A. Increase in RBC Count

Increase in the RBC count is known as polycythemia. It occurs in both physiological and pathological conditions. When it occurs in physiological conditions it is called physiological polycythemia. The increase in number during this condition is marginal and temporary. It occurs in the following conditions:

1. Age

At birth, the RBC count is 8 to 10 million/cu mm of blood. The count decreases within 10 days after birth due to destruction of RBCs causing physiological jaundice in some newborn babies. However, in infants and growing children, the cell count is more than the value in adults.

2. Sex

Before puberty and after menopause in females the RBC count is similar to that in males. During reproductive period of females, the count is less than that of males (4.5 million/cu mm).

3. High altitude

Inhabitants of mountains (above 10,000 feet from mean sea level) have an increased RBC count of more than 7 million/cu mm. It is due to hypoxia (decreased oxygen supply to tissues) in high altitude. Hypoxia stimulates kidney to secrete a hormone called erythropoietin.  The erythropoietin in turn stimulates the bone marrow to produce more RBCs.

4. Muscular exercise

There is a temporary increase in RBC count after exercise. It is because of mild hypoxia and contraction

of spleen. Spleen stores RBCs. Hypoxia increases the sympathetic activity resulting in secretion of adrenaline from adrenal medulla. Adrenaline contracts spleen and RBCs are released into blood.

5. Emotional conditions

RBC count increases during the emotional conditions such as anxiety. It is because of increase in the sympathetic activity as in the case of muscular exercise .

6. Increased environmental temperature

Increase in atmospheric temperature increases RBC count. Generally increased temperature increases all the activities in the body including production of RBCs.

7. After meals

There is a slight increase in the RBC count after taking meals. It is because of need for more oxygen for

metabolic activities.

B. Decrease in RBC Count

Decrease in RBC count occurs in the following physiological conditions:

1. High barometric pressures

At high barometric pressures as in deep sea, when the oxygen tension of blood is higher, the RBC count


2. During sleep

RBC count decreases slightly during sleep and immediately after getting up from sleep. Generally all the

activities of the body are decreased during sleep including production of RBCs.

3. Pregnancy

In pregnancy, the RBC count decreases. It is because of increase in ECF volume. Increase in ECF volume, increases the plasma volume also resulting in hemodilution. So, there is a relative reduction in the RBC count.

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