If humans can die from drinking seawater, why can't whales and dolphins die?

But don’t give in to temptation. The world of sailors knows all too well the danger of giving in to this irresistible desire. Drinking seawater, far from hydrating us, dehydrates us… And at a dizzying speed.

What happens when we lose water?

From a chemical point of view, human beings (like all other living organisms on the planet) are unstable systems and are basically composed of water with, among other things, dissolved salts. Water is the medium in which all our biochemical reactions take place and, therefore, the essential element to guarantee our metabolic subsistence.

Since we are in a terrestrial (dry) environment, water tends to escape from our internal environment, leading to dehydration and, consequently, death. If this does not happen, it is because evolution has selected, throughout our lineage, a magnificent shell that, like a mackintosh, does not allow water to pass through. It is called skin, and its waterproofing capacity is due to a protein located in its outermost layers: keratin.

However, the human body is far from being a watertight compartment. In fact, water evaporates continuously in areas that need to be kept moist to be functional (eyes, nostrils, mouth, urethra, anus and vagina). On the other hand, we eliminate our toxic nitrogenous waste (resulting from protein catabolism) in the form of urine. And this is basically urea diluted in water.

Finally, the “keratin mackintosh” needs to have pores so that we can sweat, as this is our way of cooling down in hot climates. Whatever the cause, the reality is that we are continually losing this precious and essential fluid.

Recovering lost water means “stealing” it from our main water reservoir, the blood, which reduces volemia (blood volume) and, consequently, blood pressure. This dangerous situation, detected by cardiopulmonary receptors and baroreceptors, activates the renin-angiotensin system (RAS) and reduces atrial natriuretic peptide. Both actions are dipsogenic, that is, they cause the sensation of thirst in the brain.

Once alerted, we react: we drink water, absorb it through the intestine into the bloodstream via capillaries, restore blood volume and everything returns to balance.

What if the water contains salt?

If we drink seawater, the intestine will absorb it as is. This means that water, but also salts, mainly sodium chloride or common salt, will enter the bloodstream. The kidneys will try to maintain osmotic balance at all costs and will tend to eliminate excess salt through urine.

In other words, the human kidney can remove up to about 6 grams of sodium from the blood in each liter of urine excreted. Since seawater contains about 12 grams of sodium per liter, drinking a liter of saltwater will accumulate 6 grams more of salt without the equivalent diluent water. In other words, to eliminate the salt from a glass of seawater, we would have to excrete two glasses of urine, which would leave us more dehydrated than before drinking.

What is worse is that, in addition to sodium chloride, seawater contains magnesium sulfate, a molecule that binds to water in the intestine and prevents its absorption. In fact, this is the basic component of a very popular type of laxative.

Poor castaway! He is thirstier than before and, on top of that, he has diarrhea.

How do fish, turtles and crocodiles deal with this problem?

Evolution solved this osmotic problem with very different strategies. In principle, we might think that fish, which live “in water”, do not need to fight against dehydration. This is not true. Although depending on the osmotic characteristics of each group, and always in smaller quantities than a terrestrial vertebrate, their physiology also involves the need to replenish water. And this means that they also need to eliminate excess sodium ions.

Bony fish do not urinate: they do so through their gills. Sharks, although they also have gills, are more original and eliminate salts through feces. They do this by filtering the blood twice: first in the kidneys (like any other vertebrate) and then in the rectal gland, a contractile diverticulum near the anus (cloaca).

These salt-concentrating and salt-secreting glands are also found in other vertebrates that feed and live in the sea, although they are located in other anatomical areas. So, while seabirds and some marine reptiles have them in their nostrils, some sea turtles have them in their eye sockets, while sea snakes have them under their tongues, and Asian marine crocodiles and North American alligators have them on their tongues.

The choice of whales and dolphins

Of this plural and colorful array of ultra-salty poop, snot, tears and saliva, what is the method used by marine mammals?

Well, surprisingly, they have no salt glands of any kind. In fact, they have no extrarenal organs that secrete salt. We might think, then, that they must have very efficient kidneys, capable of producing very salty urine.

Well, despite their urine being very hypertonic (concentrated), sea lions, seals, whales, porpoises, orcas and dolphins have opted for a very curious workaround: not drinking water. Their surprisingly different strategy is to “steal” the osmoregulation efforts of their prey. And they do this in two ways. On the one hand, the fluids of the animal they have just hunted (mainly blood) are their main source of water. On the other hand, they generate water biochemically from the “meat” of the animal they are eating. We could say that this is a “metabolic water” generated as the main product of their biochemistry.

The process is simple. Carbohydrates, fats and proteins from the prey are digested in the stomach of the cetacean (or pinniped, if we think of a seal instead of a dolphin), absorbed in its intestine and distributed by the blood to all the cells of the body. There, already degraded into tricarboxylic acids, they enter the prodigious biological machines that are the mitochondria to obtain energy and something else: precious hydrogens (H⁺). All that remains is to add the H+ to the oxygen they breathe (O₂) to achieve the miracle: H₂0.

Although this process, called cellular respiration, occurs widely in animals (as aerobic organisms that we are), it does not have the same relative value in all of them. For an animal that “drinks,” the water molecules generated are “leftover” elements that it eliminates directly by generating more urine. For marine mammals, on the other hand, the Mitochondria would be true “biochemical philosopher’s stones” capable of generating the most precious of treasures water.

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