Question

O Reported blood pressures are typically assumed 115/73 to be the blood pressure at the level of the heart. Imagine a doctor measured the blood pressure of a patient to be 115/73 mm Hg, when his arm is outstretched and the cuff is placed at the heart level, as shown in Figure A. If instead the patient's arm were at his side and the cuff were 15 cm below heart level, as in Figure B, what blood pressure would the doctor measure in mm Hg? Figure A Figure B 1 atm-760 mm Hg, Pblood 1055 kg/m3

Answer 1

The excess pressure measured by doctor when the cuff is 15 cm below heart level-

$\mathbf{P=\rho _{blood}\times g\times h}$

where h=`5cm=0.15 meter

$\mathbf{P=1055\times 9.8\times 0.15=1550.85 Pa}$    

$\Rightarrow \mathbf{P=\frac{1550.85 }{1.01\times 10^{5}}atm}$

$\Rightarrow \mathbf{P=0.01535atm}$

since 1atm=760mm

So 0.01535atm=0.01535x760=11.67mm

So pressure measured by doctor

$\mathbf{(115+11.67)/(73+11.67)}$

$\mathbf{(126.67)/(84.67)\approx 127/85}$