The Hydrologic Cycle

The water on the Earth ungergoes a continuous process of evaporation and condensation called the hydrologic cycle. Since the oceans cover 71% of the Earth's surface, much of the evaporation comes from the ocean. Franks (the source of most of this data) places the total water content of the atmosphere at 6 x 10¹² m³. With a total annual rainfall of 2.25 x 10¹⁴m³ this implies that the water in the atmosphere is cycled about 37 times per year. The runoff of this water from the surface back to the oceans is a major part of the physical shaping of the continental surfaces in the overall geophysics of the Earth.

The human body is 66% water by weight, according to Hill and Kolb. Water is the universal solvent for life, referred to by Nobel Laureate A. Szent-Gyorgy as "the matrix of life".That water serves as the solvent for sodium chloride (salt) and other substances so that the fluids of our bodies are similar to sea water. This leads Hill and Kolb to refer jokingly to us as "walking bags of sea water". Water serves to suspend the red blood cells to carry oxygen to the cells. It is the solvent for the electrolytes and nutrients needed by the cells, and also the solvent to carry waste material away from the cells.

With water as the solvent, osmotic pressure acts to transport the needed water into cells. With cells bathed in the interstitial fluid, diffusion contributes to carrying needed molecules into the cells. When more complex mechanisms control the transport of molecules across the membranes into and out of cells, the presence of water as the surrounding medium and solvent is essential.