The Action Of The Heart And Blood Circulation

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The heart acts as a muscular pump and beats about 70 times every minute. The heart pumps blood around the body at a rate of 5 L/min, that is, about 180 million gallons during a lifetime's pumping. The blood pressure is dependent upon the heart's constant pumping of both blood and the size of all its blood vessels through which the blood passes.

The pumping of the heart or the heartbeat is caused by alternating contractions and relaxations of the myocardium. These contractions are stimulated by electrical impulses from the sinoatrial or SA node located in the muscle of the right auricle. An impulse from the SA node causes the two auricles to contract, forcing blood into the ventricles. Contraction of the ventricles is controlled by impulses from the atrioventricular or AV node located at the junction of the two auricles (Fig. 1).

Following contraction, the ventricles relax and allow the pressure to fall. Blood again flows into the auricles and an impulse from the SA node repeats the cycle again; this process is called the cardiac cycle. The period of relaxation is called a diastole and the period of contraction is called a systole. Diastole is the longer of the two phases so that the heart can rest between the contractions. Hence, systolic pressure and diastolic pressure are observed. The rate of SA impulse is subconsciously regulated by the autonomic nervous system. In this way the heartbeat is either accelerated or slowed down in response to physical activity and other factors.

The force of blood pumped through the arteries exerts its pressure on the arterial walls. The body monitors blood pressure by means of receptors present in the main arteries of the heart and then controls it through changes in heartbeat and the flow of blood. Vasomotor effectors increase or decrease the diameter of the blood vessels and also regulate blood pressure. Physical exertion and digestion of food places a heavy demand on the heart's blood output, which can increase as much as ten-fold to meet the special needs.

There are two main systems which operate to maintain arterial blood pressure. These are the sympathetic nervous system and the renin-angion-tensin-aldosterone system. One of the processes that tends to worsen hypertension is renal glomerular sclerosis. The control system fails because narrowing of the renal vessels upsets the normal relationship between renal blood flow and arterial pressure.

Blood pressure is governed by the following five elements:

1. cardiac output

2. volume of blood

3. viscosity of blood

' Right bundle branch

Atrioventricular (AV) node Figure 1 The conducting system of the heart.

4. resistance of the arterioles

5. elasticity of the arterial walls.

Abnormality of any of these elements can cause blood pressure to be too high or too low.

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