The Cardiac Cycle

Pulmonary Artery  - Carries deoxygenated blood from right side of heart to lungs Pulmonary Vein - Carries oxygenated blood from lungs to left side of heart Vena Cava - Carries deoxygenated blood from body (not lungs) to right side of heart Aorta - Carries oxygenated blood from left side of heart to body (not lungs)

HOW DOES THE BLOOD MOVE THROUGH THE HEART? (Looking at the left side)1. Blood enters the left atrium from the pulmonary vein. As the atrium fills, its pressure increases 2. The increase in pressure causes the bicuspid (atrioventricular) valve to open and blood begins to trickle through to the ventricle.3. The atrium enters systole (it contracts), and all the blood is pushed through into the ventricle.4. The ventricle contracts, increasing the pressure. Pressure is now higher in the ventricle than in the atrium and so the atrioventricular valve snaps shut to prevent back-flow into the atrium. 5. Pressure is now higher in the ventricle than in the aorta, and so the semi-lunar valve opens and the contraction forces blood into the artery. 6. The atrium and the ventricle enter diastole (they relax). The pressure is now higher in the aorta than in the ventricle, so the semi-lunar valve snaps shut to prevent the blood from flowing back into the ventricle. 7. The atrium begins to fill again from the pulmonary vein. Pressure in the atrium increases and the atrioventricular valve opens to begin the cycle again.

The heart controls its own contractions without the need for stimulation from the brain - it is myogenic 

1. The Sino-Atrial Node (SAN) acts as a pacemaker. It sends waves of electrical activity across the atrial walls, causing them to contract (simultaneously).2. The electrical impulses reach a band of non-conductive tissue (collagen) which prevents the impulses from travelling directly from the atria to the ventricles. 3. Instead, the waves of electrical activity are transferred to the Atrio-Ventricular Node (AVN). This causes a slight delay to ensure that the atria have fully emptied before the ventricles contract. 4. The AVN passes the electrical waves down the Bundle of His (a group of fibres). The Bundle of His passes the electrical impulses through the left and right walls of the ventricles, sand so causes them to contract simultaneously and from the bottom up. 

Passage of blood

co-ordination of heartbeat