Water Conservation and Economy Species Differences - Physiology
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Species Differences in the Ability To Conserve Water
Mammals and birds can produce urine both more and less concentrated than plasma in order to conserve or excrete water. The excretion is done without interfering with the levels of solute excretion. The ability to concentrate urine depends greatly on a species access to water. Animals with access to lots of water have less of a need and therefore have not evolved great mechanisms to concentrate urine for example the beaver. However animals who have limited access to water and say live in more arid environments have evolved very good mechanisms to reabsorb lots of water and produce a very concentrated urine. An example of this is the kangaroo rat which lives in a desert habitat.
Nephron Species Differences
The variation in nephron structure is pivotal to the species differences with regard to the ability to concentrate urine. Broadly there are two structures.
Juxtamedullary Nephrons
These nephrons have glomeruli in the cortex and loops of henle which descend into the medulla. These are the commonly described nephrons. Significant urine concentration is achieved thanks to the hyperosmolar medulla achieved via the counter current multiplier drawing water out of the collecting ducts.
Cortical Nephrons
These nephrons have glomeruli in the outer cortex and their loops of henle barely penetrate the medulla. They have very limited concentrating ability.
The length of the loop of henle is the determining factor with regard to how concentrated urine an animal can produce. In species which produce very concentrated urine almost if not all of their nephrons are juxtamedullary in type and they tend to have very long loops of henle. However animals such as beavers, where dehydration is not really an issue, have very short loops of henle and excrete dilute urine. However one surprise is that the kangaroo rat which can concentrate urine to the equivalent of a 20% saline solution actually has relatively shorter nephrons than some larger species with less ability to concentrate urine. This has been attributed to it having a much higher metabolic rate and thus more active transport meaning greater gradients can occur on any given length of its loop of henle.
The following table shows a comparison between the maximum concentrating abilities of the kidneys of various mammals. Data from Physiology of Domestic Animals - Sjaastad, Hove and Sand. For full text reference see the Reference Material section.
| Beaver | ||
| Calf | ||
| Pig | ||
| Humans | ||
| Cow | ||
| Sheep | ||
| Horse | ||
| Dog | ||
| Cat | ||
| Kangaroo Rat |
Species Differences in the Ability To Conserve Water - Physiology