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right atrium via the coronary sinus. The coronary sinus drains into the
The grooves between the four heart chambers represent the sites that
right atrium to the left of and superior to the opening of the inferior vena
offer the least stretch during systole and, for this reason, are where most
of the vessels supplying the heart are situated.
cava. The great cardiac vein follows the anterior interventricular
branch of the left coronary and then sweeps backwards to the left in the
The arterial supply of the heart (Fig. 8.1) atrioventricular groove. The middle cardiac vein follows the posterior
The coronary arteries are responsible for supplying the heart itself with interventricular artery and, along with the small cardiac vein which fol-
oxygenated blood. lows the marginal artery, drains into the coronary sinus. The coronary
The coronary arteries are functional end-arteries and hence follow- sinus drains the vast majority of the heart’s venous blood.
ing a total occlusion, the myocardium supplied by the blocked artery is • The venae cordis minimi: these are small veins which drain directly
deprived of its blood supply (myocardial infarction). When the vessel into the cardiac chambers.
lumen gradually narrows due to atheromatous change of the walls, • The anterior cardiac veins: these are small veins which cross the atri-
patients complain of gradually increasing chest pain on exertion oventricular groove to drain directly into the right atrium.
(angina). Under these conditions the increased demand placed on the
myocardium cannot be met by the diminished arterial supply. Angina The conducting system of the heart (Figs 8.3 and 8.4)
that is not amenable to pharmacological control can be relieved by • The sinu-atrial (SA) node is the pacemaker of the heart. It is situated
dilating (angioplasty), or surgically bypassing (coronary artery bypass near the top of the crista terminalis, below the superior vena caval
grafting), the arterial stenosis. The latter procedure is usually per- opening into the right atrium. Impulses generated by the SA node are
formed using a reversed length of great saphenous vein anastomosed to conducted throughout the atrial musculature to effect synchronous
the proximal aorta and then distally to the coronary artery beyond the atrial contraction. Disease or degeneration of any part of the conduc-
stenosis. Ischaemic heart disease is the leading cause of death in the tion pathway can lead to dangerous interruption of heart rhythm.
western world and consequently a thorough knowledge of the coronary Degeneration of the SA node leads to other sites of the conduction path-
anatomy is essential. way taking over the pacemaking role, albeit usually at a slower rate.
The origins of the coronary arteries are as follows: • Impulses reach the atrioventricular (AV) node which lies in the
• The left coronary artery arises from the aortic sinus immediately interatrial septum just above the opening for the coronary sinus. From
above the left posterior cusp of the aortic valve (see Fig. 7.5). here the impulse is transmitted to the ventricles via the atrioventricular
• The right coronary artery arises from the aortic sinus immediately bundle (of His) which descends in the interventricular septum.
above the anterior cusp of the aortic valve (see Fig. 7.5). • The bundle of His divides into right and left branches which send
There is considerable variation in size and distribution zones of the Purkinje fibres to lie within the subendocardium of the ventricles. The
coronary arteries. For example, in some people the posterior interven- position of the Purkinje fibres accounts for the almost synchronous
tricular branch of the right coronary artery is large and supplies a large contraction of the ventricles.
part of the left ventricle whereas in the majority this is supplied by the
anterior interventricular branch of the left coronary. The nerve supply of the heart
Similarly, the sinu-atrial node is usually supplied by a nodal branch The heart receives both a sympathetic and a parasympathetic nerve
of the right coronary artery but in 30–40% of the population it receives supply so that heart rate can be controlled to demand.
its supply from the left coronary. • The parasympathetic supply (bradycardic effect): is derived from the
vagus nerve (p. 25).
The venous drainage of the heart (Fig. 8.2) • The sympathetic supply (tachycardic and positively inotropic effect):
The venous drainage systems in the heart include: is derived from the cervical and upper thoracic sympathetic ganglia by
• The veins which accompany the coronary arteries and drain into the way of superficial and deep cardiac plexuses (p. 25).

The heart II 23