Hemoptysis

Two water pipes, one ruptures

Imagine your lungs as a house with two separate plumbing systems running side by side. The pulmonary arteries are like thin, low-pressure supply pipes (the garden hose system at 15–25 mmHg) that bring dark, stale water to the air-conditioning unit (alveoli) so it can get refreshed and sent back out bright and clean. The bronchial arteries are like the thick, high-pressure pipes from the main street (120/80 mmHg) that feed water to the walls of the pipes themselves and the basement (pleura). Normally the bronchial pipes are tiny and you never notice them. But if chronic damage thickens and weakens the thin basement pipes (like in bronchiectasis) or if a hole erodes through an old scarred section (like a TB cavity), the high-pressure water from the aorta-fed bronchial system suddenly gushes into the house's air ducts (airways). The water floods the ducts, clots block the flow of fresh air, and the house starts drowning from the inside—not from running out of water, but from water blocking the very passages where air is supposed to move. The faster the gush, the quicker the ducts fill, and the quicker the house suffocates.