RGI_2_Regulation_of_Respiration_and_Gas_Transport

• Outline gas transport and the partial pressures in blood and alveolar air.

• Show how oxygen is transported in the blood and explain the oxygen-haemoglobin dissociation curves.

• Discuss the oxygen utilization coefficient.

• Describe carbon dioxide transport in the blood.

• Explain the carbon dioxide dissociation curve and the Haldane effect.

Gas Transport

• Gases in the alveoli reach equilibrium with blood through diffusion across the pulmonary epithelium and capillary walls.

• Diffusion occurs due to differences in partial pressures of gases between the alveoli and blood.

Gas Pressure

• Definition: The pressure exerted by an individual gas in a mixture is known as its partial pressure.

• Dalton’s Law of Partial Pressure: Total pressure in a gas mixture is the sum of the individual partial pressures. Each gas exerts its own partial pressure independently.

Atmospheric Context

• At sea level, barometric pressure supports a column of mercury 760 mm high.

• Oxygen's partial pressure: 21% of air at 760 mm Hg yields 160 mm Hg (0.21 x 760).

• Carbon Dioxide's partial pressure: 0.04% at sea level equals 0.3 mm Hg (0.0004 x 760).

Alveolar Gas Pressures

• Gradient of PO2 from inspired air (160 mm Hg) to alveolar air (104 mm Hg) due to:

  • Increase of water vapor partial pressure

  • Residual air in lungs and continuous O2 diffusion into blood.

• Air gets warmed and moistened during inhalation.

Fick’s Law of Diffusion

• Gas diffusion across an alveolar membrane depends on:

  • Differences in partial pressures

  • Surface area of the membrane

• Greater pressure differences or surface area results in faster diffusion.

Oxygen Transport

• Oxygen is transported in the blood in two forms:

  1. Physical: Dissolved in plasma.

  2. Chemical Combination: >98% bound to haemoglobin.

Interaction with Tissues

• PO2 in arterial blood = 100 mm Hg, in tissues = 40 mm Hg.

• Oxygen diffuses from capillaries into tissues due to pressure gradient.

  • Venous blood will reflect the same PO2 as the cells it just passed due to diffusion.

Hemoglobin Characteristics

• O2 solubility in blood is poor (0.25 mL / 100 mL blood) and insufficient for respiration.

• About 20 mL O2 / 100 mL blood is present, with only 1.5% being dissolved.

• 98.5% is transported bound to haemoglobin (140 - 180 g/L for men, 120 to 160 g/L for women).

Hemoglobin Structure

• Composed of four peptide chains with heme rings containing iron atoms, allowing oxygen binding (up to four molecules per hemoglobin).

• Terms:

  • Oxyhaemoglobin (bound to O2)

  • Deoxyhaemoglobin (not bound to O2).

Oxygen-Haemoglobin Dissociation Curve

• Describes the relationship between oxygen partial pressure and haemoglobin saturation.

• Increased oxygen concentration leads to higher binding rates (highest in pulmonary capillaries).

• The curve is sigmoidal due to cooperative binding.

  • Oxygen Carrying Capacity: Maximum O2 haemoglobin can transport.

  • Oxygen Content: Actual amount of O2 bound.

  • % Oxygen Saturation: Ratio of oxygen content to carrying capacity.

Factors Influencing Binding

• Factors affecting hemoglobin's oxygen binding include:

  • pH

  • Carbon dioxide concentration

  • 2,3-DPG (2,3-diphosphoglycerate)

  • Temperature

• Right Shift: Decreased affinity for oxygen, easier to release O2.

• Left Shift: Increased affinity for oxygen, harder to release O2.

Bohr Effect

• Describes how pH changes affect oxygen binding.

• Increased CO2 correlates with higher acidity (lower pH), resulting in easier oxygen unloading in tissues.

• Deoxyhemoglobin has higher H+ affinity, promoting O2 release.

Muscle Activity and pH

• Lactic acid from active muscles also contributes to decreased blood pH, promoting oxygen release.

Oxygen Utilization Coefficient

• Percentage of blood's O2 delivered to tissues:

  • Normal arterial oxygen level: ~20 mL O2 / 100 mL blood.

  • 5 mL O2 / 100 mL is typically released (25% utilization).

• During exercise, oxygen delivery can increase to approximately 75% due to lower partial pressures in cells.

Carbon Dioxide Transport

• CO2 produced (200 mL/min at rest) is transported in three forms:

  1. Dissolved in plasma (as carbonic acid).

  2. Bound to proteins (particularly hemoglobin).

  3. As bicarbonate ions (HCO3-).

• CO2 solubility is higher in blood than O2, with blood carrying approximately 50 mL CO2 / 100 mL.

Mechanisms of CO2 Transport within Erythrocytes

1. Carbaminohaemoglobin: CO2 binds hemoglobin (20% approx); released in lungs due to lower CO2 concentrations.

2. Bicarbonate: 75% of CO2 forms carbonic acid via the enzyme carbonic anhydrase, dissociating into H+ and HCO3-.

Chloride Shift

• Converts CO2 into bicarbonate ions, maintains CO2 uptake in blood.

• Excess H+ produced is buffered by hemoglobin, preventing pH shifts.

• Bicarbonate (HCO3-) transported out of cells in exchange for Cl- ions (the chloride shift).

Reverse Chloride Shift

• At the lungs, bicarbonate is transported back into RBCs for CO2 release.

• H+ dissociates from hemoglobin and combines with bicarbonate to produce carbonic acid, which converts back to CO2 for exhalation.

Carbon Dioxide Dissociation Curve

• During capillary exchange, ~4 mL CO2 / 100 mL blood is exchanged.

• PCO2 levels: Arterial – 40 mm Hg, Venous – 45 mm Hg.

Haldane Effect

• Binding of oxygen to hemoglobin displaces CO2, aiding its release in lungs and uptake in tissues.

• Increased H+ from deoxygenation enhances CO2 production for release.

• Deoxygenated blood carries more CO2 efficiently.