The cardiovascular system includes the heart and the blood vessels (the veins and the arteries). The function of the heart is to pump blood. The right side of the heart pumps blood to the lungs, where oxygen is added to the blood and carbon dioxide is removed from it. The left side pumps blood to the rest of the body, where oxygen and nutrients are delivered to tissues, and waste products (including carbon dioxide) are transferred to the blood for removal by other organs (such as the lungs and kidneys). The heart is a hollow, muscular organ which, in mammals and birds, is divided into 4 chambers. The muscular tissue is called the myocardium. There are upper chambers on both the left and ride sides of the heart called the left and the right atria (the plural form of atrium). There are also 2 lower chambers called the left and right ventricles.
A series of valves keep blood flowing in one direction through the heart. The atrioventricular valves are valves between the atria and the ventricles. The semilunar valves are valves between the heart and the aorta and between the heart and the pulmonary artery. Each ventricle has an inlet and an outlet valve. In the left ventricle, the inlet valve is called the mitral valve, and the outlet valve is called the aortic valve. In the right ventricle, the inlet valve is called the tricuspid valve, and the outlet valve is called the pulmonary valve.
Blood from the body flows through the 2 largest veins, called the venae cavae, into the right atrium. When the right ventricle relaxes, blood in the right atrium pours through the tricuspid valve into the right ventricle. When the right ventricle is nearly full, the right atrium contracts, pushing additional blood into the right ventricle. The right ventricle then contracts, pushing blood through the pulmonary valve into the pulmonary arteries, which lead to the lungs. In the lungs, blood absorbs oxygen and gives up carbon dioxide. The blood then flows through the pulmonary veins into the left atrium. When the left ventricle relaxes, the blood in the left atrium pours through the mitral valve into the left ventricle. When the left ventricle is nearly full, the left atrium contracts, pushing additional blood into the left ventricle. The left ventricle then contracts, pushing blood through the aortic valve into the aorta, the largest artery in the body. This blood carries oxygen to all of the body except to the lungs.
Each heartbeat consists of 2 parts: diastole and systole. The first half of a heartbeat is the sound of the mitral and tricuspid valves closing. The second half is the sound of the aortic and pulmonary valves closing. During diastole, the ventricles relax and fill with blood. During systole, they contract and pump blood out to the body.
The rate and force of contraction of the heart and the degree of narrowing or widening of blood vessels are controlled by different hormones and by the autonomic nervous system (the part of the nervous system that controls involuntary activity).
The heart beats because of a tiny electrical current that originates in the heart's pacemaker called the sinoatrial node. Rhythmic electrical impulses or discharges cause the contraction of muscle fibers in the heart. While an animal is at rest, the sinoatrial node discharges many times each minute: about 15 times per minute in the horse, more than 200 times per minute in the cat, and 60 to 160 times per minute in the dog. In general, the larger the species, the slower the rate of sinoatrial node discharge and the slower the heart rate.
In quiet, healthy dogs, the heart rate is usually irregular. It increases while breathing in and decreases while breathing out. This is called respiratory sinus arrhythmia. A change in heart rate coinciding with breathing in and out is a good indicator of health. This change in heart rate usually does not occur during excitement or during heart diseases that may reduce the quality or duration of life.
Heart rate is also inversely related to blood pressure. When blood pressure increases, heart rate decreases; when blood pressure decreases, heart rate increases. In heart failure, nerve endings that are sensitive to blood pressure changes, called baroreceptors, mistakenly report to the brain that blood pressure is too low and begin mechanisms (such as narrowing the blood vessels and increasing heart rate) that are designed to increase blood pressure. Unfortunately, these mechanisms also injure the heart.
Heart Sounds and Murmurs
Heart sounds are produced by the rapid acceleration and deceleration of blood and the resulting vibrations in the heart due to the circulation of blood. They can be heard using a stethoscope. In dogs, 2 heart sounds can normally be distinguished.
Heart murmurs are vibrations that can be heard coming from the heart or major blood vessels and generally are the result of turbulent blood flow or vibrations of heart structures such as part of a valve. Murmurs are typically described by their timing (that is, whether they occur continuously or only intermittently), their intensity (that is, whether they can be heard easily or with difficulty), and their location. Not every murmur indicates a heart disorder; for example, murmurs are commonly detected in puppies less than 6 months of age.
Arrhythmias are abnormalities of the rate, regularity, or site of heartbeat formation. An arrhythmia does not necessarily indicate heart disease. Many arrhythmias are functionally insignificant and require no specific treatment. Some arrhythmias, however, may cause severe signs such as loss of consciousness due to lack of blood flow to the brain or lead to sudden death. Many disorders are associated with abnormal heart rhythms.
A pulse is the rhythmic expansion of an artery that can be felt with the fingertips during physical examination. In dogs, pulses are typically felt at the femoral artery (in the thigh). A jugular pulse in the neck can be noted in normal animals. A pulse may be absent, increased (strong), or decreased (weak)—each of which may indicate a specific type of heart disease or defect.
Last full review/revision July 2011 by Davin Borde, DVM, DACVIM; Clay A. Calvert, DVM, DACVIM; Benjamin J. Darien, DVM, MS, DACVIM; Jorge Guerrero, DVM, PhD, DEVPC (Ret); Michelle Wall, DVM, DACVIM