Lecture 16: Gas Transport and Respiration

• Chapter 22 covers gas transport, oxygen use, cellular respiration, and respiratory disorders.

Gas Exchange

• Occurs in the alveoli and systemic tissues.
• Focus is on the exchange of oxygen (O₂) and carbon dioxide (CO₂).

Oxygen in Blood

• 98.5% of oxygen is bound to hemoglobin within red blood cells (RBCs).
• Only 1.5% of oxygen is dissolved in blood.
• Hemoglobin consists of 4 globin chains and 4 heme groups.
• Each heme group binds one O₂ molecule to an iron atom.
• 100% saturation means all available oxygen binding sites on hemoglobin are occupied.
• Binding of O₂ causes hemoglobin to change shape, enhancing its affinity for more O₂.

Carbon Dioxide in Blood

• 90% of carbon dioxide is transported as carbonic acid.
• Carbonic acid forms from CO₂ and H₂O, which then dissociates into H+ and HCO₃-.
• 5% of carbon dioxide is transported as carbamino compounds bound to plasma proteins, including hemoglobin.
• Hemoglobin can bind CO₂ (HbCO₂), but it binds to a polypeptide rather than heme.
• Carbon dioxide can bind along with oxygen inhibiting the oxygen.
• 5% of carbon dioxide is transported as dissolved gas.
• Blood gives up dissolved and HbCO₂ more readily.
• 70% of exchanged CO₂ is from H. CO.

Systemic Circulation

• Oxygen is unloaded from hemoglobin in systemic circulation, where CO₂ enters the blood.
• CO₂ combines with H₂O to form H₂CO₃, which dissociates into HCO₃- and H+.
• H+ can bind to hemoglobin inside RBCs.
• This process lowers hemoglobin's affinity for O₂.
• Tissues using O₂ undergo aerobic respiration.
• This maintains a low O₂ level of about 40 mm Hg.
• O₂ diffuses into tissues from hemoglobin and dissolved oxygen.

Differential Oxygen Delivery

• PO₂ affects oxygen delivery.
• Cells using more O₂ maintain a steeper partial pressure gradient, which promotes more oxygen unloading.
• Temperature influences oxygen release.
• Higher temperatures in metabolizing tissues result in more O₂ unloading.
• pH affects oxygen affinity.
• More CO₂ means more H+, which weakens the O₂-Hb bond, meaning lower saturation.
• BPG (Bisphosphoglycerate) influences oxygen delivery.
• RBCs have no mitochondria, and anaerobic glycolysis produces BPG.

Respiratory Rhythm

• Rate and depth of breathing are influenced by pH, PCO₂, and PO₂.
  • pH is first and should be between 7.35-7.45.
  • PCO₂ is second with a level of 40 mm HG.
  • PO₂ is third with a level of 95 mm Hg.
• Chemoreceptors detect changes in pH, CO₂, and O₂.
• 75% of chemoreceptors are central receptors.
• H+ does not cross the brain-barrier, thus C02 becomes H₂CO₃ in CSF.
• There are few buffers to hind with H= and they are more sensitive to pH.
• 25% are peripheral receptors.
• Integration occurs in the Dorsal Respiratory Group (DRG) and VRG.

Respiratory Rate and Rhythm Disorders

• Acidosis occurs when pH is below 7.35.
• Respiratory acidosis is caused by hypoventilation and can be corrected by hyperventilating.
• CO₂ is expired + H₂O becomes H₂CO₃, then HCO₃ + ↓ H+.
• Alkalosis occurs when pH is above 7.45.
• Respiratory alkalosis is caused by hyperventilating and can be corrected by hypoventilating.
• CO₂ + H₂O becomes H₂CO₃, then HCO3- + ↑H+.
• It can also be corrected by rebreathing from a bag.

Exercise

• Exercise leads to heavy breathing, due to an increased CO2 level, by maintaining normal conditions.
• Anticipatory effect: The brain sends motor commands to muscles. Signals are also sent to respiratory centers. Increase respiration anticipating activity.

Oxygen Imbalances

• Hypoxia is an oxygen deficiency.
• Hypoxemic hypoxia: This occurs due to inadequate pulmonary exchange, often due to high elevation, impaired ventilation and/or lung disease.
• Ischemic hypoxia: Poor blood circulation, and/or heart failure.
• Anemic hypoxia: Low RBC count, and/or hemoglobin deficiency.
• Histotoxic hypoxia: Metabolic poison. Prevents tissues from using oxygen.

Pulmonary Hypertension (PH)

• High blood pressure in pulmonary vessels.
• Causes include narrowed, blocked or destroyed blood vessels in the lungs, COPD, blood clots and genetics.
• It also causes disproportionate effect on heart, more specifically the right side. Increased heart failure.

Pneumothorax- Spontaneous

• Air in the pleural cavity.
• Visceral pleura punctured, and blebs.