Biology · Breathing and Exchange of Gases · NEET
The PERCENTAGE of oxygen in air stays about 21% at every altitude. What drops is the total atmospheric (barometric) pressure. Since partial pressure of O2 = 21% of total pressure, a lower total pressure means a lower pO2. So there are fewer O2 molecules per breath even though the percentage is unchanged. NEET tests this exact idea: low pressure, not low percentage.
Oxygen binds haemoglobin based mainly on the partial pressure of O2 (pO2), not the amount you inhale in litres. NCERT states binding of O2 with haemoglobin is primarily related to pO2. When pO2 in the alveoli is low, haemoglobin cannot get fully saturated (loaded), so blood carries less O2 to the tissues. This oxygen shortage is called hypoxia.
These are the body trying to fix the low oxygen. Breathing rate speeds up (you feel breathless) to pull in more air, and heart rate rises (palpitations) to push the available oxygen to tissues faster. Both are compensation responses to hypoxia, so both symptoms appear together in altitude sickness.
Hypoxia means the tissues are getting an insufficient supply of oxygen. At high altitude it happens because low pO2 stops haemoglobin from saturating properly. NCERT even sets an activity asking students to gather information about hypoxia, so it is examinable terminology, not just a side note.
The specific 'adaptations to altitude' extension content was rationalised out for some recent years, but the core cause-and-effect (low pressure to low pO2 to less O2 binding to hypoxia) is still built on Chapter 14 concepts of partial pressure and oxyhaemoglobin, which ARE in the syllabus. Assertion-Reason PYQs on this appeared in NEET 2021 and 2023, so learn the reasoning chain.
Assertion (A): A person goes to high altitude and experiences 'altitude sickness' with symptoms like breathing difficulty and heart palpitations. Reason (R): Due to low atmospheric pressure at high altitude, the body does not get sufficient oxygen. In the light of the above statements, choose the correct answer:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Low atmospheric pressure at high altitude lowers the partial pressure of oxygen (pO2). Because O2 binds haemoglobin according to pO2, less oxygen loads onto blood, causing hypoxia (insufficient oxygen to tissues).
Breathing difficulty (faster breathing) and heart palpitations (faster heart rate). Both are the body compensating for the low oxygen supply.
No. Air is still about 21% oxygen. Only the total atmospheric pressure drops, which reduces pO2 and therefore the amount of O2 available to the blood.
Hypoxia is an insufficient supply of oxygen to the body tissues. At high altitude it results from low pO2 preventing full saturation of haemoglobin.
It links three syllabus ideas: partial pressure, oxygen-haemoglobin binding, and disorders of breathing. It has appeared as an Assertion-Reason PYQ (NEET 2021 and 2023).
