Maintaining the Internal Sea
An aquatic animal can be thought of as a bag of salty fluid separated from the external environment by a membrane that is a partial barrier to the passage of dissolved materials. Although elegantly subdivided into cells, tissues, and organs bathed in a watery medium, this living bag is subject to the same chemical laws that dictate the behavior of sugar or milk in a cup of coffee. Large molecules (proteins, fatty acids) that cannot pass across the outer membrane remain where they are, irrespective of any concentration gradient, much as tea leaves remain neatly within the porous bag (unless it is ruptured). In contrast, any concentration gradient of small molecules (gases, water, salts, simple sugars, amino acids, and so on) on either side of the membranous skin will induce them to diffuse across it from areas of high concentration to areas of low until a dynamic equilibrium is reached.
Like other animals, the White Shark needs a supply of water and dissolved materials inside its body to carry out the exquisite chemical choreography needed to maintain its life processes. Dissolved in the Great White's bodily fluids is a complex mixture of salts, including sodium, potassium, and calcium chloride, sulfates, and many others. Outside the shark's body, the seawater contains a similar but more concentrated mixture of dissolved salts. Since water is among the smallest of biologically-important molecules, it requires less energy to move water than salt. Thus, if relative salt concentrations were the only important factor, a White Shark would continually lose its vital supply of internal water to the outside marine environment through the relentless process of osmosis. But there is more to saltiness than just salt.
Like other sharks, the Great White's bodily fluids also contain small organic molecules. Among the most important of these from an osmotic standpoint are urea and trimethylamine oxide (or TMAO, for short). Both urea and TMAO are nitrogen-containing break-down products of protein metabolism. Urea is highly toxic to living tissue at moderate to high concentrations, causing proteins to de-stabilize and thus cease to function properly or at all. That is why, although we can tolerate 'holding it' for a little while, we must eventually excrete urea or face dire physiological consequences. Yet sharks routinely retain bodily concentrations of urea that would kill most other vertebrates. This is largely due to the presence of even higher bodily concentrations of TMAO, which counters the protein-de-stabilizing effects of urea. Together, urea and TMAO add substantially to a shark's osmotic pressure, effectively rendering the internal fluids slightly (about 5%) 'saltier' than the external environment. As a result, sharks do not need to invest any metabolic effort toward obtaining the water their bodies need. A constant supply of fresh water osmoses passively into a shark's body through the gills and other exposed membranes.
Contrast this with a typical marine teleost. We've all heard the phrase, "drink like a fish". Fact of the matter is, most marine fishes must drink seawater continually. Without retention of high concentrations of urea and TMAO, the bodily fluids of a typical teleost have a far lower osmotic pressure than the surrounding seawater. Thus, most marine teleosts do have to drink copious amounts of seawater in order to replace the water lost to the marine environment by passive osmosis. The teleost physiologically separates the dissolved compounds from the water, absorbs the now fresh water at the kidneys, and excretes the excess salts via special 'chloride cells' in the gills. Human kidneys can perform the same seawater desalination process as those of teleosts. But we all know that, even if lost at sea and desperately thirsty, one should not drink seawater. Without chloride cells or gills, the human body uses more fresh water to excrete the excess salts than is obtained from the seawater in the first place. As a result, drinking seawater promotes rather than alleviates dehydration.
Because ocean salinity changes with such factors as temperature, depth, and proximity to shore, wide-ranging creatures like the White Shark must continually balance its internal osmotic pressure with that of the external marine environment. As in other sharks, excess salts are removed from the Great White's bloodstream by the kidneys, which extend the length of the body cavity on either side of the backbone. These salts are then excreted as a thick, whitish paste from a small, salami-shaped organ called the rectal gland, located just inside the cloaca. Excess bodily water is filtered by the kidneys and excreted out the cloaca as urine. But there are firm limitations on the extent to which a White Shark can adjust its internal osmotic pressure. In water of greatly reduced salinity, for example, a White Shark's high internal osmotic pressure works against it, so that its kidneys must work overtime to combat the massive influx of fresh water. The resultant metabolic cost is so high, it is unlikely that a White Shark that found its way into a freshwater river or lake would long survive.
That some sharks and rays have adapted to low-salinity aquatic environments is testimonial to the marvelous environmental plasticity of elasmobranchs. But - just as every human monarch has limitations to his or her reign - even the mighty Great White Shark cannot circumvent the limitations of its osmoregulatory physiology.