Physiology of Urine Formation Notes

Mechanism Of Urine Formation

It includes 3 basic processes

• Glomerular filtration is the ultrafiltration of the blood through the glomerulus into the bowmen capsule.
• Tubular reabsorption is the process that moves solutes and water from the nephron into peribulbar capillaries.
• Tubular secretion is the process that moves substances from peribulbar capillaries to the renal tubular lumen; this process is the opposite of reabsorption.

Urinary excretion: Filtration – absorption + secretion

Glomerular filtration

It is determined by

• Glomerular capillary filtration coefficient (Kf).
• Starling forces across Glomerular capsule and bowmen space.
• Glomerular capillary filtration coefficient (Kf) depends on.
  • Glomerular membrane hydraulic permeability (conductivity).
  • Effective filtration surface area.
• Starling forces across Glomerular capsule and bowmen space.

Tubular reabsorption and secretion

There are two pathways of reabsorption and secretion, paracellular and Transcellular. In the paracellular pathway, solute passes from tight junction between two adjacent cells while in the Transcellular pathway, the solute has to cross the luminal membrane and basolateral membrane.

Reabsorption takes place by 2 basic mechanisms, active and passive transport:

Active transport – energy is required to transport substances against a concentration gradient (uphill).

• Primary active transport – energy is utilized in form of ATP
  • Example:  Na+ K+ pump.
• Secondary active transport – energy is utilized in the form of an electrochemical gradient of one substance to transport another substance.
  • Co-transport – when both substances move in the same direction.
    • Example: Na+ Glucose
  • Counter transport – when two substances move in opposite direction
    • Example:  Na+ H+.

Passive transport – energy is not required, transport occurs along a concentration gradient (downhill).

• Osmosis – when solvent moves along a concentration gradient.
• Diffusion – when solute moves along a concentration gradient.
  • Simple – transport of solute without the aid of transmembrane integral proteins.
  • Facilitated – transport of solute with the aid of transmembrane integral proteins.