NORMAL BLOOD VOLUME

NORMAL BLOOD VOLUME

Total amount of blood present in the circulatory system, blood reservoirs, organs and tissues together constitute blood volume. In a normal young healthy adult male weighing about 70 kg, the blood volume is about 5 L. It is about 7% of total body weight. It ranges between 6% and 8% of body weight. In relation to body surface area, blood volume is 2.8 to 3.1 L/sq M.

VARIATIONS IN BLOOD VOLUME

PHYSIOLOGICAL VARIATIONS

1. Age

Absolute blood volume is less at birth and it increases steadily as the age advances. However, at birth, the blood volume is more when compared to body weight and less when compared to the body surface area. At birth and at 24 hours after birth, the blood volume is about 80 mL/kg body weight. At the end of 6 months, it increases to about 86 mL/kg. At the end of one year, it is about 80 mL/kg. It remains at this level until 6 years of age. At 10 years, it is about 75 mL/kg. At the age of 15 years, the blood volume is about 70 mL/kg body weight, which is almost the adult volume.

2. Sex

In males, the blood volume is slightly more than in females because of the increase in erythropoietic

activity, body weight and surface area of the body. In females, it is slightly less because of loss of blood through menstruation, more fats and less body surface area.

3. Surface Area of the Body

Blood volume is directly proportional to the surface area of the body.

4. Body Weight

Blood volume is directly proportional to body weight.

5. Atmospheric Temperature

Exposure to cold environment reduces the blood volume and exposure to warm environment increases the blood volume.

6. Pregnancy

During early stage of pregnancy, blood volume increases by 20% to 30% due to the increased fetal mass and sodium retention. However, it reduces in later stages.

7. Exercise

Exercise increases the blood volume by increasing the release of erythropoietin and production of more RBCs.

8. Posture

Standing (erect posture) for long time reduces the blood volume by about 15%. It is because the pooling of blood in lower limbs while standing increases the hydrostatic pressure. This pressure pushes fluid from blood vessels into the tissue spaces; so blood volume decreases.

9. High Altitude

Blood volume increases in high altitude. It is because of hypoxia, which stimulates the secretion of erythropoietin. It induces the production of more RBCs, which leads to increase in blood volume.

10. Emotion

Excitement increases blood volume. It is because of sympathetic stimulation, which causes splenic contraction and release of stored blood into circulation.

PATHOLOGICAL VARIATIONS

Abnormal increase in blood volume is called hypervolemia and abnormal decrease in blood volume is called hypovolemia.

HYPERVOLEMIA

Increase in blood volume is called hypervolemia. It occurs in the following pathological conditions:

1. Hyperthyroidism

Blood volume increases because thyroxine increases the RBC count and plasma volume.

2. Hyperaldosteronism

In hyperaldosteronism, excess retention of sodium and water leads to increase in the ECF volume and blood volume.

3. Cirrhosis of the Liver

In this condition, the blood volume is more because of increase in the plasma volume.

4. Congestive Cardiac Failure

Retention of sodium occurs in this condition. Sodium retention leads to water retention and increase in ECF volume, plasma volume and blood volume.

HYPOVOLEMIA

Decrease in blood volume is called hypovolemia. It occurs in the following pathological conditions:

1. Hemorrhage or Blood Loss

Acute hemorrhage occurs due to cuts or accidents. The chronic hemorrhage occurs in ulcers, bleeding

piles and excessive uterine bleeding in females during menstruation.

2. Fluid Loss

Fluid loss occurs in burns, vomiting, diarrhea, excessive sweating and polyuria.

3. Hemolysis

Excessive destruction of RBCs occurs because of the presence of various hemolytic agents and other factors such as, hypotonic solution, snake venom, acidity or alkalinity, mismatched blood transfusion, hemorrhagic smallpox and measles.

4. Anemia

Blood volume decreases in various types of anemia because of decrease in RBC count. In some cases,

the quantity (volume) of blood remains the same but the quality of the blood alters. Blood becomes dilute

(hemodilution) because of the entrance of fluid into the blood vessel.

5. Hypothyroidism

During hypothyroidism, the blood volume is decreased because of reduction in plasma volume and RBC

Count.

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