Overview Of Cardiovascular System In Dogs

The cardiovascular system consists of the heart and blood vessels, including the veins, the arteries and the capillaries. The main purpose of the heart is to pump blood around the body, ensuring that every organ is catered for adequately. The heart is an organ that is not only hollow, but quite muscular. Within the bodies of mammals and birds, it is separated into 4 chambers; two upper and two lower chambers. The upper left side chamber is called the left atrium, while the upper right side one is called right atrium. The lower left and lower right side chambers are known as left and right ventricles respectively. The muscular cardiac tissue found within the heart is known as the myocardium.

The right side of the heart is responsible for pumping blood to the lungs, where blood acquires oxygen (oxygenation) and carbon dioxide is expelled from the blood. The function of the left side of the heart is to pump blood to the other parts of the body, where oxygen and nutrients are transported to the tissues. It is also during this process that waste products, such as carbon dioxide, are shifted from the tissues into the blood for elimination by organs, like the lungs and kidneys.

There is a sequence of valves whose sole purpose is to ensure that blood flows in one direction via the heart. The valves located between the atria and the ventricles are called atrioventricular valves. The valve found between the heart and the aorta, and between the heart and the pulmonary artery is known as the semilunar valve. Both ventricles have one inlet and outlet valve. The inlet valve found within the left ventricle is called the mitral valve, while the outlet valve of the left ventricle is known as the aortic valve. On the other hand, the inlet valve within the right ventricle is called the tricuspid valve, while the outlet valve is known as the pulmonary valve.

The blood that flows from the body to the heart goes through the two biggest veins, known as venae cavae, into the right atrium. The relaxation of the right ventricle leads to blood in the right atrium pouring into the right ventricle, through the tricuspid valve. As the right ventricle almost fills up, the right atrium squeezes and pushes extra blood into the right ventricle. Then the right ventricle contracts and squeezes blood via the pulmonary valve into the pulmonary arteries, which transport it to the lungs. The lungs provide oxygen to the blood, and take in carbon dioxide from the blood. The blood then proceeds through the pulmonary veins to end up in the left atrium. The relaxation of the left ventricle causes the blood in the left atrium to pour into the left ventricle, via the mitral valve. When the left ventricle is almost filled with blood, the left atrium squeezes and pushes more blood into the left ventricle. Contraction of the left ventricle then occurs, causing blood to be pushed via the aortic valve and into the aorta, the body’s biggest artery. This oxygenated blood delivers oxygen to every organ in the body, with the exception of the lungs.

The heartbeat is made up of two phases: diastole and systole. The diastole phase is when the ventricles relax and fill up with blood. The systole phase is where the ventricles contract and push blood to the body’s organs. The sound made by the mitral and tricuspid valves closing form the initial half of a heartbeat. The second half of the pulse consists of the sound of the aortic and pulmonary valves closing.

The pace and power of heart contractions, and the extent to which blood vessels narrow or widen, are all under the control of different hormones and the autonomic nervous system. The autonomous nervous system is responsible for controlling reflex action.

Heart Beat Or Pulse

A heartbeat is the result of minute electrical currents that begin in the heart’s pacemaker, known as the sinoatrial node. These electrical impulses are periodic in nature, and result in the contraction of cardiac muscle fibres. Whenever an animal is at rest, the sinoatrial node generates numerous electrical discharges per minute: around 15 times every minute for horse, over 200 times every minute for cats, and between 60 to 160 times per minute in dogs. The general rule is that the bigger the species, the slower the pulse rate and the lower the rate of sinoatrial node discharge.

Dogs that are calm and in good health will naturally have a heart rate that is irregular i.e. it speeds up while breathing in and slows down while breathing out. This is known as respiratory sinus arrhythmia. This variation in heart rate synchronising with inspiration and expiration is a sign of good health. This variation in heart rate does not usually take place when an animal is exhilarated, or is suffering from heart diseases that may diminish the quality or length of life.

The heart rate and blood pressure are also inversely connected to each other. This means that when the blood pressure goes up, the heart rate drops; when the blood pressure drops, heart rate goes up. During heart failure, nerve endings that are receptive to variations in blood pressure (known as baroreceptors) erroneously transmit messages informing the brain that blood pressure is too low. This automatically triggers responses that are intended to increase blood pressure e.g. constricting the blood vessels and raising the heart rate. Regrettably, these responses tend to cause damage to the heart.

Heart Sounds and Heart Murmurs In Dogs

Heart sounds are generated by the quick increase and decrease of blood flow rate, and the ensuing cardiac vibrations due to blood circulation. These sounds are audible through a stethoscope. Dogs naturally produce two distinct heart sounds.

Heart murmurs are audible vibrations that are produced by the heart or main blood vessels, and are normally the consequence of disorderly blood flow or vibrations of heart structures (e.g. part of a valve). The accurate description of murmurs is dependent on their timing (whether they take place continuously or sporadically), their intensity (whether they are easily audible or only heard with difficulty), and their location. It is important to note that not every murmur is indicative of a heart disorder. They are commonly diagnosed in puppies who are under 6 months of age.

Abnormal Heart rhythm (Arrhythmias)

Arrhythmias are irregularities detected in the pace, uniformity, or position of heartbeat formation. Just like a heart murmur, an arrhythmia does not automatically point to heart disease. A lot of arrhythmias are functionally inconsequential and do not necessitate treatment. However, there are some arrhythmias that may display severe symptoms such as fainting due to shortage of blood flow to the brain or even sudden death. There are numerous disorders that could be linked to abnormal heart rhythms.


A pulse is the periodic expansion of an artery that is detected using the fingertips during a physical exam. The pulse of a dog is normally felt in the thigh i.e. femoral artery, with normal animals exhibiting a jugular pulse in the neck. A particular type of heart disease or defect can be diagnosed using the pulse, which can either be absent, increased (strong), or decreased (weak).

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