The Heart

note: in diagrams, the heart will be drawn as though you are looking at someone's heart i.e. the left side of the heart will be on the right

note: anything pulmonary is lung related

- 4 chambers; 2 upper atria and 2 lower ventricles. Right and left sides are divided by septum.

- Right atrium recieves deoxygenated blood from the body via the vena cavae (superior and inferior)

- Left atrium recieves oxygenated blood from the lungs via the pulmonary vein 

- Right ventricle supplies the pulmonary circulation via the pulmonary artery

- Left ventricle supplies the whole body circulation via the aorta

note: muscle wall of the left ventricle is 3x as thick as the right ventricle

- Blood entering the aorta from the left ventricle is at a higher pressure than blood entering the       pulmonary artery from the right ventricle (3x as thick; contracts more strongly; greater         force/pressure generated)

- Left atrium separated from left ventricle by the bicuspid valve. 

- Right atrium is separated from the right ventricle by the tricuspid valve.

   both are examples of atrio-ventricular valves

- Fibrous cords are attatched to the ventricular side of the valves (chordae tendinae) -> they prevent the valves from turning inside out when ventricles contract; they are taut when ventricles contract and relaxes when the atrio-ventricular valves are open

- At the base of the aorta and pulmonary artery are semi lunar valves -> these prevent the back flow of blood into the ventricles

The Cardiac Cycle - sequence of events in one heart beat (0.8s)

DIASTOLE: (relaxation)

- Atria and ventricles relaxed

- Semi lunar valves closed (initially atrio-ventricular valves closed)

- Blood enters left atrium and right atrium

(Some blood flows into the left and right ventricles)

ATRIAL SYSTOLE: (contraction)

- Atria contract, blood is forced into the ventricles

- Atrio-ventricular valves (bi/tricuspid) closed to prevent back flow into atria

- Semi lunar valves closed

VENTRICULAR SYSTOLE: (contraction)

- Ventricles contract, blood flows into aorta and pulmonary artery

- Semi lunar valves open

- Ventricles contract from the base upwards to force blood up and out of the heart

Heartbeat is myogenic - is self perpetuating (keeps going); each beat is the stimulus for the next

- Stimulus for contraction originates in the right atrium in the sino-atrial node (pace maker)

- The sino-atrial node initiates heartbeat (rate varied by Autonomic Nervous System)

- Na+ diffuse into cells of the sino-atrial node producing a depolarisation (change of charge). A wave of depolarisation then passes across the cardiac muscle fibres of the atria, causing them to contract.

- Impulses produced by the sino-atrial node (SA node) are conducted through the atria, stimulating the atria to contract: atrial systole

- Impulses stimulate the atro-ventricular (AV node) at the base of the atria (AV node delays electrical impulse)

- There is a layer of non-conductile tissue between the atria and ventricles

- A delay of 0.15s in conduction from the SA node to the AV node allows the atrial systole to be complete before ventricular systole begins

- Impulses from the AV node now pass down the bundle of His fibres to the apex (bottom)

- The bungle of His give rise to Purkyne fibres which spread through the ventricles

- Impulses from the Purkyne fibres cause the ventricles to contract from the base upwards - ventricular systole