Biology · Breathing and Exchange of Gases · NEET
It DECREASES the affinity. When pH drops, H+ concentration rises. High H+ makes haemoglobin hold oxygen less tightly, so O2 is released. This is the exact point NEET 2016 tested: reduction in pH decreases the affinity of haemoglobin for oxygen. Do not confuse 'decrease affinity' with 'less oxygen delivered' — lower affinity means MORE oxygen is unloaded to the tissues.
The two sites have opposite conditions. Tissues have high pCO2, high H+ (low pH) and higher temperature — these favour dissociation, so O2 leaves haemoglobin. Alveoli (lungs) have high pO2, low pCO2, low H+ and lower temperature — these favour formation of oxyhaemoglobin, so O2 binds. NCERT states this clearly: O2 gets bound to haemoglobin on the lung surface and gets dissociated at the tissues.
A right shift means haemoglobin's affinity for O2 is lower, so at the same pO2 it releases more oxygen. High CO2, high H+ (low pH) and high temperature all shift the curve to the RIGHT (more unloading, good for tissues). The opposite conditions shift it LEFT (tighter binding, loading at lungs). NCERT does not use the exact phrase 'Bohr effect', but it describes this shift, so learn both the factors and the curve direction.
It is mainly about UNLOADING oxygen at the tissues. The Bohr effect explains how the very waste products of active tissue (CO2 and H+) automatically trigger more O2 release exactly where oxygen is needed most. So an actively respiring muscle, which makes more CO2 and heat, gets more oxygen delivered — a self-adjusting system.
NCERT lists three: high pCO2, high H+ concentration (low pH) and higher temperature. A fourth linked point is low pO2 (as in tissues). Remember them as tissue conditions: tissues are warm, full of CO2, acidic and oxygen-poor — perfect for releasing O2. The alveoli have the exact opposite set, which is why O2 loads there.
Reduction in pH of blood will:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT does not use the exact words 'Bohr effect', but it describes it fully: high pCO2, high H+ concentration and higher temperature favour dissociation of oxygen from oxyhaemoglobin. NEET can ask it either as the named effect or as these factors, so learn both.
High pCO2, high H+ concentration (low pH), higher temperature and low pO2. All four are the conditions found in active tissues, which is why oxygen is released there.
High pO2, low pCO2, low H+ concentration (higher pH) and lower temperature. These are the alveolar conditions that favour formation of oxyhaemoglobin.
About 5 mL of O2 per 100 mL of oxygenated blood under normal physiological conditions. This delivery is made possible by the Bohr effect unloading oxygen at the tissues.
Higher temperature helps RELEASE (dissociation) of oxygen. That is why exercising muscles, which are warmer and produce more CO2, receive more oxygen automatically.
