© Diffomédia/Royal Canin

In adult dogs, the heart is fully compartmentalised. Oxygenated and deoxygenated blood cannot come into contact with each other, because the blood flow in the various chambers is regulated by the heart valves.

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© Diffomédia/Royal Canin

If it develops normally, the heart should not be in the middle of the body, but slightly to the left (4/7ths). It is flat transversally, which positions the right heart cranially (towards the front) and the left heart caudally (towards the back).

Thus, the bottom of the heart, where the vessels are, is in the cranial and dorsal position, whereas the top is in caudal and ventral position. The heart is situated between the third and sixth rib, and its weight varies greatly depending on the breed of dog.


In physiological terms, the heart works to a regular cardiac cycle. The heart chambers, which have contractile properties, work in two phases, of which systole is the contraction phase and diastole is the relaxation phase.

These phases are not synchronised in all chambers, because the atria contract before the ventricles do. The number of cardiac cycles per minute is called the heart rate, which is between 70 and 160 in dogs, depending on the breed (the heart of a small dog beats faster) and physical activity.

The heart rate falls (brachycardia) during sleep and rises (tachycardia) in the event of physical exertion or great stress, like a visit to the veterinarian. These are purely physiological phenomena.

The cardiac cycle

© Diffomédia/Royal Canin

The cardiac cycle follows a well-established sequence. In atrial systole, due to low venous pressure and the opening of the atrioventricular valves, the ventricles fill up passively (as the arterial valves are closed). The contraction of the atria completes the process.

Ventricular systole then occurs. The ventricle is completely full, the intraventricular pressure increases, which closes the atrioventricular valves. The contraction of the ventricles is intensified as the intraventricular pressure rises above the arterial pressure, which opens the arterial valves.

The cardiac muscles then relax, allowing the closure of the atrioventricular valves. The atria fill up again, the atrioventricular valves open, the ventricles fill up and a new cycle begins.

During auscultation with a stethoscope, the veterinarian will only hear the sounds caused by these phases. The cardiac cycle produces two sounds in dogs: a long sound followed by a short interval, then a short sound followed by a long interval. The long sound is caused by a number of different phenomena: closing of the atrioventricular valves, pressurisation of the blood in the ventricles, turbulent flow of the blood into the large arterial vessels. The short sound is caused exclusively by the closing of the arterial valves. Any additional sounds may be regarded as pathological.

Modern techniques like electrocardiography and echocardiography mean that the study of the full cardiac cycle can be much more in-depth. Interpreting the data is a complicated process, however, and should be carried out by specialists.

So how does the heart beat in the first place? The muscle wall contains three so-called nodal tissues, which consist of cells which are able to slowly and spontaneously depolarise, producing an action potential that spreads to all heart cells, causing the heart to contract. The nodal tissues in the atria set the rate, acting as the heart’s natural pacemaker.

The heart rate can be adjusted by various factors, both external (the sight of a stressful object, for instance) and internal, by acting on the nerve pathways, which are made up of accelerator and inhibitory fibres. The lungs and blood gases also influence the heart rate through baroreceptors (sensory nerve endings) in the arch of the aorta. Too much oxygen slows the heart down, whereas too much carbon dioxide speeds it up.

© Diffomédia/Royal Canin
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