Kinesiology Notes on Respiratory Physiology

Overview of Pulmonary Circulation

  • Concentration of oxygen and carbon dioxide in systemic arterial blood is maintained due to:
    • Oxygen moves from alveolar air into the blood at the same rate it is consumed by the tissues.
    • Carbon dioxide moves from the blood into alveolar air at the same rate it is produced in the tissues.

Diffusion of Gases

  • Gases diffuse down pressure gradients, moving from high pressure to low pressure.
  • Presence of other gases affects the concentration of individual gases but does not affect the diffusion process of gas itself.
  • Partial Pressure:
    • The pressure that an individual gas contributes to a mixture of gases, measured in atmospheres (atm).

Dalton’s Law of Partial Pressures

  • States that the pressure exerted by a gas mixture equals the sum of the individual pressures exerted by each gas.
    • P{total} = P1 + P2 + P3 + … + P_n
    • Calculation of Partial Pressure:
    • P{gas} = ext{%gas} imes P{total}
  • Composition of air at sea level:
    • Nitrogen: 79%
    • Oxygen: 20.93% (rounded to 21%)
    • Carbon Dioxide: 0.03%

Alveolar Gas Pressures

  • Alveolar gas pressures differ from atmospheric pressures due to:
    1. Continuous gas exchange between alveolar air and capillary blood.
    2. Mixing of fresh atmospheric air with CO2-rich air during inspiration.
    3. Air in alveoli being saturated with water vapor.
  • Important values:
    • P_{O2} at 100 mmHg
    • P_{CO2} at 40 mmHg

Gas Exchange in Alveoli

  • The exchange of oxygen and carbon dioxide occurs via diffusion down partial pressure gradients between alveoli and blood.
  • Factors influencing gas exchange rates:
    • Solubility: CO2 is about 25 times more soluble than O2.

Fick’s Law of Diffusion

  • The rate of gas transfer (V_{gas}) is proportional to:
    • Tissue area (A)
    • Diffusion coefficient of the gas (D)
    • Difference in partial pressure of gas (P1 - P2)
    • Inversely proportional to tissue thickness (T)
  • Formula:
    • V{gas} = A imes D imes rac{(P1 - P_2)}{T}
  • Larger surface area and thinner membrane in lungs facilitate gas transfer.

Transport of Oxygen in Blood

  • O2 is not very soluble in plasma; therefore, hemoglobin plays a crucial role in oxygen transport.
  • Oxygen binding to hemoglobin is reversible:
    • Hb + O2 ightleftharpoons Hb ullet O2
  • Capacity of hemoglobin:
    • 1g of hemoglobin carries 1.34 mL of O2.
    • Normal hemoglobin levels:
    • Males: 13-18 g/dL
    • Females: 12-16 g/dL
    • At 15 g/dL, oxygen-carrying capacity is 1.34 imes 15 = 20 mL/dL or 200 mL O2/L blood.

Oxyhemoglobin Dissociation Curve

  • Shows the relationship between oxygen saturation of hemoglobin and oxygen partial pressure.
  • Factors influencing the curve:
    • pH (Bohr effect): A decrease in pH shifts the curve to the right, indicating a lower affinity for O2, thus facilitating oxygen unloading during exercise.
  • Cooperative binding effect: As more O2 binds to hemoglobin, its affinity for O2 increases.

Transport of Carbon Dioxide

  • CO2 is transported in 3 forms:
    • Dissolved in plasma (5-6%)
    • Bound to hemoglobin as carbaminohemoglobin (5-8%)
    • Converted to bicarbonate (≈90%) in red blood cells:
    • Carbonic anhydrase facilitates the conversion:
    • CO2 + H2O
      ightleftharpoons H2CO3
      ightleftharpoons HCO_3^- + H^+

Central Regulation of Ventilation

  • Neural control involves the brainstem, which houses respiratory centers:
    • Breathing is controlled by motor neurons:
    • Phrenic nerve stimulates diaphragm for inspiration.
    • Internal intercostal nerves also play a role in respiration.
  • The peripheral and central chemoreceptors monitor blood gases and pH to adjust the breathing rate accordingly.

Chemoreceptors and Breathing Regulation

  • Peripheral chemoreceptors: Located in carotid bodies; respond to changes in blood O2, CO2, and pH.
  • Central chemoreceptors: Located in the medulla, respond indirectly to CO2 via pH changes in the cerebrospinal fluid.

Important Terms in Respiratory Physiology

  • Hyperpnea: Increased ventilation to meet metabolic demands.
  • Dyspnea: Difficult or labored breathing.
  • Apnea: Temporary cessation of breathing.
  • Tachypnea: Rapid, shallow breathing.
  • Hyperventilation: Ventilation exceeds metabolic demand.
  • Hypoventilation: Ventilation insufficient to meet metabolic demand.
  • Hypoxia: Oxygen deficiency in tissues.
  • Hypoxemia: Oxygen deficiency in blood.
  • Hypercapnia: Excess carbon dioxide in blood.
  • Hypocapnia: Deficiency of carbon dioxide in blood.