9E Urinary System Filtration Pressure and Rate

Components of Filtration Pressure

• Glomerular Hydrostatic Pressure (GHP): High blood pressure across capillaries; approximately $55\,mmHg$.

• Capsular Hydrostatic Pressure (CsHP): Opposes GHP; approximately $15\,mmHg$.

• Net Hydrostatic Pressure (NHP): Calculated as $NHP = GHP - CsHP = 40\,mmHg$. Drives movement out of capillaries.

• Blood Colloid Osmotic Pressure (BCOP): Result of plasma proteins; opposes NHP; approximately $30\,mmHg$. Drives movement back into capillaries.

• Filtration Pressure (FP): Calculated as $FP = NHP - BCOP = 10\,mmHg$.

Glomerular Filtration Rate (GFR)

• Definition: Amount of filtrate produced by the kidney per minute.

• Values: Approximately $125\,ml/min$ or $180\,L/day$.

• Measurement: Measured by creatinine clearance.

• Control Mechanisms: Autoregulation (local response), Autonomic regulation, and Hormonal regulation.

GFR Regulation Mechanisms

• Autonomic Regulation: Involves sympathetic innervation; vasoconstricts the afferent arteriole to decrease GFR (e.g., during exercise).

• Hormonal Regulation: Controlled via the Renin-angiotensin system and Natriuretic peptides.

Renin-Angiotensin-Aldosterone System (RAAS)

• Activation: The Macula densa senses low fluid flow or low $Na^+$ concentration, triggering Juxtaglomerular cells to secrete renin.

• Enzymatic Pathway:

  • Liver releases angiotensinogen into the blood.

  • Renin converts angiotensinogen to Angiotensin I.

  • Angiotensin-converting enzyme (ACE) in pulmonary blood converts Angiotensin I to Angiotensin II.

• Angiotensin II Actions:

  • Causes widespread vasoconstriction.

  • Stimulates the Adrenal cortex to secrete Aldosterone.

• Fluid and Ion Reabsorption:

  • Aldosterone: Stimulates $Na^+$ uptake in the distal convoluted tubule and collecting ducts.

  • ADH: Causes aquaporins to move to the collecting duct plasma membrane to increase $H_2O$ reabsorption.