Blood is an important part of the human body. It is the liquid that transports nutrients throughout the body and removes waste. It makes up about 8 percent of the total weight of the human body.
Blood cells make up about half the volume (amount) of blood. These cells form inside bone marrow, which is a soft tissue inside bones. There are three main types of blood cells: red cells, white cells, and platelets.
Red Blood Cells
Red blood cells are the most numerous kind of blood cell. Each drop of blood contains about 300 million of them. Their main job is to transport oxygen. In each cell, an iron-rich substance called hemoglobin carries the oxygen. Hemoglobin and oxygen together give blood its red color.
White Blood Cells
White blood cells, or leukocytes, help to keep the body healthy. There are three different types of white blood cells, but they all help protect the body from disease. Some swallow up tiny living things called bacteria or other foreign substances. Others release proteins that attack invading substances. Still others help to break down and remove dead cells.
Platelets
Platelets are the smallest of the blood cells. They are able to stick to one another and form blood clots. Clots plug holes that may develop in the walls of blood vessels. This helps stop bleeding.
Plasma
The watery part of the blood is the plasma. Most of the thousands of chemicals carried by the blood are found there. These chemicals include minerals, such as sodium and potassium, as well as complex vitamins, hormones, and enzymes.
The circulatory system carries blood to all parts of the body. It is a complex system composed of a powerful pump—the heart—and a vast network of different kinds of blood vessels, such as arteries, veins, and capillaries. The walls of these vessels are made up of living cells through which substances, oxygen, and nutrients, as well as waste products, can pass into and out of the blood.
As blood passes through the lungs it picks up oxygen. The oxygen-rich blood then travels throughout the body. The body’s cells take in this oxygen. In exchange, they send a gas called carbon dioxide into the blood. The blood carries the carbon dioxide back to the lungs, which push the gas out of the body.
Blood also carries nutrients (proteins, fats, sugars, salts, vitamins, and minerals) to the body’s cells. It takes away various wastes made by the cells. The kidneys, spleen, and liver clear the blood of wastes.
Doctors can learn a lot about a patient by testing the patient’s blood. Tests on the blood can tell how well the body is working and whether it has all the nutrients and chemicals that it needs. If the test shows something unusual in the blood, that can mean that there is something wrong in the body. The doctor can tell if the patient has a particular disease.
To get blood for a test, the doctor or another health care worker can use a needle to draw a little bit of blood from the patient. It is most commonly taken from an arm vein. The blood is then examined in a laboratory. Some blood is tested automatically in machines. For some tests, the blood is examined under a microscope.
