Question

There are many variables that affect the calculation of a patient’s total blood volume. For the purposes of this question, weight and age will be the only variables considered. The Australian Red Cross states that “Blood volume can be estimated as approximately 70mL/kg for adults, 80mL/kg in children…”. Work on the assumption that an adult is a person aged 16 years or over and a child is a person aged 2 to 16 years.

Let:
W = Weight (kg)
P = Plasma volume (mL) and
T = Total blood volume (mL)

Write 2 equations for calculating Total blood volume (T) given Weight (W), expressing T in terms of W, one for adults and one for children:

Answer 1

The two equations are;

For adults, T = W kg × 70 mL/kg ............Equation (1)

For children, T = W kg × 80 mL/kg.............Equation (2)

Explanation:

We are told that

Blood volume = 70 mL/kg for adults

Blood volume = 80 mL/kg in children

Therefore, the total blood volume,T of an adult of weight, W is given by the following relation;

T = W kg × 70 mL/kg ............Equation (1)

The total blood volume,T of a child of weight, W is given by the following relation;

T = W kg × 80 mL/kg.............Equation (2).