The Circulatory System

The circulatory system in anatomy and physiology is the course taken by
the blood through the arteries, capillaries, and veins and back to the heart. In
humans and the higher vertebrates, 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

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. Contraction of this 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, that
is, 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 it goes
to the heart, completing the circuit.

In addition to the pulmonary and systemic circulations described above,
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

Coronary circulation is the means by which the heart tissues themselves
are supplied with nutrients and oxygen and are freed of wastes. Just beyond the
semilunar valves, two coronary arteries branch from the aorta. These then break
up into an elaborate capillary network in the heart muscle and valve tissue.
Blood from the coronary capillary circulation enters several small veins, which
then enter directly into the right auricle without first passing into the vena

The action of the heart consists of successive alternate contraction
and relaxation of the muscular walls of the auricles and ventricles. During the
period of relaxation, the blood flows from the veins into the two auricles,
gradually distending them. At the end of this period, the auricles are
completely dilated then their muscular walls contract, forcing almost the entire
contents through the auriculoventricular openings into the ventricles. This
action is sudden and occurs almost simultaneously in both auricles. The mass of
blood in the veins makes it impossible for any blood to flow backward. The force
of blood flowing into the ventricles is not powerful enough to open the
semilunar valves, but it distends the ventricles, which are still in a condition
of relaxation. The tricuspid and mitral valves open with the blood current and
close readily at the beginning of ventricular