the correct answer is:
This portion of a nephron functions to control the amount of water in urine so that concentrated urine is produced when one is dehydrated distal convoluted tube-(low blood volume/pressure) or dilute urine is produced when one is overhydrated ascending and descending limbs of the Loop of Henle (high blood volume/pressure).
Explanation:
The kidneys are responsible for the production of urine through the mechanisms of ultrafiltration, tubular reabsorption and tubular secretion. Depending on the need for water, either dehydrated or hydrated state, a concentrated and small volume or dilute and large volume urine is produced, in order to maintain the relative water balance of the kidneys.
The nephron is the functional unit of the kidneys.
Now, let me explain how concentrated and dilute urine are produced;
Dilute Urine: Dilute urine is a urine with a high water content. The need to excrete a large volume of water occurs when the water content of the blood is high resulting in high blood volume (hypervolemia) or high blood pressure, hence, to bring back the pressure and volume of blood to normal level, the body gets rid of excess water through the urine. The process of dilution of urine is brought about by the counter-current mechanism in the renal medulla. The concentration of the Renal Medulla increases progressively along with the descending limb of the Loop of Henle. The tubular filtrate flow in opposite directions as the Loop descends and ascends, creating a counter-current mechanism. as the fluid in the compartment of the Henle's Loop ascends, NaCl is excreted into the interstitial spaces, increasing the interstitial solute concentration, at the same time the loop is impermeable to water, making the fluid in the distal ascending limb of the loop to be dilute, and the remaining part of the nephron is impermeable to water hence a large volume of dilute urine is excreted.
Concentrated urine: The need for concentrated urine arises when the water content of the blood is low, resulting in low blood volume/pressure. Hence, the need to excrete a small volume of water in the urine, resulting in a small urine volume that is concentrated. The concentration is brought about by the reabsorption of water in the distal convoluted tubule and the collecting duct. These regions become permeable to water under the influence of the hormone Vasopressin which is secreted in a dehydrated state.
