Don’t Skip A Beat: A Feline’s Circulatory System

A feline’s circulatory system, or cardiovascular system, is responsible for circulating blood throughout the body. Its role is to transport oxygen, nutritive substances, immune substances, hormones, and chemicals to the tissues and organs of the body necessary for normal function. It also carries away waste products and carbon dioxide, helps to regulate body temperature, and helps to maintain normal water and electrolyte balance.

The circulatory system consists of the heart and blood vessels, namely arteries, veins, and capillaries. The blood vessels leave the heart and form a conduit system throughout the body, carrying blood and other organic materials to all organs, tissues, and cells.

The Heart

the_blood_flow_of_the_human_heartLike in all other mammals, the heart is located in the chest between the right and left lungs, contained in a very thin sac called the pericardial sac. The sac allows the heart to retain its shape and stabilize its position. It extends approximately from the 3rd and to the 4th ribs. It is the central organ that contracts rhythmically until the cat’s death to pump blood continuously through the blood vessels.

The heart consists of four chambers:

The right atrium is the collecting chamber for blood from distant parts of the body. Blood is carried back to this upper right chamber of the heart in various veins. The oxygen levels in the blood in this chamber are very low. As the right atrium contracts, blood flows through the tricuspid valve into the right ventricle.

The right ventricle is the pumping chamber of the lower right heart. As it contracts, the right ventricle sends blood it receives from the right atrium into the pulmonary artery.

The left atrium is a collecting chamber that sends oxygenated blood to the left ventricle. The mitral valve separates the left atrium to the left ventricle.

The left ventricle is the biggest pumping chamber of the heart as it is responsible for pumping oxygen-rich blood to the rest of the body. Due to the distance, the walls of the left ventricle are three times thicker than those of the right ventricle.

A muscular wall called the septum separates the left side of the heart from the right side of the heart.

Blood Vessels

The largest of the blood vessels are arteries, strong, muscular blood vessels that carry oxygen-rich blood from the heart to various parts of the body. Small blood vessels that branch off the arteries are called arterioles.

The main arteries are:

  • The pulmonary arteries, which transports the blood to the lungs
  • The coronary arteries, which transport oxygen-rich blood back to the heart
  • The aorta, which transports oxygen-rich blood to the rest of the body

e5ac71048844c421c559df654de25ba3Veins are usually much thinner, carrying blood from various parts of the body back towards the heart. Due to their thin walls, the veins are very compliant, with their volume and size varying with blood pressure. Small blood vessels that lead from the capillaries to the larger veins are called venules.

They also contain valves, which allow blood to flow in only one direction, towards the heart, and preventing leaking, or blood flowing backwards towards the organs.

The main veins are:

  • Superior and inferior vena cavas,the major veins that brings blood to the heart
  • The pulmonary veins, which transport oxygen-rich blood from the lungs back to the heart. Despite carrying oxygen-rich blood, it is still considered a vein due to carrying blood back to the heart.

Capillaries are the smallest of all blood vessels. These blood vessels are so small that in many instances, only a few red blood cells can pass at a time. They usually lie between the arterioles and venules. The walls act as a membrane that allows various substances, like oxygen, carbon dioxide, water, electrolytes, nutrients, and minerals, to travel between the blood and the tissues. The capillaries are the site of the greatest exchange of material between the blood and the tissues of the body.

Blood Pathway

Blood from the body returns through the veins, the vena cavas and the pulmonary veins. Once they reach the heart, the blood enters the right atrium, passes the tricuspid valve, and flows into the right ventricle. When the right ventricle contracts, the blood is pushed into the pulmonary artery. At the opening of the pulmonary artery is the pulmonary valve, preventing blood from moving backwards into the right ventricle.

From there, the blood is carried into the lungs, where the red blood cells can get of carbon dioxide and oxygenate themselves. The carbon dioxide leaves the body during expiration, the action of breathing out, and oxygen is taken in during inspiration, the action of breathing in.


Once the oxygenated blood cells pass through the lungs, it reenters the heart through the pulmonary veins arteries to the left atrium, passes through the mitral valve, and into the left ventricle. When the left ventricle contracts, blood is transferred through the aorta to circulate throughout the rest of the body.

