The circulatory system has as main function the rapid transport of substances to cells throughout the body, but is also involved in other processes that are essential to the normal functioning of organisms.

The basic functions of the cardiovascular system are, as follows:

• Rapid substance transport to and from the tissues and organs of the body. This is done by convection over the long distances and by diffusion at the cellular level. The most important advantage convective transport has to offer is speed of transport. Oxygen, glucose, amino-acids, fatty acids, water, vitamins, drugs are carried along blood vessels, in a stream of fluid that is pumped through them by the heart. Oxygen is carried by the red cells of the blood, in the form of oxyhemoglobin, a stable compound formed of oxygen and hemoglobin, the characteristic pigment that gives red cells their colour.

• Rapid removal of metabolic waste (carbon dioxide, urea, creatinine). The circulatory system collects the metabolic waste products and delivers them to the excretory organs – e.g., the kidneys, which then filter wastes (especially urea) from the blood and excrete them and water in urine. CO₂ is carried to the lungs by the red cells as well, through hemoglobin, which is capable of attaching carbon dioxide as well.

• Distribution of hormones to the tissues. Hormones are secretions of the endocrine glands that have specific effects on the functioning of other tissues/organs. The circulatory system is part of this control system, by distributing, through blood, the hormones to their tissue/organ of destination. It is interesting to note that the cardiovascular system also secrets some hormones of its own (e.g. the atrial natriuretic peptide).

• Immune protection. Blood circulation plays an important part in theimmune system of defense against infection. The immune system protects the body from foreign substances and pathogenic microorganisms by producing the immune response. The circulatory system carries out its role of protection through blood cells: leucocytes or white cells are specialized for antibody production and destruction of toxins of microbial origin; they function by means of fagocitosis (ingestion and destruction); thrombocytes (platelets) have a role in immune reactions, in the endocytosis of smaller particles (engulfment of bacteria or molecules by forming a vesicle around them), as well as in wound healing, by stimulating the growth of smooth muscle fibers. Protection against pathogens is also carried out by means of blood clotting, process in which platelets have the essential role as well.

• Temperature regulation. Through the circulatory system, that covers the entire body, heat is delivered from the core to the peripheral areas. Temperature regulation is done by the alteration of the blood flow through the skin. Vasodilation (dilation of arterioles and small arteries) and vasoconstriction (the opposite process) determine temperature locally; the arterioles act as “taps” of the circulation, allowing blood flow increase or turning down the blood flow, to match local needs. By contracting hard, terminal arterioles can even prevent blood from flowing through the capillaries they feed. The arterio-venous anastomosis also helps local temperature regulation of skin. The anastomosis (interconnection) occurs in skin tissues and is in fact the connection of arterioles to venules directly, bypassing the capillaries, by means of shunt vessels having a diameter of 20-135 µm.

• Reproduction: The role the circulatory system in the reproductive process is to provide a mechanism for penile erection. The male reproductive organ contains two chambers, called the corpora cavernosa, filled with spongy tissues; this type of tissue contains smooth muscles, fibrous tissues, spaces, veins, and arteries. Impulses from the brain and local nerves cause the muscles of the corpora cavernosa to relax, allowing blood to flow in and fill the open spaces. The blood creates pressure in the corpora cavernosa, making the penis expand; the surrounding membrane helps to trap the blood in the corpora cavernosa, thereby sustaining erection. The reverse process starts when muscles in the penis contract, stopping the inflow of blood and opening outflow channels.