Zoology · Ch 6 — Respiration
Exchange of Gases
Exchange of Gases
When a mixture of gases (like air) is in contact with a liquid (like blood), each gas in the mixture contributes its own share of the total pressure - this is called its partial pressure, written pO2 for oxygen and pCO2 for carbon dioxide. Gases always diffuse from a region of higher partial pressure to one of lower partial pressure, exactly as a solute diffuses down a concentration gradient.
At the alveoli, pO2 is high (about 104 mm Hg) compared with the pO2 of deoxygenated blood arriving from the tissues (about 40 mm Hg), so oxygen diffuses from the alveolar air into the blood. At the same time, pCO2 in deoxygenated blood (about 45 mm Hg) is higher than pCO2 in the alveolar air (about 40 mm Hg), so carbon dioxide diffuses out of the blood into the alveoli to be exhaled. The same principle governs exchange at the tissues, but in reverse: oxygenated blood arriving at the tissues has a relatively high pO2 and low pCO2 compared with the metabolically active cells, so oxygen diffuses out of the blood into the tissues while CO2 diffuses from the tissues into the blood. …
| Respiratory gas | Atmospheric air | Alveoli | Deoxygenated blood | Oxygenated blood | Tissues |
|---|---|---|---|---|---|
| pO2 (mm Hg) | 159 | 104 | 40 | 95 | 40 |
What this figure shows. A schematic showing the partial pressures of O2 and CO2 at each stage of gas transport - inspired air, alveolar air, expired air, the pulmonary arteries and veins, and the systemic arteries and veins reaching the tissues - with arrows showing O2 moving from alveoli into blood and on to tissues, and CO2 moving from tissues into blood and on to the alveoli, around a central heart pumping deoxygenated blood to the lungs and oxygenated blood …
Respiratory Pigments
Haemoglobin is a conjugated protein found inside red blood cells. It is made up of a colourless protein portion called globin (about 96% of the molecule) combined with an iron-containing pigment portion called haem (about 4%). A single haemoglobin molecule has a molecular weight of roughly 68,000 and carries four iron atoms, each capable of binding one molecule of oxygen - so one haemoglobin molecule can carry up to four O2 molecules at a time. This reversible binding is what allows haemoglobin to pick up oxygen at the lungs, where pO2 is high, and release it at the tissues, where pO2 is low. …