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A test tube filled with centrifuged (separated) blood. The yellow liquid is plasma, the transparent is white blood cells and platelets, and the red liquid is red blood cells. Credit to original author

Blood is a liquid in humans and many animals except insects. Blood is pushed through the organism by the heart, and brings nutrients and oxygen to our tissues. It also takes away waste and carbon dioxide from tissues.

Blood of vertebrates is made up of blood plasma and various cells: red blood cells, white blood cells and platelets. Platelets help blood to clot. Hemoglobin is in red blood cells. White blood cells help fight infections and heal wounds. Plasma is the yellow liquid in which blood cells float. Plasma is made up of nutrients, electrolytes, salts, non-protein hormones, lipids, and proteins.

There are five types of White Blood Cells:
Name Microscopic Appearance Diameter in Micrometers


Main targets[1]
Neutrophil in a blood smear.jpg
10 - 12 Bacteria, Fungi
Eosinophil Eosinophil blood smear.JPG Larger parasites and modulate allergic inflammatory responses
Basophil Basophil.jpg 12 - 15 Inflammatory responses

(releases histamine)

Lymphocyte Lymphocyte GL.jpg Natural killer cells like virus-infected and tumor cells.
Monocyte Monocyte sanguin.jpg 15 - 30 Monocytes migrate from the bloodstream to other tissues and differentiate into macrophages (White Blood Cells that digest non-blood content) and «Kupffer cells» in the liver.

Blood cells are made in the bone marrow and in the spleen. The bone marrow is the soft material in the middle of bones. Special cells in the bone marrow make most of the blood cells in your body.


  1. ISBN 0-443-01657-7