Excretion of dilute and concentrated urine
Excretion of dilute urine
When there is excess of water in the body and a reduction in plasma osmolarity, kidneys can excrete a dilute urine with a concentration as low as 50 mOsm/L. This can be achieved by reabsorbing only the solutes and not water in the distal parts of the nephron.
The total amount of solute excreted remains constant but the urine formed is very dilute and urine osmolarity decreases from 600 to about 100 mOsm/L.
The glomerular filtrate initially formed has the osmolarity similar to that of plasma (300 mOsm/L). Inorder to excrete excess water it is necessary to dilute the filtrate as it passes along the tubule which is achieved by reabsorption of solutes to a greater extent than water.
As the fluid flows through the proximal tubule solutes and water are reabsorbed in equal amount and there is a little change in osmolarity but when it reaches down the descending limb of loop of Henle water is reabsorbed by osmosis and the tubular fluid reaches the osmolarity of renal medulla, i.e. it becomes hyperosmotic.
In the ascending limb of loop of Henle sodium, potassium and chloride are actively reabsorbed. Therefore, the fluid becomes dilute as it flows to distal tubule with osmolarity decreasing to 100 mOsm/L (osmolarity is one third of that of plasma).
When the fluid passes to late distal tubule and collecting tubule there is little reabsorption of sodium chloride. In the absence of ADH, collecting tubule is impermeable to water and additional reabsorption of solutes causes tubular fluid to become even more dilute, decreasing its osmolarity to as low as 50 mOsm/L.
Therefore, failure of reabsorption of water and continued reabsorption of solutes leads to a large volume of dilute urine.
Excretion of a concentrated urine
- For formation of a concentrated urine two basic requirements are essential
- High level of ADH which increases the permeability of distal tubule and collecting tubule to water.
- High osmolarity of the renal medullary fluid which provides osmotic gradient for reabsorption of water in the presence of ADH.
When the tubular fluid leaves the loop of Henle and flows into the distal convoluted tubule in the renal medulla, fluid is dilute with a osmolarity of 100 mOsm/L.
As fluid flows into the cortical collecting tubule, the amount of water reabsorption is dependent on plasma concentration of ADH. In the presence of high concentration of ADH the cortical collecting tubule becomes highly permeable to water and large amounts of water are reabsorbed.
As the fluid flows into the medullary collecting tubule there is further water reabsorption and when the fluid reaches the end of the collecting ducts the osmolarity reaches 1200 mOsm/L which is similar to renal medullary osmolarity.
Therefore, by reabsorbing as much as water as possible, kidneys form a highly concentrated urine excreting normal amount of solutes in urine and increasing the ECF volume.