Page 359 - Biology_F5

Biology for Advanced Level Secondary Schools

7.4.3 Hormonal control of This hormone promotes the excretion
osmoregulation of potassium ions and the reabsorption
As water moves in the loop of Henle, it of sodium ions at the distal convoluted
leaves the loop of Henle with the water tubules. The reabsorption of sodium ions
potential greater than that of blood plasma. is followed by the reabsorption of water.
This creates a concentration gradient Therefore, blood volume and blood
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between fluid in the distal convoluted pressure increase.
tubule and the surrounding tissue fluid.
The concentration gradient is enhanced 7.4.4 Adaptations of mammals to arid
and semi-arid conditions
when salts are actively pumped out of the
distal convoluted tubule. Reabsorption of High temperature and low rainfall
water in the distal convoluted tubule and characterise some areas of the world. It
the collecting duct will depend on their becomes hard to believe that animals can
survive in arid and semi-arid conditions.
permeability, which is controlled by the However, these animals survive because
hormone called Antidiuretic hormone they have adaptations that allow them
(ADH). This hormone is released by the to live in the hot, dry conditions.
posterior lobe of the pituitary gland in These adaptations help to balance
response to an increased concentration of thermoregulation with water gain and
salts in the blood. When ADH is present, loss. For instance, many mammals that
more water is reabsorbed (blood volume live in the desert obtain much or all of
and pressure rise) and the urine produced their water from the food they consume.
is more concentrated. The reduced water intake is partially
balanced through concentrated urine and

The regulation of salt and blood pressure dry faeces. Evaporative cooling helps to
are closely connected. If the sodium ion regulate temperature.
concentration in the blood is low, the
blood water potential increases and water To limit the water loss through evaporative
moves by osmosis into the tissue, slightly cooling, some mammals are nocturnal;
lowering the blood pressure. When blood have light coloration and other body
pressure is not sufficient (below the set features to help them dissipate heat and
point), secretory cells near the glomerulus, use micro-environments to reduce heat
juxtaglomerular apparatus secrete renin. gain. This is only a short list of the many
The later is an enzyme that changes amazing adaptations. Characteristically,
angiotensinogen (angiotensinogen is a arid regions receive 100 mm - 250 mm
large plasma protein produced by the liver) of rain per year and semi-arid regions
into Angiotensin I. Later Angiotensin receive 250 mm - 500 mm of rain per year.
I is converted to Angiotensin II. When The following are some of adaptations of
Angiotensin II reaches the adrenal cortex, mammals to the life in arid or semi-arid
it stimulates the secretion of aldosterone. conditions:

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