Understanding the Heart: Anatomy, Function, and Circulation
The heart is the engine that drives the blood. It is a hollow, tapered organ located in the thoracic cavity between the lungs. The heart is wrapped by a membrane called the pericardium, and its walls are formed by the heart muscle, or myocardium, which enables the heart to contract and relax.
Heart Structure
The heart is divided into two halves, separated by a partition. Each half has two chambers: an atrium and a ventricle. Each atrium communicates only with its corresponding ventricle. The mitral and tricuspid valves allow blood flow from the atria to the ventricles and prevent backflow in the opposite direction. At the beginning of the pulmonary artery and aorta are semilunar valves, which prevent backflow of blood into the ventricles.
How the Heart Works
The myocardium contracts and relaxes. The period of contraction is called systole, and the period of relaxation is called diastole. A cardiac cycle has several stages:
- Ventricular Diastole: Venous blood fills the atria and ventricles passively. In this phase, the mitral and tricuspid valves are open, and the pulmonary and aortic valves are closed.
- Atrial Systole: Sends extra blood to the ventricles.
- After atrial systole, the mitral and tricuspid valves close.
- Ventricular Systole: Causes the opening of the aortic and pulmonary valves and pumps blood into the arteries.
- The ventricles relax, the aortic and pulmonary valves shut, the mitral and tricuspid valves open, and blood enters the atria and ventricles. This marks the beginning of a new cardiac cycle.
Blood Circulation
The blood, driven by the heart in systole, circulates through the arteries, capillaries, and veins. The elasticity of the aorta and pulmonary arteries allows the discontinuous flow produced by the heart to be transformed into a continuous flow. The arteries are exits from the ventricles, while the veins are pathways to the atria.
The circulation is double because blood travels throughout the body via two circuits connected in series: the pulmonary circuit and the systemic circuit.
Pulmonary Circulation
The pulmonary circulation is located between the heart and lungs. Its function is to effect the exchange of respiratory gases, oxygen, and carbon dioxide. It begins with the contraction of the right ventricle, which sends blood to the pulmonary artery. The artery divides into two branches, which penetrate the lungs and branch to form capillaries. The oxygenated blood returns through the pulmonary veins to the left atrium, which is relaxed.
Systemic Circulation
The systemic circulation is located between the heart and the rest of the body. It carries oxygen and nutrients to all tissues and removes wastes produced by the cells. It begins with the contraction of the left ventricle, which pushes oxygenated blood from the pulmonary circulation into the aorta. Because the left ventricle has to overcome much resistance, its wall is thicker than the right ventricle’s. The blood circulates through the arteries and arterioles throughout the body, facilitating capillary exchange. The capillary flow enters the veins that carry blood back to the heart. This blood, deoxygenated and full of carbon dioxide, from the vena cava enters the right atrium.