Physiology Math - Problem Set #1 - Answer Key
1. A female subject weighs 60 kilograms and is composed of 60% water by weight. Assuming
that one liter of water weighs one kg, how many liters of total body water does she have?
60 kg x 0.6 = 36 kg = 36 L
2. In the example above, if the extracellular fluid (ECF) makes up one third of her total body
water, how many liters of ECF does she have?
36 L x (1/3) = 12 L
3. Suppose the subject in the example above drinks three liters of water and that, initially, all this
water is absorbed into her extracellular fluid.
a) What is the percent change (increase or decrease) in her ECF volume?
New ECF volume = 12 L + 3 L = 15 L
% change
= 100 x (15 L – 12 L) / (12 L)
= 100 x (0.25) = 25% increase
b) What is the percent change in her total body water?
New TBW volume = 36 L + 3 L = 39 L
% change
= 100 x (39 L – 36 L) / (36 L)
= 100 x (0.083) = 8.3% increase
4. In this 60 kg woman, the left ventricle of her heart pumps 5 liters of blood per minute at rest.
Her kidneys have a blood flow of 1.2 L/min and the kidneys have a combined weight of 0.3 kg.
a) What percent of her body mass do the kidneys make up?
(0.3 kg / 60 kg) x 100 = 0.5% of body mass
b) What percent of the total blood flow do the kidneys receive?
(1.2 L/min / 5 L/min) x 100 = 24% of total blood flow
5. Suppose the 60 kg female requires 1,500 calories per day to meet her energy needs. If you
assume that caloric requirement is directly proportional to body mass*, how many calories would
a 90 kg woman require?
1,500 calories / 60 kg = x / 90 kg
60 x = (1,500 x 90)
x = (1,500 x 90) / 60 = 2,250 calories