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.