HEART AND BLOOD

OUR BODIES CONTAIN about 8 pints (4.5 liters) of blood. Throughout life, the heart, an organ inside the chest, pumps blood to every part of the body, keeping us alive. The heart muscle contracts and relaxes about once every second, pushing blood out of the heart and back again.

Traveling along tubes called blood vessels, blood carries oxygen and nourishment from digested food to every part of the body. Blood also carries away waste products such as carbon dioxide. Blood consists of red and white blood cells, platelets, and a watery liquid called plasma. A drop of blood the size of a pinhead contains millions of red cells and thousands of white cells.

About once every second, the muscular walls of the heart contract, squeezing blood out of the heart and into blood vessels called arteries. The arteries divide many times until they form a network of tiny blood vessels called capillaries. The capillaries gradually join up again to form veins, which carry the blood back to the heart.

INSIDE THE HEART

The heart consists of two pumps, left and right, that work together. Each side has two chambers, an upper atrium and a lower ventricle. Oxygen-poor blood from the body enters the right atrium through two veins, the superior and posterior venae cavae. Blood passes into the right ventricle, which pumps it through the pulmonary artery to pick up oxygen in the lungs.

The oxygen-rich blood returns to the heart’s left atrium through the pulmonary veins. From here, it passes to the left ventricle, which pumps it along the aorta and its branches to all parts of the body before returning to the right atrium. Valves inside the heart ensure that blood flows in one direction only.

BLOOD CELLS

There are three types of blood cells. Red blood cells carry oxygen from the lungs to the rest of the body. White blood cells protect the body against illness and fight infection. Platelets, which are actually small cell fragments, help the blood clot. All blood cells are produced in the bone marrow inside the bones.

HOW BLOOD CLOTS

When you cut yourself and blood flows out of the wound, platelets in the blood stick together and form a fine meshwork of fibers. This meshwork traps more blood cells and forms a clot to seal the wound.

ARTERIES

These blood vessels carry blood away from the heart to the body. Arteries have thick walls that can resist the high blood pressure produced when the heart beats. The coronary arteries deliver oxygen-rich blood to the walls of the heart itself.

CAPILLARIES

These tiny blood vessels branch off from arteries and join together to form veins. Their walls are only one cell thick, allowing oxygen, nutrients, and waste products to pass between the blood and body tissues.

VEINS

These blood vessels return blood to the heart. Veins have thinner walls than arteries and contain valves that prevent blood from flowing backward.

HEARTBEAT

On average, an adult’s heart beats about 70 times each minute. This adds up to more than 100,000 beats every day. Each heartbeat has three phases: atria fill with blood, ventricles contract to pump blood into arteries, and the heart relaxes and refills.

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