This entire cycle, which continues without pause, is regulated by the sinoatrial node. It acts as a pacemaker, sending out electrical impulses that trigger the heart muscle to contract. It is rare for cats to have problems with the sinoatrial node.

Blood Type

Unlike humans, cats only have three known primary blood types: A, B, and AB. Among these, type A is the most common. In fact, roughly 99% of all domesticated short hair cats in the world have type A blood.

Type B blood is found primarily in purebred cat breeds. The breeds with the highest incidence of type B blood are British Shorthair, Himalayan, Persian, Devon Rex, Cornish Rex, and Sphynx breeds.

Both blood types have a natural antibody against the other so these blood types cannot be successfully mixed in a transfusion.

The third type of blood occurring in cats is type AB, which is exceeding rare. AB blood cats are universal receives as a result of it not containing antibodies against either of the other two blood types. Therefore, it may receive blood transfusions from all other cats.

There is no universal donor blood type in cats.


Heart or Blood Diseases

  • Hypertrophic cardiomyopathy: It is a disease where the heart muscle becomes thickened. It most often affects the muscles of the left ventricle and septum of the heart. As the walls increase in size, the size of the heart chambers become smaller, which reduces the amount of blood that could flow through the heart.
  • Dilated cardiomyopathy: It is a disease where the heart muscle becomes weak and flabby. As the muscles weaken, the chambers of the heart enlarge in size, becoming dilated. It primarily affects the left side of the heart, decreasing its ability to pump blood to the rest of the body.
  • Heart failure: It is the inability of the heart to maintain a circulation sufficient to meet the body’s needs. It may arise for several reasons, including: severe forms of cardiomyopathy, inflammation of the heart, secondary to pericardial diseases, and tumors.
  • Arrhythmias: They are disturbances in heart rate or rhythm. When the electrical activity of the heart is altered, it greatly affects the ability of the heart to coordinate the contractions of its different chambers. It may be mild and insignificant or serious enough to cause heart failure and sudden death. Typically develops with other diseases in the body.
  • Infectious endocarditis: It is an inflammation of the heart caused by some sort of infectious agent, such as bacteria, protozoa, and viruses. Uncommon in the cat, but may arise with bacterial infections or infections with toxoplasmosis.
  • Valvular disease: Various diseases affect the values and alter the normal function of the valves. Congenital valvular defects are uncommon in cats, but pulmonic stenosis (narrowing of the pulmonary valve) and malformation of the tricuspid and mitral valves may occur. Acquired valvular disease might occur and usually causes the valves to leak. As the chambers of the heart contract, blood may leak backward through an abnormal valve.
  • Pericarditis and pericardial effusion:  It is the inflammation of the pericardium, the fibrous sac that encloses the heart. Pericardial effusion is the accumulation of fluid within the pericardial sac. As fluid accumulates, it applies pressure to the heart, decreasing the ability of the heart to pump blood.
  • Heartworm: It is caused by a parasite that enters the body through the bite of an infected mosquito. Adult worms prefer to live in the pulmonary vessels that lead from the right hurt to the lungs. Uncommon in cats, difficult to diagnose, and sometimes only found after death.
  • Thromboembolism: A thrombus is a blood clot that develops within the heart or a blood vessel. An embolus is a blood clot that arises in one area of the circulatory system, transported to a distant site where it become lodged in a blood vessel.
  • Arteritis or vasculitis: It is an inflammation of arteries or veins that may be caused by viruses, bacteria, parasites, or immune-mediated diseases. This type of inflammation is rare in cats.

References + For More Reading

Structure and Function of the Cardiovascular System in Cats

Cat Health: Cardiovascular

Feline Cardiovascular System

Cat Blood Types Explained

Feline Blood Groups and Blood Incompatibility

Great Vessels of the Heart

Featured Image: Jennifer Fairman, “Feline Circulatory System”, Client: Veterinary Learning Systems, Icon Custom Communications, © 2004 Fairman Studios, LLC

For Others In This Series

Getting On A Cat’s Nerves: A Look Into the Feline Nervous System

The Muscles Behind the Cat

Feline Skeletal Similarities From Head to Tail


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