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Question

To regulate the osmotic balance in the human body, the kidneys

A
increase the volume of urine in summer
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B
decrease the volume of urine in summer
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C
decrease the volume of urine in winter
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D
keep the volume of urine constant during every season
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Solution

The correct option is B decrease the volume of urine in summer
All organisms respond to the changes in the abiotic factors (temperature, pH) in their environment.

Regulators are organisms that respond by regulating their internal environment to maintain a constant stable state. Examples include mammals and birds.

Conformers are organisms that change their internal environment according to the changes in the external environment. Examples include most invertebrates and plants.

Humans are considered as regulators who respond to any changes in their external environment by maintaining a constant internal environment (homeostasis). This can be achieved by various mechanisms such as thermoregulation (to regulate constant body temperature) and osmoregulation (to regulate a steady osmotic balance of the body).

Osmoregulation refers to the maintenance of the osmotic pressure of the cells by regulating a stable concentration of water and solutes in the blood. The kidneys play an important role in this function.

During summers the body tries to cool itself by losing water in the form of sweat. As the sweat evaporates from the body, it takes away body heat and cools it. But excessive sweating leads to decrease in the concentration of water in the blood, thus reducing blood volume and pressure. In such circumstances, under the influence of ADH (antidiuretic hormone) from the pituitary gland, the kidneys increase the reabsorption of water from the nephrons during urine formation. This results in the decrease of the volume of urine and helps to increase blood volume and blood pressure to restore the ideal osmotic balance of the body. Hence option b is correct and option a is incorrect.

In winters, due to lack of sweating the concentration of water in blood is high. This causes the pituitary to restrict the release of ADH and hence reabsorption of water in the nephrons is also reduced. Excess water is allowed to be lost along with urine, resulting in an increase in urine volume. Hence both option c and d are incorrect.

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