HUMAN HEART

Heart is a muscular organ responsible for pumping blood through blood vessels by repeated contractions. The term "cardiac" means "related to the heart". The bulk of the walls of heart chambers is made of cardiac muscles.

In human body, heart is situated between lungs, in the middle of chest cavity (thorax) under breastbone.

PERICARDIUM

Heart is enclosed in a sac known as pericardium. There is a fluid, known as pericardial fluid, between pericardium and heart walls. It reduces friction between pericardium and heart, during heart contractions.

FOUR CHAMMBERS

Human heart consists of four chambers, like the heart of birds and other mammals. The upper thin-walled chambers are called left and right atria (singular 'atrium'), and the lower thick-walled chambers are called left and right ventricles. Left ventricle is the largest and strongest chamber in heart.

DOUBLE PUMP

Human heart works as a double pump. It receives deoxygenated (with less oxygen) blood from body and pumps it to lungs. At the same time, it receives oxygenated (with more oxygen) blood from lungs and pumps it to all body. Inside heart chambers, the deoxygenated and oxygenated bloods are kept separated. Here is a brief description of the circulation of blood inside heart to show its double-pump mechanism.

Right atrium receives deoxygenated blood from body via the main veins i.e. superior and inferior vena cavae. When right atrium contracts it passes the deoxygenated blood to right ventricle. The opening between right atrium and right ventricle is guarded by a valve known as tricuspid valve (because it has 3 flaps). When right ventricle contracts, the blood is passed to pulmonary trunk, which carries blood to lungs. Tricuspid valve prevents the backflow of blood from right ventricle to right atrium. At the base of pulmonary trunk, pulmonary semilunar valve is present which prevents the backflow of blood from pulmonary trunk to right ventricle.

OXYGENERATED BLOOD

The oxygenated blood from lungs is brought by pulmonary veins to left atrium. Left atrium contracts and pumps this blood to left ventricle. The opening between left atrium Both atria a simultaneo contract tog and left ventricle is guarded by a valve known as bicuspid the blood to valve (because it has two flaps). When left ventricle contracts, it pumps the oxygenated blood in aorta, which carries blood to all parts of body (except lungs). Bicuspid valve prevents the backflow of blood from left ventricle to left atrium. At the base of aorta, aortic semilunar valve is present which prevents the backflow of blood from aorta to left ventricle.
Pulmonary and systemic circulation We see that right side of heart collects deoxygenated blood from body and distributes it to lungs while left side collects oxygenated blood from  lungs and distributes it to body. The pathway on which deoxygenated blood is carried from heart to lungs and in return oxygenated blood is carried from lungs to heart is called pulmonary circulation or circuit. Similarly, the pathway on which oxygenated blood is carried from heart to body tissues and in return deoxygenated blood is carried from body tissue to heart is called systemic circulation or circuit.