BIOLOGY Volume 3 - A Guide to General Biology - 2004

20. EXCRETION AND OSMOREGULATION

20.3. Nitrogen Excretion and Osmoregulation in Selected Animals

It should always be borne in mind that The Nature of excretory products and the mechanisms of osmoregulation are heavily influenced by the environment. The elimination of nitrogenous catabolites is usually closely linked to the problems of Water intake and loss, and hence to the structures involved in osmoregulation. Consequently, it is logical to consider both processes together.

20.3.1. Environmental Influences on Osmoregulation

The internal environment of many aquatic organisms is characterized by a higher water potential than their external surroundings (i.e., their Body Fluids are more dilute). As a result, There is a continuous inward "pumping" of water by osmosis. Water losses are compensated in various ways, including drinking and feeding. If the internal water potential is higher than that of the environment (making the internal medium more concentrated), water enters via osmosis. To minimize such fluctuations, many organisms are enclosed in waterproof integuments.

All terrestrial organisms face The Challenge of losing water from their body fluids to the surrounding environment. The constancy of the intracellular fluid composition is maintained in these organisms by regulating the extracellular fluid via specialized osmoregulatory-excretory Organs, such as Malpighian tubules and Kidneys. The amounts of water molecules and ions gained and lost must be carefully balanced. Water balance issues are discussed in detail in Section 20.6.

Osmoregulation also involves maintaining optimal concentrations of dissolved solutes in a given fluid through diffusion and Active Transport.

Adaptation to Arid Environments

The kangaroo rat (Dipodomys) stands out among mammals for its remarkable ability to endure the hyper-arid conditions typical of the North American deserts. It thrives in these environments thanks to a unique combination of morphological, physiological, and behavioral adaptations. Respiratory water loss is minimized because exhaled air has a lower Temperature than the internal body core. During inhalation, the incoming air absorbs heat in the nasal passages, cooling them down. Upon exhalation, the water vapor contained in the warm air condenses on the nasal mucosa, thereby conserving water. The kangaroo rat feeds on dry seeds and other dry vegetation and drinks no water at all. Its only sources of water are metabolic water produced during cellular Respiration and the very small amounts of water present in its food. Schmidt-Nielsen, in his classic studies summarized in Table 20.2, measured the water balance of a kangaroo rat weighing 35 g and consuming 100 g of barley under experimental conditions (at 20 °C and 20% relative humidity). The barley grains served as its sole source of water throughout the test period.

Class="center">Table 20.2. Water balance in the kangaroo rat under experimental conditions. The animal received water exclusively from its food

Finally, in its natural habitat, the kangaroo rat avoids evaporative water loss by spending much of its time within the relatively humid atmosphere of an underground burrow.

Another striking example of water conservation is the water balance of the camel, whose physiological adaptations are discussed in Section 19.5.7.