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


Direct method is employed only in animals because it involves sacrificing the life. The animal is killed by

decapitation and the blood is collected. The blood vessels and the tissues are washed thoroughly with

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 is advantageous because, it is used to measure the blood volume in human beings without causing any discomfort or any difficulty to the subject. Measurement of total blood volume involves two steps:

1. Determination of plasma volume

2. Determination of blood cell volume.

Determination of Plasma Volume

Plasma volume is determined by two methods:

i. Indicator or dye dilution technique

ii. Radioisotope method.

i. Determination of plasma volume by indicator or dye dilution technique


10 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 control sample in the procedure. The other portion is used to determine the hematocrit value. Then, a known volume of the 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 the 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.


Plasma volume is determined by using the formula,

Amount of dye injected – Amount excreted Volume =

Average concentration of dye in plasma

ii. Determination of plasma volume by radioisotope method

Radioactive iodine (131I or 132I) is injected. After sometime, a sample of blood is collected. The radioactivity is determined by using appropriate counter. From this, the plasma volume is determined.

Determination of Blood Cell Volume

Blood cell volume is determined by two methods:

i. By hematocrit value

ii. By radioisotope method.

i. Determination of blood cell volume by hematocrit value

This is usually done by centrifuging the blood and measuring the packed cell volume. 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.

100 × Amount of plasma Blood volume =

100 – PCV

ii. Determination of blood cell volume by radioisotope method

Volume of blood cell is measured by 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. 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 the hematocrit value.


Various mechanisms are involved in the regulation of blood volume. The important ones are the renal and

hormonal mechanisms. Hypothalamus plays a vital role in the activation of these two mechanisms during the regulation of blood volume. When blood volume increases, hypothalamus causes loss of fluid from the body. When the blood volume reduces, hypothalamus induces retention of water. Hypothalamus regulates the extracellular fluid (ECF) volume and blood volume by acting mainly through kidneys and sweat glands and by inducing thirst.  Hormones which are involved in the maintenance of

ECF volume are:

1. Antidiuretic hormone

2. Aldosterone

3. Cortisol

4. Atrial natriuretic peptide

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