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The Blood


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    Blood is a viscous liquid composed of cells and plasma; about half and half. Depending on body weight, we have 3 to 4 liters of blood for a woman, and 4 to 6 liters for a man. Most of the cells, over 99% of the cells, are the red blood cells, the cell type that carries the oxygen to our tissues. Remaining cells are the white blood cells, being so few, they do not participate in the physical characteristics of blood. The white blood cells are essential to the immune system, our defense system against micro-organisms.

Short video clip about the red blood cells
(in French).

Here is the translation of the narration:
"Essential to the maintenance of life, red blood cells or erythrocytes are blood cells whose role is to transport oxygen and carbon dioxide. They are produced in the bone marrow from stem called erythroblasts. The erythroblasts synthesize hemoglobin which stains them in red. Red blood cells then become reticulocytes, and lose their nucleus as well as the machinery necessary for the synthesis of hemoglobin. When erythrocytes leave bone marrow to reach the blood, they start a lifetime of about 120 days. Paradoxically, these erythrocytes appear, here, in white. Red blood cells travel in a complex network of capillaries to distribute its oxygen content."

Red blood cell formation.
Red blood cell formation.

    The plasma is the non cellular portion of blood. The composition of plasma is complex, it contains: 90% water, blood platelets and fibrinogen, nitrogenous substances (urea, creatinine, albumin, globulin, amino acids), lipids (cholesterol, fatty acids) , carbohydrates (sugar, mainly glucose), minerals (chlorine, sodium, potassium, calcium, magnesium, phosphorus, iodine, iron), enzymes (phosphatase, lipase, etc..), hormones, and various metabolites. And, after you let the plasma clot, a process which involve the platelets and the fibrinogen, the liquid that remains is the serum.

    The red blood cells (erythrocytes) are the most abundant cell type in our body. Their number can reach , 4 to 5 million cells per milliliter of blood, in women, and 4.5 to 5.5 millions in man. Erythrocytes are enucleated cells, meaning they have lost their nucleus during maturation. Their primarily role is to transport the hemoglobin which is the protein that can fix the oxygen molecule. Erythrocytes also contain a significant amount of an enzyme (carbonic anhydrase) that helps transport carbon dioxide (CO 2) from tissues to the lungs where it will be exhaled. By exchanging oxygen and carbon dioxide, the hemoglobin is also involved in acid-base regulation.

     The red cells are produced from stem cells (hemocytoblasts), which specialize to become red blood cells. In the first weeks of embryonic life, the erythrocytes are produced in the yolk (egg yolk equivalent). A little later during gestation, they are produced by the liver, spleen and lymph nodes. Towards the end of the gestation and for the rest of our lives, they are produced inside the bone marrow. As we age, our bones become less and less able to produce stem cells. If oxygen is in short supply, like when we go to high altitude where oxygen is rare, the production of erythrocytes is accelerated.


Tertiary structure of the hemoglobin.
Tertiary structure of the hemoglobin.

Molecular structure of the heme,
Molecular structure of the heme,
with the iron in its center.
Top of the page.      TEXT© 2000-2015 René St-Jacques