02.1 Glomerular filtration and blood flow

Glomerular Filtration and Renal Blood Flow

• Renal blood flow (RBF) is approximately 1100 mL/min, 22% of cardiac output. Kidneys are highly perfused for metabolic needs and use oxygen at twice the rate of the brain.
• Renal plasma flow (RPF) is calculated as RBF x (1 - hematocrit). A typical hematocrit is 45%, leading to an RPF around 605 mL/min.
• Glomerular Filtration Rate (GFR) is the rate at which plasma is filtered into Bowman's capsule, typically 125 mL/min in normal adults.
• The filtration fraction is about 20% of RPF.
• Ultrafiltration occurs as fluid moves across the glomerular filtration barrier into Bowman's space. It lacks proteins and blood cells. The process is influenced by Starling forces.

Key Concepts

• Starling Forces are the balance of hydrostatic and oncotic pressures influencing glomerular filtration.
  • Glomerular hydrostatic pressure (PGC) promotes filtration.
  • Bowman's space hydrostatic pressure (PBS) opposes filtration.
  • Glomerular oncotic pressure (πGC) opposes filtration.
• Net Filtration Pressure (NFP) is the combined effect of these forces driving fluid into Bowman's space.

Pathophysiology

• Filtration Barrier: The glomerular capillaries filter based on size and charge, allowing small, positively charged molecules to pass more readily.
• Starling Forces influence the direction and rate of fluid movement across the glomerulus.

Pharmacology

• Vasodilators: Drugs like ACE inhibitors or angiotensin II receptor blockers reduce efferent arteriolar resistance, decreasing glomerular pressure and GFR.
• Diuretics: Used to manage fluid balance but high doses might reduce renal perfusion.
• NSAIDs: These drugs can constrict afferent arterioles which reduces GFR and RBF.

Clinical Applications

• Case Study: Prolonged hypotension due to hemorrhage can lead to renal dysfunction and needs assessment of GFR and RBF dynamics.
• Diagnostic Approach: Serum creatinine is measured to estimate GFR, and ultrasound assesses renal blood flow.
• Treatment options include restoring blood pressure with fluids or vasopressors and using diurétics to balance fluids.
• Complications/Management: Acute kidney injury (AKI) can arise from prolonged ischemia, resulting in reduced GFR and RBF. Management focuses on addressing the underlying cause and optimizing renal perfusion.

Differential Diagnosis

• Acute Kidney Injury (AKI): Reduced GFR and oliguria often from hypotension or nephrotoxins.
• Chronic Kidney Disease (CKD): Gradual loss of GFR over time often related to electrolyte and fluid overload.
• Nephrotic Syndrome: Characterized by proteinuria related to damage in the glomerular filtration barrier.

Investigations

• Serum Creatinine: A marker for GFR, with higher levels indicating reduced kidney function.
• Creatinine Clearance Test: Estimating GFR based on urine creatinine levels.
• Imaging (Ultrasound): Useful for assessing renal blood flow and identifying blockages.

Questions for Clarification

• Question 1: How does efferent arteriole constriction affect glomerular filtration rate (GFR) in short-term and long-term conditions?
• Question 2: What pathological conditions lead to increased Bowman's capsule hydrostatic pressure?

This quiz covers the key concepts of glomerular filtration and renal blood flow, including the calculations for renal plasma flow and filtration rate. Understand the influence of Starling forces on the filtration process and the physiological significance of these parameters.