RS 2

Urine Formation

  • Urine formation comprises three basic renal processes:

    • Glomerular filtration: Movement of fluid from the glomerulus to Bowman's capsule.

    • Tubular reabsorption: Movement from the tubular system back to the vascular system.

    • Tubular secretion: Movement from the vascular system to the tubular system.

Glomerular Filtration

  • Glomerular filtration rate (GFR) measures the filtration produced by kidneys:

    • Daily output ≈ 180 L/day, but only 1-2 L become urine.

    • Approximately 99% of filtrate is reabsorbed.

    • Filtrate production rate = 125 ml/min.

Parameters Affecting Glomerular Filtration

  1. Structural Integrity

    • Filtration barrier composed of three layers:

    1. Glomerular capillary wall:

      • Single layer of endothelial cells with pores (fenestrae).

      • 100 times more permeable to water and solutes than other capillaries.

    2. Basement membrane:

      • Composed of collagen (provides strength) and glycoproteins (discourages filtration of proteins).

      • Glycoproteins are negatively charged leading to minimal protein presence in filtrate (~0.03%).

    3. Inner layer of Bowman's capsule:

      • Contains podocytes with foot processes creating narrow filtration slits.

  2. Glomerular Filtration Pressure

    • Adequate blood flow is essential for filtration:

      • Requires arterial blood pressure and effective vascular resistance (differential diameters of arterioles).

    • Three forces determine glomerular filtration pressure:

    1. Glomerular capillary blood pressure (GCP):

      • Approximately 55 mmHg, fluid pressure exerted by blood within glomerular capillaries.

      • Affected by arterial pressure and arteriole resistance.

      • Resistance due to a smaller diameter of efferent compared to afferent arterioles promotes filtration.

    2. Bowman's capsule hydrostatic pressure (CP):

      • Approximately 15 mmHg, opposes filtration.

    3. Plasma colloid osmotic pressure (COP):

      • Approximately 30 mmHg, also opposes filtration due to unequal distribution of plasma proteins.

Net Glomerular Filtration Pressure

  • Calculated as:
    Net \, Glomerular \, Filtration \, Pressure = GCP - CP - COP
    = 55 \, mmHg - 15 \, mmHg - 30 \, mmHg = 10 \, mmHg

Relation to GFR

  • Filtration pressure governs GFR: GFR \, \propto \, GCP - CP - COP

    • In further detail:
      GFR = Kf imes (GCP - CP - COP)

    • Where K_f is the filtration coefficient.