Sunday, 13 June 2010

Structure of the heart and the cardiac cycle

The heart is a muscular pump that pushes blood around the body. It is behind the breastbone and between the ribs and it is made up of cardiac muscle. It is a muscular cone shaped organ which is about the size of a clenched fist.
It is divided into sections. Right and left sections which are also divided into upper and lower compartments which are the atria and ventricles.
The heart structure has three layers. Epicardium which is a thin outer layer which gives the hearts surface a smooth, slippery texture. Endocardium is the smooth inner lining of the heart and it is continuous with the large blood vessels which the heart is connected too. Myocardium is the biggest part of the heart is responsible for pumping blood. The myocardium is made of strong cardiac muscle fibres which are connected by electrical synapses which allow muscle action potentials to spread from fibre to fibre.
The upper chambers (the atria) receive blood that is returning to the heart. The lower chambers (the ventricles) eject blood into your arteries to your lungs. The average human heart beats about 72 beats per minute and weighs around 9-11oz in females and 11-12oz in males.
Blood flows from the heart in one direction from the Atria to the ventricles. Blood is prevented from flowing backwards by the triscupid, mitral, aortic and pulmonary valves.
The function of the right side of the heart is to collect deoxygenated blood into the right atrium from the body and pump it through the right ventricle into the lungs so that carbon dioxide can be dropped off and oxygen can be picked up.
The left side collects oxygenated blood from the lungs into the left atrium. From the left atrium the blood moves to the left ventricle which pumps it out to the body via the aorta.
The cardiac cycle is what happens when the heart beats. There are two phases, the diastole phase where the heart ventricles are relaxed and the heart fills with blood and the systole phase where the ventricles contract and pumps blood to the arteries. In the diastole phase the atrioventricular valves are open. Deoxygenated blood flows into the right atrium. The open atrioventricular valves allow blood to pass through to the ventricles. The sino atrial (sa) node contracts triggering the atria to contract. The right atrium empties its contents into the right ventricle. The tricuspid valve prevents the blood from flowing back into the right atrium.
During the systole phase, the right ventricle receives impulses and contracts. The atrioventricular valves close and the semi lunar valves open. The deoxygenated blood is pumped into the pulmonary artery. The pulmonary valve prevents the blood from flowing back into the right ventricle. The pulmonary artery carries blood to the lungs. The blood picks up oxygen and is returned to the left atrium.
In the next diastole period the semi lunar valves close and the atrioventricular valves open. Blood from the pulmonary veins fills the left atrium. The sa node contracts again and the left atrium empties into the left ventricle.
In the next systole period the atrioventricular valves close and the semi lunar valves open. The left ventricle receives impulses and contracts. Oxygenated blood is pumped into the aorta. The aorta branches out to provide oxygenated blood to all parts of the body. The oxygen depleted blood is returned to the heart. One cardiac cycle is completed when the heart fills with blood and is then pumped from the heart. The heart beat sounds we hear are the closing of the valves, so this shows us how quickly our hearts are working inside of us.

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