Biology of Disease

contraction. Both ventricles contract together in a wringing fashion so that
blood is squeezed out from their bases, pumping blood from the heart.
These coordinated actions keep the four chambers of the heart working in
the appropriate sequence. When things go wrong people may have to have
artificial pacemakers fitted.

The part of the nervous system that regulates the heart rate automatically
is, not surprisingly, the autonomic nervous system and consists of the
sympathetic and the parasympathetic fibers of the vagus nerve (Figure 14.4).
The sympathetic system increases the heart rate and the parasympathetic
system slows it down.

The pericardium is a thin, flexible, two-layered bag that surrounds the heart.
It contains just sufficient lubricating fluid between the two layers so that they
slide easily over each other as the heart beats. It keeps the heart in position
and prevents it overfilling with blood.

14.3 Blood Vessels

The blood vessels comprise arteries, arterioles, capillaries, venules and veins
(Figures 14.5 and 14.6). Arteries are strong and flexible and carry blood away
from the heart. They are subjected to the highest pressure (Section 14.4), and
their resilience helps to maintain blood pressure while the heart is in between

BLOOD VESSELS

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

Figure 14.4 The SA and AV nodes and the control

of the heart rate.See text for details.

Left

atrium

Left

ventricle

Right

ventricle

Right

atrium Bundle of His

Sinoatrial

node

Atrioventricular

node

Left bundle

Right bundle branch

branch

Purkinje

fibers

Vagus nerve

(slows heart)

Sympathetic nerve

(accelerates heart)