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The Circulatory System of a Horse

Veins and arteries of the horse circulatory system
The circulatory system of a horse comprises the heart, blood, blood vessels, spleen and the frog.
Horse Heart Circulation
A horse's heart is made up of muscle tissues and is more rounded in shape as compared to the human heart. It pumps blood throughout the body and is divided into four chambers: the left and right atria, and the left and right ventricles. The average weight of a horse's heart is 3.6 kg or 8.5 lb, though sometimes it can be more than twice this size. The heart of a horse grows till the horse reaches an age of four years, but can also increase with proper conditioning.
Horse circulatory system
Blood and Blood Vessels
The blood is formed of erythrocytes (red blood cells), leukocytes (white blood cells) and the plasma. The red blood cells are produced in bone marrow and are responsible for carrying oxygen to tissues and removing carbon dioxide through hemoglobin. The white blood cells are present in the body's immune system and protect against pathogens and foreign infectious materials. The plasma contributes to the blood volume and suspends the blood cells which contain clotting factors. The heart and blood vessels of an adult horse contain nearly 34 liters of blood.
The spleen eliminates damaged red blood cells from circulation and holds extra blood cells. It releases these blood cells during exertion to increase blood volume and the quantity of oxygen carried to the tissues.
Each foot or hoof of a horse contains a structural component known as the frog. This frog covers the deeper structure of the foot called the digital cushion which is a vessel-filled tissue. When a horse puts weight on his leg, the ground pushes upward on the frog and compresses it along with the underlying digital cushion. This squeezes the blood out of the digital cushion and pumps it back up the leg which helps the heart to function against the gravity.