Non-oxygenated blood arrives at the right atrium via the venae cavae, from the head and arms (upper vena cava) and from the abdomen and legs (lower vena cava). This blood passes to the right ventricle through the tricuspid valve. The right ventricle pumps this blood, through the pulmonary valve, into the lungs through the pulmonary arteries, which is where the blood gets it oxygen.
This oxygenated blood returns to the left atrium via the pulmonary veins. From the left atrium it is directed to the left ventricle through the mitral valve. The left ventricle pumps the blood to the aorta through the aortic valve to distribute it to all the organs and tissues in the body.
The heart is irrigated by the coronary arteries, right and left. These coronary arteries divide into several branches to carry oxygenated blood throughout the heart tissue.
The heart contracts due to an electric stimulus triggered by the conduction system. The cardiac conduction system is made up of a series of cells that have the capacity to create this stimulus and determine heart rate. This stimulus begins in the sinus node, which is found where the superior vena cava enters the right atrium. This stimulus causes the atrium to contract. This stimulus then propagates the ventricle through another structure called the atrioventricular node. This conduction system is capable of increasing the heart rate when necessary, such as for example during exercise, when you have a fever, when you feel emotions, etc., or decreasing the heart rate when you are sleeping. This system is regulated by the action of different hormones or in response to nervous stimuli in the cardiac plexus.
The cardiac cycle has two phases: systole and diastole. In systole, the heart contracts to send blood to the major arteries and during diastole it relaxes to fill with blood to later be ejected.