Respiratory System Physiology and Anatomy

Comprehensive vocabulary flashcards covering the anatomy, mechanics, lung capacities, gas laws, gas exchange, and neural/chemical regulation of the human respiratory system.

Ventilation ($V$)

The mechanical movement of air into and out of the lungs, delivering oxygen and removing carbon dioxide from the alveoli.

Perfusion ($Q$)

The passage of blood through the blood vessels of an organ or tissue (named with the variable '$Q$' from the French word 'quantité').

External Respiration

The exchange of gases between the external environment, the alveoli of the lungs, and the bloodstream within pulmonary capillaries.

Internal Respiration

The exchange of respiratory gases between the systemic capillary bloodstream and the metabolizing tissues of the body.

Hyoid Bone

The 'floating bone' suspended in a muscular hammock that does not articulate with any other bone; it serves as an anchor point for muscles of swallowing and speech.

Larynx

A cartilaginous airway structure that provides a patent (open) airway, acts as a switching mechanism routing air and food, and houses the vocal folds for vocalization.

Trachea

A flexible tube in the mediastinum extending from the larynx at vertebra $C_6$ to $T_4/T_5$ where it bifurcates at the carina; supported by $15\text{--}20$ U-shaped hyaline cartilages and the posterior trachealis muscle.

Ciliary Escalator

The coordinated rhythmic beating of cilia in a periciliary fluid layer that propels mucus sheets upward from the lower respiratory tract toward the pharynx to be swallowed or expectorated.

Club Cells

Specialized secretory cells situated in the bronchioles that replace goblet cells to trap particulates, clear debris, and provide airway defence.

Visceral Pleura

The serous membrane that adheres tightly to the outer surface of each lung, protecting lung tissue from friction and transmitting pressure changes from the chest wall.

Parietal Pleura

The serous membrane lining the internal thoracic wall, the superior surface of the diaphragm, and the lateral aspect of the mediastinum.

Intrapleural Pressure ($P_{\text{ip}}$)

The subatmospheric pressure within the pleural cavity (normally $-4\,\text{mmHg}$ at rest) created by the opposing forces of inward lung elastic recoil and outward chest wall recoil.

Pneumothorax

The presence of air within the pleural space caused by a breach in the thoracic wall or lung, which abolishes the negative intrapleural seal and results in lung collapse.

Boyle's Law

A gas law stating that at a constant temperature, the pressure ($P$) and volume ($V$) of a gas are inversely proportional ($P \propto \frac{1}{V}$).

Intrapulmonary Pressure ($P_{\text{pul}}$)

The pressure exerted within the alveoli and airways of the lungs that fluctuates above and below atmospheric pressure during breathing and equilibrates to $0\,\text{mmHg}$ ($760\,\text{mmHg}$ absolute) between breaths.

Transpulmonary Pressure ($P_{\text{tp}}$)

The pressure difference between intrapulmonary and intrapleural pressure ($P_{\text{tp}} = P_{\text{pul}} - P_{\text{ip}}$), typically around $4\,\text{mmHg}$ at rest, which keeps the lungs inflated and determines lung volume.

Quiet Inhalation

An active process wherein contraction of the diaphragm (flattening downwards) and external intercostals (moving ribs up and out) increases thoracic volume, dropping intrapulmonary pressure below atmospheric pressure to draw air inward.

Active Exhalation

A forced respiratory movement produced by contraction of the internal intercostal and abdominal muscles (external oblique, internal oblique, transversus abdominis, rectus abdominis), rapidly decreasing thoracic volume to push air out.

Tidal Volume (TV)

The volume of air inspired or expired into and out of the lungs during a single relaxed, resting respiratory cycle (typically approximately $500\,\text{mL}$).

Inspiratory Reserve Volume (IRV)

The maximum extra volume of air that can be forcefully inhaled beyond a normal tidal inspiration (approximately $1900\,\text{mL}$ in females and $3000\,\text{mL}$ in males).

Expiratory Reserve Volume (ERV)

The extra volume of air that can be forcefully exhaled from the lungs after completing a normal, resting tidal expiration ($1000\text{--}1200\,\text{mL}$ in healthy adults).

Residual Volume (RV)

The volume of air remaining inside the lungs following a maximal forced exhalation (approximately $1200\,\text{mL}$), preventing alveolar collapse and maintaining continuous gas exchange between breaths.

Vital Capacity (VC)

The total volume of air that can be forcefully exhaled following a maximal inhalation, calculated as $\text{VC} = \text{IRV} + \text{TV} + \text{ERV}$.

Total Lung Capacity (TLC)

The absolute maximum amount of air contained within the lungs following a maximal, deepest possible inhalation ($\text{TLC} = \text{VC} + \text{RV}$).

$\text{FEV}_1/\text{FVC}$ Ratio

The proportion of forced vital capacity exhaled within the first second of forced exhalation; normally $75\text{--}80\%$ in healthy adults, with a value below $0.70$ ($70\%$) indicating obstructive lung disease.

