2Physiology of gaseous exchange and mechanisms of ventilation

What does the oxygen cascade describe?

How oxygen levels drop from the site of binding to haemoglobin to delivery at the tissues

What is the relationship between PaO₂ and SpO₂?

• Essentially interchangeable

• Majority of oxygen transport occurs via haemoglobin

What systems make up the body’s oxygen delivery system?

• Lungs

• Cardiovascular system

• What does Dalton’s Law state about the total pressure of a gas mixture?

Total pressure = sum of the partial pressures of all constituent gases

What is the partial pressure of a gas in a mixture? daltons law

• The pressure the gas would exert if it alone occupied the volume of the mixture

How is the total pressure of a gas mixture calculated? daltons

• By summing the partial pressures of each individual gas in the mixture

What does Henry’s Law state about gas solubility in a liquid?

• The amount of gas dissolved in a liquid is directly proportional to the gas’s partial pressure in contact with the liquid

• Gas tension = partial pressure of gas in equilibrium with the solution

What factors affect the solubility of a gas in a liquid?

• Each gas-liquid pair has a specific proportionality constant (Henry’s constant)

• Solubility is inversely proportional to temperature for a given partial pressure

Why is alveolar PO₂ lower than atmospheric PO₂?

• Only ~15% of alveolar air is replaced with each breath

• Explains why alveolar gas composition remains relatively constant

Why is arterial PO₂ (PaO₂) lower than alveolar PO₂?

• Some pulmonary blood bypasses alveoli (shunting)

• Reduces arterial oxygen tension compared to alveolar air

Why is dissolved O₂ and CO₂ in blood alone insufficient?

• Amount dissolved is too low for adequate tissue O₂ delivery and CO₂ removal

• Mechanisms exist to increase carrying capacity

How is the carrying capacity of O₂ and CO₂ increased in blood?

• O₂: bound to haemoglobin (~70× more than dissolved)

• CO₂: converted to bicarbonate and other forms (~17× more than dissolved)

Why are PO₂ and PCO₂ important despite these mechanisms?

• Determine how much gas is carried by haemoglobin and other systems

• Drive gas transfer in alveoli and tissues via partial pressure differences

• Dissolved O₂ important in therapies like hyperbaric 100% O₂ for CO poisoning

How quickly does gas diffusion occur between alveoli and blood?

• Pulmonary capillary blood equilibrates rapidly with alveolar air

• O₂ and CO₂ diffusion across alveoli is very fast

What factors determine the rate of gas diffusion across alveoli?

• Large surface area

• Thin membrane

• Partial pressure differences (alveolar ↔ blood)

• Gas solubility