In this article we will discuss about:- 1. Definition of Blood Volume 2. Variations in Blood Volume 3. Methods of Determination 4. Regulation 5. Causes of Decrease and Increase.
Contents:
- Essay on the Definition of Blood Volume
- Essay on the Variations in Blood Volume
- Essay on the Methods of Determination of Blood Volume
- Essay on the Regulation of Blood Volume
- Essay on the Causes of Decrease and Increase of Blood Volume
1. Definition of Blood Volume:
The term blood volume means the total amount of blood in circulation, as well as in the blood stores.
Normal Blood Volume:
It can be expressed in two ways:
(a) In relation to body weight—78 to 97 ml (average 90 ml) per Kg of body weight; or about 1/11th (9%) of the total body weight. That of plasma is 50 ml per Kg body wt. or 1/20th (5%) of the total body weight,
(b) In relation to body surface 2.5 to 4 litres (average 3.3) per sq. metre of body surface. A man, weighing 70 Kg, has about 5 litres of blood in his circulation.
2. Variations in Blood Volume:
i. Age:
In infants the blood volume is greater in proportion to the body weight but is lesser in proportion to the body surface. [The body surface of infants is proportionally larger than their body weight]. This larger volume is due to greater number of corpuscles as well as larger amount of plasma.
ii. Sex:
In males the blood volume is 7.5% higher (per sq. metre of body surface) than in females. This is due to greater number of red cells in the males. The plasma volume is same in both.
iii. Body Weight and Surface Area:
Body weight and surface area (vide above)
iv. Pregnancy:
Blood volume rises due to increase of both cells and plasma but the rise in plasma volume is much greater than the rise in cell volume. It falls after delivery.
v. Muscular Exercise:
Raises blood volume probably due to contraction of spleen.
vi. Posture:
In erect posture there is about 15% diminution of total plasma. It passes out into the tissue spaces.
vii. Blood Pressure:
Rise of blood pressure lowers blood volume by pressing out more fluid into the tissue spaces. Lowered blood pressure draws in more fluid from the tissue spaces and raises the blood volume.
viii. Altitude:
At higher altitude the blood volume rises, due to anoxia produced, the number of red cells increase in such condition.
ix. Anoxia:
Anoxia due to any other cause will raise blood volume.
x. Adrenaline injection:
Raises blood volume probably by splenic contraction.
3. Methods of Determination of Blood Volume:
Direct Method:
i. Welcker:
An animal is bled to death and the blood is collected. Then its blood vessels are washed out by pumping saline solution into the vessels. The saline washings are added to the already collected blood. The colour of this mixture is matched against the sample of normal blood of the same animal. From these data the volume may be calculated. In dogs the total blood volume is 7.7% of body weight.
ii. Bischoff:
The above method was applied upon decapitated criminals. It is obvious that the direct methods have got no place in clinical medicine, for which one of the following indirect methods is used.
Indirect Method:
In this method known amount of a particular substance is introduced into the blood stream. After some time a sample of blood is drawn out and the concentration of the injected substance is determined in it. From this, the degree of dilution is calculated. Total blood volume can be found out from these data.
The substance to be used must have certain special qualities. It must not be toxic, must not alter blood volume, must not easily pass into the tissue spaces or be excreted, and must not be taken up by the phagocytic cells of the blood. Also it must not change its colour and chemical composition while in the body.
Usually, two classes of substances are used for this purpose:
1. Dye stuff like Congo red, Evans blue, etc.
2. Radioactive substances.
1. Dye Method:
For clinical purposes the blood volume can be determined by dye method. The dye Evans’ blue (T -1824) which is non-toxic and escapes slowly from the blood vessels, is mostly used in present times. 10 ml of venous blood from the subject is taken in a heparinised tube.
This serves as the control sample. 5 ml of a 5% solution of Evans blue in distilled water is then injected intravenously. 10 minutes after beginning of the injection another 10 ml sample is withdrawn from the vein of the opposite side into another heparinised tube. The Haematocrit of both samples are determined. The optical density of the dye stained plasma is estimated. 0.01 ml of the dye is then diluted to 5 ml (dilution 1: 500) with control plasma and its optical density is determined.
2. Radioactive Methods:
(a) Radio-iodine Plasma Albumin Method:
To a sample of plasma, iodine-containing radioactive isotope 131I or 132I are added, and allowed to incubate for some time. This plasma is slowly injected intravenously. The degree of dilution of its radioactivity is determined, which is a measure of plasma volume.
