Phys: Gas Exchange

Diffusion

The process by which oxygen and carbon dioxide move across the alveolar-capillary membrane from areas of high concentration to low concentration.

Partial Pressure

The pressure exerted by an individual gas within a mixture, which drives gas movement across biological membranes.

Alveolar Oxygen Pressure (PAO2)

The partial pressure of oxygen inside the alveoli.

Arterial Oxygen Pressure (PaO2)

The partial pressure of oxygen dissolved in arterial blood.

Arterial Carbon Dioxide Pressure (PaCO2)

The partial pressure of carbon dioxide in arterial blood.

Alveolar-Capillary Membrane

The thin structural barrier separating alveolar air from pulmonary capillary blood, across which oxygen and carbon dioxide diffuse.

Anatomic Dead Space

The volume of the conducting airways where air enters but no gas exchange occurs due to the absence of alveoli.

Physiologic Dead Space

The total volume of inhaled air that does not participate in gas exchange, combining anatomical dead space with unperfused alveolar dead space.

Wasted Ventilation

Ventilation supplied to alveoli that lack sufficient blood perfusion, resulting in air movement without gas exchange.

Respiratory Shunt

A condition where pulmonary capillary blood flows past unventilated alveoli (perfusion without ventilation), leaving the blood unoxygenated.

Anatomic Shunt

Vascular pathways where deoxygenated venous blood bypasses the pulmonary capillaries entirely and flows directly into systemic arterial circulation.

Physiologic Shunt

The total portion of cardiac output that enters systemic arterial circulation without undergoing complete oxygenation.

Ventilation-Perfusion Ratio (V/Q)

The ratio of alveolar ventilation ($V$) to pulmonary capillary blood flow ($Q$).

V/Q Mismatch

A regional imbalance between alveolar airflow and capillary blood flow, acting as a primary mechanism of clinical gas exchange impairment.

Low V/Q Ratio

A state where blood perfusion exceeds ventilation, creating a shunt-like effect that results in poorly oxygenated arterial blood.

High V/Q Ratio

A state where ventilation exceeds perfusion, creating a dead-space effect where air is brought into the lungs but cannot be picked up by blood flow.

Alveolar-Arterial (A-a) Gradient

The calculated difference in partial pressure between alveolar oxygen ($PAO_2$) and arterial oxygen ($PaO_2$), evaluated to detect impaired gas transfer across the lungs.

Hypoxemia

An abnormally low partial pressure or concentration of oxygen in arterial blood.

Hypercapnia

An elevated partial pressure of carbon dioxide in arterial blood, typically caused by alveolar hypoventilation or marked dead space.

Hypoxic Pulmonary Vasoconstriction

A vascular response in which low alveolar oxygen tension causes constriction of local pulmonary arterioles to divert blood flow to better-ventilated regions.

Acclimatization

The set of physiological compensations—including hyperventilation, renal bicarbonate excretion, and increased erythropoietin production—that restore tissue oxygenation upon exposure to high altitude.

Respiratory Exchange Ratio

The relationship between carbon dioxide production and oxygen consumption, reflecting body substrate metabolism and matching ventilation to cardiac output