1.3.7 Describe the cardiac cycle (atrial systole, ventricular systole and diastole) and relate the structure and operation of the mammalian heart to its function, including the major blood vessels.

key idea: maintaining pressure gradient.

- if blood pressure is higher behind a valve the valve opens; if blood pressure is higher in front of the valve the valve closes. this ensures that blood flows in one direction throughout the cardiovascular system.

atrial systole: atria contract, decreasing their volume and increasing their pressure, this forces open the atrioventricular valves due to the pressure difference and pushes blood into the ventricles, there's a slight increase in ventricular pressure and chamber volume as the ventricles receive the ejected blood from the contracting atria. 

ventricular systole: the ventricles contract, decreasing their volume and increasing their pressure, as the pressure becomes higher in the ventricles than in the atria the atrioventricular valves shut, preventing the backflow of blood from the ventricles to the atria. the pressure in the ventricles is also higher than the pressure in the aorta and the pulmonary artery; the semilunar valves force open, and blood is forced out into these arteries.

cardiac diastole: the ventricles and atria relax, the higher pressure in the pulmonary artery and the aorta closes the semilunar valves, preventing backflow of blood from these arteries to the ventricles. blood returns to the heart, the atria refill due to the higher pressure in the pulmonary vein and vena cava, the veins’ pressure then falls below the atrial pressure, opening the atrioventricular valves, so some blood moves passively into the ventricles from the atria. the atria then contract and the cycle begins again.

the structure is related to function:

• the left ventricle pumps blood around the body so it has thicker, more muscular walls that allow it to contract powerfully to pump blood with enough pressure to reach all parts of the body, than the right ventricle, which only needs to pump blood a shorter distance to the lungs (so it has thinner walls).
• the ventricles push blood out of the heart, so they have thicker walls than the atria, who only need to push blood a shorter distance into the ventricles. 
• cords attach the atrioventricular valves to the ventricles to stop them being forced up the atria when the ventricles contract.