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Partial Pressure of O2 and CO2 (Values at Alveoli, Blood and Tissues)

Biology · Breathing and Exchange of Gases · NEET

Partial pressure is the pressure of ONE gas inside a mix of gases. We write it as pO2 for oxygen and pCO2 for carbon dioxide. The key NCERT numbers (Table 14.1, in mm Hg) are: at the ALVEOLI pO2 = 104 and pCO2 = 40; in oxygenated (arterial) blood pO2 = 95 and pCO2 = 40; at the TISSUES pO2 = 40 and pCO2 = 45. Memory hook: O2 falls "104 to 40" (alveoli to tissue) and CO2 rises "40 to 45", so each gas simply flows downhill from high pressure to low pressure.
Partial Pressures (mm Hg) — Table 14.1AlveolipO2 = 104pCO2 = 40Blood (oxygenated)pO2 = 95pCO2 = 40TissuespO2 = 40pCO2 = 45O2 flows high to low (104 to 40)CO2 flows opposite (45 to 40)
NCERT Table 14.1 values: oxygen (green) diffuses from alveoli (104) down to tissues (40); carbon dioxide (red) moves the opposite way, from tissues (45) to alveoli (40). Each gas simply follows its own pressure gradient.

Your doubts, answered

What exactly is 'partial pressure' in simple words?

Air is a mixture of many gases. The pressure made by only ONE of those gases is its partial pressure. For oxygen we call it pO2 and for carbon dioxide we call it pCO2. So pO2 tells you how much 'push' oxygen alone is making. A higher pO2 means more oxygen is present and pushing harder, so it will move to a place where pO2 is lower.

Why is pO2 at the alveoli (104) higher than pO2 in the blood (40) coming to it?

The blood arriving at the alveoli is deoxygenated (it came back from the body after giving oxygen away), so its pO2 is only about 40 mm Hg. The alveoli are full of fresh air with pO2 = 104 mm Hg. Because 104 is much more than 40, a concentration gradient exists and oxygen diffuses FROM alveoli INTO blood. This is why NCERT says alveolar pO2 is 'more than that in the blood' (NEET 2016).

What are the exact values I must memorise for NEET (Table 14.1)?

Atmospheric air: pO2 = 159, pCO2 = 0.3. Alveoli: pO2 = 104, pCO2 = 40. Oxygenated (arterial) blood: pO2 = 95, pCO2 = 40. Deoxygenated blood entering alveoli: pO2 = 40, pCO2 = 45. Tissues: pO2 = 40, pCO2 = 45. Notice pCO2 stays low (40) in the lung side and rises to 45 at tissues.

In which direction does each gas move, and why?

Gases always diffuse from HIGH partial pressure to LOW partial pressure. O2 gradient: alveoli (104) to blood to tissues (40), so O2 goes into tissues. CO2 gradient is the opposite: tissues (45) to blood to alveoli (40), so CO2 leaves the body. Same rule, opposite directions.

Why does pO2 drop from 104 in alveoli to only 95 in oxygenated blood?

When blood picks up oxygen at the alveoli it does not reach the full 104. A small amount of deoxygenated blood mixes in and diffusion is not 100 percent perfect, so arterial blood settles at about pO2 = 95 mm Hg. This 95 is still far above the tissue value of 40, so oxygen easily unloads at the tissues.

Does CO2 need a big pressure gap to diffuse like O2 does?

No. CO2 only needs a small gradient (45 at tissues vs 40 at alveoli, a gap of just 5 mm Hg). This works because CO2 is 20 to 25 times more soluble than O2, so even a tiny partial pressure difference moves a large amount of CO2. This solubility point is a favourite NEET trap.

⚠️ The NEET trap
✗ Students pick pO2 = 95 for the alveoli because they confuse alveolar air with oxygenated blood.
✓ At the ALVEOLI pO2 = 104 mm Hg (and pCO2 = 40). The value 95 is for oxygenated BLOOD, not the alveoli. 159/0.3 is atmospheric air and 40/45 is the tissue.
🧠 Say it as a ladder: Air 159, Alveoli 104, Blood 95, Tissue 40. Oxygen only goes DOWN the ladder.

Real NEET questions

2021

The partial pressures (in mm Hg) of oxygen (O2) and carbon dioxide (CO2) at alveoli (the site of diffusion) are:

A · pO2 = 95 and pCO2 = 40
B · pO2 = 159 and pCO2 = 0.3
C · pO2 = 104 and pCO2 = 40 ✓
D · pO2 = 40 and pCO2 = 45
Solution: From NCERT Table 14.1, the partial pressures at the alveoli are pO2 = 104 mm Hg and pCO2 = 40 mm Hg. 159/0.3 belongs to atmospheric air, 95/40 is oxygenated blood, and 40/45 is the tissues.
2016

The partial pressure of oxygen in the alveoli of the lungs is:

A · Equal to that in the blood
B · More than that in the blood ✓
C · Less than that in the blood
D · Less than that of carbon dioxide
Solution: Alveolar pO2 is 104 mm Hg while the deoxygenated blood reaching the alveoli has pO2 of only 40 mm Hg. Since 104 is more than 40, a concentration gradient drives O2 from alveoli into blood, so alveolar pO2 is MORE than that in the blood.

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Frequently asked

What is the partial pressure of O2 at the alveoli?

104 mm Hg. The partial pressure of CO2 (pCO2) at the alveoli is 40 mm Hg.

What are pO2 and pCO2 values in atmospheric air?

In atmospheric air pO2 = 159 mm Hg and pCO2 = 0.3 mm Hg, according to NCERT Table 14.1.

What is pO2 and pCO2 at the tissues?

At the tissues pO2 = 40 mm Hg and pCO2 = 45 mm Hg. Low O2 and high CO2 here cause O2 to unload and CO2 to be picked up.

Why is only a small pressure gradient enough for CO2?

Because CO2 is 20 to 25 times more soluble than O2. A small partial pressure difference of about 5 mm Hg is enough to move a large volume of CO2 across the diffusion membrane.

Do O2 and CO2 move in the same direction?

No. O2 moves from alveoli to blood to tissues (high to low pO2). CO2 moves the opposite way, from tissues to blood to alveoli (high to low pCO2).

Related concepts:
Exchange of Gases at the Alveoli and TissuesDiffusion of Gases and the Diffusion MembraneFactors Affecting Diffusion of Gases in the LungsFavourable Conditions for Oxyhaemoglobin Formation at AlveoliTransport of Oxygen: Formation of Oxyhaemoglobin
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Breathing and Exchange of Gases PYQs