The circulatory system includes the heart, blood, blood vessels (arteries,
capillaries, and veins), and lymphatics. The function of the circulatory system is to
circulate blood to the tissues of the body. These tissues supply oxygen and nutrients to
the cells and remove carbon dioxide and other wastes.
a. Heart. The heart is a muscular, cone-shaped organ (see figure 1-7). It
continuously pumps blood to all parts of the body. It is divided into two separate halves,
each forming a pump. Each half is divided into an upper chamber (atrium) and a lower
chamber (ventricle) by a valved partition. The valves in these partitions allow blood to
flow in only one direction (into the ventricle). The right atrium receives deoxygenated
blood from the body tissues, which is then transferred to the right ventricle. The right
ventricle pumps the deoxygenated blood to the lungs where the blood is oxygenated.
From the lungs, the blood returns to the left atrium, goes to the left ventricle, and is
finally pumped again out into the arteries. The heart beats in a cycle (called the cardiac
cycle), never stopping this regular rhythmic beat. This cycle consists of alternate
contractions and dilations of the heart. The phase in which the heart contracts is called
systole. The phase in which it dilates is called diastole.
(1) Systole. Systole starts with a simultaneous contraction of both atria.
This is followed by a simultaneous contraction of both ventricles. As ventricular
contraction starts, the atrioventricular valves close. At the end of the ventricular
contraction, the valves close between the ventricles and the large arteries into which the
blood was pumped.
(2) Diastole. Diastole starts after the closing of the valves. During diastole,
the heart muscle of all chambers relaxes while blood flows into the atria. When the atria
are filled, the atrioventricular valves open, the atria contract, and the cycle starts again.
(3) Heartbeat. Normally, the heart contracts 70 to 80 times a minute. This
varies depending on such factors as age, sex, physical fitness, and emotional state of
the individual. The heartbeat causes a wavelike expansion of the arteries called pulse.
The pulse may be felt and counted at points where arteries are close to the surface of
the body, such as the inner surface of the wrist.
b. Blood. Blood has many important functions besides the oxygen/carbon
dioxide exchange between lungs and cells. Blood carries food absorbed from the
digestive tract to the cells. It also carries wastes of cell metabolism to the excretory
organs for elimination from the body. Other functions of the blood include the circulating
of chemical substances (hormones) secreted by endocrine glands, helping to maintain
body temperature, protecting the body against infection, and keeping the proper
balance between chemicals in the body. There are about 11 to 12 pints of blood in the
average adult body. The blood is composed of plasma, red blood cells, white blood
cells, and platelets. See figure 1-8 for a simplified diagram of blood circulation.