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Tour of the Human Body Systems

The Tour Starts Here!

The Immune System
The Nervous System
The Excretory System
The Respiratory System
The Circulatory Syatem
The Digestive System
The Skeletal System
The Muscular System
Glossary of Terms
The Modes of Transportation

Red Blood Cells


The red blood cells are tiny, round, biconcave disks, averaging about 0.003 in in diameter. A normal-sized man has about 5L of blood in his body, containing more than 25 trillion red cells. Because the normal life span of red cells in the circulation is only about 120 days, more than 200 billion cells are normally destroyed each day by the spleen and must be replaced. Red blood cells, as well as most white cells and platelets, are made by the bone marrow.




White Blood cells

The leukocytes, or white blood cells, are of three types; granulocytes, lymphocytes, and monocytes. All are involved in defending the body against foreign organisms. There are three types of granulocytes: neutrophils, eosinophils, and basophils, with neutrophils the most abundant. Neutrophils seek out bacteria and engulf, them.


Platelets, or thrombocytes, are much smaller than the red blood cells. They are round or biconcave disks and are normally about 30 to 40 times more numerous than the white blood cells. The platelets' primary function is to stop bleeding. When tissue is damaged, the platelets aggregate in clumps to obstruct blood flow.


The plasma is more than 90 percent water and contains a large number of substances, many essential to life. Its major solute is a mixture of proteins. The most abundant plasma protein is albumin. The globulins are even larger protein molecules than albumin and are of many chemical structures and functions. The antibodies, produced by lymphocytes, are globulins and are carried throughout the body, where many of them fight bacteria and viruses