Memory tricks for kidneys and urine formation

Kidneys produce urine → Ureters (peristalsis carries urine down) → Urinary Bladder (stores 400-600 mL) → Urethra (eliminates urine). Female urethra is 3-4 cm — short, which is why UTIs are more common in women. Male urethra is 20 cm and has three parts: prostatic, membranous, and spongy. Each kidney contains approximately 1 million nephrons — the functional units of the kidney.

Filtrate flows through: Glomerulus (filtration) → Bowman's capsule → Proximal Convoluted Tubule (most reabsorption) → Loop of Henle (descending and ascending, concentration gradient) → Distal Convoluted Tubule (fine-tuning, hormone regulated) → Collecting Duct (final water reabsorption, ADH dependent) → Renal pelvis → Ureter. Cortical nephrons handle most filtration. Juxtamedullary nephrons have long loops of Henle — concentrate urine.

Filtration: blood pressure forces water, ions, glucose, amino acids, urea through glomerular membrane into Bowman's capsule (180 L/day). Reabsorption: 99% of filtrate is returned to blood — glucose, amino acids, water, Na+ all reabsorbed. Secretion: additional waste products actively added to filtrate — H+, K+, drugs, toxins. Excretion: remaining filtrate (~1.5 L/day) becomes urine and is excreted. What remains in the tubule = urine.

Renin (from kidney juxtaglomerular cells) → activates angiotensin → aldosterone released → Na+ reabsorption → BP increases. ADH (antidiuretic hormone, from posterior pituitary) → increases water reabsorption in collecting duct → concentrated urine. Erythropoietin (EPO) → produced by kidneys → stimulates RBC production in bone marrow. Chronic kidney disease → low EPO → anemia. Athletes illegally use synthetic EPO to boost RBC count.

Renal cortex (outer): contains glomeruli and convoluted tubules — most of the nephron is here. Renal medulla (inner): contains renal pyramids and loops of Henle. Renal pyramids: cone-shaped structures — tips (papillae) drain urine into minor calyces. Minor calyces → major calyces → renal pelvis → ureter. The hilum is the medial indentation where the renal artery, vein, and ureter enter/exit. Adrenal glands sit on top of each kidney.

GFR measures how well the kidneys filter blood. Normal GFR = 125 mL/min = 180 L/day filtered. Only ~1.5 L/day becomes urine — 99% is reabsorbed. GFR decreases with age (normal decline). CKD staging is based on GFR: Stage 1 = GFR >90, Stage 5 = GFR <15 (kidney failure, dialysis needed). Creatinine is the standard lab measure of GFR — high creatinine = low GFR = kidneys not filtering well. Three forces determine GFR: glomerular blood pressure (promotes), capsule pressure (opposes), osmotic pressure (opposes).

ADH (vasopressin) from the posterior pituitary controls water reabsorption in the collecting duct. High ADH → more aquaporins inserted → more water reabsorbed → concentrated urine (dark, small volume). Low ADH → few aquaporins → little water reabsorbed → dilute urine (light, large volume). Diabetes insipidus = absent ADH → massive dilute urine output. SIADH = too much ADH → water retention → hyponatremia. Alcohol suppresses ADH → diuresis → dehydration.

Bladder fills with urine → stretch receptors in bladder wall activate at ~300 mL → signal sent to sacral spinal cord (S2-S4) → parasympathetic signals cause detrusor muscle to contract → internal urethral sphincter relaxes (involuntary, smooth muscle). External urethral sphincter is skeletal muscle — under voluntary control. Brain can suppress the reflex until appropriate. Spinal cord injury above S2 → loss of voluntary control → reflex voiding. Damage at S2-S4 → flaccid bladder (retention).

Kidney stones (nephrolithiasis) travel from the renal pelvis down the ureter to the bladder. Three anatomical narrowings where stones most commonly lodge: UPJ (ureteropelvic junction) where renal pelvis meets ureter, the pelvic brim where the ureter crosses the iliac vessels, and UVJ (ureterovesical junction) where ureter enters the bladder — the narrowest point and most common site. Colicky flank pain radiating to groin = classic presentation. Most stones pass spontaneously if <5 mm.