According to merriam-webster.com, the heart is “a hollow muscular organ of vertebrate animals that by its rhythmic contraction acts as a force pump maintaining the circulation of the blood.” The heart is an important part of the circulatory system, aside from the blood and the blood vessels. These structural circulatory elements work together to convey the oxygen and the nutrients to the other body parts. It also carries the unnecessary byproducts, like the carbon dioxide, out of the body. Heart has the major role to make the circulation of blood around the body possible.
Heart is an involuntary cardiac muscle which is as big as a clenched fist. It lies in the chest cavity between two lungs. It is surrounded by a fluid-filled sac called pericardium that prevents friction when the heart beats. A human heart has four chambers; the upper two chambers are called the left and the right atria, and the lower two chambers are called the left and right ventricles. The inner surfaces of the heart’s chambers are lined with the muscle called endothelium. There are four valves inside the heart: the left and the right atrioventricular valves, and the left and right semilunar valves.
The left and right atria are also called auricles. These atria receive the blood from veins and blood vessels. The left and right ventricles push the blood into the arteries and blood vessels, away from the heart.
The four valves prevent the blood from flowing back into the heart. The atrioventricular valves are located between the atria and ventricles. The left atrioventricular valve has two flaps, which is why it is called the bicuspid or mitral valve. The right atrioventricular valve has three flaps, which is why it is called the tricuspid valve. The semilunar valves have three half-moon-shaped flaps. The semilunar valve located between the left ventricle and aorta is called the aortic valve. The semilunar valve located between the right ventricle and the pulmonary artery is called the pulmonary valve.
The heart’s continuous cycle and heart beat are called the cardiac cycle. The heart muscle cells can create impulses that cause the heart to contract with a consistent beat. The heart beat starts from the sinoatrial node. The sinoatrial node (SA) is located in the upper right corner of the right atrium. The cells in this area create impulses more often compared to the other heart cells located in other areas-the reason the SA node is regarded as the heart’s pacemaker. The cells in the SA node generate electrical impulses that spread throughout the atria. This will cause the atria to contract as they push the blood down to the ventricles. Before the blood reaches the ventricles, the atrioventricular node (AV) slows the impulse, giving the atria time to contract before the ventricles do. The pathway of fibers called the Purkinje fibers causes the ventricles to contract as they push the blood away from the heart. The cycle then repeats.
The cardiac cycle has two phases: the systole (the heart contracts) and the diastole (the heart is relaxed). The systole begins when the atria contract as the blood fills the ventricles. The ventricles contract as they push the blood out the semilunar valves and arteries. This time, the atrioventricular valves are closed to prevent the blood from going back to the atria. The pressure rises in the arteries, making the semilunar valves shut to prevent the blood from going back to the ventricles. The diastole begins when the atria relax, followed by the ventricles; this is also the time when the atrioventricular valves are open.