Chronic Kidney Disease (CKD) is a progressive, irreversible deterioration of kidney function lasting >3 months. The most common causes are: Diabetes mellitus (diabetic nephropathy - leading cause worldwide), Hypertension (hypertensive nephrosclerosis - second most common), Glomerulonephritis, Polycystic kidney disease, recurrent UTIs. CKD stages by GFR (G1-G5). Stage 5 = End Stage Renal Disease (ESRD) - requires dialysis or kidney transplant. Complications: anaemia (reduced EPO), hypertension, bone disease, cardiovascular disease, hyperkalaemia. Early detection: urine albumin:creatinine ratio (ACR), eGFR.

Bowman's Capsule (Glomerular capsule) is the cup-shaped, double-walled structure at the beginning of each nephron that completely surrounds the glomerulus. Together, the Bowman's capsule and glomerulus form the Renal Corpuscle - the filtration unit of the nephron. Blood is filtered from the glomerular capillaries into the Bowman's space (between the two layers of the capsule), forming the glomerular filtrate (primary urine). Named after Sir William Bowman (1842). The inner layer contains specialised cells called podocytes that form filtration slits.

Micturition (Urination/Voiding) is the process of expelling urine from the bladder through the urethra. The micturition reflex: Bladder fills (~300 mL) - stretch receptors in bladder wall activated - Signals sent to sacral spinal cord (S2-S4) - Parasympathetic stimulation - detrusor muscle contracts, internal sphincter relaxes - Somatic nerve - voluntary relaxation of external urethral sphincter - Urine expelled. Voluntary control is learned during childhood (toilet training). Loss of bladder control = Urinary incontinence. Inability to urinate = Urinary retention (common in prostate enlargement in men).

Osmoregulation is the homeostatic regulation of the osmotic pressure (solute concentration) of body fluids. The kidneys are the primary osmoregulatory organs. Key mechanisms: When blood osmolarity rises (too concentrated - dehydration) = Osmoreceptors in hypothalamus detect change > ADH released > water reabsorbed > thirst stimulated > dilute blood. When blood osmolarity falls (too dilute - overhydration) = ADH suppressed > water excreted > concentrated blood. The hypothalamus acts as the osmoreceptor centre and controls ADH release. Normal plasma osmolarity = 280-295 mOsm/kg. This ensures cells neither shrink nor swell due to osmotic imbalances.

The Glomerulus is a tiny ball-shaped network (tuft) of capillaries located inside the Bowman's capsule. It receives blood from the afferent arteriole and drains into the efferent arteriole. The glomerulus is the site of ultrafiltration - blood pressure forces small molecules (water, glucose, urea, salts, amino acids) out of the blood into the Bowman's capsule. Large molecules like proteins and blood cells are retained in the blood. The glomerular filtration rate (GFR) is approximately 125 mL/min or 180 L/day in healthy adults - the gold standard measure of kidney function.

Urethra differences between sexes: Female urethra = ~3-4 cm long, carries urine only, opens between clitoris and vaginal opening. Short length = higher risk of UTIs (Urinary Tract Infections) - bacteria from anal region easily reach bladder. Male urethra = ~18-20 cm long, divided into Prostatic (passes through prostate), Membranous (shortest, passes through urogenital diaphragm), and Spongy/Penile (longest, passes through penis). Carries both urine and semen (but not simultaneously). The male's longer urethra is protective against ascending UTIs.

The Renal Threshold for Glucose is the plasma glucose concentration above which the tubular maximum (Tm) for glucose reabsorption is exceeded and glucose spills into urine (glucosuria). Normally: All filtered glucose is reabsorbed in the PCT via SGLT2 (sodium-glucose cotransporter 2) active transport. Renal threshold = ~180 mg/dL (10 mmol/L). When blood glucose exceeds 180 mg/dL (as in uncontrolled diabetes mellitus): SGLT2 transporters are saturated, glucose appears in urine (glucosuria), glucose in tubule draws water with it (osmotic diuresis) causing polyuria and polydipsia. SGLT2 inhibitors (e.g., empagliflozin, dapagliflozin) lower blood glucose by blocking glucose reabsorption - used in diabetes treatment.

Normal urine composition: Water (~95%), Urea (main nitrogenous waste ~2% - product of protein catabolism), Creatinine (waste product of muscle metabolism), Uric acid (from nucleic acid metabolism), Salts (ions) - Sodium, potassium, chloride, phosphates, sulphates, and Ammonia (small amount). Normal urine is pale yellow (due to urochrome pigment from bilirubin breakdown), slightly acidic (pH 4.6-8.0, average 6.0), specific gravity 1.001-1.035. Abnormal findings: Glucose (glucosuria) = diabetes, Protein (proteinuria) = kidney disease, Blood (haematuria) = kidney stones/infection/cancer.

Kidney Stones (Renal Calculi / Nephrolithiasis) are hard, crystalline deposits that form in the kidneys or urinary tract when urine becomes supersaturated with certain minerals. Types: Calcium oxalate (most common - ~80%), Calcium phosphate, Uric acid stones (in gout patients), Struvite stones (infection-related - Proteus bacteria), and Cystine stones (rare - genetic). Symptoms: severe colicky flank pain (renal colic), haematuria, nausea, vomiting. Diagnosis: CT scan (gold standard), ultrasound. Treatment: hydration, pain relief, lithotripsy (sound waves to break stones), surgery. Prevention: high fluid intake.

Proteinuria is the abnormal presence of protein (mainly albumin) in urine, indicating that the glomerular filtration barrier is damaged. Normally, the glomerulus prevents large proteins from passing into the filtrate. Normal urine protein 3.5g/day), Diabetic nephropathy (microalbuminuria = earliest sign of diabetic kidney disease - 30-300 mg/day), Glomerulonephritis, and Hypertensive nephropathy. Proteinuria causes: Hypoalbuminaemia > reduced oncotic pressure > oedema (pitting oedema of legs, face, ascites - due to low oncotic pressure). Diagnosis: Dipstick urinalysis, 24-hour urine collection, ACR (albumin:creatinine ratio).