A 23-year-old hiker loses a foothold and falls several feet during a mountain climb. He sustains no major injury but loses his backpack. He walks in the heat without water for the next 8 hours; by the time he reaches a shelter, he is extremely thirsty. Fluid from which of the following nephron segments is likely to be the most hypoosmotic in this individual?

Question

A 23-year-old hiker loses a foothold and falls several feet during a mountain climb.  He sustains no major injury but loses his backpack.  He walks in the heat without water for the next 8 hours; by the time he reaches a shelter, he is extremely thirsty.  Fluid from which of the following nephron segments is likely to be the most hypoosmotic in this individual?

Quizplus · Accepted Answer

11ec160b_3cfe_de87_9249_7fdafcc82db1_MD0001_00 The following changes in osmolarity occur as glomerular filtrate transits the nephron: Glomerular capillaries are highly permeable to water and other solutes, producing filtrate in the Bowman space that is roughly isoosmolar with plasma (~275-300 mOsm/L). The proximal convoluted tubule actively reabsorbs electrolytes, glucose, and amino acids; water follows passively, and lumenal osmolarity remains unchanged. In the descending limb of the loop of Henle, water is reabsorbed passively into the hypertonic renal interstitium; tubular fluid osmolarity increases (~1,200 mOsm/L). The thin and thick ascending limbs of the loop of Henle are impermeable to water, but electrolytes (primarily sodium and chloride) are actively reabsorbed in large quantities, leading to increasingly dilute tubular fluid. When the fluid reaches the initial part of the distal convoluted tubule, it will be at its lowest osmolarity (~100 mOsm/L) up to that point in the nephron (Choices A, B, C, and D). Reabsorption of water in the latter part of the distal tubule and collecting duct is variable and regulated by antidiuretic hormone (ADH). At baseline (ie, well hydrated), water permeability of the ductal epithelium is low, leading to production of dilute urine. However, in the dehydrated state (eg, hypotension, high plasma osmolarity), secretion of ADH increases, greatly enhancing permeability of the collecting duct to water and rapidly concentrating the urine. This dehydrated individual likely has high circulating ADH levels to help minimize further fluid loss. Therefore, the fluid leaving the early distal tubular segment will be the most hypoosmotic; in the latter part of the distal tubule and collecting duct, ADH will facilitate diffusion of water into the hypertonic medullary interstitium and urine concentration will rise up to 1,200-1,400 mOsm/L (Choices F and G). Educational objective: Dehydration (eg, hypotension, high plasma osmolarity) increases circulating levels of antidiuretic hormone, increasing the water permeability of the collecting duct and allowing production of highly concentrated urine. In the dehydrated state, tubular fluid osmolarity will be lowest at the beginning of the distal convoluted tubule (ie, terminus of loop of Henle).

A 23-Year-Old Hiker Loses a Foothold and Falls Several Feet