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Human Body
the nourishment required by the other organs and tissues. The nourishment is not, however, obtained from the blood passing through its chambers, but from its own blood supply. It is fed by two (right and left) coronary arteries which arise from the aorta just after its emergence from the left ventricle. Both branch out again and again sorrounding the heart-muscle and nourish it adequately. Heart-attacks or -coronaries are caused by narrowing or partial blockade of these arteries. Cardiac Output
Under resting conditions each systole pumps about 70 ml. of blood into the aorta. This is called stroke volume. "The cardiac output is defined as-stroke volume x heart rate.
With an average heart-rate of 70-72 beats per minute, the cardiac output comes to about five litres. Thus in the space of just one minute, a volume equivalent to all the blood in the body passes through the heart. But during beavy exercise, the amount pumped can increase to over 25 litres per minute.
The cardiac output does not remain constant but is adjusted to meet the varying needs of the body. Thc heart is capable of increasing its output by fivefold or even more. It has been estimated that during a man's life-time a total of 500,000,000 litres blood is pumped. Under normal conditions the liver receives 28% of the total cardiac output; the kidneys 24%, the skeletal muscles 15%, the brain 14% and the heart 5%.
Heart-rate steadily slows down as one gets older ; it is 140 before birth, 90 as a child and 70-72 as an adult. The Blood-Stream
Blood is a red-coloured fluid with a salty taste and a characteristic odour. Its specific gravity is only slightly more than water, about 1.055, but its viscosity is about five times as high and hence it is not as free-flowing as water.
1. Every adult has total blood volume of 4.5 to 5 litres.
2. Actually the normal range extends from 60 to 100 Incidentally heart-rate of a mouse is 700, that of a rabbit 150, while an elephant's is only 25 beats per minute.
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