You can live with one kidney, but not without its function. Every heartbeat sends blood to these bean-shaped organs, where waste is filtered, electrolytes are balanced, and blood pressure is quietly regulated.
🫀 1️⃣ Overview of the Urinary System
The renal (urinary) system includes: 2 kidneys, 2 ureters, 1 urinary bladder, and 1 urethra.
Main Functions
- Excretion of waste products (urea, uric acid, creatinine)
- Regulation of water, electrolytes, and acid-base balance
- Maintenance of blood pressure and osmolarity
- Production of hormones (erythropoietin, renin, calcitriol)
🫘 2️⃣ The Kidneys — Structure and Location
Shape: Bean-shaped, reddish-brown organs
Location: Retroperitoneal, on either side of the vertebral column (T12–L3 level)
Size: ~11 cm long, 6 cm wide, 3 cm thick
Right kidney slightly lower (due to liver)
Gross Anatomy
| Part | Description / Function |
|---|---|
| Hilum | Entry/exit of renal artery, vein, lymphatics, ureter |
| Cortex | Outer layer; contains glomeruli and convoluted tubules |
| Medulla | Inner layer; made up of renal pyramids |
| Renal pyramids | Contain loops of Henle and collecting ducts |
| Renal pelvis | Funnel-shaped chamber → narrows to form ureter |
⚙️ 3️⃣ The Nephron — Functional Unit of the Kidney
Each kidney has ~1 million nephrons, each acting as a miniature filter and reabsorber.
| Part | Structure / Function |
|---|---|
| Renal corpuscle | Made up of Bowman’s capsule + glomerulus (site of filtration) |
| Proximal convoluted tubule (PCT) | Reabsorbs most water, Na⁺, glucose, amino acids |
| Loop of Henle | Creates osmotic gradient (descending = water-permeable; ascending = salt-permeable) |
| Distal convoluted tubule (DCT) | Selective reabsorption; under hormonal control |
| Collecting duct | Final urine concentration (regulated by ADH) |
🧠 Types of nephrons:
- Cortical nephrons (85%) — short loops, mainly filtration
- Juxtamedullary nephrons (15%) — long loops; crucial for concentrating urine.
🧬 4️⃣ The Blood Supply of the Kidney
The kidneys receive 20–25% of cardiac output (~1.2 L/min).
Pathway of Renal Circulation:
Renal artery → Segmental → Interlobar → Arcuate → Interlobular → Afferent arteriole → Glomerulus → Efferent arteriole → Peritubular capillaries (or vasa recta) → Veins → Renal vein → IVC
💡 Key difference:
- Afferent arteriole → brings blood to glomerulus
- Efferent arteriole → drains it (still arterial!)
🧩 5️⃣ The Juxtaglomerular Apparatus (JGA)
This is the nephron’s control center for regulating blood pressure and filtration.
| Component | Function |
|---|---|
| Macula densa (in DCT) | Senses NaCl concentration |
| Juxtaglomerular (granular) cells | Secrete renin → activates RAAS |
| Extraglomerular mesangial cells | Communication between the two |
🧠 When BP or Na⁺ drops → JGA releases renin → RAAS activation → ↑ BP, ↑ Na⁺ reabsorption.
🚰 6️⃣ Ureters, Bladder, and Urethra
| Structure | Function |
|---|---|
| Ureters | Muscular tubes (~25 cm) → transport urine from renal pelvis to bladder via peristalsis |
| Urinary bladder | Hollow muscular organ (detrusor muscle) → stores urine (~500 mL capacity) |
| Urethra | Carries urine outside the body; sphincters regulate flow (internal = involuntary, external = voluntary) |
💡 Micturition reflex: Stretch receptors in bladder wall → signal to spinal cord → detrusor contracts → internal sphincter relaxes → voluntary control by external sphincter.
🧠 7️⃣ Clinical Correlations
| Condition | Key Issue | Effect |
|---|---|---|
| Hydronephrosis | Obstruction of urine flow | Dilation of renal pelvis & calyces |
| Renal stones (calculi) | Crystallization of salts | Flank pain, hematuria |
| Polycystic kidney disease | Cystic dilation of nephrons | Enlarged kidneys, renal failure |
| Pyelonephritis | Bacterial infection of renal pelvis | Fever, back pain |
| Renal failure | Loss of nephrons | Uremia, acidosis, electrolyte imbalance |
🧩 8️⃣ Summary Table — Key Structures and Functions
| Structure | Main Function |
|---|---|
| Glomerulus | Filtration of plasma |
| PCT | Reabsorption of solutes & water |
| Loop of Henle | Concentrates urine |
| DCT | Fine-tunes electrolyte balance |
| Collecting duct | Final water reabsorption (ADH-controlled) |
| JGA | BP regulation via renin |