Skip to Content

Components of Blood


Ravindra Sarode

, MD, The University of Texas Southwestern Medical Center

Last full review/revision Feb 2021| Content last modified Feb 2021

The main components of blood include

  • Plasma
  • Red blood cells
  • White blood cells
  • Platelets

(See also Overview of Blood.)


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 drugs. Other proteins in plasma include antibodies (immunoglobulins), which actively defend the body against viruses, bacteria, fungi, and cancer cells, and clotting factors, which control bleeding.

Plasma has other functions. It acts as a reservoir that 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 also 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. Plasma circulation also plays a role in regulating body temperature by carrying 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. When the number of red blood cells is too low (anemia), blood carries less oxygen, and fatigue and weakness develop. When the number of red blood cells is too high (erythrocytosis, as in polycythemia vera), blood can become too thick, which may cause the blood to clot more easily and increase the risk of heart attacks and strokes.

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. White blood cells are responsible primarily for defending the body against infection. There are five main types of white blood cells.

Neutrophils, the most numerous type, help protect the body against infections by killing and ingesting bacteria and fungi and by ingesting foreign debris.

Lymphocytes consist of three main types: T cells (T lymphocytes) and natural killer cells, which both help protect against viral infections and can detect and destroy some cancer cells, and B cells (B lymphocytes), which develop into cells that produce antibodies.

Monocytes ingest dead or damaged cells and help defend against many infectious organisms.

Eosinophils kill parasites, destroy cancer cells, and are involved in allergic responses.

Basophils also participate in allergic responses.

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 accomplish this by engulfing and digesting organisms and by producing antibodies that attach to organisms so that they can be more easily destroyed.

When the number of white blood cells is too low (leukopenia), infections are more likely to occur. A higher than normal number of white blood cells (leukocytosis) may not directly cause symptoms, but the high number of cells can be an indication of an underlying disorder such as an infection, inflammatory process, or leukemia.


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. When the number of platelets is too low (thrombocytopenia), bruising and abnormal bleeding become more likely. When the number of platelets is too high (thrombocythemia), blood may clot excessively and can block blood vessels causing disorders such as a transient ischemic attack. When the number of platelets is extremely high, the platelets can absorb clotting proteins and paradoxically cause bleeding.

Copyright © 2022 Merck & Co., Inc., known as MSD outside of the US, Kenilworth, New Jersey, USA. All rights reserved. Merck Manual Disclaimer