BIOLOGY Volume 3 - A Guide to General Biology - 2004

20. EXCRETION AND OSMOREGULATION

20.3. Nitrogen Excretion and Osmoregulation in Selected Animals

20.3.5. General Principles of Water Balance

For Cells to function properly within an animal's body, a steady-state condition of the intracellular fluid must be maintained. The homeostatic exchange of Water between cells, tissue fluid, Lymph, Blood Plasma, and the external environment poses a challenge for both aquatic and terrestrial organisms. Aquatic forms lose or gain water through osmosis across all permeable areas of the body surface, depending on whether their environment is hypotonic or hypertonic. Terrestrial organisms face The Challenge of water loss and employ various mechanisms to maintain a stable water balance, as listed in Table 20.3. This steady state is achieved through a balance between water loss and water intake, as illustrated in Fig. 20.9.

Class="right">Table 20.3. Water conservation mechanisms in two groups of terrestrial animals: insects and mammals

Animals

Mechanisms and Adaptations

Insects

Impermeable cuticle; Trachea and spiracles; Malpighian tubules; uric acid as a nitrogenous waste product; cleidoic eggs

Mammals (including humans)

Keratinized Skin and Hair; hypertonic urine containing urea; viviparity; behavioral responses to excess heat; restricted habitat range; physiological tolerance to dehydration

Fig. 20.9. Daily human water balance.



Last update: 06/08/2026

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