Structure of Human Heart


Human heart moves blood into the body. It pumps its entire blood volume about five litres every minute; about 8000 litres of blood move through 96000 km; of blood vessels every day.

The heart of average adult beats about 70 times per minute. Most of the human heart is composed of cardiac muscle tissue called myocardium (myo=muscle). Outer protective covering of heart is fibrous connective tissue called pericardium. Connective tissue and endothelium from the inside of the heart, the endocardium.
Left and right halves of heart are two separate pumps each containing two chambers. In each half blood first flows into a thin walled atrium then into thick walled ventricle.

Valves:
Tricuspid valve is between right atrium and right ventricle. Bicuspid valve is between left atrium and left ventricle. They are collectively called Atrio-vascular values. Pulmonary semilunar valve is at the exit of right ventricle. The aortic semilunar valve is at the exit of left ventricle (collectively these are referred to as semilunar valves). All of these valves open and close due to blood pressure changes when the heart contracts during each heart beat. Heart valves prevent backflow of blood.

Heart cycle:
The heart beat is a sequence of muscle contractions and relaxation called cardiac cycle.
A ‘pacemaker’, a small mass of tissue called sinoatrial node (SA node) at the entrance to the right atrium initiates each heart beat. The SA node initiates the cardiac cycle by producing an action potential that spreads over both atria, causing them to contract simultaneously and eject blood into the pulmonary and systemic circulations. The action potential moving over the surface of the heart causes current flow which can be recorded. 

During each cycle the atria and ventricles go through a phase of contraction called systole and a phase of relaxation called diastole. Specifically while the atria are relaxing and filling with blood, the ventricles are also relaxed. As more and more blood accumulates in the atria, blood pressure rises and the atria contract, forcing AV values open and causing blood to rush into the ventricles. When the ventricles contract the AV values close and the semilunar values open, allowing blood to be pumped into the pulmonary arteries and aorta. After the blood has been ejected from the ventricles they relax and start the cycle again.

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