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Friday, 17 August 2018

What is the HUMAN HEART and describe their Cardiac Cycle?




The HUMAN HEART and Their Cardiac Cycle


The human heart is a hard-working pump that moves blood through the body. It pumps its entire blood volume about 5 liters every minute; about 8,000 liters of blood move through 96,000 km of blood vessels daily. The heart of an average adult beats about 70 times per minute––more than 100,000 times per day.

Composition

The human heart is composed of cardiac muscle tissue that is called myocardium. The outer protective covering of the heart is composed of fibrous connective tissue called the epicardium. Connective tissue and endothelium form the inside of the heart, the endocardium.  

Part of Heart

The left and right halves of the heart are two separate pumps, each containing two chambers (See fig). In each half, blood first flows into a thin-walled atrium then into a thick-walled ventricle. Valves are between the upper (atria) and lower (ventricles) chambers. The tricuspid valve is presen b/w  right atrium and right ventricle, and the bicuspid valve is between the left atrium and left ventricle. Collectively, these are referred to as the AV valvesatrioventricular valves. The pulmonary semilunar valve is at the exit of the right ventricle, and the aortic semilunar valve is at the exit of the left ventricle.
All of these valves open and close when blood pressure changes when the heart contracts during each heartbeat. The function of valves in veins, heart valves keep blood moving in one direction, preventing backflow.



Cardiac Cycle

The heartbeat is a sequence of muscle contractions and relaxations called the cardiac cycle. A pacemaker a small mass of tissue called the sinoatrial node (SA node) at the entrance to the right atrium, initiates each heartbeat. Because the pacemaker is in the heart, nervous innervation is not necessary. The SA node initiates the cardiac cycle by producing an action potential that spreads along both atria, causing them to contract simultaneously. The action potential then passes to the atrioventricular node (AV node), near the interatrial septum.

From here, the action potential continues through the atrioventricular bundle), at the tip of the interventricular septum. The atrioventricular bundle divides into right and left branches, which are continuous with the Purkinje fibers in the ventricular walls. Stimulation of these fibers causes the ventricles to contract almost simultaneously and eject blood into the pulmonary and systemic circulations.

Process of cardiac cycle

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

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