The cardiovascular system consists of the heart and blood vessels. The heart has four chambers and uses valves to ensure one-way blood flow. It is divided into a pulmonary and systemic circuit. The heart wall has three layers. It contains cardiac muscle cells that are branched and connected, allowing electrical signals to spread rapidly. The conducting system includes the sinoatrial node, which initiates each heartbeat, and pathways that coordinate ventricular contraction. An electrocardiogram monitors the heart's electrical activity through different waves. The cardiac cycle involves alternating periods of atrial and ventricular relaxation and contraction. This pumps blood through the heart and vessels in a continuous circuit.