Kidney Function and Blood Flow Regulation

Kidney Basics

• Primary Function: Filter blood to remove waste.
• Blood Supply: Receives ~25% of cardiac output per heartbeat.
• Blood Volume: Heart pumps ~5 liters/minutes, kidneys receive ~1.25 liters/minute.
• Functional Unit: Nephron (1 million per kidney)
  • Components: Renal corpuscle (glomerulus + Bowman's capsule) and renal tubule.

Blood Flow and Filtration

• Flow Path:
  1. Renal artery to smaller arteries to afferent arterioles.
  2. Afferent arterioles to glomerulus.
  3. Glomerulus to efferent arterioles.
• Glomerulus: Forms filtrate (precursor to urine), filters plasma.
• Glomerular Filtration Rate (GFR): ~125 mL/min, 20% of plasma filtered.

Renal Tubule Structure

• Parts:
  • Proximal convoluted tubule.
  • Loop of Henle (ascending and descending limbs).
  • Distal convoluted tubule.
• Function: Exchange of waste, ions, water with peritubular capillaries.

Venous Return

• Peritubular capillaries form venous vessels, ultimately forming renal vein.
• Renal vein drains into the inferior vena cava.

Factors Affecting Renal Blood Flow

• Proportional to:
  • Pressure gradient (renal artery pressure - renal vein pressure) / Resistance.
• Increased Flow: High systemic BP + low renal arteriolar resistance.
• Decreased Flow: Low BP + high resistance.

Hormonal Regulation of Renal Blood Flow

• Adrenaline (Epinephrine):
  • Secreted by adrenal glands (fight-or-flight response).
  • Binds to alpha-1 adrenergic receptors in arterioles → constriction → ↓ Blood flow.
• Angiotensin II:
  • Synthesized in response to low BP; involves renin and ACE.
  • Constricts arterioles → ↓ Blood flow but keeps GFR constant at low levels.

Natriuretic Peptides

• Atrial Natriuretic Peptide (ANP) & Brain Natriuretic Peptide (BNP):
  • ANP: Secreted by atria.
  • BNP: Secreted by ventricles.
  • Respond to cardiac stretch → Dilate afferent arterioles & constrict efferent arterioles → ↑ Blood flow.

Other Molecules Influencing Renal Blood Flow

• Prostaglandins (E2 and I2): Dilate arterioles slightly under sympathetic stimulation.
• Dopamine: Low concentrations → Dilate arterioles → ↑ Blood flow.

Autoregulation

• Definition: Maintain constant renal blood flow and GFR over a range of BP (80-200 mmHg).
• Mechanisms:
  • Myogenic Mechanism: Smooth muscle cells contract when stretched by high BP.
  • Tubuloglomerular Mechanism: Macula densa cells sense increased filtrate flow, release adenosine → constriction of afferent arteriole → ↓ GFR.

Summary

• Increase Resistance/Decrease Blood Flow: Adrenaline, Angiotensin II.
• Decrease Resistance/Increase Blood Flow: ANP, BNP, Prostaglandins, Dopamine.
• Autoregulation Mechanisms: Myogenic reaction, tubuloglomerular feedback.