AB

Gas Transport and Respiratory Control - Lecture 19

Oxygen Transport

  • Oxygen is carried in two forms: dissolved $O_2$ and bound to haemoglobin (Hb) in red blood cells (RBCs).
  • Dissolved $O2$ is limited: about 3\ \mathrm{mL\ O2\ per\ L\ blood}; total content is roughly 15\ \mathrm{mL\ O2\ per\ L}. The body needs ~250\ \mathrm{mL\ O2\ min^{-1}}.
  • Hb binds up to four oxygen molecules per Hb molecule (four heme units).
  • Most oxygen is transported bound to Hb; only a small fraction is dissolved in plasma.

Hemoglobin Binding Curve

  • The Hb-O2 saturation curve is sigmoidal (not linear) due to cooperative binding of O2 between heme sites.
  • The curve shows how many Hb sites are occupied as alveolar PO2 increases.
  • The shape enables efficient loading in the lungs and unloading in tissues that need O2.

Factors Shifting the Curve

  • The curve shifts to the right (lower affinity, easier O2 release) when conditions favor delivery to tissues:
    • Lower pH (acidic)\n - Higher temperature\n - Higher CO2 (PCO2)\n- Exercise causes these conditions in active tissues, promoting O2 unloading to muscles.

Oxygen Delivery and Tissue Uptake

  • The sigmoidal shape and its shifts help deliver O2 preferentially to tissues with high demand (lower local PO2).

Carbon Dioxide Transport

  • CO2 is transported in three forms:
    • Dissolved in plasma
    • Bound to Hb
    • Converted to bicarbonate (HCO3-)
  • Majority is transported as bicarbonate; a portion is bound to Hb or dissolved.

Formation of Bicarbonate

  • CO2 + H2O ⇌ H2CO3 ⇌ HCO3^- + H^+
  • This reaction lowers blood pH when CO2 rises (more acidic blood).

Control of Breathing

  • Purpose: keep arterial O2 and CO2 within narrow limits for metabolic stability.
  • Ventilation is tightly regulated despite variable O2 use and CO2 production.

Central Controller, Sensors, and Effectors

  • Sensors: chemoreceptors, baroreceptors, lung stretch receptors, protective reflex receptors.
  • Controller: brain regions (pons, medulla, others).
  • Effectors: respiratory muscles (drive ventilation via afferent/efferent signals).

Chemoreceptors

  • Monitor O2 and CO2 (and pH) levels in blood.
  • If O2 drops or CO2 rises, they increase breathing frequency and tidal volume.
  • They are the primary regulator of ventilation and can override voluntary control.
  • CO2 receptors play the primary role; O2 receptors are secondary.
  • Locations: carotid bodies/arteries and the medulla oblongata.

Baroreceptors

  • BP sensors located in carotid sinus and aortic arch.
  • Provide feedback to respiratory control via afferent signaling (via vagus/X nerve).
  • If arterial BP falls, respiratory minute volume tends to rise; if BP rises, ventilation tends to fall.

Inflation/Deflation and Protective Reflexes

  • Lung stretch receptors provide input to prevent over-inflation/over-deflation (Hering-Breuer reflex).
  • Irritation triggers protective reflexes (sneeze, cough).

Summary (Essentials)

  • O2 transport: mainly Hb-bound; a small portion dissolved in plasma.
  • Hb binding curve: sigmoidal, enabling efficient O2 delivery; shifts with pH, temperature, CO2 to aid delivery where needed.
  • CO2 transport: mainly as bicarbonate; some bound to Hb or dissolved.
  • Respiratory control: chemoreceptors, baroreceptors, stretch receptors, and protective reflexes provide afferent input to brain, which regulates ventilation via efferent output.

Quick Exam Focus

  • Pulmonary fibrosis (thickened blood–air barrier) tends to reduce arterial O2 due to slower diffusion.