Osmoregulation

An important evolutionary adaptation that allowed animals to survive on land was the development of a kidney that would produce concentrated (hypertonic) urine. The need for water conservation is particularly well illustrated in desert mammals such as the kangaroo rat. A major adaptation that allows the kangaroo rat to conserve water is the ability to form very hypertonic urine twenty times more concentrated than its blood plasma. The kidneys of the Kangaroo rat are able to accomplish this because the loop of Henle of their nephrons is much longer and more efficient than that of most other mammals. Terrestrial mammals need to drink water at least occasionally to compensate for the water lost from the skin and respiratory passages and through urination.

In the loop of Henle, a counter current exchange mechanism is combined with the active secretion of solutes. A system that uses this combined type of exchange is called counter current multiplier system. The loop of Henle functions as a counter current multiplier due to its close proximity of ascending and descending limbs, permeability of the descending limb to water, impermeability of the descending limb to solute, permeability of the ascending limb to solute, passive transport of solute in thin ascending limb, and active transport mechanism for the thick ascending limb.

These features enable the loop of Henle to create a very high concentration gradient between the tissue fluid and blood in the medulla of the kidney and the urine in the collecting ducts. The loop of Henle is connected at one end to the proximal convoluted tubule and at the other end to the distal convoluted tubule. It first descends deep into the medulla and then bends and ascends into the cortex again.

Throughout its length, it is surrounded by fine looped blood vessels called the Vasa recta. These vessels carry blood from the glomerulus to the renal vein.

Salts like sodium and chloride ions, diffuse passively out of the thin ascending limb and pumped actively out of the thick ascending limb into the surrounding tissue fluids. This pumping of salts out of the limb creates an osmotic gradient which draws water out of the descending limb into the medulla. This is because the ascending limb is impermeable to water; therefore, water moves out of the limb only to the descending limb. When water in the descending limb is pumped out, it causes the fluid in the descending limb to have a slightly higher salt concentration compared to the ascending limb. The process continues down the length of the loop so that this concentration effect is multiplied. The counter current multiplier means that the fluid in and around the loop of Henle becomes saltier as it goes down the loop, and it is saltiest at the bottom end of the loop. In contrast, it becomes less salty as it goes up the ascending limb Therefore, the final salt concentration depends on the length of the loop, the longer the loop; the higher the final salt concentration in the tissues.

As water moves in the loop of Henle, it leaves the loop of Henle with the water potential greater than that of blood plasma.

This creates a concentration gradient between fluid in the distal convoluted tubule and the surrounding tissue fluid.

The concentration gradient is enhanced when salts are actively pumped out of the distal convoluted tubule. Reabsorption of water in the distal convoluted tubule and the collecting duct will depend on their permeability, which is controlled by the hormone called Antidiuretic hormone (ADH). This hormone is released by the posterior lobe of the pituitary gland in response to an increased concentration of salts in the blood. When ADH is present, more water is reabsorbed (blood volume and pressure rises) and the urine produced is more concentrated.

The regulation of salt and blood pressure are closely connected. If the sodium ion concentration in the blood is low, the blood water potential increases and water moves by osmosis into the tissue, slightly lowering the blood pressure. When blood pressure is not sufficient (below the set point), secretory cells near the glomerulus, juxtaglomerular apparatus secrete, renin. The later is an enzyme that changes angiotensinogen (angiotensinogen is a large plasma protein produced by the liver) into Angiotensin I. Later Angiotensin I is converted to Angiotensin II. When Angiotensin II reaches the adrenal cortex, it stimulates the secretion of aldosterone.

This hormone promotes the excretion of potassium ions and the reabsorption of sodium ions at the distal convoluted tubules. The reabsorption of sodium ions is followed by the reabsorption of water. Therefore, blood volume and blood pressure increase.

High temperature and low rainfall characterise some areas of this world. It becomes hard to believe that animals can survive in arid and semi-arid conditions.

However, these animals survive because they have adaptations that allow them to live in the hot, dry conditions. These adaptations help to balance thermoregulation with water gain and loss. For instance, many mammals that live in the desert obtain much or all of their water from the food they consume.

The reduced water intake is partially balanced through concentrated urine and dry faeces. Evaporative cooling helps to regulate temperature.

To limit the water loss through evaporative cooling, some mammals are nocturnal; have light coloration and other body features to help them dissipate heat and use micro-environments to reduce heat gain. This is only a short list of the many amazing adaptations. Characteristically, arid regions receive 100-250 mm of rain a year and semi-arid regions receive 250-500 mm of rain per year. The following are some of adaptations of mammals to the life in arid or semi-arid conditions:

1. Water and food consumption

   Human beings obtain about 60% of the water they need from ingested liquids, 30% from ingested food, and 10% from metabolism. Rodents and camels adapted to arid conditions obtain approximately 90% of their water from metabolism and 10% from ingested food.

2. Excretory adaptations

   The ability to excrete concentrated urine and dry feces is an important adaptation to arid conditions. Mammals adapted to the desert have a very long loop of Henle compared to animals that live in less arid regions and aquatic environments. A longer loop of Henle allows urine to become very concentrated due to osmotic gradients in the kidneys.

   Desert rodents produce concentrated urine about five times as concentrated as that of humans. A longer loop of Henle increases the efficiency of water reabsorption, thereby conserving water.

3. Behavioural adaptations

   Behavioural adaptations help animals reduce the amount of heat gained, thereby reducing the need for evaporative cooling. One basic behavioural adaptation is the timing of activity rhythms. Nocturnal animals regulate their heat load by resting during the day, as night-time temperatures are 15-20°C lower than daytime temperatures.

   Examples of nocturnal animals include the quoll, bilby, and the spinifex hopping mouse.

4. Torpor and metabolic rate

   Many mammals, such as rodents and squirrels, enter a period of torpor in response to severe heat. During torpor, metabolism decreases, and the heartbeat and respiratory system slow down based on a circadian rhythm.

   Torpor serves as a water-conserving mechanism as the animal's body temperature lowers, reducing the reliance on evaporation. If the period of torpor becomes extended, it is called aestivation or summer dormancy.