The blood vessels in the medulla (vasa recta) form hairpin loops that run parallel to the loop of Henle and medullary collecting ducts. The blood enters the top of the vessel loop at an osmolarity of 300 mOsmol/L, as the blood flows down the loop deeper and deeper into the medulla, sodium and chloride do indeed diffuse into, and water out of the vessel. However, after the bend in the loop is reached, the blood then flows up the ascending vessel loop, where the process almost completely reversed. Thus, the hairpin loop structure of the vasa recta minimizes excessive loss of solute from interstitium by diffusion. At the same time, both the salt and water being reabsorbed from the loop of Henle and collecting ducts are carried away in equivalent amounts of bulk flow, as determined by the usual capillary Starling forces. This maintains the steady-state counter current gradient set up by the loops of Henle. Because of NaCl and water reabsorbed from the loop of Henle and collecting ducts, the amount of blood flow leaving the vasa recta is at least twofold higher than the blood flow entering the vasa recta.finally the total blood flow going through all of the vasa recta is a small % of the total of the renal blood flow. This helps to maintain or minimize the washout of the hypertonic interstitium of medulla.
As urea passes through the remainder of the nephron, is reabsorbed, secreted into the tubule and then reabsorbed again. This traps urea, an osmotically active molecule, in the medullary interstitium, thus increasing its osmolarity, in fact, urea contributes to the total osmolarity of the small medulla.
Urea is freely filtered in the glomerulus. Approx. 50% of the filtered urea is reabsorbed in the proximal tubule, and the remaining 50% enters the loop of Henle. So the thin descending and ascending limbs of the loop of Henle, urea that was accumulated in the medullary interstitium is secreted back into the tubular lumen of facilitated diffusion, virtually all of the urea that was originally filtered in the glomerulus is present in the fluid that enters the distal tubule. Some of the original urea is reabsorbed from the distal tubule and cortical collecting duct. Thereafter, about half of the urea is reabsorbed from the medullary collecting duct, whereas only 5% diffuses into the vasa recta.
One remaining amount is secreted back into the loop of Henle. 15% of the urea originally filtered remains in the collecting duct and is excreted in the urine. This recycling of urea through the medullary interstitium and minimal uptake by the vasa recta traps urea there and contributes to the high osmolarity.
As urea passes through the remainder of the nephron, is reabsorbed, secreted into the tubule and then reabsorbed again. This traps urea, an osmotically active molecule, in the medullary interstitium, thus increasing its osmolarity, in fact, urea contributes to the total osmolarity of the small medulla.
Urea is freely filtered in the glomerulus. Approx. 50% of the filtered urea is reabsorbed in the proximal tubule, and the remaining 50% enters the loop of Henle. So the thin descending and ascending limbs of the loop of Henle, urea that was accumulated in the medullary interstitium is secreted back into the tubular lumen of facilitated diffusion, virtually all of the urea that was originally filtered in the glomerulus is present in the fluid that enters the distal tubule. Some of the original urea is reabsorbed from the distal tubule and cortical collecting duct. Thereafter, about half of the urea is reabsorbed from the medullary collecting duct, whereas only 5% diffuses into the vasa recta.
One remaining amount is secreted back into the loop of Henle. 15% of the urea originally filtered remains in the collecting duct and is excreted in the urine. This recycling of urea through the medullary interstitium and minimal uptake by the vasa recta traps urea there and contributes to the high osmolarity.
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