The pleura: a vacuum-sealed connection
Each lung is wrapped in a double layer of slippery membrane, the pleura. The inner layer hugs the lung; the outer layer lines the chest wall. Between them is a whisper-thin film of fluid — not empty space, but a sealed, slightly suction-y gap. That suction, the negative intrapleural pressure, is what glues the lung to the moving chest wall. When the wall expands, the lung is dragged open with it. The fluid also lets the two layers glide silently over each other thousands of times a day.
The muscles that drive a breath
The star of breathing is the diaphragm, a broad dome of muscle stretched across the bottom of the chest. During inspiration it contracts and flattens, dropping like a piston and pulling the chest cavity larger; the external intercostal muscles between the ribs lift the rib cage outward at the same time. The bigger cavity drops the pressure inside, and air rushes in. Expiration at rest is the easy part — the muscles simply relax, and the stretched, elastic lung recoils on its own, pushing air back out. No work needed for a quiet breath out.
- Diaphragm contracts and flattens; external intercostals lift the ribs.
- Chest cavity enlarges, so pressure inside drops below the outside air.
- Air flows down the pressure gradient into the alveoli — inspiration.
- Muscles relax, elastic lung recoils, air flows back out — quiet expiration.
Two blood supplies, one hilum
Oddly, the lung has two blood supplies with two different jobs. The pulmonary circulation carries oxygen-poor blood from the right heart to the alveoli to get refreshed — this is the blood that gas exchange is for. The bronchial circulation is much smaller and branches off the aorta to feed the airway walls and lung tissue with oxygen-rich blood, like any other organ. Both, along with the main bronchus and nerves, enter and leave each lung through one doorway: the hilum, the root of the lung on its inner face.
Two circulations, two purposes:
PULMONARY circulation (the big one, for gas exchange)
Right heart --> pulmonary arteries --> alveolar capillaries
--> oxygen loaded, CO2 unloaded --> pulmonary veins --> left heart
Carries: deoxygenated blood TO the lungs
BRONCHIAL circulation (the small one, for feeding the tissue)
Aorta --> bronchial arteries --> airway walls & lung tissue
Carries: oxygenated blood, like to any organ
Both enter/exit at the HILUM, alongside the main bronchus & nerves.