Gas Exchange & Transport – Dr. Lee Siew Keah (UTAR)

Question-and-Answer flashcards covering definitions, mechanisms, and regulatory factors in pulmonary gas exchange, oxygen and carbon dioxide transport, and related physiological effects.

According to Dalton’s Law, how is the total pressure of a gas mixture determined?

It equals the sum of the partial pressures exerted by each individual gas in the mixture.

Define partial pressure (Pgas).

The pressure a single gas in a mixture would exert if it occupied the entire volume alone; it is proportional to that gas’s percentage in the mixture.

State Henry’s Law in the context of gases in contact with a liquid.

Each gas dissolves in a liquid in proportion to its own partial pressure until partial pressures in the gas and liquid phases are equal.

Which two variables determine how much of a gas dissolves in a liquid under Henry’s Law?

Its solubility in the liquid and its partial pressure; temperature also modulates solubility (higher temperature decreases solubility).

Why does CO₂ diffuse as rapidly as O₂ across the respiratory membrane despite a smaller pressure gradient?

CO₂ is about 20 times more soluble in plasma than O₂, compensating for its shallower gradient.

List the three factors that influence the rate of external respiration.

1) Thickness & surface area of the respiratory membrane, 2) Partial-pressure gradients & gas solubilities, 3) Ventilation–perfusion coupling.

What happens to gas exchange if the respiratory membrane thickens (e.g., pulmonary edema)?

Diffusion slows and gas exchange efficiency decreases.

Define ventilation-perfusion coupling.

Matching airflow (ventilation) to blood flow (perfusion) by local autoregulation of bronchiolar diameter (via PCO₂) and arteriolar diameter (via PO₂).

Give the normal alveolar PO₂ and pulmonary venous PO₂ values that create the steep O₂ gradient in the lungs.

Alveolar PO₂ ≈ 104 mm Hg; venous blood PO₂ ≈ 40 mm Hg.

In what forms is oxygen transported in the blood, and in what proportions?

≈1.5–2 % dissolved in plasma; ≈98–98.5 % bound to hemoglobin (Hb).

How many O₂ molecules can one hemoglobin (Hb) molecule carry?

Up to four, one per heme group.

Differentiate oxyhemoglobin and deoxyhemoglobin.

Oxyhemoglobin (HbO₂) is Hb bound to O₂; deoxyhemoglobin (HHb) has released its oxygen.

Describe the Bohr effect.

Increased CO₂ or H⁺ (lower pH) weakens the Hb-O₂ bond, enhancing O₂ unloading in metabolically active tissues.

Name five factors that regulate O₂ loading/unloading from Hb.

PO₂, PCO₂, blood pH, temperature, and BPG (2,3-bisphosphoglycerate) concentration.

How does a rise in temperature affect the O₂-Hb dissociation curve?

It shifts the curve to the right, decreasing Hb affinity for O₂ and promoting unloading.

What is ‘venous reserve’?

The 75 % Hb saturation (≈15 vol % O₂) that remains in venous blood at rest, available for use during increased demand.

State the three main forms in which CO₂ is transported in blood and their approximate percentages.

≈70 % as bicarbonate ions (HCO₃⁻) in plasma, ≈20 % bound to Hb as carbaminohemoglobin, ≈7–10 % dissolved in plasma.

Write the carbonic anhydrase-catalyzed reaction that forms bicarbonate in red blood cells.

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

What is the chloride shift?

The exchange of Cl⁻ ions into RBCs for HCO₃⁻ ions moving out (systemic capillaries) or vice-versa (pulmonary capillaries) to maintain electrical neutrality.

Explain the Haldane effect.

Deoxygenated Hb binds CO₂ more readily; thus lower PO₂ (less Hb saturation) increases CO₂ transport, facilitating CO₂ uptake in tissues and release in lungs.

Does CO₂ compete with O₂ for the heme iron binding site on Hb?

No; CO₂ binds to globin amino groups, whereas O₂ (and CO) bind to heme iron.

Why is carbon monoxide (CO) dangerous even at low concentrations?

Its affinity for Hb is >200× that of O₂, so it out-competes O₂ and causes hypoxic tissue injury.

List four types of hypoxia and a key cause for each.

Anemic (low Hb/RBC), ischemic (reduced blood flow), histotoxic (cells can’t use O₂, e.g., cyanide), hypoxemic (low arterial PO₂ from lung disease).

How does slow, shallow breathing affect blood pH?

CO₂ accumulates, carbonic acid rises, pH drops (respiratory acidosis).

What immediate therapy is given for CO poisoning and why?

Hyperbaric oxygen; very high PO₂ displaces CO from Hb faster and restores O₂ delivery.

During vigorous exercise, what combined effects facilitate greater O₂ unloading to muscles?

↑Temperature, ↑PCO₂, ↓pH, and ↑BPG all shift the dissociation curve right, lowering Hb affinity for O₂.

At high altitude, why does Hb remain almost fully saturated despite lower atmospheric PO₂?

The upper flat portion of the S-shaped dissociation curve provides a ‘safety margin’, so saturation stays high until PO₂ falls markedly.

What is the main physiological role of BPG in red blood cells?

It binds Hb and decreases its affinity for O₂; levels rise when chronic hypoxia is present, aiding O₂ unloading.

Why does emphysema reduce gas exchange efficiency?

Destruction of alveolar walls reduces total respiratory membrane surface area.

Which two laws of physics underpin the principles of gas exchange discussed in this lecture?

Dalton’s Law (partial pressures) and Henry’s Law (gas solubility in liquids).