49 Urine Formation
The process of osmosis plays a major role in the regulation of urine concentration. Recall that osmosis refers to the flow of a liquid across a semipermeable membrane to equalize the solute concentration on both sides of the membrane. The ability of the solution to cause osmosis, its osmotic activity, depends on the amount and not the type of solute particles that cannot pass through the semipermeable membrane. For example, in the same volume of water, the osmotic activity of 10 sodium ions is the same as that of 10 glucose molecules, and the same as 10 proteins, and so on. Thus, the osmotic activity of a solution is based on the osmolarity of that solution, a quality defined as the number of solute particles dissolved in one kilogram of water. Osmotic activity is frequently measured in units of 0.001 mole of particles, a milliosmole (mOsm). Osmolarity is measured in mOsm/liter.
Formation of Dilute Urine
The ratio of water and solutes in glomerular filtrate is the same as that in blood, with an osmolarity of around 300 mOsm/liter (one mOsm is translated as a milli osmole or one thousandth of an osmole). The osmolarity of the filtrate that enters the proximal convoluted tubule is still the same. And when filtrate enters the nephron loop’s descending limb, its osmolarity is the same as both blood plasma and cortical interstitial fluid. Because tubular fluid is already dilute as it flows through the ascending limb of Henle’s loop, the formation of dilute urine is a relatively simple process. When the hypothalamus (via the posterior pituitary gland) is not secreting ADH, additional water is not reabsorbed in the collecting ducts and dilute urine is produced. Before urine enters the renal pelvis, it is further diluted by the removal of ions, including sodium ions, in the distal convoluted tubule and collecting duct. As a result, the osmolarity of urine can be reduced to nearly one fourth of that in the glomerular filtrate or plasma, dropping to as low as 70 mOsm.
Formation of Concentrated Urine
If a person does not drink enough water, sweats a lot, or has diarrhea, water conservation is required. In such cases, the kidneys will produce a small amount of concentrated urine to preserve water, while still eliminating wastes and surplus ions. Antidiuretic hormone (ADH), as its name suggests, inhibits diuresis (urine production). ADH helps the kidneys generate urine that is up to four times as concentrated as glomerular filtrate or blood plasma (up to 1,200 mOsm/liter for urine versus 300 mOsm/liter for glomerular filtrate). ADH can do this only when there is an osmotic gradient of solutes in the renal medulla’s interstitial fluid, in other words, when the solute concentration in the interstitial fluid is higher than that in the tubular fluid. This osmotic gradient, coupled with the insertion of aquaporins into the luminal membrane of the collecting ducts’ principal cells, causes water and urea to pass from the filtrate into the interstitial space to equalize osmolarity. At maximum ADH secretion, as much as 99 percent of the water in the tubular filtrate is reabsorbed, and the kidneys produce about half a liter of highly concentrated urine per day. The production of concentrated urine is key to our ability to survive for an extended period of time without water.
