Physiology Chapter 19: Gas Exchange and Transport

A complete set of 50 vocabulary flashcards reviewing gas exchange, hypoxia classifications, hemoglobin kinetics, and respiratory gas transport from Chapter 19.

Hypoxia

A condition characterized by inadequate oxygen available to tissues.

Hypercapnia

An increased concentration of $CO_2$ in the blood.

Hypoxic hypoxia

Inadequate tissue oxygen caused by low arterial $PO_2$, resulting from causes such as high altitude, hypoventilation, impaired diffusion, or mismatched ventilation and perfusion.

Anemic hypoxia

Inadequate oxygen delivery caused by reduced oxygen bound to hemoglobin (e.g., from anemia, blood loss, or carbon monoxide poisoning) while arterial $PO_2$ can remain normal.

Ischemic hypoxia

Inadequate oxygen delivery to tissues resulting from reduced blood flow, such as in heart failure, shock, or a blood clot blocking circulation.

Histotoxic hypoxia

A condition in which cells receive adequate oxygen but cannot use it because a poison (such as cyanide) interferes with cellular respiration.

Arterial blood gas values

Normal reference values in arterial blood: $PO_2$ of $95\,mmHg$ (range $85\text{–}100\,mmHg$), $PCO_2$ of $40\,mmHg$ (range $35\text{–}45\,mmHg$), and pH of $7.40$.

Venous blood gas values

Normal reference values in venous blood: $PO_2$ of $40\,mmHg$, $PCO_2$ of $46\,mmHg$, and pH of $7.37$.

Partial pressure

The pressure contributed by a single gas in a mixture; the total gas pressure equals the sum of the individual partial pressures.

Alveolar gas exchange requirements

The three prerequisites for effective alveolar exchange: oxygen must reach the alveoli, gases must diffuse effectively across the alveolar-capillary barrier, and the alveoli must receive adequate blood flow.

Hypoventilation

Ventilation that is insufficient for metabolic needs, resulting in decreased arterial $PO_2$ and increased arterial $PCO_2$.

Causes of hypoventilation

Three major slide causes: decreased lung compliance, increased airway resistance, and CNS depression (such as from alcohol poisoning or drug overdose).

Alveolar-capillary barrier

The thin diffusion barrier separating alveolar air and blood, composed of type I alveolar epithelium, basement membranes, and capillary endothelium.

Factors influencing gas diffusion rate

Physical variables determining diffusion across the respiratory membrane: surface area, barrier permeability, diffusion distance, and the partial pressure gradient.

Partial pressure gradient in healthy lungs

The primary factor that normally changes to influence gas exchange, as surface area, barrier properties, and diffusion distance remain relatively constant under normal conditions.

Emphysema

A disease that impairs gas exchange through the destruction of alveolar walls, thereby reducing the total surface area available for diffusion.

Fibrotic lung disease

A condition in which scar tissue thickens the alveolar barrier, slowing gas diffusion and decreasing lung compliance.

Pulmonary edema

An accumulation of excess fluid that increases the diffusion distance between alveolar air and blood, and in severe cases can flood the alveoli.

Determinants of gas dissolving in liquid

The three factors that determine how much gas dissolves in a liquid: the gas's partial pressure, its solubility in that liquid, and temperature.

Gas equilibrium between air and liquid

A state where gas partial pressures are equal across air and liquid phases, while concentrations can differ due to differing gas solubilities.

Carbon dioxide vs. oxygen solubility

$CO_2$ is approximately $20\times$ more soluble in water than oxygen, meaning oxygen depends heavily on hemoglobin for blood transport and is more susceptible to diffusion problems caused by lung fluid.

Dissolved oxygen in plasma

The initial form of oxygen entering the blood, comprising less than $2\%$ of total transported oxygen, with more than $98\%$ carried bound to hemoglobin.

Hemoglobin structure

A protein consisting of four globin chains and four heme groups; each heme iron atom can bind one $O_2$ molecule, allowing a maximum capacity of four $O_2$ molecules per hemoglobin.

Oxyhemoglobin

Hemoglobin with oxygen bound to it, abbreviated $HbO_2$.

Law of mass action in oxygen binding

The chemical equilibrium principle governing $Hb + O_2 \rightleftharpoons HbO_2$, where increased dissolved oxygen shifts the reaction toward binding and decreased dissolved oxygen promotes release.

