Around 4000 BC, humans began to domesticate horses, and since then, besides being a great companion, they are being used for different purposes throughout the world. The horse belongs to the family Equidae and subspecies Equus ferus caballus. It has evolved from a small multi-toed creature to a large single-toed animal today. It has a well-developed sense of balance, and a strong fight or flight instinct. A horse has a lifespan of 25 to 30 years, and reaches adult development by the age of five. It is able to sleep, both standing up and lying down. Horses are a grazing animal, and mainly feed on good-quality forage from hay and pasture. There are nearly 300 breeds of horses, which can be differentiated into three categories on the bases of their temperament - cold bloods or draft horses like Belgian, Clydesdale for slow and heavy work; hot bloods or oriental horses like Akhal-Teke, Barb, Arabian Horse for speed and endurance; and warm breeds (cross between cold bloods and hot bloods) like Trakehner, Hanoverian for specific riding purposes. The interaction between humans and horses is not restricted to sport competitions and recreational pursuits only. Horses are also beneficial in a number of other important activities, including police work, entertainment, agriculture, assisted learning, and therapy. Horses have been used for their by-products like milk, meat, hair, bones, hide, as well as the pharmaceutical extracts from the urine of pregnant mares (female horses). The circulatory system of a horse comprises the heart, blood, blood vessels, spleen, and the frog. The following is a brief description of each of these. Heart 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 an adult horse's heart is 3.6 kg or 8.5 lb, though sometimes, it can be more than twice this size. The heart grows till the horse reaches an age of four years, but can also increase slightly with proper conditioning later in its life. Blood and Blood Vessels The blood is formed of erythrocytes (red blood cells), leukocytes (white blood cells), and plasma. Red blood cells are produced in bone marrow, and are responsible for carrying oxygen to tissues and removing carbon dioxide through hemoglobin. 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 weight approximately 450 kg or 990 lb, and contains nearly 34 liters or 9.6 U.S. gal. of blood. The blood performs a number of important functions in a horse's body. 1) Transporting oxygen from the lungs to vital organs. 2) Distributing hormones and antibodies. 3) Transporting lymph fluid from the tissues to the blood. 4) Removal of waste products, such as lactic acid, urea, and dead blood cells. 5) Regulating the horse's body temperature. 6) Transporting white blood cells to a site of infection. The blood vessels which include the veins, arteries and capillaries, are used to transfer blood to and from the heart to the rest of the body of the horse. Arteries transport oxygenated blood from the heart to the body. Veins transport the deoxygenated blood from the body back to the heart. The capillaries join together the veins and the arteries and allow the exchange of oxygen and carbon dioxide molecules through their thin walls. Spleen 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 thus the quantity of oxygen carried to the tissues. Frog 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 its 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. The circulatory system of a horse comprises the heart, blood, and blood vessels. Any disruption in its normal functioning can lead to minor or even major issues. Some of the common disorders that affect the circulatory system include bruises, excessive bleeding, anemia, arrhythmia, heart failure, etc. It is important that a vet is consulted for proper treatment.