The human heart is a muscular pump. It is the second most important organ in the body, second only to the brain since the brain has a command centre for controlling the heart as well. It pumps about 75 times per minute for a healthy adult. The heart rate tends to be lower in athletes than in the average adult since the hearts of athletes is able to pump blood more efficiently requiring less number of pumps per minute to deliver sufficient nutrients to the whole body.
The heart consists of four chambers, two atria and two ventricles. The ventricles are at the bottom and the atria are at the top. The walls of the atria tends to be thinner than the ventricles since the atria has to pump at a lower pressure than the ventricles. The left ventricles has the thickest walls since it has the biggest burden of pumping blood to the whole body except the lungs.
The blood first enters at right atrium. The right atrial pressure generally tends to be low so that the blood can easily flow from the vena cava and into the right atrium. Having a high right atrial pressure is a sign of heart failure and can lead to systemic oedema. As the right atrium fills up with blood, the pressure of the right atrium will increase and this will open up the atrioventricular valve between the right atrium and ventricle and push blood down through this gap and into the ventricle. The first 2/3 of the blood volume will flow through passively and the last third will be pumped into the ventricle by the atrium.
The left ventricle will then pump the blood into the pulmonary circulation. The blood will return to the left atrium which will then pump it into the left ventricle which will pump to the systemic circulation. The two atria pumps together at the same time. There is also a valve between the left atria and the left ventricle which will open when the pressure in the left atrium has increased. There are semi lunar valves at the aorta and the pulmonary artery to prevent the backflow of blood back into the heart.
The heart sounds consists of a lub and a dub. The lub sounds occurs when the atria contract and the dub sounds occurs when the ventricle contracts. The heart sounds is due to the vibration of the blood on the heart walls when the valves open.
The contraction of the chambers of the heart is by electrical impulses that are generated at the SA node (pacemaker). It will depolarise the atrial muscle fibres and then pass into the ventricular purkinje fibres through the AV node and bundle of His. The Av nodal delay ensures that the atria is able to carry out its function as the primer pump.