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HUMAN BLOOD

Human blood is the fluid circulated by heart through the human vascular system. There are 3 cellular components of human blood: red blood cell, white blood cell and platelets. RBC transport oxygen to other cells of body packed with hemoglobin.RBC are produced in bone marrow and have a life span of 120 days. WBC (purple) protects body from infection, attacking and destroying foreign particles like dust, pollen and viruses. Platelets (blue) defend the body against excessive body loss. Platelets flow freely in the blood in an inactive state, but when an injury is sustained, platelets become sticky to plug the injured area.

 

 Normally, 7-8% of human body weight is from blood.  In adults, this amounts to 4-5 quarts of blood. Also it plays a vital role in our immune system and in maintaining a relatively constant body temperature. Blood is a highly specialized tissue composed of many different kinds of components.    All humans produce these blood components—there is no population or regional differences.

 

RED Cells


Red cells, or erythrocytes, are relatively large microscopic cells without nuclei.  In this latter trait, they are similar to the primitive prokaryotic cells of bacteria.  Red cells normally make up 40-50% of the total blood volume.  They transport oxygen from the lungs to all of the living tissues of the body and carry away carbon dioxide.  The red cells are produced continuously in our bone marrow from stem cells  at a rate of about 2-3 million cells per second.   Hemoglobin is the gas transporting protein  molecule that makes up 95% of a red cell.  Each red cell has about 270,000,000 iron-rich hemoglobin molecules.  People who are anemic generally have a deficiency in red cells.  The red color of blood is primarily due to oxygenated red cells.  Human fetal hemoglobin molecules differ from those produced by adults in the number of amino acid chains.  Fetal hemoglobin has three chains, while adults produce only two.  As a consequence, fetal hemoglobin molecules attract and transport relatively more oxygen to the cells of the body

WHITE CELLS


 White cells, or leukocytes , exist in variable numbers and types but make up a very small part of blood's volume--normally only about 1% in healthy people.  Leukocytes are not limited to blood.  They occur elsewhere in the body as well, most notably in the spleen, liver, and lymph glands.  Most are produced in our bone marrow from the same kind of stem cells that produce red blood cells.  Others are produced in the thymus gland, which is at the base of the neck.  Some white cells (called lymphocytes ) are the first responders for our immune system.  They seek out, identify, and bind to alien protein on bacteria, viruses  and  fungi so that they can be removed.  Other white cells (called granulocytes and macrophages ) then arrive to surround and destroy the alien cells.  They also have the function of getting rid of dead or dying blood cells as well as foreign matter such as dust and asbestos.  Red cells remain viable for only about 4 months before they are removed from the blood and their components recycled in the spleen.  Individual white cells usually only last 18-36 hours before they also are removed, though some types live as much as a year.  The description of white cells presented here is a simplification.  There are actually many specialized sub-types of them that participate in different ways in our immune responses.
   

Platelets
Platelets , or thrombocytes , are cell fragments without nuclei that work with blood clotting chemicals at the site of wounds.  They do this by adhering to the walls of blood vessels, thereby plugging the rupture in the vascular wall.  They also can release coagulating chemicals which cause clots to form in the blood that can plug up narrowed blood vessels.  There are more than a dozen types of blood clotting factors and platelets that need to interact in the blood clotting process.  Recent research has shown that platelets help fight infections by releasing proteins that kill invading bacteria and some other microorganisms.  In addition, platelets stimulate the immune system.  Individual platelets are about 1/3 the size of red cells.  They have a lifespan of 9-10 days.  Like the red and white blood cells, platelets are produced in bone marrow from stem cells.

Plasma
Plasma is the relatively clear, yellow tinted water (92+%), sugar, fat, protein and salt solution which carries the red cells, white cells, platelets, and some other chemicals.  Normally, 55% of our blood's volume is made up of plasma.  About 95% of it consists of water.  As the heart pumps blood to cells throughout the body, plasma brings nourishment to them and removes the waste products of metabolism.  Plasma also contains blood clotting factors, sugars, lipid, vitamins, minerals, hormones, enzymes, antibodies, and other proteins.  It is likely that plasma contains some of every  protein produced by the body--approximately 500 have been identified in human plasma so far.