In-depth Notes on Renal Physiology

Renal Physiology Overview

• Dr. Jeremy Pullicino's Lecture on renal function encompasses several key topics related to kidney physiology.

Learning Objectives

• Overview of Renal Function.

• Major features of the nephron (the kidney's functional unit).

• Renal blood flow and glomerular filtration.

• Mechanisms of fluid and solute reabsorption in:

  • Proximal convoluted tubule.

  • Loop of Henle.

  • Distal nephron.

• Acid-base homeostasis.

Renal Function Overview:

Key Kidney Functions:

• Maintain Body Fluid Balance: Controls hydration levels.

• Regulate Electrolyte Balance: Maintains levels of Na⁺, Cl⁻, K⁺, and other ions.

• Waste Filtration: Removes metabolic waste (urea, creatinine, etc.) from the blood.

• Acid-Base Regulation: Controls blood pH by managing hydrogen ions (H⁺) and bicarbonate (HCO₃⁻).

Major Components of the Kidney:

• Nephron: Functional unit of the kidney responsible for urine formation and homeostasis.

• Renal Blood Flow Pathway:

  • Renal Arteries → Segmental Artery → Interlobular Artery → Arcuate Arteries → Afferent Arteriole → Glomerulus → Efferent Arteriole → Peritubular Capillaries → Arcuate Vein → Renal Vein.

  • Filtration Pressure: Higher pressure in the afferent arteriole compared to the efferent arteriole helps in filtration (Net filtration pressure: 10 mmHg).

Renal Blood Flow and Filtration:

• Glomerular Filtration Rate (GFR):

  • Normal GFR (Adult): >90 ml/min.

  • Decreases with age even in healthy individuals.

  • eGFR: Estimated GFR used to calculate kidney function.

The Nephron:
The nephron is where all the important functions of the kidney occur, and it is made up of different segments:

1. Glomerulus:

   • Function: Filtration of blood to produce filtrate (fluid that enters the renal tubules).

   • Process: Blood pressure forces fluid and solutes from the blood into the nephron.

2. Proximal Convoluted Tubule (PCT):

   • Function: Reabsorption of key substances like water, sodium, glucose, and amino acids.

   • Mechanism: Sodium is actively reabsorbed via Na⁺/K⁺ ATPase pumps, creating a gradient for other solutes. Approximately 70% of water is reabsorbed here.

3. Loop of Henle:

   • Function: Concentrates urine and creates a hyperosmotic environment in the medulla.

   • Descending Limb: Permeable to water (aided by aquaporins).

   • Ascending Limb: Impermeable to water but actively pumps Na⁺ and Cl⁻ to create a concentrated medullary interstitium.

   • Countercurrent Multiplication: Process where the fluid moves in opposite directions in the descending and ascending limbs to concentrate urine.

4. Distal Convoluted Tubule (DCT):

   • Function: Secretion and reabsorption of ions (Na⁺, K⁺, H⁺, HCO₃⁻).

   • Regulated by Hormones: Aldosterone and parathyroid hormone (PTH) influence calcium and phosphate levels.

5. Collecting Duct:

   • Function: Final adjustments to urine concentration and volume.

   • Regulated by ADH (Antidiuretic Hormone): Stimulates water reabsorption by inserting aquaporins into the collecting duct walls, which concentrates the urine. Without ADH, the collecting duct remains impermeable to water, leading to dilute urine.

Acid-Base Homeostasis:
The kidneys play a critical role in maintaining blood pH within the range of 7.35–7.45 by regulating the balance of H⁺ and HCO₃⁻.

• HCO₃⁻ Reabsorption: The kidneys reabsorb bicarbonate from urine to prevent acidosis.

• H⁺ Secretion: The kidneys secrete hydrogen ions into the urine to help balance the pH.

• Excretion of Titratable Acids and Ammonium: The kidneys excrete H⁺ in the form of titratable acids and ammonium to buffer pH.

Conditions of Acid-Base Imbalance:

• Acidosis (pH < 7.35):

  • Metabolic Acidosis: Caused by decreased bicarbonate (HCO₃⁻).

  • Respiratory Acidosis: Caused by increased CO₂.

• Alkalosis (pH > 7.45):

  • Metabolic Alkalosis: Caused by increased bicarbonate (HCO₃⁻).

  • Respiratory Alkalosis: Caused by decreased CO₂.

Summary:
The kidneys, through the nephron, perform critical functions such as filtration, reabsorption, secretion, and regulation of fluid, electrolyte, and acid-base balance. The nephron's structure allows for efficient urine formation and homeostasis, ensuring that waste is removed while vital substances are retained.