03.1 Physiology of the renal tubular system

Renal Tubular System Physiology

• The renal tubular system is crucial for urine formation, modification, and excretion.
• It involves reabsorption, secretion, and excretion of water, solutes, and ions in nephron segments.
• Membrane permeability and transport mechanisms maintain fluid and electrolyte balance.

Learning Objectives

• Describe reabsorption and secretion processes along the nephron.
• Explain segment-specific permeability in the nephron.
• Discuss transport maximum and its effect on solute reabsorption (e.g., glucose).

Key Concepts and Definitions

• Glomerular Filtration: Initial urine formation by filtering plasma through the glomerulus into the nephron.
• Tubular Reabsorption: Moving solutes and water from the nephron back into the bloodstream.
• Tubular Secretion: Transferring waste and excess substances from blood to the nephron for excretion.
• Transport Maximum (Tm): Maximum reabsorption rate of a solute; excess beyond this is excreted in urine.

Clinical Applications

• Case Study: Diabetic patients with glucosuria due to elevated plasma glucose exceeding the transport maximum.
• Diagnostic Approach: Measuring plasma glucose and urine glucose to diagnose hyperglycaemia.
• Treatment Options: Managing blood glucose levels with insulin or oral hypoglycaemics to prevent glucosuria.
• Complications/Management: Monitoring for dehydration due to osmotic diuresis (caused by glucosuria).

Pathophysiology

• Sodium Reabsorption in Proximal Tubule: Sodium-potassium ATPase drives sodium reabsorption, leading to passive water reabsorption, maintaining isosmotic conditions.
• Countercurrent Mechanism in Loop of Henle: Thick ascending limb reabsorbs sodium, impermeable to water, establishing a concentration gradient for water reabsorption in the collecting duct.

Pharmacology

• Loop Diuretics: Inhibit sodium-potassium-2 chloride transporter in the thick ascending limb, increasing urine output.
• Thiazide Diuretics: Block sodium-chloride symporter in the distal convoluted tubule, resulting in mild diuresis and reduced blood pressure.

Differential Diagnosis

• Diabetes Mellitus: Hyperglycaemia and glucosuria due to exceeding renal glucose threshold.
• Diabetes Insipidus: Polyuria and polydipsia due to impaired ADH secretion or renal response.
• Acute Kidney Injury (AKI): Reduced urine output and azotaemia due to decreased glomerular filtration or tubular damage.

Investigations

• Glomerular Filtration Rate (GFR): Measures kidney function by assessing plasma filtration rate.
• Urinalysis: Detecting substances (glucose, proteins, ions) indicating impaired tubular reabsorption or secretion.

Summary and Key Takeaways

• Proximal tubule reabsorbs solutes and water; loop of Henle establishes osmotic gradient.
• Sodium and water reabsorption is regulated by hormones (aldosterone, ADH) in distal nephron.
• Transport maximum limits solute reabsorption; exceeding it leads to solute loss in urine.