What are the Components of Blood?

Rebecca Partington

Blood is primarily composed of three things: blood cells, plasma, and platelets. The cells are predominantly either white or red, and together these do most of the essential work performed by the fluid as a whole. Red cells transport oxygen, for instance, and white cells help ward off infection and destroy bacteria. Without plasma, though, both cell types would have a hard time circulating and getting from place to place. Plasma is made mostly of water and gives blood its familiar fluid consistency. Platelets, in turn, help keep everything inside the body; this element is primarily responsible for clotting, which prevents people from bleeding to death and allows wounds and incisions to heal. It’s often the case that blood also contains small amounts of other things, including certain nutrients, proteins, and electrolytes. These things are known collectively as “secondary elements.” In general, all blood compnents must work together to maintain ideal health. Although each has its own identity, when any one is weakened or strained the entire balance can be disturbed, which often leads to serious health consequences.

Red blood cells.
Red blood cells.

Red Blood Cells

The red blood cells, known scientifically as erythrocytes, are probably what most people think of when they think about the main components of blood. These cells use hemoglobin to transport oxygen throughout the body. It's this hemoglobin, or more specifically the iron in the hemoglobin, that gives blood its red color. Red blood cells are relatively round with indentations in the center, and are often described as donuts with the centers just slightly filled in.

A bag of blood plasma.
A bag of blood plasma.

Oxygen distribution is a critical function of health, and these cells do the bulk of that work. When the blood goes from the heart to the lungs, the hemoglobin molecules bind with the oxygen in the lungs. After the blood passes through the lungs, it briefly returns to the heart to be pumped to the rest of the body, gradually delivering the oxygen to the rest of the body's cells.

A diagram showing different types of white blood cells.
A diagram showing different types of white blood cells.

White Blood Cells

White blood cells, also more formally known as leukocytes, are another of the key components of blood. These cells fight infection, and usually come in three types: granulocytes, lymphocytes, and monocytes . There are a further two different types of cells in the lympocyte family: T cells and B lymphocytes. T cells are responsible for directing what the immune system does, while the B lymphocytes manufacture antibodies. Monocytes pass through the cell walls and become macrophages, which then eat both harmful bacteria and the damaged and dead cells that are native to the body.

Platelets, one component of blood, make up blood clots.
Platelets, one component of blood, make up blood clots.


Platelets, or thrombocytes, are also technically blood cells, but they are much smaller than the reds and whites and also play a very different role. The primary function of platelets is to make the blood clot. When platelets are exposed to the air, which happens when a wound bleeds, they break down to release a special clotting substance into the blood. This substance indirectly causes fibrinogen, a protein, to become fibrin, which is a clotting agent. The fibrin turns into long strings, which mat together to form a clot that keeps red blood cells from escaping the body through the wound. This makes platelets one of the most important blood components, as they prevent excessive blood loss and promote speedy healing that can lessen the chance for infection.


Blood plasma makes up more than half of the total composition of blood, and itself is about 90% water. Plasma's main purpose is to transport the other components of the blood throughout the body. While doing this, it also transports various proteins, nutrients, electrolytes, hormones, cholesterol, vitamins, and chemicals like iron.

Secondary Elements

These proteins and other nutrients are known broadly as “secondary elements” of blood chemistry. They aren’t always present, and don’t always occur in predictable or otherwise routine concentrations, either. Some of this has to do with a person’s lifestyle or health needs. The blood often responds to crises in the body or excesses in diet by flushing nutrients either to or from some specific location.

Fibrin is the substance that allows blood to clot.
Fibrin is the substance that allows blood to clot.

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Discussion Comments


All three components of blood are formed in the bone marrow of the body, most commonly in the thighbones, hips, and sternum. On average, red blood cells live 120 days, so new blood is being made continually.


@tmacsgirl, Sickle-cell anemia can cause serious health problems for those suffering from it. Sickle-cell trait is when a person only has one gene and not both required for sickle-cell disease to be present. In tropical areas where malaria is a problem, having sickle-cell trait can increase resistance to malaria. Sickle-cell trait is also much more common in these areas.


Sometimes the red blood cells are abnormal, rigid, and sickle-shaped. This sickling of the cells is considered a genetic blood disorder called sickle-cell disease. Sickling results in inflexibility of the cells and can cause various other complications. The cells are misshapen due to a mutation in the hemoglobin gene.

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