Respiratory gases, and when control systems break down

Regulating respiratory gases

• Purpose: to supply respiring tissue with oxygen and remove carbon dioxide.

  • Oxygen is needed as the final electron acceptor in aerobic respiration. Without it, cells respire anaerobically, producing far less ATP.
  • Carbon dioxide must be removed because it lowers the pH of blood and tissue fluid, and enzymes have a narrow optimum pH.

• The system is driven primarily by carbon dioxide, not oxygen. This surprises students, and explaining why is worth marks.

The chemistry that makes CO₂ detectable

• Carbon dioxide is detected indirectly, through the pH change it causes. This is the equilibrium reaction the Excellence criterion names.

• Dissolved CO₂ reacts with water, catalysed by carbonic anhydrase in red blood cells:

$$\text{CO}_2 + \text{H}_2\text{O} \rightleftharpoons \text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^-$$

• CO₂ — carbon dioxide, produced by respiring tissue

• H₂CO₃ — carbonic acid

• H⁺ — hydrogen ions, which determine pH

• HCO₃⁻ — hydrogencarbonate ions

• How the equilibrium responds:

  • Rising CO₂ shifts the equilibrium to the right, producing more H⁺, so pH falls.
  • Falling CO₂ shifts it to the left, removing H⁺, so pH rises.

• So blood pH is a direct and rapid indicator of CO₂ concentration, which is what makes CO₂ easy to monitor.

Components and mechanism

• Receptors:

  • Central chemoreceptors in the medulla oblongata, which detect H⁺ concentration in the cerebrospinal fluid. These are the most important in normal conditions.
  • Peripheral chemoreceptors in the carotid bodies and aortic bodies, which detect H⁺, CO₂ and — unlike the central ones — low oxygen.

• Control centre: the respiratory centre in the medulla oblongata.

• Effectors: the diaphragm and the external intercostal muscles.

• When CO₂ rises, as during exercise:

  • The equilibrium above shifts right, so H⁺ rises and pH falls.
  • Chemoreceptors detect the fall in pH.
  • The respiratory centre increases the rate and depth of ventilation.
  • More CO₂ is exhaled, so blood CO₂ falls.
  • The equilibrium shifts left, H⁺ concentration falls, and pH returns toward the set point.

• When CO₂ falls, ventilation rate and depth decrease, CO₂ accumulates, and pH falls back toward the set point.

Why CO₂ rather than oxygen

• Two reasons, and giving both shows real understanding:
  • CO₂ is the more sensitive indicator. Haemoglobin is still largely saturated with oxygen across a wide range of partial pressures, so blood oxygen changes relatively little in ordinary circumstances. Blood CO₂ and pH change quickly and measurably as soon as respiration rate changes, so CO₂ gives an earlier warning.
  • CO₂ is the more urgent threat. Accumulating CO₂ lowers pH, and a pH change disrupts the hydrogen and ionic bonds maintaining every enzyme's tertiary structure — affecting all metabolism at once, not just aerobic respiration.
• Low oxygen does drive ventilation, through the peripheral chemoreceptors, but only once oxygen falls substantially — as at high altitude. It is a backup rather than the normal control.

When control systems break down

• The standard requires the potential effect of disruption, and analysing a breakdown is one of the three Excellence routes. Causes are grouped as:

  • External — extreme environmental conditions, disease or infection, drugs, toxins.
  • Internal — genetic conditions, metabolic disorders.

• A control system can fail at any of its components, and identifying which component has failed is the most useful analytical step:

  • The receptor fails to detect the deviation.
  • The control centre fails to process it or issue a response.
  • The effector fails to respond, or the response is inadequate.
  • The signal between them is blocked or absent.

Breakdown of glucose regulation

• Type 1 diabetes is an internal cause: the body's own immune system destroys the β cells of the islets of Langerhans, so little or no insulin is produced.

  • The effector arm for lowering glucose is missing. Detection still works; the response cannot be made.
  • Blood glucose therefore remains high after a meal. Because it is not taken into cells, blood water potential falls, drawing water out of cells by osmosis; glucose appears in the urine, taking water with it osmotically and causing increased urine volume and thirst.
  • Cells are simultaneously short of glucose despite the blood being full of it, because uptake depends on insulin.

• Type 2 diabetes involves cells becoming less responsive to insulin. Here insulin is produced, but the effector does not respond adequately — a failure at a different point in the same loop.

• The analytical point: both produce high blood glucose, but for different reasons — one a failure of hormone production, the other a failure of target-cell response. Identifying which component failed is what turns a description into an analysis.

Breakdown of thermoregulation

• Hypothermia is an external cause: heat loss exceeds heat production for long enough that core temperature falls substantially.
• What makes it dangerous is that the failure reverses the feedback:
  • As core temperature falls, enzyme-controlled reactions slow, because molecules have less kinetic energy and fewer successful collisions occur.
  • Slower metabolism means less heat is generated by respiration.
  • Less heat generated means core temperature falls further.
  • Shivering, the main heat-generating response, eventually stops, because it depends on muscle metabolism that has itself slowed.
• This is now positive feedback — the change amplifies itself rather than being opposed. A control system that fails does not simply stop working; it can begin driving the deviation it was built to correct.
• This is why recovery generally requires external warming: the internal mechanism that would normally correct the deviation is precisely what has failed.

Selective advantage

• Regulating respiratory gases allows an animal to increase its metabolic rate enormously — during escape or pursuit — without its internal pH moving outside the range its enzymes tolerate. Without it, sustained activity would be self-limiting, because the CO₂ produced would disable the metabolism generating it.
• Using CO₂ as the signal is itself adaptive: it provides an early indicator, so ventilation increases as demand rises rather than after oxygen has already fallen. The system is anticipating the shortage rather than reacting to it.