Urine formation

Filtration

  • The first step in urine formation is filtration, in which water and solutes, including ions and nutrients, are filtered from the blood to produce ultrafiltrate.
  • Filtration is a passive process, and relies on pressures within the renal blood vessels and nephron.
  • Ultrafiltrate passes through the nephron tubule as tubular fluid.

Reabsorption

  • Removes solutes and water from the tubular fluid and returns them to the blood;
  • It reclaims much much of the water, ions, and nearly all of the nutrients that are filtered are reclaimed via reabsorption.

Secretion

  • Moves solutes from the blood and nephron tubule cells into the tubular fluid;
  • Secretion is important for removal of substances that aren’t filtered (such as drugs and metabolites) and for fine-tuning the final urine composition.

Nephron segments

  • Renal corpuscle is where the blood is filtered and tubular fluid is formed. .
  • Proximal tubule is the primary site of reabsorption of water, ions, and nutrients.
    Specifically: sodium, potassium, water, and nutrients are reabsorbed from the proximal tubule; hydrogen, which plays a major role in acid/base balance, is secreted into the proximal tubule.
  • The nephron loops is the U-shaped segment:
    It comprises descending and ascending limbs.
    Water is reabsorbed from the descending limb; solutes are reabsorbed from the ascending limb. .
    This is where urine is either diluted or concentrated, depending on the body’s needs.
  • Distal tubule is the distal portion of the nephron.
  • The collecting duct delivers urine to the renal pelvis.
    In these last two segments, reabsorption and secretion are hormonally regulated to maintain water and ion homeostasis.

Specifically:

  • Sodium and water are reabsorbed from the distal tubule.
  • Hydrogen and potassium are secreted into it.
  • Sodium and water are reabsorbed from the collecting duct;
  • Potassium and hydrogen are either reabsorbed or secreted, depending on the body’s needs. It is also a primary site of secretion of substances that weren’t filtered but need to be excreted in the urine.
  • The nephron loop either dilutes or concentrates the tubular fluid, depending on the body’s needs.
  • Activity of the the distal tubule and collecting duct is hormonally regulated to maintain ECF volume and osmolarity homeostasis.

Key ways that reabsorption and secretion are regulated:

  • First, it’s important to recognize that, in general, water and other solutes follow sodium: when sodium is reabsorbed, they usually are, too (the exception is the ascending limb of the nephron loop).
  • Therefore, altering sodium reabsorption is an effective way to alter the reabsorption of other solutes and water.
  • This may be necessary, for example, in the case of high blood pressure:
  • To help reduce blood volume, diuretics reduce sodium reabsorption to facilitate increased excretion of solutes and water.
  • On the other hand, in cases of low blood pressure, the body needs to conserve solutes and water.
  • Two key hormones that facilitate this are:
  • Aldosterone increases reabsorption of sodium within the distal nephron; since water follows sodium, it also increases blood volume.
  • Anti-diuretic hormone directly increases the rate of water absorption in the distal nephron, and, therefore, blood volume.

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