Question

How does the structure of a nephron encourage concentration of urine? What is the name of this process and how does it work?

Answer 1

Answer :

The ascending limb of the nephron loop is impermeable to water, but Na + and Cl – are pumped out into the surrounding fluids by active transport. As fluid travels up the ascending limb, it becomes less and less concentrated because Na + and Cl – are pumped out.The result is concentrated urine

When there is a water deficit in the body, the kidney forms a concentrated urine by continuing to excrete solutes while increasing water reabsorption and decreasing the volume of urine formed. The human kidney can produce a maximal urine concentration of 1200 to 1400 mOsm/L, four to five times the osmolarity of plasma.

Process :

The high concentration of salt in the medullary fluid is believed to be achieved in the loop by a process known as countercurrent exchange multiplication.

How it works :

The urine concentrating mechanism plays a fundamental role in regulating water and sodium excretion. When water intake is large enough to dilute blood plasma, a urine more dilute than blood plasma is produced; when water intake is so small that blood plasma is concentrated, a urine more concentrated than blood plasma is produced. In both cases, the total urinary solute excretion rate and the urinary sodium excretion rate are small and normally vary within narrow bounds.

In contrast to solute excretion, urine osmolality varies widely in response to changes in water intake. Following several hours without water intake, such as occurs overnight during sleep, human urine osmolality may rise to ∼1,200 mOsm/kg H2O, about 4-times plasma osmolality (∼290 mOsm/kg H2O). Conversely, urine osmolality may decrease rapidly following the ingestion of large quantities of water, such as commonly occurs at breakfast, human (and other mammals) urine osmolality may decrease to ∼50 mOsm/kg H2O. Most physiologic studies relevant to the urine concentrating mechanism have been conducted in species (rodents, rabbits) that can achieve higher maximum urine osmolalities than humans. For example, rabbits can concentrate to ∼1,400 mOsm/kg H2O, rats to ∼3,000 mOsm/kg H2O, mice and hamsters to ∼4,000 mOsm/kg H2O, and chinchillas to ∼7,600 mOsm/kg H2O.