The Juxtaglomerular Apparatus (JGA) is a specialised structure located at the junction of the afferent arteriole and the distal convoluted tubule (DCT). It consists of: Juxtaglomerular (JG) cells (in afferent arteriole wall) - secrete renin when blood pressure falls, Macula densa cells (in DCT wall) - sense NaCl concentration in tubular fluid - if low, stimulate renin release, and Mesangial cells (supporting cells). The JGA is critical for the RAAS (Renin-Angiotensin-Aldosterone System) and autoregulation of GFR. It maintains kidney blood flow and filtration within a normal range despite changes in systemic blood pressure.

Glomerular Filtration Rate (GFR) is the volume of plasma filtered by all the glomeruli per minute. It is the best measure of kidney function. Normal GFR: ~125 mL/min (180 L/day). GFR is estimated clinically using serum creatinine (eGFR formula) or creatinine clearance. GFR staging (CKD): G1 = >=90 mL/min (normal), G2 = 60-89 (mildly reduced), G3 = 30-59 (moderately reduced), G4 = 15-29 (severely reduced), G5 = <15 (kidney failure - dialysis/transplant needed). GFR decreases with age (~1 mL/min/year after age 30).

The kidney has two distinct regions: Renal Cortex (outer region) = contains glomeruli, Bowman's capsules, PCT, DCT, site of filtration and most reabsorption, reddish-brown in colour due to rich blood supply, contains cortical nephrons (85%). Renal Medulla (inner region) = contains loops of Henle and collecting ducts, organised into renal pyramids (8-18 triangular structures), tips of pyramids = renal papillae - drain urine into minor calyces, site of urine concentration. Renal Pelvis = funnel-shaped structure that collects urine from calyces and drains into the ureter.

Urea (CO(NH2)2) is the primary nitrogenous waste product in humans, formed in the liver through the Urea Cycle (Ornithine Cycle): Amino acids are deaminated (amino group -NH2 removed), the -NH2 group becomes ammonia (NH3) - highly toxic, ammonia is converted to urea in the liver - much less toxic, water-soluble, urea is transported in blood to the kidneys and excreted in urine. Normal blood urea: 7-25 mg/dL. Elevated blood urea = Uraemia (kidney failure). The urea cycle involves: Ornithine > Citrulline > Argininosuccinate > Arginine > Ornithine + Urea.

Nephritis is inflammation of the kidneys, most commonly affecting the glomeruli (glomerulonephritis). Types: Acute Glomerulonephritis (often follows Group A Streptococcal throat or skin infection - post-streptococcal glomerulonephritis. Symptoms: haematuria - cola-coloured urine, proteinuria, oedema, hypertension, reduced urine output), Chronic Glomerulonephritis (progressive destruction of glomeruli - CKD), Nephrotic Syndrome (massive proteinuria >3.5g/day, hypoalbuminaemia, oedema, hyperlipidaemia), and Interstitial Nephritis (inflammation of renal tubules and interstitium - drug reactions, infections). Diagnosis: urine analysis, renal biopsy. Treatment: immunosuppressants, blood pressure control.

The Urinary System consists of: Kidneys (x2 - filter blood and produce urine), Ureters (x2 - muscular tubes ~25-30 cm long that carry urine from each kidney to the bladder by peristalsis), Urinary Bladder (x1 - muscular sac that stores urine, capacity ~400-600 mL, lined with transitional epithelium/urothelium), and Urethra (x1 - tube that carries urine from the bladder to outside the body, length ~4 cm in females, ~20 cm in males - passes through prostate gland and penis). This is why UTIs (urinary tract infections) are more common in females - shorter urethra.

Creatinine is a waste product formed from the breakdown of creatine phosphate in muscle tissue. It is produced at a constant rate proportional to muscle mass and is freely filtered by the glomerulus and not reabsorbed - making it an excellent marker of GFR. Normal serum creatinine: Males = 0.7-1.2 mg/dL, Females = 0.5-1.0 mg/dL (lower due to less muscle mass). Elevated creatinine = impaired kidney function (CKD, acute kidney injury). Creatinine clearance is approximately equal to GFR - calculated using Cockcroft-Gault formula. eGFR (estimated GFR) is calculated from serum creatinine, age, sex, and race - used to stage CKD.

ADH (Antidiuretic Hormone / Vasopressin) is produced by the hypothalamus and released from the posterior pituitary gland. It acts on the collecting ducts and distal tubules of the nephron: ADH increases = collecting duct becomes more permeable to water = more water reabsorbed = concentrated urine (small volume). ADH decreases = less water reabsorbed = dilute urine (large volume). ADH is released when: blood osmolarity increases (dehydration), blood volume drops, stress. Diabetes Insipidus: ADH deficiency = massive production of dilute urine (polyuria up to 20 L/day), excessive thirst (polydipsia).

Normal urine is pale to deep yellow in colour. The yellow colour is primarily due to Urochrome (Urobilin) - a pigment produced from the breakdown of urobilinogen (itself a product of bilirubin metabolism from haemoglobin breakdown). Urine colour variations: Colourless (dilute urine - high fluid intake, diabetes insipidus), Dark yellow/amber (concentrated urine - dehydration), Orange (dehydration, certain drugs - rifampicin, liver disease), Red/Pink (blood/haematuria, beetroot ingestion, rifampicin), Brown/Cola (glomerulonephritis, myoglobinuria - muscle breakdown), Green (Pseudomonas infection, certain medications), and Foamy (excess protein - proteinuria).

Nephrotic Syndrome is characterised by the classic tetrad: 1) Massive proteinuria (>3.5 g/day - nephrotic range), 2) Hypoalbuminaemia (low serum albumin - because albumin is lost in urine), 3) Oedema (generalised - periorbital oedema in mornings, pedal oedema, ascites - due to low oncotic pressure), and 4) Hyperlipidaemia (elevated cholesterol/triglycerides) + Lipiduria (fat in urine - oval fat bodies, fatty casts). Causes in children: Minimal Change Disease (most common - responds well to steroids). Causes in adults: Membranous nephropathy, Focal Segmental Glomerulosclerosis (FSGS), Diabetic nephropathy. Treatment: steroids, ACE inhibitors (reduce proteinuria), manage oedema.