Why humans can’t drink seawater unlike whales and dolphins

The question of whether or not to drink seawater is the most torturous diatribe a castaway can face.

And it’s no wonder. It must be awful to be dying of thirst when you can’t even take a sip of the tons of water around you.

But we must not fall into temptation.

The marine world knows only too well the danger that would entail succumbing to this imperious desire.

Drinking seawater, far from hydrating us, dehydrates us… And at dizzying speeds.

From a chemical point of view, humans (like the rest of the living organisms on the planet) are unstable systems basically made up 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.

As we are in a terrestrial (dry) environment, water tends to escape from our internal environment, which leads to dehydration and, consequently, death.

If this does not happen, it is because evolution has selected, throughout our lineage, a magnificent covering that, like a raincoat, does not allow water to pass through.

It is the 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 sealed compartment.

In fact, water is continually evaporating through areas that must be kept moist to be functional (eyes, nostrils, mouth, urethra, anus, and vagina).

On the other hand, we eliminate our poisonous nitrogenous remains (resulting from protein catabolism) in the form of urine. And that, basically, is urea diluted in water.

Finally, the “keratinic raincoat” has to have pores so that we can sweat, since it is our way of cooling ourselves when it is hot.

Whatever the cause, the reality is that we continually lose our precious and essential liquid.

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 decreases atrial natriuretic peptide.

Both actions are dipsogenic, that is, they trigger the sensation of thirst in the brain.

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

If we drink seawater, the intestine will absorb it as is.

This implies that water will reach the blood, but also salts, mainly sodium chloride or common salt.

The kidneys will try to maintain the osmotic balance at all costs and will tend to eliminate excess salt through urine.

If we translate it into figures, 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 one liter of salt water will accumulate 6 more grams of salt without its equivalent diluent water.

In other words, to eliminate salt from a glass of seawater we would have to excrete two glasses of urine, which would make us more dehydrated than before drinking.

The serious thing is that, in addition to sodium chloride, seawater contains magnesium sulfate, a molecule that retains water inside the intestine, preventing its absorption.

In fact, it is the basic component of a very popular type of laxatives.

Poor castaway! He is thirstier than before and also has diarrhea.

Evolution has solved this osmotic problem with very different strategies.

In principle, we might think that fish, living “in water” do not have to fight dehydration.

It isn’t true. Although depending on the osmotic particularities of each group, and always in fewer quantities than a terrestrial vertebrate, their physiology also involves the need to replenish water.

And that means they also need to eliminate excess sodium ions.

Bony fish do not urinate: they do so through the gills. Sharks and similar animals, although they also have gills, are more original and eliminate salts through feces.

They achieve this by filtering their 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 secreting glands are also found in other vertebrates that feed and live in the sea, although located in other anatomical areas.

Thus, while seabirds and some marine reptiles place them nasally, some sea turtles have them in the eye sockets, while sea snakes place them under the tongue and, above it, Asian and North American marine crocodiles.

From this plural and varied sample of ultra-salty poop, mucus, tears and saliva, what modality is used by marine mammals?

Well, surprisingly, they do not have any type of salt gland.

In fact, they do not have extrarenal salt secretion organs.

We might think, then, that they must have very efficient kidneys capable of producing very salty urine.

Well, despite the fact that their urine is actually very hypertonic (concentrated), sea lions, seals, whales, porpoises, killer whales and dolphins have opted for a very curious alternative solution: not drinking water.

Their strikingly different strategy is to “scrounge” (borrow) the osmoregulatory efforts of their prey. And they do it twice.

On the one hand, the fluids of the animal they have just hunted (mainly its 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 it is a “metabolic water” that is generated as the star product of its biochemistry.

The process is easy. Carbohydrates, fats and proteins from the prey are digested in the stomach of the cetacean (or the pinniped, if instead of a dolphin we think of a seal), absorbed in its intestine and distributed through its blood to all the cells in its body.

There, already degraded to tricarboxylic acids, they enter the prodigious biological machines that are the mitochondria to obtain energy and something else: invaluable hydrogen ions (H⁺).

All that remains is to add the H+ with 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.

On the contrary, for marine mammals mitochondria would be authentic “biochemical philosopher’s stones” capable of generating the most precious of treasures: water.