Реферат: Heart Essay Research Paper HEARTThe human heart

and a right half by the cardiac septum. Each half contains two separate spaces: the atrium (pl. atria),

or auricle, and the ventricle. The upper reservoirs, or collecting chambers, are the thin-walled atria,

and the lower pumping chambers are the thick-walled ven!

tricles. The total thickness of the ventricular walls is about three times that of the atria; the wall of

the heart’s left half is approximately twice as thick as that of the right half. The thickness of the

heart muscle varies from 2 to about 20 mm (0.1 to 0.8 in). This thickness is correlated with the maximum

pressure that can be attained in each chamber.

FLOW OF BLOOD THROUGH THE HEART

The right atrium receives oxygen-poor blood from two major veins: the superior and inferior vena cava,

which enter the atrium through separate openings. From the right atrium the blood passes through the

tricuspid valve, which consists of three flaps, or cusps, of tissue. This valve directs blood flow from

the right atrium to the right ventricle. The tricuspid valve remains open during diastole, or ventricular

filling; however, when the ventricle contracts, the valve closes, sealing the opening and preventing

backflow into the right atrium. Five cords attached to small muscles (papillary muscles) on the

ventricles’ inner surface prevent the valves’ flaps from being pushed backward. From the right ventricle

blood is pumped through the pulmonary, or semilunar, valve, which has three half-moon-shaped flaps, into

the pulmonary artery. This valve prevents backflow from the artery into the right ventricle. From the

pulmonary artery, blood is pumped to the lungs, where it gives up ca!

rbon dioxide and receives oxygen, and then is returned to the heart’s left side through four pulmonary

veins (two from each lung) to the left atrium and then through the mitral valve, a two-flapped valve also

called a bicuspid valve, to the left ventricle. As the ventricles contract, the mitral valve prevents

backflow of blood into the left atrium, and blood is driven through the aortic valve into the AORTA, the

major artery, which supplies blood to the entire body. The pulmonary valve, like the aortic valve, has a

semilunar shape and a unidirectional function.

CORONARY CIRCULATION

The blood supply to the heart muscle is furnished mainly by the CORONARY ARTERIES, which originate from

the aorta immediately after the aortic valve. These vessels pass through the fatty tissue beneath the

pericardium and then branch out into the heart muscle.

The coronary veins transport the deoxygenated blood from the heart muscle to the right atrium. The