The heart is a complex organ with pumping and endocrine functions. The heart keeps blood moving in a clockwise direction through two connected circulatory pathways and secrets neurohormonal substances (peptides) that stimulate the excretion of sodium and water by the kidneys, thereby helping to regulate blood pressure and blood volume. The circulatory pathway originating from the left side of the heart (systemic circulation) carries oxygenated blood to all parts of the body except the lungs, whereas the pathway originating from the right side of the heart (pulmonary circulation) carries deoxygenated blood to the lungs via the pulmonary artery and returns oxygenated blood to the left atria via the pulmonary veins. Arteries carry blood away from the heart, whereas veins carry blood back to the heart. The smallest arteries are called capillaries, whereas the smallest veins are called venules.

The heart chambers and blood vessels secrete natriuretic peptides, which promote diuresis.