12.3 Gas Exchange and Transport

4 min read•august 28, 2024

Gas exchange and transport are vital processes in the respiratory system. They ensure oxygen reaches our cells and carbon dioxide is removed from our bodies. Understanding these processes helps us grasp how our lungs work with our blood to keep us alive and functioning.

The alveoli, tiny air sacs in our lungs, play a crucial role in gas exchange. Oxygen and carbon dioxide move between the air in our lungs and our blood through these structures. This exchange is driven by differences in gas concentrations and relies on special proteins in our blood.

Gas exchange in the alveoli

Alveolar structure and function

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  • Gas exchange occurs between the alveoli and the pulmonary capillaries in the lungs
  • The alveoli are lined with type I and type II alveolar cells
    • Type I cells are thin and facilitate gas exchange
    • Type II cells secrete surfactant to reduce surface tension
  • The alveoli are surrounded by a dense network of pulmonary capillaries, creating a large surface area for efficient gas exchange
  • The alveolar-capillary membrane is extremely thin, allowing for rapid diffusion of gases between the alveoli and the blood

Ventilation and perfusion

  • Oxygen diffuses from the alveoli into the blood, while carbon dioxide diffuses from the blood into the alveoli
  • Ventilation and perfusion must be matched for optimal gas exchange
    • Ventilation is the flow of air into and out of the alveoli
    • Perfusion is the flow of blood through the pulmonary capillaries
  • Examples of ventilation-perfusion mismatch include:
    • Pulmonary embolism (decreased perfusion)
    • Pneumonia (decreased ventilation)

Partial pressures in gas exchange

Partial pressure gradients

  • Partial pressure is the pressure exerted by an individual gas in a mixture of gases
    • It is determined by the concentration of the gas and the total pressure of the mixture
  • Gas exchange is driven by partial pressure gradients between the alveoli and the blood
    • Oxygen has a higher partial pressure in the alveoli compared to the blood, causing it to diffuse into the blood
    • Carbon dioxide has a higher partial pressure in the blood compared to the alveoli, causing it to diffuse into the alveoli

Alveolar and blood gas partial pressures

  • The partial pressure of oxygen in the alveoli (PAO2) is approximately 100 mmHg
    • The partial pressure of oxygen in the pulmonary capillaries (PaO2) is around 40 mmHg, creating a gradient for oxygen diffusion
  • The partial pressure of carbon dioxide in the alveoli (PACO2) is approximately 40 mmHg
    • The partial pressure of carbon dioxide in the pulmonary capillaries (PaCO2) is around 45 mmHg, creating a gradient for carbon dioxide diffusion
  • Examples of factors affecting partial pressures:
    • High altitude (decreased PAO2)
    • Hypoventilation (increased PACO2)

Oxygen and carbon dioxide transport

Oxygen transport

  • Oxygen is primarily transported in the blood by binding to hemoglobin in red blood cells
    • Each hemoglobin molecule can bind up to four oxygen molecules
  • A small amount of oxygen is also dissolved in the plasma, but this accounts for only a small fraction of the total oxygen content in the blood
  • Examples of factors affecting oxygen transport:
    • Anemia (decreased hemoglobin)
    • Carbon monoxide poisoning (decreased oxygen-carrying capacity)

Carbon dioxide transport

  • Carbon dioxide is transported in the blood in three main forms:
    1. Dissolved in plasma (7-10%)
    2. Bound to hemoglobin as carbaminohemoglobin (20-30%)
    3. As bicarbonate ions (60-70%)
  • The enzyme carbonic anhydrase in red blood cells converts carbon dioxide to bicarbonate ions
  • The chloride shift (Hamburger effect) occurs when bicarbonate ions move out of red blood cells and chloride ions move in, maintaining electrical neutrality and facilitating the transport of carbon dioxide
  • Examples of factors affecting carbon dioxide transport:
    • Respiratory acidosis (increased PaCO2)
    • Metabolic alkalosis (decreased bicarbonate)

Oxygen-hemoglobin dissociation curve

Sigmoidal shape and P50

  • The oxygen-hemoglobin dissociation curve represents the relationship between the partial pressure of oxygen (PO2) and the percentage of hemoglobin saturated with oxygen (oxygen saturation)
  • The curve has a sigmoidal shape, indicating that hemoglobin's affinity for oxygen changes depending on the PO2
    • At high PO2 levels (such as in the lungs), hemoglobin has a high affinity for oxygen and becomes fully saturated
    • At low PO2 levels (such as in the tissues), hemoglobin has a lower affinity for oxygen and releases it to the tissues
  • The P50 is the PO2 at which hemoglobin is 50% saturated with oxygen
    • A higher P50 indicates a lower affinity for oxygen
    • A lower P50 indicates a higher affinity for oxygen

Factors affecting the dissociation curve

  • The dissociation curve can shift to the right or left due to various factors:
    • Temperature (increased temperature causes a right shift)
    • pH (decreased pH causes a right shift)
    • CO2 levels (increased CO2 causes a right shift)
    • 2,3-bisphosphoglycerate (2,3-BPG) in red blood cells (increased 2,3-BPG causes a right shift)
  • A right shift reduces hemoglobin's affinity for oxygen, facilitating oxygen release to the tissues
  • A left shift increases hemoglobin's affinity for oxygen, promoting oxygen loading in the lungs
  • The oxygen-hemoglobin dissociation curve is significant because it allows for efficient oxygen loading in the lungs and unloading in the tissues, ensuring adequate oxygen delivery to meet metabolic demands
  • Examples of factors affecting the dissociation curve:
    • Exercise (right shift due to increased temperature and CO2)
    • Fetal hemoglobin (left shift due to higher affinity for oxygen)
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