I have to study, but

I miss you again tonight.

You seem to be dissolved in my blood, cycling through my throbbing head, the lump in my throat, and my freezing toes at a rate of 5.6 liters per every minute which feels like sixty. I can feel my heart forcing you out to fill every artery and feed every inch of my body the oxygen it needs to survive and

I am bursting with you.

There is a similar cycle on both sides of the heart, but the pressures in the right ventricle and pulmonary arteries are less than those in the left ventricle and aorta.

Systole refers to contraction, while diastole refers to relaxation. Both contraction and relaxation can be isometric, when changes in intraventricular pressure occur without a change in length of the muscle fibres.

The cycle starts with depolarisation at the sinoatrial node leading to atrial contraction. Until this time blood flow into the ventricles has been passive, but the atrial contraction increases filling by 20-30%

Ventricular systole causes closure of the atrioventricular valves (1st heart sound), and contraction is isometric until intraventricular pressures are sufficient to open the pulmonary and aortic valves, when the ejection phase begins.

The volume of blood ejected is known as the stroke volume. At the end of this phase ventricular relaxation occurs, and the pulmonary and aortic valves close (2nd heart sound). After isometric relaxation ventricular pressures fall to less than atrial pressures. This leads to opening of the atrioventricular valves and the start of ventricular diastolic filling. The whole cycle then repeats following another impulse from the sinoatrial node.