Cardiovascular System Health Article

Definition

The cardiovascular system includes the heart and the blood vessels and is responsible for the transport of blood throughout the body.

Description

The main components of the cardiovascular system are the heart, arteries, arterioles, capillaries, venules, and veins. Adults have approximately 60,000 miles (96,000 km) of blood vessels. By moving blood throughout this network of vessels, the cardiovascular system supplies all cells of the body with oxygen and nutrients and removes carbon dioxide and other waste products.

The heart

The heart is the focal point of the cardiovascular system. It supplies the driving force for the movement of blood. The heart functions as a pump, actively forcing blood out of its chambers and passively relaxing to allow the next quantity of blood to enter. On refilling, the blood does not get actively sucked into the heart, but moves into the chambers due to the underlying pressure of the cardiovascular system as a whole.

The heart is cone-shaped, pointing down and to the left, and is located left of center of the chest between the lungs. The organ is made of three types of tissue: the myocardium (middle layer), the epicardium (outer layer), and the endocardium (thin inner layer). A fluid-filled sac called the pericardium surrounds the heart, helping to reduce friction during contraction. When the myocardium applies force on the blood by contracting, the cells of the tissue become short and thick. The contraction phase of the myocardium is called systole. This is followed by relaxation of the cells, where they become thinner and longer. The relaxation phase is called diastole.

The heart functions as a double pump, with both the right and left heart having a structure to receive blood and a structure to pump the blood. The blood-receiving structures are called the atria and the blood-pumping structures are called the ventricles. During a heartbeat, the two atria contract together, moving the blood from the atria to the ventricles. Then, while the two atria relax and refill, the two ventricles contract, moving the blood out of the heart. This system means that blood leaves the heart in pulsed waves.

The right atrium and ventricle pump blood from the heart to the lungs using a subset of the blood vessels called the pulmonary circulation system. The blood travels to the lungs where it gives off waste carbon dioxide and receives oxygen, then returns to the left side of the heart to be pumped to the rest of the tissues and organs of the body. The blood vessels that carry blood to the body are called the systemic circulation system. In a healthy heart, blood does not pass directly between the left and right sides of the heart. The two atria are separated by a wall known as the atrial septum, and the wall separating the two ventricles is known as the ventricular septum.

Valves within the heart ensure that the blood travels in the right direction. On the right side of the heart, the tricupsid valve allows blood to travel only from the right atrium to the right ventricle. The mitral valve performs the same function on the left side of the heart. As the blood leaves the right ventricle to go to the lungs, the pulmonary valve controls the direction of the blood flow, while the aortic valve functions between the right ventricle and the aorta, the largest artery.

During diastole, when the ventricles relax, the mitral and tricupsid valves open, allowing blood to flow into the ventricles. At the same time, the aortic and pulmonary valves are closed to prevent reentry of the blood that had been pumped from the heart. During systole, when the ventricles contract, the mitral and tricupsid valves close to prevent backflow, and the aortic and pulmonary valves open to allow the blood to leave the heart. There are no valves at the atrial inputs, part of what ensures consistent blood inflow into the ventricles.

The heart works on an electrical conduction system, as the cells contract in response to electrical signals. All cells of the heart can contract spontaneously, with the beginning of the heartbeat dependent on the cells with the most rapid innate rate. These cells are located in the sinoatrial (SA) node of the heart, sometimes called the heart's natural pacemaker. The electrical signal moves from the SA node to the atrium, in a cluster of conducting cells called the atrio-ventricular (AV) node. The slowing of the signal at this point allows the atria to contract slightly before the ventricles, giving the ventricles more time to fill before they contract. The signal passes on to the electrical network of the ventricles, called the His-Purkinje system, which causes the ventricles to contract. The electrical workings of the heart can be visualized using an electrocardiography unit.

Overall, heart rate is controlled by signals from the autonomous nervous system to the SA node. The autonomic nervous system automatically controls the heart rate as well as many other functions of the body including breathing, blood pressure, and excretion. The system is extremely flexible and can double the heart rate in as fast as three to five seconds.