Renal system

1. Role of the Renal System in Homeostasis

The kidney maintains homeostasis by regulating:

  • Blood Volume and Pressure: Adjusting the amount of water and electrolytes reabsorbed.

  • Ionic Composition and pH: Regulating Na+, K+, Cl−, HCO3−, and H+ to maintain acid-base balance.

  • Osmolarity: Keeping the solute concentration in extracellular fluids constant.

  • Excretion of Wastes: Such as urea, creatinine, and drugs.

  • Hormone Production:

    • Calcitriol: Active form of vitamin D, regulates calcium absorption.

    • Erythropoietin: Stimulates red blood cell production in response to hypoxia.

2. Glomerular Function and Filtration

Renal Corpuscle

  • Glomerulus: Blood enters the glomerulus via the afferent arteriole and exits via the narrower efferent arteriole, creating high pressure that drives filtration.

  • Bowman’s Capsule: Collects the filtrate through its double-walled structure.

  • Filtration Membrane: Comprises endothelial cells, the basement membrane, and podocyte filtration slits.

Filtration Dynamics

  • Net Filtration Pressure (NFP): Determines the amount of filtrate formed.NFP=GBHP−(CHP+BCOP)NFP=GBHP−(CHP+BCOP) Example:NFP=55−(15+30)=10 mmHgNFP=55−(15+30)=10mmHg This small positive pressure allows 180L/day of fluid to be filtered.

Knowledge Check 1: How might a high-protein diet affect kidney function and why?

  • A high-protein diet increases the production of urea (a waste product of protein metabolism), which places extra stress on the kidney to excrete it. Additionally, high protein levels can increase glomerular blood pressure, raising the filtration load and potentially causing long-term damage to the filtration membrane.

3. Glomerular Filtration Rate (GFR)

Definition:

  • GFR measures the volume of filtrate formed per minute (~105–125 mL/min in healthy adults).

Factors Affecting GFR:

  • Glomerular Blood Pressure (GBHP): Increased BP raises GFR; reduced BP lowers it.

  • Capsular Hydrostatic Pressure (CHP): Increased backpressure reduces GFR.

  • Plasma Osmotic Pressure (BCOP): Increased osmotic pressure (e.g., from dehydration) reduces GFR.

Knowledge Check 2: If blood pressure decreases slightly, how is renal filtration pressure maintained?

  • Autoregulation Mechanisms:

    • Myogenic Mechanism: Afferent arterioles dilate when BP drops, maintaining consistent blood flow.

    • Tubuloglomerular Feedback: Macula densa cells sense reduced flow and signal afferent arterioles to dilate.

4. Tubular Function: Reabsorption and Secretion

Proximal Tubule

  • Reabsorbs: 67% of Na+, Cl−, K+, and water. Nearly all glucose and amino acids are reabsorbed.

  • Mechanisms:

    • Na+ is actively transported via Na+-K+ ATPase.

    • Glucose is reabsorbed via Na+-glucose symporters.

  • Secretion: Organic cations, anions, and drugs.

Knowledge Check 3: Why was p-aminohippuric acid (PAH) infused with penicillin during WWII?

  • Penicillin is quickly excreted by the kidneys. PAH competes with penicillin for the same transporters, slowing penicillin’s clearance and prolonging its therapeutic effect.

5. Loop of Henle

Function:

  • Descending Limb: Permeable to water, impermeable to solutes; water is reabsorbed.

  • Ascending Limb: Impermeable to water, actively reabsorbs NaCl.

Knowledge Check 4: Why do we reabsorb 99% of the filtrate?

  • Reabsorbing most of the filtrate allows the kidneys to fine-tune water and solute levels for homeostasis while removing waste products efficiently.

6. Regulation of Water Balance

Antidiuretic Hormone (ADH):

  • Released when plasma osmolarity increases.

  • Promotes water reabsorption in the collecting duct by inserting aquaporins into the apical membrane.

Knowledge Check 5: Why does ethyl alcohol increase urine output?

  • Alcohol inhibits ADH secretion, reducing water reabsorption in the collecting duct. This leads to dilute urine and increased volume (diuresis).

7. Regulation of Sodium Balance

Key Mechanisms:

  1. RAAS:

    • Low BP → Renin release → Angiotensin II → Aldosterone.

    • Aldosterone increases Na+ reabsorption and K+ secretion.

  2. Natriuretic Peptides:

    • Released in response to high blood volume.

    • Promote Na+ excretion (natriuresis) and reduce GFR.

Knowledge Check 6: What happens if sodium transport is blocked in the renal tubule?

  • Urine volume increases as water follows Na+ into the tubule.

  • Clinical Use: Diuretics like loop diuretics block Na+ transport to treat hypertension and fluid overload.

8. Clinical Relevance

Diabetic Nephropathy:

  • Elevated angiotensin II causes efferent arteriole vasoconstriction, increasing glomerular pressure. Over time, this damages the filtration membrane, allowing proteins to enter the urine (proteinuria).

Knowledge Check 7: How does angiotensin II contribute to proteinuria in diabetic nephropathy?

  • Increased pressure from efferent arteriole constriction damages the glomerular capillaries, increasing permeability and allowing proteins to leak into the filtrate.