Many species fall into the class of vertebrates to include fish, reptiles, mammals, and birds. The main function of the heart is to pump blood throughout the body, supplying the body with oxygen and removing carbon dioxide and other waste products. While, the heart functions similarly among vertebrates it is structurally different, particularly in the number of chambers found in it. Vertebrates are distinguished by their multi chambered hearts can be categorized depending on the number of chambers (Bettex, 2014). Two chambered hearts are found in fish, three chambered hearts are found in reptiles and amphibians, and four chambered hearts are found in birds and mammals. The number of chambers determine how efficiently blood rich in oxygen is separated from oxygen deficient blood. Fish generally have two chambered hearts, which consist of one atrium and one ventricle separated by the rudimentary valve. Fish breath by taking in water through their mouth and out through their gill. The conus arteriosus function in the same way as the aorta found in higher level vertebrates. Large capillary networks allow oxygen to diffuse from the water into their circulatory system. The circulation of oxygen is quite simplistic, blood is pumped from the ventricle through the conus arteriosus to the gills. At the gills the blood is oxygenated and carbon dioxide is expelled, once the blood is oxygenated it flows to through the atrium via the sinus venosus and the cycle starts over again (Icardo, 2016).Most reptiles, excluding crocodiles have a three-chambered heart. Their heart consists of two atria that are separated completely by an interatrial septum and two ventricles that are partially separated. The pathway of blood through the heart starts with oxygenated blood flowing from the lungs through the pulmonary veins and into the left atrium, into the left ventricle. Simultaneously, deoxygenated blood travel into the heart through the sinus venosus, and enter the right atrium. Mixing of oxygenated and deoxygenated blood is minimized before ventricular ejection through differential in pressure that occur during beating of the heart (Stephenson, 2017). A reptile’s heart is not as efficient as a four-chambered system found in a mammal, which keeps the two circulations completely separate.Mammals on the other hand have a four-chambered heart that contains two atria and two ventricles. This four-chambered heart is the power house for the circulatory system in mammals. Deoxygenated blood enters the heart through the inferior and superior vena cava where it then flows into the right atrium. It then passes through the tricuspid valve into the right ventricle, from the right ventricle blood is expelled into the pulmonary circulatory system. In the lungs, the deoxygenated blood becomes oxygenated circulated back to the heart by means of the pulmonary veins. Oxygenated blood flow into the left atrium, through the mitral valve into the left ventricle where it is circulated out of the heart via the aortic arch to the rest of the body throughout the body (Holmes, 1975). The number one advantage to having four chambers of the heart are that there is no mixing of deoxygenated blood with oxygenated, enabling mammals to efficiently acquire oxygen from the blood. The first appearance of the heart or a structure that resembles the heart appeared 500 million years ago. Over millions of years the heart has evolved from a single layered tube, to a muscular powerful four-chambered pump responsible for supplying the body with oxygen and removing carbon dioxide as well as other waste products (Olson, 2006). Without this vial organ life would cease to exist.