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What is blood?

Blood is the fluid that passes through the circulatory system within blood vessels. In humans, it includes plasma, blood cells (red and white variants), and cell debris called platelets. Cells and platelets account for 45% of human blood, while plasma accounts for 55%.


Plasma is a liquid component of blood in which red blood cells, white blood cells, and platelets are suspended. It accounts for more than half of the blood, mainly composed of water containing dissolved salts and proteins. The main protein in plasma is albumin. Albumin helps prevent fluid from leaking from the blood vessels into the tissues. Albumin can bind and carry substances, such as, hormones and certain drugs. Other proteins in plasma, including antibodies, are effective defenses against the body's resistance to viruses, bacteria, fungi, and cancer cells, as well as clotting factors that control bleeding.

Red blood cells

Red blood cells accounts for about 40% of blood volume. Red blood cells contain hemoglobin, which makes the blood red, allowing it to carry oxygen from the lungs and deliver it to all body tissues. Cells use oxygen to generate the energy needed by the body and leave carbon dioxide as waste. Erythrocytes carry carbon dioxide away from the tissues and return to the lungs. When the number of red blood cells is too small, oxygen carried by the blood will be reduced, and fatigue will occur. When the number of red blood cells is too high, the blood becomes too thick, which may cause the blood to coagulate more easily and increase the risk of heart attack and stroke

White blood cells

The number of white blood cells is less than the number of red blood cells, the ratio of one white blood cell to every 600 to 700 red blood cells. White blood cells are mainly responsible for defending the body against infection. There are five main types of white blood cells:
  1. Neutrophils are the most diverse species and help protect the body from infection by killing and ingesting bacteria and fungi and ingesting foreign debris.
  2. Lymphocytes consist of three major types: T-lymphocytes and natural killer cells, both of which help protect against virus infection and detect and destroy some cancer cells. B-lymphocytes develop antibodies that produce antibodies. 

  3. Monocytes take up dead or damaged cells and help defend against many infectious organisms

  4. Eosinophil kill parasites, destroy cancer cells, and participate in allergic reactions.

  5. Basophils are also involved in allergic reactions


Platelets are smaller cell-like particles than red blood cells or white blood cells. The number of platelets is less than that of red blood cells, and the proportion of platelets is about 1 red blood cell to about 20 red blood cells. Platelets help the clotting process by collecting and condensing together at the bleeding site to form an embolus that helps seal the blood vessel. At the same time, they release substances that help promote further coagulation. When the number of platelets is too small, bruising and abnormal bleeding become more likely. When the platelet count is too high, the blood may coagulate excessively and cause transient ischemic attacks. When platelet counts are very high, platelets can absorb clotting proteins and abnormally cause bleeding.

Here is the video about the components of blood:


Raven, P. H., Johnson, G. B., Mason, K. A., Losos, J. B., and Singer, S. R. (2014). The components of blood. In Biology (10th ed., AP ed., pp. 1018-1021). New York, NY: McGraw-Hill.

Strode, R (2017) Components of Blood

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