Sodium homeostasis... was Re: tears

J. Moore (
Wed, 25 Oct 95 17:55:00 -0500

AD> >salt proportion of the body a "salt-excretion mechanism" if you wish.
AD> >It doesn't change the fact that tears are not an effective
AD> >mechanism for ridding the body of excess salt.

IB> Anything that gets rid of salt can be a salt excretion mechanism. I
IB> agree that unless they wept a lot it would be ineffective, certainly
IB> not as effective as kidneys or sweating but so what.

The problem for the body, in terms of excreting of sodium (or
potassium, etc.), is maintaining a balance (homeostasis) of water
and sodium (or potassium, etc.). If an excreted solution has the
same ratio of sodium to water as is in plasma, it doesn't change
this balance, and the organ excreting this solution is not going
to be able to rid the body of excess sodium. The solution itself
is said to be isotonic. This is the situation with human tears.

Human sweating excretes a substance which is hypotonic, having a
lower ratio of sodium to water than is in plasma. Here too, the
solution is not useful at ridding the body of excess sodium --
quite the opposite -- since the body is eliminating proportionately
more water than sodium from its plasma as it sweats.

Mammalian urine, on the other hand, is hypertonic, having a higher
ratio of sodium to water than is in plasma. It is therefore
effective in getting rid of excess sodium from the body's plasma.
Also, in mammals, the renal system is well-regulated to respond to
differences in sodium levels in the body. This is why mammalian
kidneys provide the regulation of sodium-water balance, and why in
marine mammals, the kidneys are extremely large and heavily
lobulated. In marine mammals, as in some arid desert mammals such
as kangaroo rats and very *unlike* humans, the kidneys produce urine
which contains a higher concentration of sodium to water than is
found in seawater. For the desert-dwelling mammal, this feature
allows them to conserve fresh water, enabling them to get all their
water from the plants they eat. For marine mammals, this feature
allows them to eat marine foods and even makes it possible for them
to drink seawater:

"A seal that drinks a liter of seawater, after excreting the salts,
ends up with a slight net gain of pure water. A human, however,
experiences a net water loss of about a third of a liter of water after
drinking the same amount of seawater. Human kidneys are less powerful
than those of seals or whales, and even the most concentrated urine they
can produce is always more dilute than salt water" (Riedmann 1990:31,
*The Pinnipeds: Seals, Sea Lions, and Walruses* by Marianne Riedmann.
University of California Press: Berkeley, L.A., and Oxford).

The kidneys of birds and reptiles are generally not as effective as
mammalian kidneys in the necessary task of ridding the body of excess
sodium and providing a sodium-water balance. The salt glands of marine
birds and reptiles are strongly hypertonic in regard to sodium --
very *unlike human tears* -- and provide a great deal of this function.

Jim Moore (

* Q-Blue 2.0 *