Gas Exchange and Breathing: Pulmonary Ventilation, Gas Transport, and Regulation

Gas Exchange and Breathing

Overview of Pulmonary Ventilation and Gas Exchange

Gas exchange and breathing are essential physiological processes that ensure the delivery of oxygen to tissues and the removal of carbon dioxide from the body. These processes involve the coordinated function of the respiratory and cardiovascular systems, including the movement of gases across the respiratory membrane and their transport in the blood.

Pulmonary and Systemic Circulation

Blood Flow and Gas Transport

• Pulmonary circulation carries deoxygenated blood from the right ventricle to the lungs for gas exchange and returns oxygenated blood to the left atrium.

• Systemic circulation delivers oxygenated blood from the left ventricle to body tissues and returns deoxygenated blood to the right atrium.

• Oxygen and carbon dioxide are exchanged between alveoli and blood, and between blood and tissues, following their respective partial pressure gradients.

Gas Exchange at the Respiratory Membrane

Structure and Function

• The respiratory membrane consists of the alveolar epithelium, fused basement membranes, and capillary endothelium.

• Gas exchange occurs by diffusion across this thin barrier, allowing O2 to enter the blood and CO2 to exit.

Partial Pressures and Gas Diffusion

Dalton’s Law of Partial Pressures

Dalton’s Law states that the total pressure exerted by a mixture of gases is equal to the sum of the pressures exerted by each individual gas.

• The partial pressure of a gas is calculated as the product of its fractional concentration and the total pressure of the mixture.

Example: For atmospheric air at sea level (760 mmHg):

• Oxygen (O2): 20.93% → PO2 = 0.2093 × 760 mmHg = 159 mmHg

• Nitrogen (N2): 79.04% → PN2 = 0.7904 × 760 mmHg = 601 mmHg

• Carbon Dioxide (CO2): 0.03% → PCO2 = 0.0003 × 760 mmHg = 0.23 mmHg

Effect of Humidity

• Water vapor contributes to the total pressure in humid air, reducing the partial pressures of other gases.

Solubility of Gases in Liquids

Gas Dissolution and Equilibration

• Gases dissolve in liquids in proportion to their partial pressures and solubility coefficients.

• CO2 is about 20 times more soluble in water than O2.

• At equilibrium, the partial pressure of a gas in the liquid equals that in the air, but concentrations differ due to solubility.

Example: At PO2 = 100 mmHg, [O2]air = 5.2 mmol/L, [O2]water = 0.15 mmol/L; [CO2]air = 5.2 mmol/L, [CO2]water = 3.0 mmol/L.

Partial Pressures at Different Body Sites

Typical Values

The partial pressures of O2 and CO2 vary at different sites in the body, reflecting ongoing gas exchange.

Site	Oxygen (mm Hg)	Carbon Dioxide (mm Hg)
Atmospheric air	160	0.3
Alveolar air	100	40
Pulmonary veins	100	40
Systemic arteries	100	40
Cells	≤40	≥46
Systemic veins	40	46
Pulmonary arteries	40	46

Determinants of Alveolar Gas Pressures

Factors Affecting Alveolar PO2 and PCO2

• Partial pressures of inspired air

• Alveolar ventilation rate

• Rate of tissue O2 consumption and CO2 production

• The most important determinant is the ratio of alveolar ventilation to oxygen consumption/carbon dioxide production.

Respiratory Physiology Terms

Key Definitions

Term	Definition
Hyperpnea	Increased ventilation to meet metabolic demands
Dyspnea	Labored or difficult breathing
Apnea	Temporary cessation of breathing
Tachypnea	Rapid, shallow breathing
Hyperventilation	Ventilation exceeds metabolic demands
Hypoventilation	Ventilation insufficient for metabolic demands
Hypoxia	Deficiency of oxygen in tissues
Hypoxemia	Deficiency of oxygen in blood
Hypercapnia	Excess carbon dioxide in blood
Hypocapnia	Deficiency of carbon dioxide in blood

Transport of Gases in the Blood

Oxygen Transport

• Oxygen is transported in two forms: dissolved in plasma (3 mL/L) and bound to hemoglobin (197 mL/L).

• Hemoglobin saturation depends on the partial pressure of oxygen (PO2).

• Oxyhemoglobin (HbO2) forms at high PO2; deoxyhemoglobin predominates at low PO2.

Oxyhemoglobin Dissociation Curve

• Describes the relationship between PO2 and hemoglobin saturation.

• At PO2 of 100 mmHg (arterial), Hb is ~98.5% saturated; at 40 mmHg (venous), ~75% saturated.

• The curve is sigmoidal due to cooperative binding of O2 to hemoglobin.

Factors Affecting Hemoglobin Affinity for O2

• Increased temperature, decreased pH, increased PCO2, and increased 2,3-BPG shift the curve right (decreased affinity).

• Decreased temperature, increased pH, decreased PCO2, and decreased 2,3-BPG shift the curve left (increased affinity).

Carbon Dioxide Transport

• CO2 is transported dissolved in plasma (5%), bound to hemoglobin as carbaminohemoglobin (5%), and as bicarbonate ions (90%).

• The enzyme carbonic anhydrase catalyzes the conversion of CO2 and H2O to H2CO3, which dissociates to H+ and HCO3-.

Regulation of Ventilation

Control by Chemoreceptors

• Central chemoreceptors (medulla) and peripheral chemoreceptors (carotid bodies) detect changes in arterial PO2 and PCO2.

• Increased PCO2 or decreased PO2 stimulates increased ventilation.

• Negative feedback mechanisms maintain homeostasis of blood gases.

Acid-Base Homeostasis

pH Regulation and Buffer Systems

• Normal blood pH is 7.38–7.42; deviations can be life-threatening.

• Acidosis: pH < 7.35 (CNS depression); Alkalosis: pH > 7.45 (CNS excitation).

• Hemoglobin and bicarbonate ions act as buffers to resist changes in pH.

• The Henderson-Hasselbalch equation describes the relationship between pH, bicarbonate, and CO2:

$pH = 6.1 + \log \left( \frac{[HCO_3^-]}{0.03 \times P_{CO_2}} \right)$

• Respiratory and renal compensation mechanisms restore acid-base balance.

Summary

• Gas exchange and transport are tightly regulated to maintain homeostasis.

• Partial pressures, solubility, and hemoglobin affinity are key determinants of O2 and CO2 movement.

• Ventilation is controlled by chemoreceptor feedback to match metabolic demands and maintain acid-base balance.