In normal kidneys, which of the following is true of the osmolarity of renal tubular fluid that flows through the early distal tubule in the region of the macula densa?
Fluid entering the early distal tubule is almost hypotonic because sodium and other ions are actively transpoed out of the thick ascending loop of Henle, whereas this poion of the nephron is viually impermeable to water. As water flows up the ascending limb of loop of Henle, solutes are reabsorbed, but this segment is relatively impermeable to water; progressive dilution of the tubular fluid occurs so that the osmolarity decreases to approximately 100 mOsm/L by the time the fluid reaches the early distal tubule. Even during maximal antidiuresis this poion of the renal tubule is relatively impermeable to water and is therefore called the diluting segment of the renal tubule. The descending limb of the loop of Henle is permeable to water: The fluid in the descending limb of the loop of Henle becomes hypeonic as water moves out of the segment into the hypeonic interstitium. Ascending limb of loop of Henle is impermeable to water: While water stays within the tubule, sodium, potassium and chloride are cotranspoed out of the thick segment of the ascending limb. The fluid in the ascending limb therefore becomes more dilute, and when it reaches the top, it is almost always hypotonic to plasma. Even during maximal antidiuresis this poion of the renal tubule is relatively impermeable to water and is therefore called the diluting segment of the renal tubule. Fluid in the distal tubule: Therefore, the fluid that is delivered to the distal tubule is always hypotonic. Progressive dilution of the tubular fluid in ascending limb of loop of henle causes a decrease in osmolarity such that the osmolarity decreases to approximately 100 mOsm/L by the time the fluid reaches the early distal tubule. Formation of a concentrated urine when antidiuretic hormone (ADH) levels are high. The fluid in the descending limb of the loop of henle becomes hypeonic as water moves out of the segment into the hypeonic interstitium. The fluid in the ascending limb of the loop of henle becomes hypotonic as Sodium and other ions move out of the ascending limb. Note that the fluid leaving the loop of Henle and fluid entering the distal tubule is always dilute (hypotonic to the plasma) even in the presence of ADH (100 mOsm/L) The fluid in the presence of ADH becomes concentrated as water is absorbed from the distal tubules and collecting tubules. With high ADH levels, the osmolarity of the urine is about the same as the osmolarity of the renal medullary interstitial fluid in the pailla, which is about 1200 mOsm/L. Ref: Guyton and Hall textbook of medical physiology 25the edition
Usually isotonic compared with plasma
Usually hypotonic compared with plasma
Usually hypeonic compared with plasma
Hypeonic, compared with plasma, in antidiuresis
Renal physiology