Biology of Disease

the artery walls snap back, but the heart contracts again before enough blood
has flowed into the capillary bed to relieve the pressure. This is the peripheral
resistance. As a result, there is still a substantial blood pressure even during
diastole, the diastolic pressure, and so blood flows into the arterioles more or
less continuously.

Typically, at rest the systolic pressure is about 120 mmHg and the diastolic
pressure approximately 70 mmHg. Blood pressure can be measured using
a manual sphygmomanometer combined with a stethoscope (Figure 14.9
(A)andBox 14.1) or with an automatic electronic one (Figure 14.9 (B)).
Blood pressure is not constant because as demands on the circulatory
system change, regulatory mechanisms ensure that adequate blood flow
is maintained. Thus, during heavy exercise the arterioles in the working
muscles dilate to allow increased delivery of oxygen to the muscles. This of
course decreases the peripheral resistance, which, by itself, would cause a fall
in blood pressure. However, the cardiac output is increased to counteract this
effect.

BLOOD PRESSURE

CZhhVg6]bZY!BVjgZZc9Vlhdc!8]g^hHb^i]:YLddY (-,

A)

B)

N

EC

EC

EC

EC

EC

EC

L

BM

1μm

Figure 14.8 An electron micrograph of (A) a

capillary and (B) its tracing. BM, basement

membrane; EC, endothelial cell; L, lumen;

N, nucleus of endothelial cell. Courtesy of Dr P.

Kumar, School of Biology, Chemistry and Health

Science, Manchester Metropolitan University, UK.

Figure 14.9 Measuring blood pressure using (A)

a manual sphygmomanometer and a stethoscope

(Box 14.1) and (B) electronic sphygmomanometer.