The heart is a muscular organ roughly the size of a closed fist located towards the left side of the chest. It is responsible for pumping blood continuously throughout the life of a human. It supplies the body with oxygenated blood, hormones and other necessary substances. It also carries away metabolic waste products from the cells of the body such as carbon dioxide which is transported to the lungs where it can be exchanged for oxygen.
The human heart has 4 chambers; Two upper ones called Atria and two lower ones called Ventricles.
The heart receives blood into the right atrium and from there it passes into the right ventricle. From the right ventricle, it gets pumped to the lungs where the blood gets oxygenated and carbon dioxide is expelled. The oxygen-rich blood then comes back to the heart into the left atrium, passes through to the right ventricle from where it gets pumped out into the aorta and the rest of the body. Generally, the ventricles have thicker muscle tissue surrounding them than the atria since they need to create a higher pressure to move blood into bigger circulatory systems. Similarly, the left ventricle wall is thicker than the right one since the pressure needed to pump blood out into the aorta is greater than what's needed for the blood to reach the lungs in the pulmonary circulatory system.
Since the heart has to constantly pump for us to stay alive it is only logical that it consists of a myriad of muscle fibers. The heart muscle, myocardium, is composed of striated muscle tissue formed together into fibers branching in different directions.