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Components of Blood


Ravindra Sarode

, MD, The University of Texas Southwestern Medical Center

Reviewed/Revised Jan 2024
Topic Resources

The main components of blood include

  • Plasma

  • Red blood cells

  • White blood cells

  • Platelets


Plasma is the liquid component of blood, in which the red blood cells, white blood cells, and platelets are suspended. It constitutes more than half of the blood's volume and consists mostly of water that contains dissolved salts (electrolytes) and proteins.

The major protein in plasma is albumin. Albumin helps keep fluid from leaking out of blood vessels and into tissues, and albumin binds to and carries substances such as hormones and certain medications.

Plasma has other functions:

  • It acts as a reservoir

  • It helps maintain blood pressure and circulation

  • It helps maintain body temperature

In its function as a reservoir, plasma can either replenish insufficient water or absorb excess water from tissues. When body tissues need additional liquid, water from plasma is the first resource to meet that need.

Plasma prevents blood vessels from collapsing and clogging and helps maintain blood pressure and circulation throughout the body simply by filling blood vessels and flowing through them continuously.

To help regulate body temperature, plasma circulating in the blood vessels carries heat generated in core body tissues through areas that lose heat more readily, such as the arms, legs, and head.

Red Blood Cells

Red blood cells (also called erythrocytes) make up about 40% of the blood's volume. Red blood cells contain hemoglobin, a protein that gives blood its red color and enables it to carry oxygen from the lungs and deliver it to all body tissues. Oxygen is used by cells to produce energy that the body needs, leaving carbon dioxide as a waste product. Red blood cells carry carbon dioxide away from the tissues and back to the lungs.

White Blood Cells

White blood cells (also called leukocytes) are fewer in number than red blood cells, with a ratio of about 1 white blood cell to every 600 to 700 red blood cells Red Blood Cells The main components of blood include Plasma Red blood cells White blood cells Platelets read more Red Blood Cells .

White blood cells are responsible primarily for defending the body against infection.

There are 5 main types of white blood cells:

  • Neutrophils

  • Lymphocytes

  • Monocytes

  • Eosinophils

  • Basophils

White Blood Cells

Some white blood cells flow smoothly through the bloodstream, but many adhere to blood vessel walls or even penetrate the vessel walls to enter other tissues. When white blood cells reach the site of an infection or other problem, they release substances that attract more white blood cells. The white blood cells function like an army, dispersed throughout the body but ready at a moment's notice to gather and fight off an invading organism White blood cells . White blood cells accomplish this by engulfing and digesting organisms (called phagocytosis, so white blood cells are sometimes called phagocytes) and by producing antibodies Antibodies One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more Antibodies that attach to organisms so that they can be more easily destroyed.

When the number of white blood cells is too low (leukopenia White Blood Cells White Blood Cells ), infections are more likely to occur.

A higher than normal number of white blood cells (leukocytosis Overview of White Blood Cell Disorders Overview of White Blood Cell Disorders ) may not directly cause symptoms, but the high number of cells can be an indication of an underlying condition such as an infection, inflammatory process, or leukemia Overview of Leukemia Leukemias are cancers of white blood cells or of cells that develop into white blood cells. White blood cells develop from stem cells in the bone marrow. Sometimes the development goes awry... read more .


Platelets (also called thrombocytes) are cell-like particles that are smaller than red or white blood cells. Platelets are fewer in number than red blood cells, with a ratio of about 1 platelet to every 20 red blood cells.

Platelets help in the clotting process by gathering at a bleeding site and clumping together to form a plug that helps seal the blood vessel. At the same time, they release substances that help promote further clotting.

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