Question #75

A dog’s systolic blood pressure is measured as 120 mmHg. Which of the following is true?

peak left ventricular pressure must be < 120 mmHg

the systolic arterial blood pressure is subatmospheric

to be a valid measurment, this pressure must have been recorded with the animal’s limb at the same vertical level as its heart

this is approximately 1.6 kPa

a manometer tube of a standard cross sectional area must have been used if this measurement is to be meaningful

In physiology and medicine pressures are often represented as the height of a column of fluid: pressure is equal to the product of the density of the fluid, acceleration due to gravity and the height of the column. If the first two quantities are standardised then column height reflects pressure.

Given than the density of mercury is approximately 13.6 x 103 kg/m3, i.e. 13.6 times more dense than water, a corresponding pressure in cmH2O or Pa can be calculated.

Pressure is a force per unit area and, therefore, does not depend on the cross sectional area of the manometer tube.

Clearly systolic blood pressure cannot be subatmospheric: 0 mmHg is taken to represent atmospheric pressure.

Since matter (just as charge) will only flow down an energy gradient, peak left ventricular pressure must be greater than systolic blood pressure if blood is to flow through the arterial limb of the circulation.

Since energy, represented by pressure in this case, is dissipated when a fluid flows against gravity, blood pressures should really be measured with the site of measurement at the same vertical level as the point of outflow from the heart.

See A Thinking Approach to Physiology, pages 2 - 6.

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