Q.Explain the transport of O2 and CO2 between alveoli and tissue with diagram.

Gas exchange at the alveoli and at the tissues both happen by simple diffusion, driven by differences in partial pressure. At the alveoli, oxygen pressure is much higher in the alveolar air than in the deoxygenated blood arriving there, so oxygen diffuses from the alveoli into the blood, while carbon-dioxide pressure is higher in the blood than in the alveolar air, so carbon dioxide diffuses from the blood into the alveoli to be exhaled. At the tissues, the situation reverses: oxygen pressure is higher in the oxygenated blood than in the tissues, so oxygen diffuses from blood into the tissues, while carbon-dioxide pressure is higher in the tissues (produced by ongoing catabolism) than in the blood, so carbon dioxide diffuses from the tissues into the blood. A rough sketch would show two facing arrows at …

Oxygen and carbon dioxide are exchanged at the alveoli, and again between the blood and the tissues, by simple diffusion, driven mainly by differences in the partial pressure of each gas. The rate of this diffusion depends on the pressure gradient across the surface, the solubility of the gas, and the thickness of the membrane the gas must cross.

At the alveoli. Oxygen pressure is high in the freshly inhaled alveolar air and low in the deoxygenated blood arriving from the tissues, so oxygen diffuses from the alveolar air into the blood. Carbon-dioxide pressure runs the opposite way — high in the deoxygenated blood and low in the alveolar air — so carbon dioxide diffuses out of the blood and into the alveolar air, ready to be exhaled.

At the tissues. Oxygenated blood arriving at the tissues carries a high oxygen pressure, while the tissues themselves have a low oxygen pressure (having been using up oxygen in catabolism), so oxygen diffuses from the blood into the tissues. Carbon-dioxide pressure is high in the tissues, generated continuously by catabolic reactions, and low in the arriving blood, so carbon dioxide diffuses from the tissues into the blood. …

Method: Tracing a gas's diffusion pathway across two successive exchange sites

This method applies to any question asking you to explain how a gas moves along a full route that passes through more than one exchange site (such as alveoli and then tissues).

Steps

Step 1: Identify the exchange sites along the pathway

List each point along the route where the gas actually crosses a membrane, rather than simply being carried — here, the alveolar membrane (between air and blood) and the tissue capillary wall (between blood and tissue cells).

Step 2: At each site, compare the gas's partial pressure on both sides

For each gas and each site separately, work out whether the pressure is higher on the "before" side or the "after" side of the membrane — this comparison, not memorised direction, is what tells you which way the gas will move.

Step 3: Apply the diffusion rule (high pressure to low pressure) independently at each site, for each gas

Since oxygen and carbon dioxide have opposite pressure patterns at each site, work them out separately rather than assuming they always move together.

Step 4: Applying to this problem …