________________
Preksha Dhyana :
Muscular walls of the heart contract and relax rhythmically. This rhythm is self-initiated requiring no nervous or chemical stimulation. The phase of contraction is called systole while that of relaxation is called diastole. A full cardiac cycle-systole and diastole-takes less than a second, normal heart-rate being 70/72 beats per minute But in the space of just one minute, a volume equivalent 10 all the blood, about 5 litres in the body, passes through the heart. Heartrate steadily slows down as one gets older; it is 140 before birth, 90 as a child and 70/72 as an adult.
Blood-vessels
Multitudinous branches of blood vessels of various sizes form a complex inter-communicating net-work that reaches practically every cell in the body. The main types of blood vessels are arteries. veins and capillaries. The blood vessels which carry the blood from the heart to the body are commonly known as arteries, while those which carry the blood in the opposite direction, i.e. towards the heart are called veins.
Capillaries are so small that the red blood cells have to pass through them in a single file. They permeate the tissues and service the body-cell directly. Oxygen, nutrients and hormones transported in the blood are transferred to the cells, while cell-produced products for export (carbon dioxide etc.) diffuse into the blood. This exchange is so rapid that each given unit of blood spends only one to three seconds in a particular capillary. The Blood-stream
Besides the oxygen, a variety of other substances must be delivered to the cells and tissues to keep them alive and vigorous. Glucose, fat, amino-acids and water are some of the more common, while copper and cobalt, jodine and phosphorus are a few of the uncommon requirements of the cells. Salts, minerals, vitamins, and hormones are also carried by the blood-stream for delivery to appropriate places. Thus blood is ever-changing, yet its overall compo
Jain Education International
For Private & Personal Use Only
www.jainelibrary.org
