combine to form a complex network from which the blood enters first small veins (venules) and then increasingly larger veins. The inner layer of the venous walls forms peculiar pocket-like valves that direct the flow of blood in one direction. The middle layer of the arterial walls contains an unusually large number of smooth muscular and elastic fibers, which enable the arteries to pulsate.
The structure of the circulatory system is most complex in vertebrate animals and man than in non-vertebrate. In mammals, birds, and reptiles, the heart has a powerful muscular wall (myocardium) and it is divided into four cavities: two atria and two ventricles. In the lung alveoli, carbon dioxyde is released and oxygen is picked by hemoglobins contained in the red blood cells and is carried to the heart left atrium by the pulmonary veins. Thus, the blood flows in one or two circulations, depending on whether the vertebrate animal breathes through lungs or gills (fish).
With the gill type of respiration (in cyclostomes and fish), there is a single circulatory system. The heart consists of two main parts—an atrium and a ventricle (bicameral); in addition there is a venous sinus and, in most fish, an arterial cone as well. The heart is filled with venous blood. From it emerges the abdominal aorta, through which the venous blood enters the afferent gill arteries. The blood is oxygenated in the gills, becomes arterial, and flows through the efferent gill arteries to the dorsal aorta, from which it is carried to all the organs of the body. Venous blood enters the heart through the anterior and posterior cardinal veins, which in cyclostomes empty directly into the venous sinus and in fish through the ducts of Cuvier.
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| Human being's circulatory system. Arteries are in red color, while veins are in blue. Lymphatic vessels are not shown here. |