Blood Volume Physiology Notes

Normal Blood Volume

• The 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 liters. 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 2.8-3.1 liters per square meter.

Read And Learn More: Medical Physiology Notes

Variations In Blood Volume

Physiological Variations

1. Age
   • Absolute blood volume is less at birth, and it increases steadily as 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
4. Body Weight: The blood volume is directly proportional to body weight.
5. Atmospheric Temperature: Exposure to cold reduces the blood volume and exposure to warm environment increases the blood volume.
6. Pregnancy: During the early stage of pregnancy, blood volume increases by 20-30% due to increased fetal mass and sodium retention. However, it reduces in later stages.
7. Exercise: Exercise increases blood volume by increasing the release of erythropoietin and the production of more RBCs.
8. Posture: Standing (erect posture) for a 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 altitudes. It is because of hypoxia, which stimulates the secretion of erythropoietin. It induces the production of more RBCs, which leads to an 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: An abnormal increase in blood volume is called hypervolemia and an abnormal decrease in blood volume is called hypovolemia. Refer to applied physiology for details.

Measurement Of Blood Volume

Blood volume is measured by two methods, the direct method, and the indirect method.

• Direct Method
  • This method is employed only in animals because it involves sacrificing their life. The animal is killed by decapitation and the blood is collected.
  • The blood vessels and the tissues are washed thoroughly with a known quantity of water or saline. And, this is added to the blood collected already.
  • The total volume is measured. From this, the volume of water or saline used for washing the tissues is deducted to obtain the volume of the blood in the animal.
  • This method was first employed by Welcker in 1854. Later, B’Schoff employed the same method on decapitated criminals to determine the blood volume in human beings.
• Indirect Method: The indirect method is advantageous because it is used to measure the blood volume in human beings without causing any discomfort or difficulty to the subject.

Measurement of total blood volume involves two steps:

1. Determination of plasma volume
2. Determination of blood cell volume.

1. Determination of Plasma Volume: Plasma volume is determined by two methods

1. Indicator or dye dilution technique
2. Radioisotope method.

• Determination of plasma volume by indicator or dye dilution technique: The principles and other details of this technique are explained. The dye, that is used to measure plasma volume, is Evans blue or T-1824.
  • Procedure: Ten ml of blood is drawn from the subject. This is divided into 2 equal portions. To one part, a known quantity of the dye is added. This is used as a control sample in the procedure. The other portion is used to determine the hematocrit value.
    • Then, a known volume of dye is injected intravenously. After 10 minutes, a sample of blood is drawn. Then, another 4 samples of blood are collected at the interval of 10 minutes.
    • All 5 samples are centrifuged and plasma is separated from the samples. In each sample of plasma, the concentration of the dye is measured by colorimetric method and the average concentration is found.
    • The subject’s urine is collected and the amount of dye excreted in the urine is measured.
  • Calculation: The plasma volume is determined by using the formula, Volume =

• Determination of plasma volume by radioisotope method: Radioactive iodine (¹³¹l or ¹³²l) is injected. After some time, a sample of blood is collected. The radioactivity is determined by using an appropriate counter. From this, the plasma volume is determined.

2. Determination of Blood Cell Volume: Blood cell volume is determined by two methods

1. By hematocrit value
2. By radioisotope method.

• Determination of blood volume by hematocrit value: This is usually done by centrifuging the blood and measuring the packed cell volume. PCV is expressed in percentage. If this is deducted from 100, the percentage of plasma is known. From this, and from the volume of plasma, the amount of total blood is calculated by using the formula.

• Determination of blood volume by radioisotope method
  • The volume of blood cells is measured by the radioisotope method also. Radioactive chromium (Cr⁵²) is added with heparinized blood and incubated for 2 hours at 37°C.
  • During this time, all the red cells in the blood are “tagged” with Cr⁵². Then, this is injected intravenously. After giving sufficient time for mixing, a sample of blood is drawn.
  • The hematocrit value is determined by measuring the radioactivity in the blood sample. Radioactive iron (Fe⁵⁹, Fe⁵⁵) or radioactive phosphorus (P³²) is also used for determining hematocrit value.

Regulation Of Blood Volume

• Various mechanisms are involved in the regulation of blood volume. The important ones are the renal and hormonal mechanisms.
• The hypothalamus plays a vital role in the activation of these two mechanisms during the regulation of blood volume.
• When blood volume increases, the hypothalamus causes a loss of fluid from the body. When the blood volume reduces, the hypothalamus induces the retention of water.
• The hypothalamus regulates the ECF volume and blood volume by acting mainly through kidneys and sweat glands and by inducing thirst. This function of the hypothalamus is described.
• The hormones also are involved in the regulation of blood volume through the regulation of ECF volume. The hormones which are involved in the maintenance of EOF volume are:

1. Antidiuretic hormone
2. Aldosterone
3. Cortisol
4. Atrial natriuretic peptide

Applied Physiology

Hypervolemia: An increase in blood volume is called hypervolemia. It occurs in the following 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 an increase in the ECF volume and blood volume.
3. Cirrhosis of the Liver: In this condition, the blood volume is higher because of an increase in the plasma volume.
4. Congestive Cardiac Failure: Retention of sodium occurs in this condition. Sodium retention leads to water retention and an increase in ECF volume, plasma volume, and blood volume.

Hypovolemia: A 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. 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 a 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 a reduction in plasma volume and RBC count.