Dilute and Concentrated Urine Formation: Renal Mechanisms

Posted on Jan 28, 2025 in Biology

Mechanisms of Formation of Dilute Urine

In extreme situations of hydration (overload):

  • Proximal tubule: Resorption is equivalent to water reabsorption. The osmolarity is constant.
  • Descending loop of Henle: The renal medulla is the only part of the body that is hyperosmolar, and it becomes more hyperosmolar the deeper we go towards the calyces. Water moves into the interstitium, and there is no solute reabsorption (increasing intratubular osmolarity).
  • Ascending loop of Henle: The osmolarity decreases by increasing the filtration of solutes, but there is no water filtration.
  • Collecting duct:
    • Aquaporin II channels are assembled, and therefore, water is removed as it does not leave the interstitium. The osmolarity decreases.

In this situation of extreme overload, urine can become as dilute as 50 mOsm/L. If this is maintained over time, one could urinate 20 liters a day (but if this happens, it is a pathologic condition).

Mechanisms of Formation of Concentrated Urine

In situations of extreme dehydration (hypovolemia):

Aquatic animals can afford to produce urine at the same concentration as body fluids, but not in the water; we are constantly losing water through mechanisms such as ventilation, sweat, etc. By losing water by other means, if we urinated at 300 mOsm/L, we would dehydrate.

Land animals need to urinate at osmolarities higher than body fluids. 60-70 g/day of solutes that have to be urinated correspond to about 700 mOsm/L.

In dehydration, humans can form urine up to 1200 mOsm/L or less. The greater the adaptation to dehydration, the greater the ability to concentrate urine. The conditions for this to occur are:

  • High rates of ADH (antidiuretic hormone).
  • Existence of large medullary hyperosmolarity:
    • This is achieved by increased interstitial sodium (greater the deeper the zone of the medulla) due to three factors: the thick portion of Henle reabsorbs a lot of sodium, the thick portion of Henle is impermeable to water, and fluids are constantly in tubular system movement.

Phases:

  • It is the same in the proximal tubule.
  • It is the same in the descending portion of Henle.
  • Ascending portion of Henle (causes hyperosmolality, the greater the more deeply into the medulla):
    • Reabsorption of solutes.
    • No water reabsorption.
    • Permanent circulation of urea. Urea cycle:
      • Hepatic urea is a substance that is produced in the deamination of amino acids.
      • It is filtered at the glomerulus at 4.5 mOsm/L. During the tubular system, its concentration increases in the descending loop of Henle and is impermeable up to it.
      • When we are in dehydration, when there is a lot of ADH, it is permeable in the collecting duct in the portion of the medulla in all structures of the nephron (more important in juxtamedullary nephrons, and introducing more in the medulla). It produces urea reabsorption, which contributes 20% to the medullary hyperosmolarity (a phenomenon of urea reabsorption).
      • In dehydration, a high-protein diet increases the concentration of urea (not recommended except in specific cases). This allows us to adapt to dehydration.

Another associated factor is the vasa recta:

  • The vascular system of the renal medulla is different from the rest of the body since there would be a constant washing, preventing the formation of interstitial hyperosmolarity of the renal medulla.
  • Features:
    • Receiving only 10% of renal vascularization (poorly vascularized).
    • The vasa recta are straight (taken from the efferent arterioles and have no branches, so that the arterioles, as you delve into the medulla (capillaries), they lose water and gain solutes. This gain of osmolarity, upon returning to the cortex, is lost (recovering all substances had, except the O2 and nutrients)).
    • Allow you to maintain the hyperosmolarity.
    • The vasa recta are found only in the renal medulla and intestine.