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is the course taken by the blood through the
arteries, capillaries, and veins and back to the heart. In humans, the
heart is made up of four chambers the right and left auricles, or atria,
and the right and left ventricles. The right side of the heart pumps
oxygen-poor blood from the cells of the body back to the lungs for new
oxygen; the left side of the heart receives blood rich in oxygen from the
lungs and pumps it through the arteries to the various parts of the body.
Circulation begins early in fetal life. It is estimated that a
given portion of the blood completes its course of circulation in
approximately 30 seconds. Pulmonary circulation is where the blood from the
entire body is transported to the right auricle through two large veins.
The superior vena cava and the inferior vena cava. When the right auricle
contracts, it forces the blood through an opening into the right ventricle.
When this contracts the ventricle drives the blood to the lungs. Blood is
prevented from returning into the auricle by the tricuspid valve, which
completely closes during contraction of the ventricle. In its passage
through the lungs, the blood is oxygenated, then it is brought back to the
heart by the four pulmonary veins, which enter the left auricle. When this
chamber contracts, blood is forced into the left ventricle and then by
ventricular contraction into the aorta. The bicuspid, or mitral valve
prevents the blood from flowing back into the auricle, and the semilunar
valves at the beginning of the aorta stop it from flowing back into the
ventricle. Similar valves are present in the pulmonary artery. The aorta
divides into a number of main branches, which in turn divide into smaller
ones until the entire body is supplied by an elaborately branching series
of blood vessels. The smallest arteries divide into a fine network of still
more minute vessels, the capillaries, which have extremely thin walls; thus,
the blood is enabled to come into close relation with the fluids and
tissues of the body. In the capillaries, the blood performs three functions
then it releases its oxygen to the tissues, it furnishes to the body cells
the nutrients and other essential substances that it carries, and it takes
up waste products from the tissues. The capillaries then unite to form
small veins. The veins, in turn, unite with each other to form larger veins
until the blood is finally collected into the superior and inferior venae
cavae from which goes to the heart, completing the circuit. In addition to
the pulmonary and systemic circulations, a subsidiary to the venous system
exists, known as portal circulation. A certain amount of blood from the
intestine is collected into the portal vein and carried to the liver. There
it enters into the open spaces called sinusoids, where it comes into direct
contact with the liver cells. In the liver important changes occur in the
blood, which is carrying the products of the digestion of food recently
absorbed through the intestinal capillaries. The blood is collected a
second time into veins, where it again joins the general circulation
through the right auricle. In its passage through other organs, the blood
is further modified.
Coronary circulation is the means by which the heart tissues
themselves are supplied with nutrients and oxygen and are freed of wastes.
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