The advantage of this method is that the albumin cannot permeate through the capillary endothelium and is not affected by lipaemia and haemolysis. The plasma and blood volume are measured in a similar way as the dye dilution method. The plasma volume determined by this method is about 2.5 liter in women and 3 liters in men.
(b) Radioactive Iron Method:
Radioactive iron (55Fe or 59Fe) is incorporated into ferric ammonium citrate which is given by intravenous injection to a person (donor) belonging to group O (vide Blood Group). The radioactive iron is taken up by the newly formed blood cells which appear in the circulation within twenty-four hours. 75 ml of blood having a radioactivity of about 3,000 counts per minute per ml (determined by Geiger) is injected intravenously to recipient, whose blood volume is to be determined.
After about 20 minutes of the injection, 15 ml of blood is withdrawn from the recipient. After interval of 20 minutes, another two samples of blood of the same quantity are withdrawn. The blood samples after proper dilution are centrifuged for half an hour. The samples taken out both from the donor and the recipient are wet ashed and the iron is deposited electrolytically on copper.
The radioactive iron present in the two samples is determined with the help of Geiger counter. The red cell volume is calculated from the number of millilitres of donor’s cell injected, the radioactivity of the donor’s cells and the radioactivity of the recipient’s cells. The blood volume is determined by measuring the plasma volume by the dye method and the result is added to the value of the red cell volume.
Similarly red cell volume can be determined either with labelled radioactive chromium (51Cr) or radioactive phosphorus (32P). So from all the above methods it is clear that the dye Evans’ blue or plasma labelled with radioactive iodine help in the measurement of plasma volume whereas radioactive iron, chromium or phosphorus is used to determine the red cell volume. No substance has yet been found which helps in measurement of the total blood volume directly. For an accurate measurement of blood volume, plasma and cell volumes must be determined simultaneously.
4. Regulation of Blood Volume:
Although it is customary to talk about a constant blood volume yet it has been already shown that total blood volume does not remain constant and varies widely under different physiological conditions. This variation is mostly due to alteration of the cells and not due to that of plasma. The plasma volume remains fairly constant under normal conditions.
The problem of regulation of blood volume is intimately linked up with that of water balance. The maintenance of blood volume depends upon a balance between water intake and water loss and also upon the adjustment of fluid interchange between plasma and tissue spaces through the capillary walls.
A number of factors are involved:
i. Physical Factors:
Blood pressure, osmotic pressure, diffusion, the state of permeability of the capillaries, etc., are the important factors concerned in the regulation of the blood volume.
ii. The Tissue Spaces:
The tissue spaces, due to their enormous capacity, act as a ready reservoir. Any increase in the blood volume will lead to passage of more fluid from the plasma to the tissue spaces. While any decrease will draw in more fluid from the tissue spaces and maintain the blood volume.
iii. Vitamins:
Some vitamins, especially C, by controlling the permeability of the capillaries—take part in the process.
iv. Endocrines:
A number of endocrine factors are also involved here:
(a) The antidiuretic factor of the posterior pituitary controls excretion of water through the kidneys. When blood is diluted the secretion of the factor is inhibited and thus more water is lost. When blood becomes concentrated, reverse changes occur,
(b) Parathyroids, by their effect on calcium metabolism, control the permeability of the blood vessels and thereby the rate of interchange between blood and tissues,
(c) Adrenal cortex is believed to exert important influence upon salt balance, kidney function and excretion of water.
v. Thirst:
Another important mechanism for replenishing the reduced blood volume is the phenomenon of thirst. When the water content of the body becomes low, ‘thirst’ is felt. The subject takes water and thus the blood volume is kept up.
5. Causes of Decrease and Increase of Blood Volume:
Blood volume is reduced in the following conditions:
i. Loss of whole blood, e.g., haemorrhage.
ii. Reduction in number of R.B.C., e.g., anaemia.
iii. Loss of plasma alone.
iv. Loss of blood water or anhydraemia.
v. Acute exposure to cold causes moderate loss.
vi. Posture: Blood volume is low in the erect position than in the recumbent state.
Blood volume is increased due to:
i. High temperature.
ii. Muscular exercise.
iii. Emotional excitement.
iv. Pregnancy.
v. Congestive heart failure.
vi. Administration of mineralocorticoids (deoxycorticosterone and aldosterone).