Respiratory Pathophysiology, Gas Transport, and Central Regulation Vocabulary

Vocabulary flashcards covering gas exchange pathophysiology, oxygen and carbon dioxide transport mechanisms, hemoglobin dynamics, clinical gas transport disorders, central control of respiration, and physiological responses to exercise and high altitude.

Ventilation-Perfusion (V/Q) Mismatch

An inefficiency in gas exchange occurring when airflow (ventilation) and blood flow (perfusion) in the alveoli are not properly matched.

Low V/Q Ratio

A physiological state where blood flow is adequate but ventilation is reduced, leading to decreased oxygen levels in arterial blood.

High V/Q Ratio

A physiological state where ventilation is adequate but blood flow is reduced, preventing oxygen from being effectively transferred into the bloodstream.

Hypoxemia

Abnormally low partial pressure of oxygen in arterial blood (PaO_2 \b-down).

Hyperoxemia

Abnormally high partial pressure of oxygen in arterial blood (PaO_2 \b-up), most commonly resulting from supplemental oxygen therapy.

Shunt

The most severe form of low V/Q mismatch where ventilation is completely absent ($V = 0$) while perfusion continues ($Q > 0$), preventing blood oxygenation.

Anatomic Shunt

A type of shunt occurring when blood bypasses the lungs entirely due to structural cardiovascular defects, such as Tetralogy of Fallot.

Capillary Shunt

An intrapulmonary shunt occurring when blood passes through capillaries surrounding non-functional alveoli that are filled with fluid or collapsed.

Hypoventilation

Ventilation that is insufficient relative to metabolic needs, resulting in reduced alveolar oxygen pressure (PAO_2 \b-down) and carbon dioxide accumulation (PaCO_2 \b-up).

Hypercapnia

An abnormally increased concentration of carbon dioxide in arterial blood (PaCO_2 \b-up).

Diffusion Impairment

Impaired gas exchange resulting from thickening of the alveolar-capillary membrane or decreased surface area, which affects oxygen diffusion first due to its lower solubility.

Hypoxia

A state of insufficient oxygen supply at the tissue level, regardless of blood oxygen concentration.

Hypoxemic Hypoxia

Tissue hypoxia caused by low arterial oxygen levels originating from pulmonary gas exchange failure, high altitude, or hypoventilation.

Anemic Hypoxia

Tissue hypoxia caused by reduced oxygen-carrying capacity of the blood due to decreased hemoglobin concentration or dysfunctional hemoglobin.

Ischemic Hypoxia

Tissue hypoxia resulting from inadequate blood flow to tissues despite normal arterial oxygen content and saturation.

Histotoxic Hypoxia

Tissue hypoxia occurring when cells are unable to utilize oxygen effectively for cellular respiration despite normal oxygen delivery.

Cyanosis

A bluish discoloration of the skin and mucous membranes caused by an elevated concentration of deoxygenated hemoglobin in the blood.

Gas Transport

The physiological mechanism by which oxygen and carbon dioxide are carried in the blood between the pulmonary capillaries and systemic tissues.

Dissolved Oxygen

The small fraction ($1\text{–}2\times 10^{-2}$ or $1\text{–}2\text{\%}$) of oxygen transported freely in blood plasma, which exerts partial pressure ($PaO_2$).

Hemoglobin (Hb)

The iron-containing protein inside red blood cells responsible for transporting $98\text{–}99\text{\%}$ of oxygen in the circulation.

Heme Group

The iron-porphyrin structure within hemoglobin that contains a central ferrous iron ($Fe^{2+}$) capable of reversibly binding one oxygen molecule.

Oxyhemoglobin

Hemoglobin bound to oxygen molecules.

Deoxyhemoglobin

Hemoglobin that is not bound to oxygen, displaying a distinct structural conformation with higher affinity for carbon dioxide.

Hemoglobin Saturation ($SaO_2$)

The percentage of oxygen-binding sites on total hemoglobin molecules currently occupied by oxygen.

T-State (Tense State)

The low-affinity quaternary conformation of hemoglobin that promotes oxygen unloading into tissues.

R-State (Relaxed State)

The high-affinity quaternary conformation of hemoglobin that promotes oxygen binding in the lungs.

Positive Cooperativity

The binding property where the attachment of one oxygen molecule to a hemoglobin subunit increases the binding affinity of the remaining subunits.

Oxygen–Hemoglobin Dissociation Curve

The sigmoidal graphical relationship between the partial pressure of oxygen ($PO_2$) and the percentage of hemoglobin saturation ($SaO_2$).

P50

The partial pressure of oxygen at which hemoglobin is $50\text{\%}$ saturated, serving as a standard measure of hemoglobin affinity for oxygen.

Right Shift

A change in the oxygen–hemoglobin dissociation curve reflecting reduced hemoglobin affinity for oxygen and enhanced oxygen unloading in metabolically active tissues.

Left Shift

A change in the oxygen–hemoglobin dissociation curve reflecting increased hemoglobin affinity for oxygen, causing hemoglobin to hold oxygen more tightly.

2,3-DPG (2,3-Diphosphoglycerate)

An erythrocyte metabolite that binds deoxyhemoglobin to lower oxygen affinity, facilitating oxygen release during hypoxia, anemia, or high altitude.

