Oceans, in effect, mimic some functions of the human circulatory system. Just as arteries
carry oxygenated blood from the heart to the extremities, and veins return blood to be
replenished with oxygen, oceans provide life-sustaining circulation to the planet. Propelled
mainly by prevailing winds and differences in water density, which changes with the
temperature and salinity of the seawater, ocean currents are critical in cooling, warming, and
watering the planet's terrestrial surfaces—and in transferring heat from the Equator to the
Poles.