Lack of the solute in the kidney marrow: the distinctive features of the Henle loop
Water diffuses out of the lower end of the branch down to the loop of Henle into the interstitial canal and the osmotic pressure of the tubular fluid gradually increases as it flows towards the tip of the Henle loop.
A major reason for the high osmotic pressure of the renal medulla region is the active transport of sodium and co-transporters of potassium, chloride, and other ions from the thickened portion to the loop of Henle into the interstitial medulla. This pump is capable of setting a 200-milliosmole gradient between the inside of the renal tubule and the interstitial fluid. Because the lower end portion of the Henle loop is almost impervious to water, the solutes are pumped out, not followed by an osmotic flow of water into the interstitial marrow. Thus, the active transport of sodium and other ions out of the dendrites adds excess water-soluble substances to the interstitial renal marrow. Passive reabsorption of sodium chloride from the lower end of the branch to the Henle loop, is also impermeable, in addition to the high solute concentration of the interstitial renal medulla.
The lower end of the descending branch of the Henle loop, in contrast to the lower end of the ascending branch, is very hydrophilic, and the tubular fluid osmolar pressure quickly becomes equal to the medullary osmotic pressure. As a result, water diffuses out of the lower end of the Henle loop into the interstitial canal and the osmotic pressure of the tubular fluid gradually increases as it flows towards the tip of the Henle loop.
Board. Summary of tubular properties - Urine concentration.
ADH, antidiuretic hormone; NaCl, sodium chloride; 0, active transport or minimum permeability; +, active transport or moderate permeability; ++, aggressive transport or high permeability; + ADH, the permeability of water or urea increases in the presence of ADH.
Figure. The system of retrograde multiplication in the loop of Henle causes the renal marrow to increase secretion (numerical values in milliosmoles per litre).
The steps involved lead to the hypertonic interstitial renal medulla region. Following the above characteristics of the Henle loop in mind, let us now discuss how the kidney marrow becomes hypertonic. First, assume that the Henle loop is filled with fluid at a concentration of 300 mOsm / L, the same as when leaving the proximal tubule (step 1). Next, the action of the ion pump at the lower end of the branch on the loops of Henle reduces the concentration inside the renal tubule and increases the concentration of the interstitial medulla; This pump establishes a gradient of 200 mOsm / L between the tubular fluid and the interstitial fluid (step 2). The limit for the gradient is about 200 mOsm / L because the "paracellular" diffusion of the ions back into the tubule eventually equilibrates with the transport of ions out of the lumen when a gradient concentration of 200 mOsm / L is reached. Step 3 is the tubular fluid in the lower end of the branch to the loop of Henle and the interstitial fluid quickly reaches an osmotic equilibrium due to the osmosis of water from the lower end of the lower branch. The interstitial fluid osmolality is maintained at 400 mOsm / L due to the continued transport of ions out of the branch thicket onto the Henle loop. Thus, by itself, the active transport of sodium chloride out of the upper end of the branch is capable of setting a gradient concentration of only about 200-mOsm / L, which is much less than that achieved. by the upstream multiplication system. Step 4 involves the outflow of extra fluid to the loop of Henle from the proximal tubule, causing the previously hypertonic fluid to be formed in the lower end of the descending branch into the lower end of the ascending arm. Once this fluid is at the lower end of the ascending branch, the newly added ions are pumped into the interstitial marrow, with the remaining water in the tubular fluid, until the 200 mOsm / L gradient osmotic pressure is established, and the interstitial fluid osmolality increases to 500 mOsm / L (step 5). Then, again, the fluid in the lower end of the branch reaches equilibrium with the hypertonic medullary fluid (step 6), and as the hypertonic tubular fluid from the lower end of the branch to the loop of Henle flows into the lower end. Branched up, the solute is continuously pumped out of the renal tubule and deposited into the interstitial marrow.
These steps are repeated over and over, with the real effect of adding more and more solute to the kidney marrow area when excess water is available; With enough time, this process gradually retains the solutes in the kidney marrow and increases several times with the gradient concentration established by the action of pumping ions out of the branch thickener onto the Henle loop, eventually raising the interstitial fluid osmolality reaches 1200-1400 mOsm / L, as shown in step 7.
Thus, the repeated reabsorption of sodium chloride by the thickened portion on the Henle loop and continued inflow of new sodium chloride from the near glider into the Henle loop is called the reverse multiplication mechanism. Sodium chloride reabsorbed from the branch to the loop of Henle continues to be added to the newly arrived sodium chloride, thereby "multiplying" its concentration in the interstitial marrow.