Pathophysiology
Clinical meaning
The kidneys are paired, bean-shaped organs located retroperitoneally in the upper abdomen that perform essential homeostatic functions: filtration of metabolic waste products (urea, creatinine, uric acid) from the blood, regulation of fluid volume and osmolality, electrolyte balance (sodium, potassium, calcium, phosphorus), acid-base balance (bicarbonate reabsorption and hydrogen ion excretion), blood pressure regulation (through the renin-angiotensin-aldosterone system and fluid volume control), erythropoietin production (stimulating red blood cell production in the bone marrow), and activation of vitamin D (calcitriol, necessary for calcium absorption). The functional unit of the kidney is the nephron, of which each kidney contains approximately one million. Each nephron consists of the glomerulus (a tuft of capillaries within Bowman capsule where blood is filtered under hydrostatic pressure -- approximately 180 liters of filtrate produced daily), the proximal convoluted tubule (reabsorption of 65% of filtered sodium, water, glucose, amino acids, bicarbonate), the loop of Henle (countercurrent mechanism establishing the medullary concentration gradient for urine concentration), the distal convoluted tubule (fine-tuning of sodium, potassium, and acid-base balance under aldosterone and PTH influence), and the collecting duct (final urine concentration under ADH...