AThe average 1,000-pound horse has approximately 45 liters (about 11 gallons) of blood that the heart pumps throughout the body. The main purpose of the cardiovascular system is to pass the blood through the lungs, where it picks up oxygen, then to move it on to all parts of the body, where the oxygen is released and utilized. The heart is the center of this system and functions as a very basic pump with a series of valves.

The average-sized horse has a heart that is approximately the size of a basketball. The horse’s heart is comprised (as in all species) of very dense muscle that differs slightly from the other muscles of the body. The heart is divided into four chambers with four valves separating the various chambers, as well as the entry and exit points. The heart is divided into the "right heart" and the "left heart," with two chambers on each side, one on the top and one on the bottom.

The top chambers are called the atria, and the lower chambers are called the ventricles. Blood that is poor in oxygen travels on the right side of the heart, and oxygen-rich blood (that has traveled through the lungs) travels on the left side of the heart. Blood (oxygen poor) enters the heart from both the front and hind directions relative to the heart. The large cranial (front) vena cava and caudal (hind) vena cava are the blood vessels through which all of the body’s oxygen-depleted blood flows. So, the oxygen-poor blood enters the right atrium from the cranial and caudal vena cava. At the same time that the right atrium is filling with poorly oxygenated blood returning from the body, the left atrium is filling with oxygen- rich blood coming from the lungs