Urine Formation - Essentials
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.
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 loop 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.
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.
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.
Reabsorption and Secretion Regulation##
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.