Airflow Equation

The physical relationship $F = \frac{\Delta P}{R}$, which states that gas flow ($F$) is directly proportional to the pressure gradient ($\Delta P$) and inversely proportional to airway resistance ($R$).

Lung Compliance

A measurement of the ease with which the lungs and thoracic wall expand during inhalation, influenced by tissue elasticity and alveolar surface tension.

Elastic Recoil

The passive tendency of stretched pulmonary elastic fibers to return to their original resting size during exhalation, reducing the muscular effort required to empty the lungs.

Type I Alveolar Cells (Pneumocytes)

Extremely thin simple squamous epithelial cells lining the alveoli that form the main cellular boundary of the respiratory membrane for gas diffusion.

Type II Alveolar Cells (Pneumocytes)

Simple cuboidal epithelial cells scattered within alveoli that synthesize and secrete pulmonary surfactant.

Pulmonary Surfactant

An amphiphilic detergent-like mixture of phospholipids, neutral lipids, and proteins that disrupts hydrogen bonds among water molecules lining alveoli, decreasing surface tension and preventing alveolar collapse.

Neonatal Respiratory Distress Syndrome (NRDS)

A condition in premature infants (especially before $28\text{--}34$ weeks gestation) caused by inadequate surfactant production by Type II pneumocytes, leading to high surface tension, alveolar collapse, and low lung compliance.

Fick's Law of Diffusion

A law stating that the rate of diffusion is directly proportional to surface area and partial pressure gradient, and inversely proportional to the thickness (distance) of the diffusion barrier.

Dalton's Law

A law stating that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the individual partial pressures exerted by each gas.

Henry's Law

A law stating that the concentration of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid and its solubility coefficient.

Graham's Law

A law stating that the diffusion or effusion rate of a gas is inversely proportional to the square root of its molecular weight (making lighter gases diffuse faster in air than heavier ones).

Cooperative Binding

The mechanism whereby oxygen binding to one heme group induces a conformational shift in hemoglobin from the low-affinity T-state (tense) to the high-affinity R-state (relaxed), increasing the binding affinity of the remaining iron sites.

Carbaminohemoglobin

The molecular form transporting approximately $20\%$ of carbon dioxide in the blood, in which $\text{CO}_2$ binds directly to the terminal amino groups of the globin polypeptide chains rather than the heme iron.

Bicarbonate Buffer System ($\text{HCO}_3^-$)

The primary transport mechanism for carbon dioxide ($\sim 70\%$), wherein $\text{CO}_2$ combines with water via carbonic anhydrase to yield carbonic acid ($\text{H}_2\text{CO}_3$), which dissociates into $\text{HCO}_3^-$ and $\text{H}^+$.

Haldane Effect

The physiological effect wherein deoxygenation of hemoglobin increases its affinity for $\text{CO}_2$ in tissues, while oxygenation in pulmonary capillaries induces the R-state, releasing $\text{CO}_2$ into the alveoli to be exhaled.

Ventral Respiratory Group (VRG)

A medullary respiratory network containing the Pre-Bötzinger complex that sets the basic rhythm of breathing and innervates inspiratory muscles via the phrenic and intercostal nerves.

Dorsal Respiratory Group (DRG)

A medullary respiratory center located in the nucleus tractus solitarius (NTS) consisting mostly of inspiratory neurons that integrate sensory signals from peripheral stretch and chemoreceptors.

Pontine Respiratory Centre (PRC)

A center in the pons that modifies, smooths, and fine-tunes the basic respiratory rhythm generated by the VRG during activities such as speech, sleep, and exercise.

Central Chemoreceptors

Receptors in the ventral medulla that monitor arterial $\text{PaCO}_2$ indirectly by detecting hydrogen ion ($\text{H}^+$) concentration changes within cerebrospinal fluid (CSF), mediating approximately $70\%$ of the ventilatory response.

Peripheral Chemoreceptors

Sensory bodies located in the carotid bodies (cranial nerve IX) and aortic bodies (cranial nerve X) that stimulate ventilation in response to arterial $\text{PaO}_2 < 60\,\text{mmHg}$, elevated $\text{PaCO}_2$, and increased $\text{H}^+$.

Hypercapnia

An abnormally high partial pressure of carbon dioxide in arterial blood ($\text{PaCO}_2 > 45\,\text{mmHg}$) that triggers central and peripheral chemoreceptors to elevate the depth and rate of ventilation.

Hering-Breuer Reflex

A protective negative-feedback reflex triggered when tidal volume exceeds approximately $1000\,\text{mL}$; pulmonary stretch receptors in airway smooth muscle send vagal afferents to inhibit the DRG, preventing lung overinflation.

Irritant Receptors

Sensory nerve endings in the airway mucosa stimulated by inhaled chemical pollutants, debris, inflammation, or cold air that transmit vagal signals to induce cough, sneeze, or bronchoconstriction.