The oxygen dissociation curve is the S-shaped (sigmoid) graph you get when you plot percentage saturation of haemoglobin with oxygen (y-axis) against the partial pressure of oxygen, pO2 (x-axis). It shows how much oxygen haemoglobin carries at each pO2: high in the lungs (high pO2 = high loading), low in the tissues (low pO2 = oxygen is released). Memory hook: "S-shape = Saturation curve, Sigmoid means Smart unloading" — the flat top holds oxygen in the lungs, the steep middle drops oxygen fast where tissues need it.
The S-shaped oxygen dissociation curve: % saturation of haemoglobin (y-axis) vs pO2 (x-axis). At the alveoli (pO2 ~104 mm Hg) haemoglobin is almost fully saturated and loads O2; at the tissues (pO2 ~40 mm Hg) the steep middle releases O2. A right shift (high CO2, H+, temperature) helps unloading.
Your doubts, answered
What exactly is plotted on the two axes?
Y-axis = percentage saturation of haemoglobin with O2 (how full haemoglobin is, 0 to 100%). X-axis = partial pressure of oxygen (pO2). NCERT states: a sigmoid curve is obtained when percentage saturation of haemoglobin with O2 is plotted against the pO2. Do not swap the axes — pO2 is the cause (x), saturation is the result (y).
Why is the curve sigmoid (S-shaped) and not a straight line?
Each haemoglobin molecule can carry a maximum of four O2 molecules. When the first O2 binds, it makes it easier for the next ones to bind (cooperative binding). This is why the middle part is steep. At very high pO2 (in the lungs) haemoglobin is almost fully saturated, so the top of the curve flattens. This S-shape is what lets haemoglobin load O2 in the lungs and unload it in the tissues.
What does the flat top and the steep middle each mean?
The flat top (high pO2, like in the alveoli, pO2 = 104 mm Hg) means haemoglobin stays nearly 100% saturated — so oxygen is loaded and held in the lungs. The steep middle (lower pO2, like in tissues, pO2 = 40 mm Hg) means a small drop in pO2 releases a large amount of O2 — exactly where tissues need it. So the same curve does two opposite jobs at two body sites.
What shifts the curve to the right, and what does a right shift mean?
A right shift means haemoglobin releases oxygen more easily (lower affinity). It is caused by high pCO2, high H+ concentration (low pH, the Bohr effect) and higher temperature — the conditions in active tissues. NCERT: in the tissues, low pO2, high pCO2, high H+ concentration and higher temperature favour dissociation of oxygen from oxyhaemoglobin. A left shift (in the alveoli: low pCO2, less H+, lower temperature) favours oxygen loading.
Is the oxygen dissociation curve the same as oxyhaemoglobin formation?
They are two views of the same thing. Oxyhaemoglobin formation (loading) happens at the alveoli — the left/upper part of the curve. Dissociation (unloading) happens at the tissues — the steep part where O2 is released. The curve simply shows the balance between these two along the pO2 axis.
⚠️ The NEET trap ✗ Thinking the curve is a straight line or hyperbolic, or plotting pO2 on the y-axis and saturation on the x-axis. ✓ The oxygen dissociation curve is SIGMOID (S-shaped). Percentage saturation of haemoglobin (y-axis) is plotted against pO2 (x-axis). The S-shape comes from cooperative binding of up to 4 O2 per haemoglobin. 🧠 Sigmoid vs hyperbolic — NTA loves this axis and shape trap.
Real NEET questions
NEET 2016
Reduction in pH of blood will:
A · Reduce the rate of heart beat
B · Reduce the blood supply to the brain
C · Decrease the affinity of hemoglobin with oxygen ✓
D · Release bicarbonate ions by the liver
Solution: Lower pH means higher H+ concentration. On the oxygen dissociation curve this shifts the curve so oxygen is released from oxyhaemoglobin (Bohr effect), i.e. haemoglobin's affinity for O2 decreases. NCERT lists high H+ concentration among the conditions favouring dissociation of oxygen.
NEET 2020
Identify the wrong statement with reference to transport of oxygen.
A · Higher H+ conc. in alveoli favours the formation of oxyhaemoglobin ✓
B · Low pCO2 in alveoli favours the formation of oxyhaemoglobin
C · Binding of oxygen with haemoglobin is mainly related to partial pressure of O2
D · Partial pressure of CO2 can interfere with O2 binding with haemoglobin
Solution: Wrong statement asked. At the alveoli the favourable conditions are high pO2, low pCO2, LESSER H+ concentration and lower temperature. So a HIGHER H+ concentration does NOT favour oxyhaemoglobin formation — statement (a) is false. Binding is mainly related to pO2, which sets the position on the dissociation curve.
NEET 2021
Select the favourable conditions required for the formation of oxyhaemoglobin at the alveoli.
A · High pO2, high pCO2, less H+, higher temperature
B · Low pO2, low pCO2, more H+, higher temperature
C · High pO2, low pCO2, less H+, lower temperature ✓
D · Low pO2, high pCO2, more H+, higher temperature
Solution: Loading of O2 (upper part of the dissociation curve) happens at the alveoli where high pO2, low pCO2, lesser H+ concentration and lower temperature are all favourable. The opposite set favours unloading at the tissues.
Solved Breathing and Exchange of Gases NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
What is the oxygen dissociation curve in one line?
It is the sigmoid (S-shaped) graph of percentage saturation of haemoglobin with O2 plotted against the partial pressure of oxygen (pO2), showing how oxygen is loaded in the lungs and unloaded in the tissues.
Why is the curve important for NEET?
It links three high-yield ideas — partial pressure, oxyhaemoglobin formation, and the Bohr effect — into one graph. NEET asks about its shape (sigmoid), its axes, and how pCO2, H+ and temperature shift it.
How many oxygen molecules can one haemoglobin carry?
A maximum of four O2 molecules. This cooperative binding of four O2 is the reason the curve is S-shaped rather than a straight line.
What is the pO2 at the alveoli and at the tissues?
pO2 is about 104 mm Hg at the alveoli (high, so O2 loads) and about 40 mm Hg at the tissues (low, so O2 unloads). These two points sit at the flat top and the steep part of the curve.
What is the difference between a right shift and a left shift?
A right shift (high CO2, high H+/low pH, high temperature) means easier oxygen release at tissues. A left shift (the alveolar conditions) means easier oxygen loading. The right shift is the Bohr effect.
