Blood enters the right atrium; it flows through a valve into the right which pumps it through which flow into capillaries in the where it gives off co2 and picks up o2. the oxygenated blood flows into back to the heart where it enters the left the blood flows through a valve into the left which pumps it through which branch into smaller where o2 is delivered to all body cells before returning through to the right of the heart.

Specialized tissue on the wall between the atria. Electrical impulses pass from the pacemaker (SA node) through the and the atrioventricular bundle (bundle of His) toward the ventricles.atrium (pl. atria)One of two upper chambers of the heart.capillarySmallest blood vessel. Materials pass to and from the bloodstream through the thin walls. They have walls that are only one endothelial cell in thickness. This delicate, microscopic vessel carries nutrient-rich, oxygenated blood from the arteries and arterioles to the body cells. There, the nutrients are burned in the presence of oxygen (catabolism) to release energy.
At the same time, waste products such as carbon dioxide and water pass out of the cells and into these blood vessels. Waste-filled blood then flows back to the heart in small venues, which combine to form larger vessels called veins.carbon dioxideGas (waste) released by body cells, transported via veins to the heart, and then to the lungs for exhalation.coronary arteriesBlood vessels that branch from the aorta and carry oxygen-rich blood to the heart muscle.deoxygenated bloodBlood that is oxygen-poor.diastoleRelaxation phase of the heartbeat.