Explain how the sinoatrial node (SAN) ensures that oxygenated blood enters the aorta.

The sinoatrial node is a region found on the right atrium which acts as a pacemaker. Cells at the SAN can depolarise (become more positively charged with respect to the outside) and generate electrical activity over the atria. This therefore forces oxygenated blood into the left ventricle. This causes the atria to contract (in a process called atrial systole). Following this, the currents pass through the atrioventricular node (AVN), from the atria to the ventricles, through the Purkinje fibres collectively making up what is known as the bundle of His. This causes the ventricles to contract from the apex upwards, forcing oxygenated blood from the left ventricle to the aorta.