The Hearts Function, Cardiac Cycle and Electrical Activity....

The heart is made of cardiac muscle. It is unique as it never gets tired. The heart contains arteries, veins and capillaries. It's function is to pump oxygenated blood around the body, and de-oxygenated blood back to the lungs.

The heart is divided into two halves by the inter-ventricular septum. There are four chambers to the heart, they consist of the atria which are upper chambers; right atrium (RA) and left atrium (LA). The lower half are the right ventricle (RV) and left ventricle (LV).
It is vital that blood flows in the right direction through the heart which is why it is structured in a precise way.
The tricupsid valve separates the right atrium from the right ventricle. The pulmonic/pulmonary valve separates the right ventricle from the pulmonary artery. The mitral (bicuspid) valve separates the left atrium from the left ventricle.
The aortic valve separates the right ventricle from the ascending aorta.

The structure of the heart walls consists of three layers. The epicardium is a thin outer layer giving the hearts surface a smooth slippery texture. The endocardium is the smooth inner lining of the heart and is continuous with the large blood vessels to which the heart connects. The myocardium makes up the bulk of the heart and is responsible for the pumping action. Strong cardiac muscle fibres make up the myocardium and they are connected by electrical synapses which allow muscle action potentials to spread from fibre to fibre.

There are four main routes for the blood in and out of the heart. These are via the vena cava, pulmonary artery, aorta and pulmonary vein.
The route a blood cell takes is round the cardiac cycle. It starts with a cardiac systole, travels round the cycle and ends with a cardiac diastole. When the myocardium contracts it generates pressure changes resulting in orderly movement of blood. This allows blood to flow from high pressure areas to low pressure areas, only being held up by valves.

When the heart is in atrial systole because it is full, it passes the blood to the ventricular diastole. The atrio-ventricular valves open due to blood pressure allowing 70% of the blood to flow passively down to the ventricles so the atria do not have to contract so much. when the heart is in ventricular systole, because the blood has just been passed down, the atria goes back into diastole. The ventricle walls contract and force the blood out.

The distinct heart beat noise 'lub' comes from the pressure of the blood forcing the atrio-ventricular valves to shut. The pressure of the blood then opens the semi-lunar valves passing blood in to the aorta and pulmonary arteries. The ventricles are now relaxed and have lower pressure than the arteries, causing the now high pressure arteries to shut the semi-lunar valve. The valve shutting creates the second distinct heart beat noise 'dub'. During diastole all the muscles in the heart relax. Blood from the vena cava and pulmonary veins enter the atria starting the cycle again.

Cardiac muscle is myogenic meaning it can contract and relax without receiving signals from nerves. This pattern of contractions controls the regular heartbeat.

The process starts in the sino-atrial node (SAN) which is in the. The SAN sets the rhythm of the heartbeat by sending out regular waves of electrical activity to the atrial walls. This causes the right and left atria to contract at the same time. A band of non-conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles. Instead these waves of electrical activity are transferred from the SAN to the atrioventricular node (AVN). The AVN is responsible for passing the waves of electrical activity to the bundles of His, or AV bundles. There is a slight delay before the AVN reacts, to make sure the ventricles contract after the atria have emptied.

The bundle of His is a group of muscle fibres responsible for conducting the waves of electrical activity to the finer muscle fibres in the right and left ventricle walls called the purkinje fibres. The purkinje fibres carry the waves of electrical activity into the muscular walls of the right and left ventricles, causing them to contract simultaneously from the bottom up.

Bibliography

CG Partnership handout, wk24