Q.28 Which of the following is NOT TRUE with respect to control of respiration?

1. Peripheral chemoreceptors respond to changes in O₂ and CO₂ concentration in the blood.
2. Respiration responds to smaller changes, in the blood concentration of O₂ than CO₂.
3. Chemoreceptors in the CNS respond to changes in CO₂ concentration in the blood.
4. Central chemoreceptors are sensitive to changes in pH of the cerebrospinal fluid.

   Option B is NOT TRUE. Respiration is far more sensitive to changes in blood CO₂ than O₂ concentration.

   Option Breakdown

   A. Peripheral chemoreceptors respond to changes in O₂ and CO₂ concentration in the blood.
   True. Located in carotid and aortic bodies, these detect low arterial PO₂ (hypoxemia), high PCO₂ (hypercapnia), and low pH, triggering rapid ventilation increases via glossopharyngeal and vagus nerves.

   B. Respiration responds to smaller changes in the blood concentration of O₂ than CO₂.
   False. This is reversed: respiration responds to larger changes in O₂ (only activates below ~60 mmHg PO₂) but to smaller changes in CO₂ (threshold ~2-5 mmHg rise), as CO₂/pH drives ~80-90% of ventilatory control.

   C. Chemoreceptors in the CNS respond to changes in CO₂ concentration in the blood.
   True. Central chemoreceptors (medulla oblongata) sense CO₂ indirectly via H⁺ rise in CSF (diffuses across blood-brain barrier), accounting for most steady-state CO₂ response.

   D. Central chemoreceptors are sensitive to changes in pH of the cerebrospinal fluid.
   True. They primarily monitor CSF pH drops from elevated PCO₂/H⁺, stimulating respiration to restore acid-base balance.

   Control of Respiration Basics

   Control of respiration relies on chemoreceptors fine-tuning ventilation to blood gases and pH. For GATE Life Sciences, knowing what is NOT TRUE about peripheral/central responses separates top scorers—especially O₂ vs CO₂ sensitivity myths.

   Why Option B Fails on Chemoreceptor Sensitivity

   Respiration senses tiny CO₂ shifts (1-2 mmHg triggers hyperpnea) but ignores mild O₂ drops until severe hypoxia (PO₂ <60 mmHg). Central chemoreceptors dominate CO₂ drive (70-80%), peripheral handle acute O₂ lows. The false claim inverts this hierarchy.

   Chemoreceptor Roles Compared

   Chemoreceptor Type	Stimuli Detected	Primary Response Trigger	Contribution to Ventilation
   Peripheral	O₂↓, CO₂↑, pH↓ in blood	Hypoxemia (PO₂<60 mmHg)	20-30% (acute)
   Central (CNS)	CO₂↑ via CSF pH↓	Hypercapnia (small ΔPCO₂)	70-80% (steady-state)

   Peripheral act fast on mixed stimuli; central focus CO₂/pH for baseline rhythm.

   GATE Prep Tip for Control of Respiration

   Master: CO₂ sensitivity >> O₂; central = CSF pH, peripheral = arterial blood. Practice PYQs to nail control of respiration NOT TRUE twists.