Plasma Maintains Salt Balance

close up of a brown frog in waters with a yellow leaf

Biological Strategy

Crab-eating frog

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Maintain Homeostasis

When a living system is in homeostasis, it means that internal conditions are stable and relatively constant. For example, a human’s internal temperature is approximately 37 degrees Celsius (98.6 degrees Fahrenheit) unless there’s an illness. The human body maintains this temperature despite external ambient temperature. However, as with all physiological processes, maintaining homeostasis requires communication and coordination. So living systems have ways to detect changes from the norm, mechanisms to cause an adjustment, and negative feedback connections between the two. A desert lizard called the Gila monster offers a good example of maintaining homeostasis. The lizard goes from eating large meals to fasting for extended time periods. To maintain its blood sugar levels at a steady level, when food is scarce, its endocrine system releases a hormone that raises its blood sugar levels.

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Amphibians

Class Amphibia (“to live a double life”): Frogs, salamanders, toads

Amphibians live a “double life” by being able to survive both on land and in water. Unlike reptiles with dry, scaly skin, amphibians have smooth skin that needs to be kept moist. Many live in the water through development and start life with gills, and then transition to breathing with lungs. They can get even more oxygen through cutaneous respiration, or breathing through their skin. This class represents the first animal group to be able to efficiently walk on land—aided by the well-defined limbs that gave them a “leg up” on their finned competitors.

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The plasma of crab-eating frogs allows them to survive in salt water by topping up the ionic concentration with non-ionic solute urea.

“Adult frogs have a different, and rather more unusual, method of osmotic regulation. Instead of being osmoregulators and maintaining an imbalance between the osmotic concentration of their internal fluids and that of the exterior, they are partial osmoconformers. Internal osmotic concentration is matched with that of the exterior, at least in a hyperosmotic medium. This is brought about not by manipulating ion levels, but by ‘topping up’ the ionic concentration of the plasma with the non-ionic solute urea CO(NH2)2 (Fig. 3.4). Other amphibia show slightly raised urea levels in conditions of water shortage, but employing high urea levels to maintain osmotic balance with the environment is a habit shared only with elasmobranch fish (sharks, skates and rays) (Gordon et al. 1961). Urea is the standard nitrogenous excretory product of ordinary adult amphibia, and is normally voided at the earliest convenient opportunity. Indeed, given the solubility of urea, it is not an easy substance for a largely aquatic animal to retain, and it is not understood how Rana cancrivora [Fejervarya raja] achieves this. Not only is it soluble and difficult to retain, urea is toxic: the concentrations of urea that occur in the frog’s plasma at exterior salinities of 80 per cent sea water, should denature enzymes and affect the binding of oxygen by haemoglobin. Somehow Rana cancrivora copes with these hazards.” (Hogarth 1999:63-64)

Last Updated August 29, 2018

References

Book

The Biology of Mangroves and Seagrasses (Biology of Habitats Series)

31/05/2007 | Peter Hogarth

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