12.2 Gas Exchange
Key Takeaways
Fish gills use countercurrent exchange, so an oxygen partial-pressure gradient remains along the whole lamella.
Human inhaled air passes from the nose or mouth through the pharynx, larynx, trachea, bronchi, and bronchioles to the alveoli.
Diffusion of oxygen and carbon dioxide requires a thin moist surface and a partial-pressure gradient.
Quiet inhalation occurs when the diaphragm contracts and moves down; quiet exhalation is largely elastic recoil.
Oxygen binds hemoglobin inside red blood cells, and most carbon dioxide travels as bicarbonate in the plasma.
12.2 Gas Exchange
Animals exchange oxygen and carbon dioxide across a surface that is thin enough and moist enough for gases to dissolve and diffuse. The driving force is a partial-pressure gradient. Each gas diffuses from the place where its own partial pressure is higher toward the place where that partial pressure is lower. Vertebrates build the surface in different organs. Fish use gills. Amphibians, reptiles, birds, and mammals use lungs, and many amphibians also exchange gases across moist skin.
Surfaces compared
A bony fish gill is built from filaments, and each filament bears thin lamellae. Most bony fish pump water across those lamellae with mouth and opercular movements. Water flows one way over a lamella while blood flows through the lamella in the opposite direction. That arrangement is countercurrent exchange. Along the whole lamella, the water still holds a slightly higher oxygen partial pressure than the blood beside it, so oxygen keeps entering the blood. Blood can leave the lamella carrying more oxygen than the water that is leaving. If blood and water ran in the same direction, the two streams would equalize partway along the lamella and net diffusion would stop. Countercurrent flow prevents that early equalizing.
Insects, by contrast, pipe air through tracheae directly to their tissues and do not depend on blood to carry respiratory gases the way a vertebrate does.
Lungs, from frog to human
Vertebrate lungs are internal exchange sacs. A frog forces air in with a buccal pump. A human expands the chest, pressure in the lungs falls, and air is pulled in. Mammalian flow is tidal, in and out through the same airways, so alveolar air is always a mixture. Birds add air sacs and a one-way path that keeps fresher air moving across the exchange surface.
The human airway and the alveolus
Air enters through the nose or the mouth, then passes the pharynx, the larynx, and the trachea. The trachea divides into bronchi, the bronchi branch into bronchioles, and the bronchioles end in clusters of alveoli. Cartilage rings hold the trachea open. Mucus and cilia in the larger airways trap particles and sweep them toward the pharynx.
A thin moist barrier
The alveolus is where the gases cross. Its wall and the wall of the neighboring capillary are each one thin cell layer, with a shared basement membrane between them. The surface stays moist, so oxygen and carbon dioxide can dissolve. Type I cells form most of that thin wall. Type II cells secrete surfactant, a mixture that lowers surface tension so the smallest alveoli do not collapse at the end of exhalation. Oxygen diffuses into the blood when its partial pressure in alveolar air is higher than its partial pressure in the blood arriving from the right heart. Carbon dioxide diffuses out when its partial pressure in that blood is higher than its partial pressure in alveolar air.
In a common resting example, oxygen in alveolar air is near 104 mm Hg and oxygen in blood arriving from the right heart is near 40 mm Hg, so oxygen diffuses in. Carbon dioxide in that blood is near 45 mm Hg and carbon dioxide in alveolar air is near 40 mm Hg, so carbon dioxide diffuses out. The gradients are modest. The thin wall and the huge combined surface of the alveoli make them sufficient.
How a quiet breath is powered
A thin film of pleural fluid couples the lungs to the chest wall, so the lungs expand when the thorax expands.
Inhalation at rest is active. The diaphragm contracts and moves down, flattening from its resting dome. External intercostal muscles lift the rib cage. Thoracic volume rises. The same amount of gas now occupies a larger space, so pressure inside the lungs falls below atmospheric pressure, and air flows in. The diaphragm does not relax to cause a normal inhalation. Relaxation lets the dome rise and makes the cavity smaller.
Exhalation at rest is largely passive elastic recoil. The diaphragm relaxes and domes upward, the rib cage settles, and stretched elastic tissue in the lung springs back. Thoracic volume falls, pressure inside the lungs rises above atmospheric pressure, and air flows out. Forced exhalation is a different, active pattern that uses internal intercostal and abdominal muscles. That push is not the mechanism of an ordinary breath out.
Carriage in the blood
Oxygen binds hemoglobin inside red blood cells. Each hemoglobin protein has four heme groups, and oxygen binds the iron in those hemes. A small fraction of oxygen travels simply dissolved in plasma. At active tissues, a lower oxygen partial pressure, a higher carbon dioxide, and a lower pH loosen hemoglobin's hold on oxygen. That Bohr effect unloads oxygen where cells are using it.
Carbon dioxide travels mostly as bicarbonate in the plasma. Inside the red blood cell, carbonic anhydrase joins carbon dioxide and water to form carbonic acid, which separates into hydrogen ion and bicarbonate. Bicarbonate leaves the cell, and chloride enters to balance the charge. A smaller share of carbon dioxide binds the protein part of hemoglobin as carbaminohemoglobin, not the same iron site that carries oxygen. A still smaller share stays dissolved as gas in the plasma. In the lung the reactions run backward, carbon dioxide diffuses into alveolar air, and the oxygen just taken on is what the left heart will send to the body.
| Structure or step | What it does | Gradient or pressure change |
|---|---|---|
| Fish lamella | Countercurrent flow of water and blood | Oxygen keeps entering blood along the whole lamella |
| Human airway | Nose or mouth, pharynx, larynx, trachea, bronchi, bronchioles, alveoli | Air reaches a thin moist alveolar surface |
| Quiet inhalation | Diaphragm contracts and moves down | Thoracic volume rises and lung pressure falls |
| Quiet exhalation | Elastic recoil as the diaphragm relaxes | Thoracic volume falls and lung pressure rises |
| Oxygen in blood | Bound to hemoglobin in red blood cells | From alveolar air into pulmonary capillary blood |
| Carbon dioxide in blood | Mostly bicarbonate in the plasma | From blood into alveolar air |
Warning
A normal inhalation happens when the diaphragm contracts and moves down. Relaxation of the diaphragm belongs to quiet exhalation. In a fish gill, countercurrent flow keeps an oxygen gradient along the whole lamella because blood and water run in opposite directions.
Ventilation moves air, and the gases themselves cross the alveolar wall by diffusion.
During a quiet inhalation in a resting person, what does the diaphragm do?
It pumps oxygen across the alveolar wall by active transport
It contracts and moves downward, thoracic volume rises, and pressure in the lungs falls
It relaxes and domes upward, thoracic volume falls, and that fall pulls air in
It stays still while the alveoli contract and force air down the trachea
How does countercurrent exchange at a fish gill keep oxygen moving into the blood?
Blood and water flow in the same direction and quickly reach the same oxygen level, which speeds diffusion farther along the lamella
Air is piped through tracheae that open on the gill lamellae and skip the blood
Blood flows opposite the water, so an oxygen partial-pressure gradient remains along the whole lamella
A diaphragm in the fish packs oxygen onto hemoglobin before water reaches the gill
How is most of the carbon dioxide in human blood carried on the way to the lungs?
As bicarbonate ions in the plasma
As gas bubbles traveling inside the pulmonary arteries
Bound to the iron of hemoglobin in the same way oxygen is bound
Dissolved only inside the alveoli, with none of it present in the plasma
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