*Kidney: Structure, Function, and Clinical Significance* 1. *Introduction* The kidney is a vital organ in the urinary system and plays a crucial role in maintaining homeostasis by filtering blood, removing waste, balancing electrolytes, and regulating blood pressure. 2. *Anatomy of the Kidney* 2.1 *Location and Structure* Paired, bean-shaped organs located in the retroperitoneal space on either side of the spine. Right kidney is slightly lower than the left due to liver position. Each kidney measures about 10–12 cm in length, 5–7 cm in width, and weighs 120–150 grams in adults. 2.2 *Gross Anatomy* *Renal Capsule:* A fibrous outer covering. *Cortex:* Outer layer containing glomeruli and convoluted tubules. *Medulla:* Inner portion with renal pyramids containing loops of Henle and collecting ducts. *Renal Pelvis:* Funnel-shaped structure that collects urine before it enters the ureter. *Hilum:* Entry and exit point for the renal artery, renal vein, and ureter. 2.3 *Microscopic Anatomy* *Nephrons:* The functional unit of the kidney (~1 million per kidney). *Glomerulus:* A tuft of capillaries where blood filtration begins. *Bowman’s Capsule:* Encases the glomerulus, initiating urine formation. *Tubular System:* Includes proximal convoluted tubule (PCT), loop of Henle, distal convoluted tubule (DCT), and collecting ducts. 3. *Functions of the Kidney* 3.1 *Filtration of Blood* Filters approximately 50–60 mL/min per kidney (glomerular filtration rate or GFR ~120 mL/min in total). Removes toxins, metabolic waste (urea, creatinine), and excess substances. 3.2 *Fluid and Electrolyte Balance* Regulates sodium (Na+), potassium (K+), calcium (Ca2+), and chloride (Cl-) levels. Maintains fluid homeostasis by adjusting water reabsorption through antidiuretic hormone (ADH). 3.3 *Acid-Base Balance* Regulates blood pH by excreting hydrogen ions (H+) and reabsorbing bicarbonate (HCO3-). 3.4 *Blood Pressure Regulation* Renin-Angiotensin-Aldosterone System (RAAS): Helps control blood pressure and fluid balance. Releases renin in response to low blood pressure, leading to vasoconstriction and sodium retention. 3.5 *Erythropoiesis Regulation* Produces erythropoietin (EPO), stimulating red blood cell production in the bone marrow. 3.6 *Vitamin D Activation* Converts inactive vitamin D into its active form (calcitriol), essential for calcium absorption. 4. *Clinical Significance of the Kidney* 4.1 *Common Kidney Diseases* *Acute Kidney Injury (AKI):* Sudden loss of kidney function due to shock, sepsis, or nephrotoxins. *Chronic Kidney Disease (CKD):* Progressive decline in kidney function due to diabetes, hypertension, or glomerulonephritis. *Nephrotic Syndrome:* Characterized by proteinuria, hypoalbuminemia, and edema. *Nephritic Syndrome:* Hematuria, hypertension, and oliguria due to glomerular inflammation. *Polycystic Kidney Disease (PKD):* Genetic disorder causing fluid-filled cysts in the kidney. *Urinary Tract Infections (UTIs):* Infection of the renal system, commonly affecting the bladder (cystitis) or kidneys (pyelonephritis). *Renal Stones (Nephrolithiasis):* Formation of calculi due to supersaturation of urine with calcium, oxalate, or uric acid. 4.2 *Diagnostic Tests for Kidney Function* *Serum Creatinine:* Increased levels indicate impaired kidney function. *Blood Urea Nitrogen (BUN):* Elevated in kidney dysfunction. *Glomerular Filtration Rate (GFR):* Determines the stage of kidney disease. *Urinalysis:* Detects protein, blood, and abnormal substances in urine. *Renal Ultrasound/CT Scan:* Identifies structural abnormalities. *Biopsy:* Helps diagnose glomerular diseases. 4.3 *Treatment and Management* *Lifestyle Modifications:* Low-sodium diet, adequate hydration, and avoiding nephrotoxic drugs. *Medications:* ACE inhibitors (for hypertension), diuretics, erythropoietin for anemia. *Dialysis:* Hemodialysis or peritoneal dialysis for end-stage renal disease. *Kidney Transplant:* Definitive treatment for end-stage renal failure.