The electrical activity of the heart is impulsed by the sinus node. When the P-cells of the sinus node give out electrical activity, the electrical activity is transmitted from the sinus node to the atria, and when the atria sense the electrical activity of the sinus node, it causes atrial contraction, which pumps blood from the atria to the ventricles, followed by ventricular contraction, which pumps blood from the ventricles out of the heart, thus completing a complete heartbeat. Blood from the right atrium will be pumped into the right ventricle, and when the right ventricle contracts, it will pump venous blood to the pulmonary artery, because oxygen is drawn in from the lungs, and venous blood and oxygen are combined in order to become arterial blood. After the left atrium collects arterial blood, when the left atrium contracts, it will pump arterial blood from the left atrium into the left ventricle, and when the left ventricle contracts, it will pump arterial blood to the aorta and its important branches to supply blood and oxygen to various organs.