The Lungs
Tennis-Court Membranes, Soap Surfactants, and Why Carbon Dioxide Drives Every Breath
Unfolding all 480 million alveoli in a pair of adult lungs would spread a gas-exchange surface across 75 to 100 square meters. That area equals half a singles tennis court, yet the entire membrane is folded inside a four-liter ribcage. Wrapping those air sacs is a capillary sheet holding a thin glass-full of blood at any second.
The blood-gas barrier separating alveolar air from capillary blood is just 0.2 micrometers thick—fifty times thinner than a red blood cell. Because Fick's law states that gas diffusion speed is inversely proportional to barrier thickness, an oxygen molecule crosses from air into a passing red blood cell in less than 250 milliseconds.
Because the inner wall of every alveolus is coated with a thin film of water, surface tension constantly tries to collapse the sacs. By Laplace's law ($P = 2T/r$), smaller bubbles generate higher inward collapse pressure than larger bubbles. Type II alveolar cells prevent collapse by secreting a phospholipid soap surfactant that slashes water surface tension.
When you hold your breath underwater, the painful urge to inhale has almost nothing to do with falling oxygen levels. Chemoreceptors in the brainstem's medulla oblongata ignore oxygen until it drops dangerously low. Instead, the brainstem monitors blood acidity: rising carbon dioxide ($\text{CO}_2$) crosses the blood-brain barrier and releases hydrogen ions ($\text{H}^+$) that trigger the breath alarm.
Free-divers who hyperventilate before plunging underwater do not add extra oxygen to their blood, because normal breathing already saturates hemoglobin to 99 percent. Rapid pre-dive breathing merely blows off carbon dioxide, silencing the medullary alarm. That trick risks shallow-water blackout, where oxygen runs out during ascent before carbon dioxide rises enough to warn the diver.
Your two lungs are not mirror images of each other. The right lung weighs roughly 600 grams and is divided by fissures into three lobes—upper, middle, and lower. The left lung weighs about 500 grams, has only two lobes, and carries a scooped-out hollow called the cardiac notch to make room for the heart.
Lung tissue contains zero skeletal muscle fibers and cannot inflate itself. The lungs expand passively because the diaphragm muscle contracts downward, enlarging the airtight chest cavity. That expansion drops intrapleural pressure 4 millimeters of mercury below atmospheric pressure ($756\text{ mmHg}$ versus $760\text{ mmHg}$), allowing outside air to push its way down your windpipe.
In every systemic organ, local oxygen starvation causes arterioles to dilate so extra blood can rush in. The lungs execute the exact opposite reflex, called hypoxic pulmonary vasoconstriction. When an air sac is blocked by mucus or fluid, its local arterioles clamp shut, rerouting blood exclusively toward well-ventilated sacs filled with fresh air.
Lining your trachea and bronchi are 10 billion microscopic cilia that beat in synchronized waves at 12 to 15 Hertz. Tipped with protein hooks, these cilia sweep a sticky conveyor belt of mucus and trapped dust upward to the throat at one millimeter per minute. Every day, this escalator dumps thirty milliliters of mucus into stomach acid.
Christian Bohr discovered in 1904 that carbon dioxide and heat warp the hemoglobin tetramer to loosen its grip on oxygen. When red blood cells enter hot, acidic exercising muscles, hydrogen ions bind hemoglobin and force it to dump extra oxygen. Returning to the cool, alkaline lungs, hemoglobin snaps back to high oxygen affinity.
At the summit of Mount Everest (8,848 meters), barometric pressure drops to 253 millimeters of mercury—one-third of sea level. At normal breathing rates, alveolar water vapor and carbon dioxide would leave no room for oxygen. Climbers survive without oxygen tanks only by hyperventilating fivefold, blowing alveolar carbon dioxide down from $40\text{ mmHg}$ to $8\text{ mmHg}$.
During a quiet resting breath, an adult inhales roughly 500 milliliters of air. However, only 350 milliliters of that fresh breath ever reaches the gas-exchange alveoli. The final 150 milliliters simply fills the rigid conducting pipes of the nose, trachea, and bronchi—anatomical dead space—and is exhaled unchanged.