Page 357 - Biology_F5

Biology for Advanced Level Secondary Schools

7.4.1 Osmoregulation in marine 7.4.2 Osmoregulation in mammals
elasmobranches An important evolutionary adaptation
Marine elasmobranches are cartilaginous that allowed animals to survive on land
fish such as sharks, rays, and skates. They was the development of a kidney that
live in sea water whose salt concentrations would produce concentrated (hypertonic)
are higher than those of their body fluids. urine. The need for water conservation
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Due to this difference in concentrations, is particularly well illustrated in desert
the fishes tend to lose water from their mammals such as the kangaroo rat. A
bodies into the sea. To overcome this major adaptation that allows the kangaroo
problem, the marine elasmobranchs have rat to conserve water is the ability to form
developed mechanisms of making their very hypertonic urine twenty times more
body fluids less hypotonic to sea water. concentrated than its blood plasma. The
Because of this, the animals face another kidneys of the Kangaroo rat are able to
problem of a natural and continuous accomplish this because the loop of Henle
diffusion of water into their bodies from of their nephrons is much longer and more
their surrounding sea water. To overcome efficient than that of most other mammals.
these problems and to make their body Terrestrial mammals need to drink water
fluids isotonic to sea water, such fishes at least occasionally to compensate for the
have developed the following adaptations: water lost from the skin and respiratory
passages and through urination.
(a) They have rectal glands which secrete
salts to increase their osmotic pressure. Counter current multiplier
This mechanism aims at balancing the
internal osmotic pressure to that of the Task 7.8
surrounding sea water. Search from internet sources the videos
(b) They retain nitrogenous wastes such as or simulations showing the concept of
urea, and trimethylamine oxide (TMAO) counter current multiplier effect.
in their body cells. These chemicals are
kept in high concentrations, and they In the loop of Henle, a counter current
change the diffusion gradient enabling a exchange mechanism is combined with
fish to absorb water instead of ingesting the active secretion of solutes. A system
it. Despite the fact that these are waste that uses this combined type of exchange
products and may be harmful to the is called counter current multiplier system.
animals at high concentrations, the The loop of Henle functions as a counter
marine elasmobranchs have been able current multiplier due to its close proximity
to produce and retain urea because their to the ascending and descending limbs,
gills are impermeable to it. Their renal permeability of the descending limb to
tubules in the kidneys are capable of water, impermeability of the descending
reabsorbing urea from the renal fluid limb to solute, permeability of the ascending
back to the body cells. In addition, their limb to solute, passive transport of solute
cells are immune to the effects of high in thin ascending limb, and active transport
concentrations of urea. mechanism for the thick ascending limb

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