It says in the Book of Genesis that the life lies in the blood, and one can see why. Blood carries oxygen and nutrients to every cell of the body, carries away waste products, and plays a central role in the body's defense against intruders.
The circulatory system keeps approximately five liters (for the average adult human) of blood circulating constantly through the body. Its most important organ is the heart, that double pump that forces the blood through the blood vessels.
The body's circulatory system really has three distinct parts: circulation through the lungs (pulmonary circulation), the heart (coronary circulation), and the rest of the system (systemic circulation).
Pulmonary circulation, in which blood goes from the heart to the lungs
and back to the heart, is when the blood exchanges carbon dioxide,
a waste product it has picked up from the rest of the body, for oxygen,
which it will carry to the rest of the body. The de-oxygenated, carbon-dioxide-laden
blood returning to the heart from the body enters the right side of
the heart through two veins (one for blood coming from the lower half
of the body, the other for blood from the upper half). The heart pumps
it to the lungs, where it drops off carbon dioxide to be excreted
(exhaled), and picks up oxygen. The blood then returns to the left
side of the heart, which pumps it out to the rest of the body.
flow of the blood from the heart to the rest of the body and back
to the heart is systemic circulation. The heart pumps the now oxygen-rich
blood out through the arteries, which are wide, thick-walled blood
vessels. Then the blood moves into smaller vessels called capillaries,
where it releases the oxygen and nutrients it carries to the body's
cells. At the same time, it picks up waste products like carbon dioxide.
The blood then flows into the veins, which carry it back to the heart.
During systemic circulation, blood flows through the kidneys, in a phase called renal circulation. The kidneys filter out waste products like urea and excess salt, which will be excreted in the urine.
Blood also passes through the small intestine, in a phase called portal circulation. It picks up sugars from digested food and then collects in the portal vein, which passes through the liver. The liver filters sugars from the blood to store for the body to use as energy later.
Like any other organ, the heart itself needs blood to supply it with nourishment and carry away wastes. The movement of blood through the capillaries of the heart for these purposes is called coronary circulation.
-- Sue Wilson
Illustration: Redrawn from Carol Ballard, THE HEART AND CIRCULATORY SYSTEM (Austin, Texas: Steck-Vaughn Company, 1997), pg.26.