Functions of the Kidneys – overview 

1. Urine Production (Renal Excretory Function) 

Urine formation occurs in the nephron and involves three key processes: 

A. Glomerular Filtration 
Occurs at the renal corpuscle (glomerulus + Bowman’s capsule) 
Blood is filtered under pressure 
Allows water and small solutes (Na⁺, K⁺, glucose, urea) to pass 
Prevents filtration of proteins and blood cells 
Glomerular Filtration Rate (GFR) ≈ 125 mL/min (normal adult) 

Clinical relevance: 
↓ GFR → renal failure; assessed via serum creatinine and eGFR 

B. Tubular Reabsorption 

Mainly in proximal convoluted tubule (PCT) 
Reabsorbs: 
~65% of water and Na⁺ 
100% of glucose and amino acids (normally) 
Hormonal control: 
ADH → ↑ water reabsorption (collecting duct) 
Aldosterone → ↑ Na⁺ reabsorption, ↑ K⁺ secretion (DCT) 

C. Tubular Secretion 
Active movement from blood → tubular lumen 
Removes: 
H⁺ (acid–base balance) 
K⁺ 
Drugs (e.g. penicillin) 
Creatinine 
Purpose: Fine-tuning of urine composition 

2. Urine Elimination 

A. Urine Transport 
Urine flows: 
Collecting ducts → renal pelvis → ureter → bladder 

B. Storage 
Urinary bladder stores urine (≈400–600 mL) 

C. Micturition (Voiding) 
Controlled by: 
Parasympathetic nerves (S2–S4) → bladder contraction 
Voluntary control of external urethral sphincter 
Coordinated reflex involving spinal cord and cerebral cortex 
Clinical relevance: 
Neurogenic bladder, urinary retention, incontinence 

3. Regulation of Electrolyte Balance 
The kidneys maintain homeostasis of electrolytes and water: 

Sodium (Na⁺) 
Major determinant of ECF volume and blood pressure 
Regulated by: 
Aldosterone → Na⁺ retention 
ANP → Na⁺ excretion 

Potassium (K⁺) 
Critical for cardiac and neuromuscular function 
Excreted mainly in DCT and collecting ducts 
Aldosterone increases K⁺ secretion 

Clinical relevance: 
Hyperkalemia → arrhythmias (life-threatening) 

Calcium (Ca²⁺) and Phosphate 

Regulated by: 
Parathyroid hormone (PTH) → ↑ Ca²⁺ reabsorption 
Kidneys activate vitamin D → ↑ intestinal Ca²⁺ absorption 

Acid–Base Balance 
Kidneys: 
Excrete H⁺ 
Reabsorb/generate HCO₃⁻ 
Maintain blood pH 7.35–7.45 (normal value) 
 
4. Renal Blood Supply:

A. Blood Flow 
Kidneys receive 20–25% of cardiac output 
Essential for effective filtration 

B. Arterial Supply (in sequence) 
Renal artery 
Segmental arteries 
Interlobar arteries 
Arcuate arteries 
Interlobular arteries 
Afferent arteriole → glomerulus 
Efferent arteriole 

C. Peritubular Capillaries & Vasa Recta 
Peritubular capillaries: reabsorption in cortical nephrons 
Vasa recta: maintain medullary concentration gradient 

D. Autoregulation of Renal Blood Flow 
Maintains constant GFR despite BP changes 
Mechanisms: 
Myogenic response 
Tubuloglomerular feedback (macula densa) 
Clinical relevance: 
NSAIDs, ACE inhibitors, shock → impaired renal perfusion 

Summary Table