Carbaminohemoglobin

Carbon dioxide bound directly to the globin amino groups of hemoglobin, transporting $10\text{–}20\text{\%}$ of total blood $CO_2$.

Bohr Effect

The decrease in hemoglobin affinity for oxygen induced by elevated carbon dioxide concentrations and reduced pH, promoting oxygen release in active tissues.

Haldane Effect

The increased affinity of deoxygenated hemoglobin for carbon dioxide, facilitating $CO_2$ pickup in systemic capillaries and release in pulmonary capillaries.

Bicarbonate ($HCO_3^-$)

The principal transport form of carbon dioxide in the blood ($70\text{–}80\text{\%}$), produced inside red blood cells.

Carbonic Anhydrase

An essential erythrocyte enzyme that reversibly catalyzes the reaction of carbon dioxide and water to form carbonic acid ($CO_2 + H_2O \rightleftharpoons H_2CO_3$).

Carbonic Acid ($H_2CO_3$)

An intermediate weak acid produced by carbonic anhydrase that rapidly dissociates into a hydrogen ion ($H^+$) and a bicarbonate ion ($HCO_3^-$).

Chloride Shift (Hamburger Phenomenon)

The electrogenic exchange of intracellular bicarbonate ($HCO_3^-$) for extracellular chloride ($Cl^-$) across red blood cell membranes to maintain electroneutrality.

Carboxyhemoglobin

Hemoglobin bound to carbon monoxide, which inhibits oxygen transport and prevents oxygen unloading by shifting the dissociation curve to the left.

Methemoglobinemia

A state in which heme iron is oxidized from ferrous ($Fe^{2+}$) to ferric ($Fe^{3+}$), preventing functional oxygen binding and delivery.

Heinz Bodies

Inclusions of denatured, precipitated hemoglobin bound to red blood cell membranes caused by oxidative toxicity, leading to hemolytic anemia.

Respiratory Control System

An integrated feedback control system comprising sensors, brainstem control centers, and motor effectors that maintains gas and pH homeostasis.

Medulla Oblongata

The lower brainstem region responsible for establishing the fundamental neural rhythm of breathing.

Pre-Bötzinger Complex

A group of rhythmic, spontaneously firing pacemaker neurons located in the medulla oblongata that initiates respiratory cycles.

Dorsal Respiratory Group (DRG)

A medullary respiratory center located in the nucleus tractus solitarius that processes sensory input and drives resting inspiration.

Ventral Respiratory Group (VRG)

A medullary respiratory region containing inspiratory and expiratory neurons that coordinates forced ventilation and recruits accessory muscles.

Pneumotaxic Center

A pontine respiratory group structure that terminates inhalation, functioning as a neural brake to regulate tidal volume and increase rate.

Apneustic Center

A pontine structure that provides excitatory input to inspiratory neurons, prolonging inhalation effort and increasing tidal volume.

Central Chemoreceptors

Medullary surface receptors that monitor cerebrospinal fluid pH changes derived from arterial carbon dioxide diffusion across the blood–brain barrier.

Peripheral Chemoreceptors

Sensory structures in the carotid and aortic bodies that rapidly detect arterial hypoxemia (PaO_2 < 60\b-thinspace mm\b-thinspace Hg), hypercapnia, and acidosis.

Pulmonary Stretch Receptors

Mechanoreceptors located in airway smooth muscle that respond to lung inflation to limit tidal volume.

Hering–Breuer Inflation Reflex

A protective vagal reflex that inhibits inspiratory drive upon excessive lung inflation, preventing overexpansion.

Irritant Receptors

Airway epithelial receptors activated by mechanical or chemical irritants that initiate protective coughing, bronchoconstriction, and hyperpnea.

Juxtacapillary (J) Receptors

Alveolar wall receptors near capillaries that detect pulmonary edema or vascular congestion, triggering rapid shallow breathing.

Proprioceptors

Mechanoreceptors located in muscles and joints that signal motor movement to stimulate ventilation at the onset of exercise.

Feedforward Control

A motor cortex and proprioceptive mechanism that increases ventilation immediately upon initiating exercise prior to blood gas changes.

Minute Ventilation ($V_E$)

The total volume of gas exhaled per minute, calculated as the product of tidal volume and respiratory rate (V_E = V_T \b-times RR).

Dalton’s Law

A physical law stating that total barometric pressure equals the sum of partial pressures of constituent gases, explaining altitude hypoxemia.

Hypocapnia

Abnormally low partial pressure of carbon dioxide in arterial blood (PaCO_2 \b-down), resulting from hyperventilation.

Respiratory Alkalosis

A elevated systemic arterial pH resulting from excessive pulmonary carbon dioxide elimination.

Acclimatization

The adaptive process during chronic high-altitude exposure, involving renal bicarbonate excretion to normalise pH and increased hematocrit.

Erythropoietin (EPO)

A glycoprotein hormone produced by renal cells during hypoxia that stimulates red blood cell production in bone marrow.

Hypoxic Pulmonary Vasoconstriction

A localized physiological vascular reflex where small pulmonary arteries constrict in response to alveolar hypoxia to match ventilation and perfusion.

Brisket Disease

A high-altitude condition in cattle caused by generalized hypoxic pulmonary vasoconstriction, leading to pulmonary hypertension, right ventricular hypertrophy, right-sided heart failure, and ventral edema.