RP - Gas Transfer Flashcards
Explain the method of gas transfer:
Gas transfer in the lungs occurs via diffusion, where gases move from areas of high partial pressure to low partial pressure across the alveolar-capillary membrane.
Oxygen - diffuses from alveolar air (high partial pressure) into pulmonary capillary blood flow (low partial pressure)
Carbon dioxide - diffuses from the blood (high partial pressure) into alveoli (low partial pressure)
The driving force is the difference in partial pressures of gases in the alveoli and blood
Describe Fick’s Law of diffusion:
Rate of gas transfer across a membrane =
(SA x partial pressure gradient x solubility in water) / membrane thickness
Increased surface area = increased rate of diffusion
Increased membrane thickness = decreased diffusion rate
Greater partial pressure gradient → Faster diffusion
Describe the equilibration of alveoli with blood:
Oxygen reacts with haemoglobin
Carbon dioxide reacts partly with haemoglobin but significantly more with water molecules, reaction sped up by carbonic anhydrase in RBCs
Normally sufficient time (3/4 of a second) for full equilibration to occur
Describe the Ventilation/Perfusion Ratio (V/Q) and the Effects of Gravity:
The V/Q ratio represents the relationship between alveolar ventilation and pulmonary blood flow
Ventilation - movement of air into and out of the lungs
Perfusion - blood flow through pulmonary capillaries
Ideal V/Q ratio = 1 (perfect matching of ventilation and perfusion), but in reality, there is variation throughout the lung
Apex V/Q = 3
Overall V/Q = 0.8
Base V/Q = 0.6
Define and describe the effects of anatomical dead space and physiological dead space:
Anatomical Dead space:
- air in conducting airways (trachea, bronchi) that does not reach alveoli
- doesn’t participate in gas exchange
Physiological dead Space:
- includes anatomical dead space plus any alveoli that are ventilated but not perfused due to vascular obstruction e.g pulmonary embolism
Increased dead space reduces overall gas exchange efficiency as perfusion is reduced
Describe the 2 types of shunts and their effects:
Right to left:
- occurs when blood bypasses the lungs without participating in gas exchange
- results in hypoxemia, as deoxygenated blood mixes with oxygenated blood, reducing arterial O₂ levels
Left to right shunt:
- increases pulmonary blood flow, potentially leading to pulmonary hypertension over time due to increased circulatory load
Describe V/Q mismatch effects:
Occurs when alveoli are ventilated but not well perfused (high V/Q) or perfused but not well ventilated (low V/Q
The most common cause of hypoxemia
describe oxygen delivery in exercise:
Oxygen consumption = arterio-venous difference × cardiac output
Increased cardiac output and ventilation rate
Right shift in oxyhemoglobin dissociation curve → more O₂ unloading
Increased capillary recruitment improves gas exchange
Compare oxygen and carbon dioxide transfer
Oxygen:
- low solubility in plasma
- Hb affinity for oxygen
- reacts with haem inside RBC
- plateau of dissociation curve
- equilibrates with reserve
- PaO2 < PAO2
- ↓PaO2 in disease of transfer (↓area, V/Q mismatch, shunts) or severe exercise
- partial pressure gradient 7 kPa
Carbon dioxide:
- high solubility in plasma
- Hb low affinity
- partial pressure gradient 0.7 kPa
- reacts with water inside RBC, plasma carries more
- no plateau in curve
- equilibrates, greater reserve
- PaCO2 = PACO2
- ─ or ↓PaCO2 in disease of transfer or severe exercise
Describe the mechanisms used to maintain homogeneity of V/Q ratios:
Hypoxic Pulmonary Vasoconstriction:
- when alveolar O₂ levels are low, pulmonary arterioles constrict, redirecting blood to better-ventilated alveoli to optimise gas exchange
- helps minimize V/Q mismatch but can contribute to pulmonary hypertension in chronic lung disease
Hypocapnic bronchoconstriction:
- When CO₂ levels drop in poorly perfused alveoli, airway smooth muscles constrict to decrease ventilation to those areas, helping to maintain V/Q matching
Compare pulmonary and tissue gas exchange
Pulmonary gas exchange:
- location is alveoli of lungs
- O₂ diffuses from alveoli → pulmonary capillaries and CO₂ opposite
- driven by partial pressure gradient ; high O₂ in alveoli, low in blood; high CO₂ in blood, low in alveoli
- function, oxygenate blood and remove CO₂
Tissue gas exchange:
- location is systemic circulation and capillaries
- O₂ diffuses from blood → tissue cells and CO₂ opposite direction
- driven by partial pressure gradients; high O₂ in blood, low in tissues; high CO₂ in tissues, low in blood
- function = Deliver O₂ for cellular respiration and remove metabolic CO₂
