FUNCTIONS OF RED BLOOD CELLS
Major function of RBCs is the
transport of respiratory gases. Following are the functions of RBCs:
1. Transport of
Oxygen from the Lungs to the Tissues
Hemoglobin in RBC combines with oxygen
to form oxyhemoglobin.
About
97% of oxygen is transported in blood in the form of oxyhemoglobin.
2. Transport of Carbon
Dioxide from the Tissues to the Lungs
Hemoglobin combines with carbon
dioxide and form carbhemoglobin.
About
30% of carbon dioxide is transported in this form. RBCs contain a large amount
of the carbonic anhydrase. This enzyme is
necessary for the formation of bicarbonate from water and carbon dioxide. Thus,
it helps to transport carbon dioxide in the form of bicarbonate from tissues to
lungs. About 63% of carbon dioxide is transported in this form.
3. Buffering
Action in Blood
Hemoglobin functions as a good buffer.
By this action, it regulates the hydrogen ion concentration and thereby plays a
role in the maintenance of acidbase balance.
4. In Blood Group
Determination
RBCs carry the blood group antigens like A antigen, B
antigen and Rh factor. This helps in determination of blood group and enables
to prevent reactions due to incompatible blood transfusion
LIFESPAN OF RED BLOOD CELLS
Average lifespan of RBC is about 120
days. After the lifetime the senile (old) RBCs are destroyed in
reticuloendothelial system.
Determination of
Lifespan of Red Blood Cells
Lifespan of the RBC is determined by
radioisotope method. RBCs are tagged with radioactive substances
like radioactive iron or radioactive
chromium. Life of RBC is determined by studying the rate of loss of radioactive
cells from circulation.
FATE OF RED BLOOD CELLS
When the cells become older (120
days), the cell membrane becomes more fragile. Diameter of the
capillaries is less or equal to that
of RBC. Younger RBCs can pass through the capillaries easily. However, because of
the fragile nature, the older cells are destroyed while trying to squeeze
through the capillaries. The destruction occurs mainly in the capillaries of
red pulp of spleen because the diameter of splenic capillaries is very small. So,
the spleen is called ‘graveyard
of RBCs’.
Destroyed RBCs are fragmented and hemoglobin is released from the fragmented
parts. Hemoglobin is immediately phagocytized by macrophages of the body, particularly
the macrophages present in liver (Kupffer cells), spleen and bone
marrow. Hemoglobin is degraded into iron, globin and porphyrin. Iron combines
with the protein called apoferritin to form ferritin, which is stored in the
body and reused later. Globin enters the protein depot for later use. Porphyrin
is degraded into bilirubin, which is excreted by liver through bile.
Daily 10% RBCs, which are senile, are
destroyed in normal young healthy adults. It causes release of about 0.6 g/dL
of hemoglobin into the plasma. From this 0.9 to 1.5 mg/dL bilirubin is formed.
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