mLcmH2O
Visceral pleura: the lung's surface, pushed in by fluidBladder: pleural effusion; bottle: fluid drainedUnder each rib: intercostal vein, artery, nerveFloats: leak in vs lymph out (lymph tops out at 20 mL/h)The pleural space normally holds about 15 mL: a trickle leaks in from the capillaries and the lymphatics drain it. Lymph flow can rise 20 to 30 times, then it tops out (the short gauge's float stops at its cap). Fluid builds only when the leak beats that ceiling: high capillary pressure in heart failure, low albumin in cirrhosis or nephrotic syndrome, leaky inflamed capillaries in pneumonia or cancer, or blocked lymph. With a leak of 53 mL/h the bladder settles near 1.9 L, pleural pressure climbs from −5 to +2 cmH2O, and the lung is squeezed to 41% of its room: that is the breathlessness. The needle goes in just over the top of a rib because the vein, artery and nerve run in a groove under each rib's lower edge, and only where there is enough fluid between chest wall and lung (here 250 mL; on ultrasound, about 1 cm). The tap is diagnostic (protein and LDH, lactate dehydrogenase, tell a pressure leak from inflammation; cells, culture and cytology find infection and cancer) and therapeutic (the lung re-expands). It stops at 1.5 L, or if pleural pressure falls below −20 cmH2O, to avoid re-expansion pulmonary edema. The fluid comes back unless the cause is treated. Readout: effusion · leak in / lymph out · pleural pressure (cmH2O) · lung room. Teaching model, one day per second.