The circulatory system is composed of heart and blood vessels including arteries, veins, and capillaries. The heart is the key organ in the circulatory system. As a muscular organ, its main function is to propel the blood throughout the body. It supplies oxygen and nutrients to the body tissues and removes carbon dioxide and other wastes.

Basic anatomy of the human heart

The human heart lies in between the lungs and to the left of-of the middle of the chest cavity.

The human heart is roughly the size of a fist and weighs about 280-340g in men and about 230-280g in women.

The heart has four chambers, two atria, and two ventricles. The right atria and right ventricles from the right heart while the left atria and ventricles from the left heart. The right and left heart are separated by muscular septa.

The four chambers are enclosed in a double-walled sac, called the pericardium. Between the outer parietal pericardium and inner serous pericardium flows the pericardial fluid. The pericardial fluid acts as a lubricant during the heart contractions and movement of lung and diaphragm.

The heart wall comprises three layers – outermost pericardium, middle myocardium, and innermost endocardium. The myocardium is the muscular layer that contracts while the endocardium is the lining in contact with blood.

The chambers are separated from each other by valves. The tricuspid and mitral valve connect the atria and ventricles. The pulmonary semilunar valves separate the right ventricle from the pulmonary artery and aortic valve separates the left ventricle from the aorta.

Blood vessels

Arteries and veins from the blood vessels. Blood vessels comprise of three layers – endothelium, providing the smooth lining for blood to flow over; tunica media, middle layer and made up of muscle elastic tissue and the tunica adventitia, tough covering for protection.

Arteries carry blood from the heart to the tissues. They are the thickest blood vessels with muscular walls that contract to keep blood flowing away from the heart.

Veins carry blood to the heart. They are not as muscular as arteries but contain valves that prevent the backflow of blood. They are thinner and flexible.

The two largest veins in the body are – superior and inferior vena cava, depending on their location above and below the heart, respectively.

The arteries and veins are connected by a network of tiny capillaries. They are important as the transport occurs across them.

Circulation of blood

The basic circulation takes place through two pathways –

  1. Pulmonary circuit
  2. Systemic circuit

Pulmonary circuit – the deoxygenated blood leaves the right ventricle via the pulmonary artery and travels to the lungs and returns as oxygenated blood to the left atrium via the pulmonary vein.

Systemic circuit – the oxygenated blood leaves the heart via the aorta from the left ventricle. Then it branches out in arteries and capillaries to reach the tissues and supply oxygen. Then the deoxygenated blood via the vena cava enters the right atrium.

Since the heart is part of the body, it also needs blood supply to meet its demands. For the blood supply of heart, there are coronary arteries.

Contraction and relaxation of the heart

One complete heartbeat makes up one cardiac cycle, which consists of systole and diastole.

In the early diastole, the heart is relaxed. Then the atrium contracts to push blood into ventricles (ATRIAL SYSTOLE).

The ventricles contract (VENTRICULAR SYSTOLE), sending the blood to pulmonary and systemic circulation.

Then the ventricles stop contractions and relax (DIASTOLE).

This leads to a repeat of the events and completion of one heartbeat.

The heart has a unique electrical conduction system which causes it to beat in regular rhythm. The sinoatrial node (SA NODE) is a tissue in the right atrium that sends out electrical signals to start contractions. This is called as the pacemaker of the heart as it maintains the rhythm of contractions.

Depending on the body’s needs, they increase the rate of contractions. For e.g. during exercise or in a frightening situation, the heart pumps faster to increase the delivery of oxygen.

Heart sounds

The two major heart sounds – lub and dub.

These heart sounds are produced as a result of the contractions and relaxation of the chambers during cardiac cycle.

During the ventricular contraction, the atrioventricular valves close to prevent the backflow into atria and produce the first heart sound – lub.

When the ventricles finish contracting, the pulmonary and aortic valves close to prevent backflow into ventricles and thus producing the second heart sound –dub.

Various other sounds can be heard in different physiological and pathological conditions.

The human heart is an important organ which has a vital role sustaining the life of an individual.

also read, Human Digestive System

by Taniya

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