A Living Pump To Keep The Blood In Motion
Clench your fist, relax it, clench your fist, relax it. You have just imitated the beating of your heart, a muscular, fist-sized organ that lies near the center of your chest, between your lungs and just behind your breastbone. The heart's constant beating pumps your blood to keep it circulating throughout your body. Your heart is truly a living pump.
The heart is a hollow, muscular organ with four chambers. The upper two are blood-receiving chambers called atria. The lower two are blood-pumping chambers called ventricles. Openings permit the flow of blood from each atrium to the ventricle below, but a thick wall divides the right side of the heart from the left side.
Although the two sides are separated, they beat at the same time. Both ventricles relax and refill (diastole) with blood flowing in, through valves, from the atria above. Then both ventricles contract and pump (systole) the blood out through another set of valves into the arteries which supply blood to the lungs and the rest of the body. One complete heartbeat consists of one diastolic phase and one systolic phase.
The heart beats between 70 and 80 times per minute, or about 100,000 times per day. While you rest or sleep, your heart pumps about 2 1/2 ounces (70 milliliters) of blood with each beat. It may not sound like much, but it adds up to nearly 5 quarts (approximately 5 liters) of blood pumped by the heart in one minute, or about 75 gallons (300 liters) per hour. The output of the heart can vary according to the body's needs. For example, during periods of vigorous exercise, when the body demands more oxygen- and nutrient-rich blood, the heart increases its output by nearly five times.
A vast web of blood vessels supplies all areas of the body with blood. Some blood vessels carry fresh, oxygen-rich blood; some carry used, oxygen-poor blood. The pumping heart keeps the blood moving through the vessels--so that blood in the heart can travel to the big toe and back in less than 60 seconds.
1. Oxygen-poor blood from the body flows down through the right atrium to fill the right ventricle which pumps the blood out through the pulmonary artery to the lungs.
2. As blood flows through the lungs it gives up the waste carbon dioxide and gains more oxygen from the inhaled air. The freshly oxygenated blood returns to the left atrium of the heart by way of the pulmonary veins.
3. Oxygen-rich blood from the lungs flows down through the left atrium to fill the left ventricle which pumps it into the aorta, the main artery to the body.
4. The blood passes from the aorta into smaller arteries which carry it to all body organs and tissues. There it flows into the smalles arterial branches, the arterioles.
5. From the arterioles, blood flows into dense networks of tiny, thin-walled blood vessels called capillaries. Oxygen and nutrients in the blood easily pass through the thin capillary walls to the cells, and carbon dioxide and other cellular waste products can pass back through the walls into the blood to be carried away.