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Blood: What’s in It, Anyway?

Though blood is often thought of as a single substance, it actually consists of many separate components. In fact, blood contains 45 percent cells and 55 percent plasma, which is mostly water. Plasma also contains dissolved proteins, sugar (glucose), carbon dioxide, hormones, and minerals. White blood cells (WBCs) and red blood cells (RBCs) are the cell types found within blood, in addition to cell fragments (platelets).

When a patient’s blood is drawn for tests, it commonly goes to a lab in a tube where it is centrifuged, meaning that a machine quickly spins it so that gravity separates the most dense parts of the blood from the least dense parts. Red blood cells, the most dense part, fall to the bottom, while plasma, the least dense part, is on the top. In between the red blood cells and the plasma is a white band that with white blood cells & platelets, making up less than one percent of blood.

A Closer Look at Blood Cells

Blood cells are produced from blood stem cells in the bone marrow of large bones, such as the pelvis, sternum, and long bones. White blood cells, red blood cells, and platelets are all produced in the marrow and then enter the bloodstream for circulation.

Red Blood Cells

Red blood cells, also known as erythrocytes, are flattened and disc-like, characteristics that result from the absence of a nucleus and other cell parts, found in most cells. This unique shape gives RBCs more surface area to take on and release oxygen. Additionally, they are small and flexible, allowing them to move, single-file, through capillaries, the smallest blood vessels.

RBCs contain a protein molecule called hemoglobin, which is designed to carry oxygen throughout the body. Each molecule of hemoglobin contains four iron atoms, and each iron atom carries one molecule of oxygen. Iron is responsible for giving RBCs their red color. However, blood can take on many different shades of red. The blood found in arteries carries oxygen from the lungs, making it a brighter shade of red. Alternatively, the blood found in veins, which released its oxygen, has a bluish color.


In blood there are more RBCs than any other kind of cell. In fact, there are two to three million RBCs made every second, and it takes only 20 seconds for an RBC to circulate throughout the body. One RBC contains 250 to 270 hemoglobin molecules; therefore it holds one billion molecules of oxygen. However, the amount of red blood cells varies between men and women. The percent of RBCs in whole blood (hematocrit) is 42 to 54 percent for men and 38 to 45 percent for women.

White Blood Cells

White blood cells, specialize in fighting infections and healing injury. When the body is injured or invaded by a germs, such as bacteria, many white blood cells quickly migrate to that location to start the infection fighting process. There are several kinds of blood cells and each one has a different role in the process in fighting invaders. For example, some cells make antibodies to the invader making it easier to destroy, others attack the invader directly and others assist in the healing process. The main kinds of white blood cells are neutrophils, lymphocytes, monocytes, eosinophils, and basophils.

A White Blood Cell count measures the number of white cells in one drop of blood. The number can increase with infection and conditions such as allergy, injury, and leukemia. The number can decrease with certain virus infections, chemotherapy and leukemia. When a WBC count is very low, a person is at more risk for infection. Tests that analyze white blood cells give important clues to the cause of many conditions.

Why Plasma Matters

Adults have 3 liters of plasma in their body, which is comprised of mostly water. However, the plasma contains many other substances, also. Plasma consists of nutrients such as glucose, amino acids, vitamins and minerals, and blood proteins such as antibodies, hormones, and factors responsible for making the blood clot. It also transports the molecules for hormones, vitamins, and minerals, and it is extremely important for the transportation RBCs, WBCs and platelets. Without the plasma, there would be no way for these components to travel throughout the body.

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Go to the Blood and the Body health topic.