How air reaches the alveoli and how oxygen and CO2 trade places

Your respiratory tract has two jobs, so it is split into two zones. The conducting zone is the plumbing: it gives air a route in and out, traps debris and germs, and warms and humidifies the air. No gas exchange happens here.

The respiratory zone is where the real work happens. It starts where a terminal bronchiole joins a respiratory bronchiole, continues into alveolar ducts, and ends in clusters of alveoli. If a question asks where oxygen enters the blood, the answer is always somewhere in this zone, usually "the alveoli."

Air passes through these structures in order. The tubes get narrower and more numerous as you go deeper.

Each alveolus is a tiny air sac wrapped in a net of capillaries. Its wall is mostly type I alveolar cells, very thin, flat cells that let gases pass easily. Together with the capillary wall they form the respiratory membrane, only about 0.5 micrometers thick. Gases cross it by simple diffusion, and the lungs' huge total surface area keeps the exchange fast.

Two other cells matter. Type II alveolar cells secrete surfactant, a mix of phospholipids and proteins that lowers the surface tension of the watery film lining each alveolus. Without surfactant, alveoli would collapse when you breathe out. Alveolar macrophages roam the walls and engulf debris and microbes.

Each gas diffuses from where its partial pressure is higher to where it is lower. Think of partial pressure as how hard one gas is "pushing" in a mixture.

In the lungs, this is called external respiration. Alveolar air has an oxygen partial pressure (PO2) of about 104 mm Hg. Blood arriving in the pulmonary capillaries has about 40 mm Hg, so oxygen rushes into the blood.

Carbon dioxide runs the other way: about 45 mm Hg in that blood versus about 40 mm Hg in the alveoli, so CO2 leaves the blood. The CO2 gradient is small, but CO2 dissolves much more easily than oxygen, so it still crosses well. Blood leaving the lungs has a PO2 of about 100 mm Hg.

In the body's tissues, internal respiration reverses the picture. Tissues have a low PO2 (about 40 mm Hg), so oxygen leaves the blood, and carbon dioxide made by cells moves into it.

How do the gases travel? Only about 1.5 percent of oxygen dissolves in plasma. Most rides on hemoglobin in red blood cells, and each hemoglobin can carry up to four oxygen molecules.

For carbon dioxide, about 70 percent travels as bicarbonate. Inside red blood cells, the enzyme carbonic anhydrase turns CO2 and water into carbonic acid, which splits into hydrogen ions and bicarbonate. In the lungs the reaction runs in reverse, freeing CO2 to be exhaled. About 20 percent of CO2 binds to hemoglobin, and 7 to 10 percent stays dissolved.

Breathing runs on Boyle's law: in a closed space, gas pressure and volume are inversely related. Make the space bigger and the pressure drops.

To inhale, the dome-shaped diaphragm contracts and moves down toward the abdomen, while the external intercostal muscles lift the ribs up and out. The chest cavity gets bigger, pressure inside the alveoli drops below atmospheric pressure, and air flows in.

Quiet exhalation is passive: the muscles relax, the elastic lung tissue recoils, alveolar pressure rises above atmospheric pressure, and air flows out. During forced exhalation, the internal intercostals and abdominal muscles help squeeze air out.

The rhythm comes from respiratory centers in the brainstem: the medulla oblongata (dorsal and ventral respiratory groups) and the pons.

The main signal is not oxygen but carbon dioxide. More CO2 in the blood means more hydrogen ions and a lower pH. Chemoreceptors in the brainstem sense this, and breathing becomes faster and deeper to blow off the extra CO2. Oxygen becomes a strong signal only when it falls very low.