The circulatory system of the horse consists of the heart, the blood vessels, and the blood. Anatomy Heart The equine heart is a muscular pump that circulates blood throughout the body. It is more glenoid in shape than the human heart and consists of four chambers: the left and right atria, and the left and right ventricles. The average adult horse has a heart, although it can be more than twice this size. The heart grows until the horse is 4 years of age, although it can increase slightly in size as a response to conditioning. Heart size does not necessarily correlate to the size of the horse. Circulatory capacity is partially determined by functional mass of the heart and spleen. Once the oxygen has entered the bloodstream it must be transported to working muscle and waste products removed. The equine cardiovascular system is hugely compliant with a heart rate range from 20 to 240 beats per minute and a splenic red cell reserve able to double packed cell volume and oxygen delivery during maximal exercise. However, studies on Thoroughbreds have shown that the proportion of skeletal muscle exceeds 50% of body weight, and so the energetic capacity of the muscular system far exceeds the capacity of the cardiovascular system to deliver oxygen. Blood and blood vessels Blood is made up of red blood cells (erythrocytes) and white blood cells (leukocytes), as well as plasma. Produced in bone marrow, red blood cells are responsible for carrying oxygen to tissue and removing carbon dioxide, all via hemoglobin. White blood cells are used for defense against pathogens in the immune system. Plasma suspends the blood cells, contains clotting factors, and contributes to the greatest volume of blood. The heart and blood vessels contain approximately of blood in a horse, which equates to about 76 mL/kg (1.2 oz/lb). Spleen The spleen removes damaged red blood cells from circulation. It also holds extra blood cells, releasing them during exertion to increase blood volume and the amount of oxygen transported to tissues. Frog The horse hoof contains a structural component known as the "frog", which covers the deeper structure of the hoof known as the digital cushion, a vessel-filled tissue. When the horse places weight on a leg, the ground pushes upward on the frog, compressing it and the underlying digital cushion. This results in squeezing blood out of the digital cushion, which then helps