Cardiovascular System

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THE HEART

Location, Function, and Structure

The pump that keeps blood moving through a closed circuit of blood vessels.

Located between the lungs in the lower portion of the mediastinum.

Is positioned In the thoracic cavity between the sternum in front and the bodies of the thoracic vertebrae behind.

Is often described as a triangular organ, shaped and sized roughly like a closed fist.

Valves:

Atrioventricular valves. The atrioventricular (AV) valves

are located in the middle of the heart between the atria and ventricles

and only allow blood to flow from the atria into the ventricles. The AV

valve on the right side of the heart is called the tricuspid valve

because it is made of three cusps (flaps) that separate to allow blood

to pass through and connect to block regurgitation of blood. The AV

valve on the left side of the heart is called the mitral valve or the bicuspid valve because it has two cusps. The AV valves are attached on the ventricular side to tough strings called chordae tendineae.

The chordae tendineae pull on the AV valves to keep them from folding

backwards and allowing blood to regurgitate past them. During the

contraction of the ventricles, the AV valves look like domed parachutes

with the chordae tendineae acting as the ropes holding the parachutes

taut.

Semilunar valves. The semilunar valves, so named for the

crescent moon shape of their cusps, are located between the ventricles

and the arteries that carry blood away from the heart. The semilunar

valve on the right side of the heart is the pulmonary valve,

so named because it prevents the backflow of blood from the pulmonary

trunk into the right ventricle. The semilunar valve on the left side of

the heart is the aortic valve, named for the fact that it prevents the aorta

from regurgitating blood back into the left ventricle. The semilunar

valves are smaller than the AV valves and do not have chordae tendineae

to hold them in place. Instead, the cusps of the semilunar valves are

cup shaped to catch regurgitating blood and use the blood’s pressure to

snap shut.

Blood Flow

Deoxygenated blood returning from the body first enters the heart from the superior and inferior vena cava.

The blood enters the right atrium and is pumped through the tricuspid

valve into the right ventricle. From the right ventricle, the blood is

pumped through the pulmonary semilunar valve into the pulmonary trunk.

The pulmonary trunk carries blood to the lungs where it releases

carbon dioxide and absorbs oxygen. The blood in the lungs returns to the

heart through the pulmonary veins. From the pulmonary veins, blood enters the heart again in the left atrium.

The left atrium contracts to pump blood through the bicuspid (mitral)

valve into the left ventricle. The left ventricle pumps blood through

the aortic semilunar valve into the aorta. From the aorta, blood enters

into systemic circulation throughout the body tissues until it returns

to the heart via the vena cava and the cycle repeats.

The Cardiac Cycle

The cardiac cycle includes all of the events that take place during

one heartbeat. There are 3 phases to the cardiac cycle: atrial systole,

ventricular systole, and relaxation.

Atrial systole: During the atrial systole phase of the

cardiac cycle, the atria contract and push blood into the ventricles. To

facilitate this filling, the AV valves stay open and the semilunar

valves stay closed to keep arterial blood from re-entering the heart.

The atria are much smaller than the ventricles, so they only fill about

25% of the ventricles during this phase. The ventricles remain in

diastole during this phase.

Ventricular systole: During ventricular systole, the

ventricles contract to push blood into the aorta and pulmonary trunk.

The pressure of the ventricles forces the semilunar valves to open and

the AV valves to close. This arrangement of valves allows for blood flow

from the ventricles into the arteries. The cardiac muscles of the atria

repolarize and enter the state of diastole during this phase.

Relaxation phase: During the relaxation phase, all 4

chambers of the heart are in diastole as blood pours into the heart from

the veins. The ventricles fill to about 75% capacity during this phase

and will be completely filled only after the atria enter systole. The

cardiac muscle cells of the ventricles repolarize during this phase to

prepare for the next round of depolarization and contraction. During

this phase, the AV valves open to allow blood to flow freely into the

ventricles while the semilunar valves close to prevent the regurgitation

of blood from the great arteries into the ventricles.

Lubb and Dubb Sounds

The sounds of a normal heartbeat are known as “lubb” and “dupp” and are

caused by blood pushing on the valves of the heart. The “lubb” sound

comes first in the heartbeat and is the longer of the two heart sounds.

The “lubb” sound is produced by the closing of the AV valves at the

beginning of ventricular systole. The shorter, sharper “dupp” sound is

similarly caused by the closing of the semilunar valves at the end of

ventricular systole. During a normal heartbeat, these sounds repeat in a

regular pattern of lubb-dupp-pause. Any additional sounds such as

liquid rushing or gurgling indicate a structure problem in the heart.

The most likely causes of these extraneous sounds are defects in the

atrial or ventricular septum or leakage in the valves.

Cardiovascular System

Cardiovascular System