Maintaining their hydration is a major problem for desert animals. Large mammals e.g. Gemsbok & small mammals e.g. rodents use different strategies to cope with arid conditions. We will discuss the special behavioral or physiological adaptations that have evolved in various animals, concentrating on those found in Africa.
We will in part 1 first explain the model we will use. We will try to simplify the adaptations used to maintain water balance, once again, using our simple analogy of a wash basin. The level of water in a washbasin depends on how much is flowing in versus how much is flowing out. Similarly the level of hydration in all animals will depend on the balance between the water input & output. (Figure 1)
Water Input:- Most of the water input usually depends on the intake from exogenous sources. This is the amount we drink or the amount ingested with food e.g. fluid present in cells or surrounding tissue of animals & plants. However water is also produced endogenously when nutrients are catabolized e.g. when glucose is converted to energy ( ATP), CO2 & water are released.
Water Output:- Although water can shift compartments e.g. move from extracellular fluid to intracellular fluid & vice versa this will not change total body water. This occurs in disease states & will not be discussed. Thus for water balance we will only discuss water lost from the body i.e. exogenously, resulting in dehydration. This can occur from body sites which are permeable to water and have a large surface area. The water output thus occurs in the kidneys, skin, lungs, & gastrointestinal tract.
Renal output: Large volumes of blood pass through the kidneys for filtration. The small molecules in blood plasma e.g. water, glucose, salts, & break down products of metabolism such as urea & uric acid pass through the small pores of the glomeruli into the renal tubules where they can be reabsorbed or pass onto the bladder for excretion. In the distal part of the tubules water can be reabsorbed depending on hormonal signals & the urine concentrated. Thus the water output via the kidneys can be regulated to maintain water balance.
Pulmonary Fluid loss: Water molecules pass back & forth over the large surface area in the lungs. The alveolar/capillary interface is very thin & permeable to water. The water forms a thin alveolar/bronchiolar interface with the inhaled air &will evaporate & be carried out on exhalation. The condensation of exhaled water into steam when we breathe out on cold winter days clearly illustrates this water loss.
Skin losses: Here as in the lung the large surface area allows a large amount of water to be lost. Since surface area varies inversely with the size of the animal small animals have relatively larger surface areas & lose water more easily. This point is illustrated by the fact that premature babies dehydrate very easily & are nursed in humidified incubators. This water loss is further aggravated if the skin is very thin e.g. preterm babies. This loss is known as insensible loss. The other water loss from the skin is via active sweating. Newborn babies and many animals are unable to sweat. This decreases their water losses but hinders temperature regulation in hot conditions.
Gastrointestinal losses: A large amount of fluid is released into the gastrointestinal tract to aid digestion. Saliva, gastric, fluid, pancreatic juice & bile, as well as secretions along the small intestine contribute to this potentially large output of water but most is normally reabsorbed in the large intestine & thus feces has a relatively firm consistency. When this mechanism is disturbed e.g. vomiting or diarrhea dehydration can rapidly follow.
We will use this simple input output model in following posts on how animals adapt to desert conditions & maintain their hydration.