Hemoglobin's role in alveolar oxygen uptake

Hemoglobin binds oxygen entering red blood cells, keeping dissolved plasma $PO_2$ low and allowing net oxygen diffusion from alveoli into blood to continue until equilibrium is reached.

Determinants of oxygen bound to hemoglobin

Plasma $PO_2$ (which directly governs hemoglobin percent saturation) and the total number of available hemoglobin binding sites.

Determinants of total available binding sites

The total number of red blood cells and the quantity of hemoglobin contained inside each individual cell.

Percent saturation of hemoglobin

The percentage of available oxygen-binding sites occupied: $(\text{O}_2\text{ bound} \div \text{maximum O}_2\text{ that could bind}) \times 100$.

Oxygen-hemoglobin binding curve

A plot showing the relationship between $PO_2$ and hemoglobin saturation whose S-shape results from cooperative binding, where binding one oxygen increases affinity for subsequent oxygen molecules.

Hemoglobin saturation at $100\,mmHg$

Approximately $97\text{–}98\%$ saturation, reflecting normal oxygen loading in pulmonary capillaries.

Hemoglobin saturation at $40\,mmHg$

Approximately $75\%$ saturation, reflecting the oxygen content of resting mixed venous blood.

Hemoglobin saturation at $20\,mmHg$

Approximately $30\text{–}35\%$ saturation, demonstrating substantial oxygen unloading in actively metabolizing systemic tissues.

Plateau portion of the binding curve

The upper flat section of the oxygen-hemoglobin dissociation curve that preserves high saturation despite moderate decreases in lung $PO_2$.

Steep portion of the binding curve

The lower sloped section of the oxygen-hemoglobin dissociation curve that allows substantial oxygen unloading as tissue $PO_2$ decreases.

Right shift of the oxygen-hemoglobin curve

A change indicating lower oxygen affinity of hemoglobin and increased oxygen unloading at a given $PO_2$.

Left shift of the oxygen-hemoglobin curve

A change indicating higher oxygen affinity of hemoglobin and decreased oxygen unloading at a given $PO_2$.

Factors causing a right curve shift

Decreased pH, increased $CO_2$, and increased temperature, all of which enhance oxygen unloading to tissues.

Bohr effect

The reduction in hemoglobin's oxygen affinity caused by a decrease in pH, encouraging oxygen unloading in active, acidic tissues that produce $CO_2$ and heat.

2,3-Bisphosphoglycerate (2,3-BPG)

A compound produced in red blood cells that reduces hemoglobin's oxygen affinity; its production increases during chronic hypoxia, shifting the dissociation curve to the right.

Fetal hemoglobin

An oxygen-carrying protein isoform with a higher oxygen affinity than adult hemoglobin, producing a left-shifted curve that facilitates oxygen uptake from maternal blood across the placenta.

Bicarbonate ($HCO_3^-$) transport

The principal form of carbon dioxide transport in the blood, accounting for approximately $70\%$ of total transported $CO_2$.

Carbaminohemoglobin

Hemoglobin carrying carbon dioxide (accounting for approximately $23\%$ of blood $CO_2$), bound to amino groups on globin chains rather than to heme iron sites.

Dissolved $CO_2$ transport

The simplest mode of carbon dioxide transport, accounting for approximately $7\%$ of $CO_2$ dissolved directly in blood plasma.

Carbonic anhydrase

An enzyme concentrated in red blood cells that accelerates the reversible reaction $CO_2 + H_2O \rightleftharpoons H^+ + HCO_3^-$.

Chloride shift

The transport process at systemic tissues in which $HCO_3^-$ exits red blood cells into plasma while $Cl^-$ enters, maintaining electrical neutrality.

Hemoglobin proton buffering

The binding of $H^+$ ions by deoxygenated hemoglobin, which limits blood pH fluctuations resulting from carbonic acid dissociation.

Respiratory acidosis

A decrease in blood pH that occurs when accumulated $CO_2$ drives excessive $H^+$ production via the carbonic acid reaction beyond what buffering and ventilation can compensate for.

Pulmonary reversal of $CO_2$ transport

The process in the lungs where $CO_2$ diffuses into alveoli, causing $HCO_3^-$ to re-enter red blood cells as $Cl^-$ exits, recombining with $H^+$ to reform $CO_2$ and water for exhalation.

High altitude hypoxia

A form of hypoxic hypoxia where total atmospheric pressure decreases; although oxygen remains about $21\%$ of ambient air, its inspired partial pressure is significantly reduced.