Respiratory System Notes (Mammals)

Structure of the mammalian respiratory system

  • Primary components: nasal passages, pharynx (shared with the digestive tract), larynx, trachea, bronchi, bronchioles, and alveoli.
  • Trachea and larger bronchi: rigid, non-muscular tubes held open by rings of cartilage.
  • Bronchioles: lack cartilage; walls contain smooth muscle controlled by the autonomic nervous system and responsive to hormones and local chemicals.
  • Alveoli: tiny, thin-walled gas-exchange air sacs at the end of terminal bronchioles; cluster in grape-like bunches; each alveolus is surrounded by an almost continuous sheet of pulmonary capillaries.
  • Gas exchange site: alveoli + pulmonary capillaries where diffusion of gases occurs (O2 into blood, CO2 into air).
  • Blood vessels: branch of the pulmonary artery brings deoxygenated blood to the lungs; branch of the pulmonary vein returns oxygenated blood to the heart.
  • Airway architecture: resembles an upside-down tree.
  • Epithelium: bronchioles lined with epithelial tissue; mucus-producing submucosal glands; cilia and saline move mucus toward the pharynx for clearance.
  • Gas exchange surface area and scale:
    • In humans, the gas-exchange surface is about S≈100 m2S\approx 100\ \text{m}^2
    • There are about N≈3×108N\approx 3\times 10^{8} alveoli.
    • The surface area is roughly equivalent to half a tennis court.
  • Alveolar-diffusion distance: the alveolus diameter is described as being about 600 times larger than the intervening space between air and blood, highlighting the diffusion interface (air to blood barrier).
  • Alveolar-blood flow: pulmonary artery carries deoxygenated blood to the alveoli; pulmonary veins return oxygenated blood to the heart.
  • Functions of the respiratory system beyond gas exchange: protection from inhaled pathogens and irritants via mucus and cilia; regulation of body pH and acid-base balance through gas exchange.

Gas exchange and respiration

  • External respiration vs cellular respiration:
    • External respiration: exchange of O2 and CO2 between external environment and tissue cells via diffusion across respiratory surfaces.
    • Cellular respiration: intracellular metabolic reactions using O2 and nutrients to produce energy, with CO2 as a byproduct.
  • Gas exchange pathway:
    • Gases exchange between alveolar air and blood across alveolar and pulmonary capillary walls by diffusion.
    • Blood transports O2 from lungs to tissues and CO2 from tissues to lungs for elimination.
  • Relationship to ventilation: ventilation (pulmonary ventilation) supplies fresh air to the lungs; diffusion across pulmonary capillaries and systemic capillaries completes the gas exchange process.
  • Interdependence: external respiration (lung-blood interface) and internal respiration (blood-tissue interface) are functionally linked with circulatory and respiratory systems working together.

Pleural anatomy and mechanics

  • Pleural cavity/space: the potential space between the visceral pleura (lung surface) and the parietal pleura (internal chest wall surface); contains intrapleural fluid.
  • Pleural membranes:
    • Visceral pleura adheres to the lung surface.
    • Parietal pleura lines the thoracic cavity.
  • Analogy: pushing a lollipop into a small water-filled balloon illustrates the interaction of double-walled pleural sacs with the lung that they surround and separate from the thoracic wall.
  • Functions of the pleural system: helps maintain lung expansion and prevents collapse via the slight negative pressure in the intrapleural space.
  • Defense and protection: respiratory system traps pathogens and irritants via mucus and saline; mucus is moved up by cilia for removal from the airway.

Ventilation and breathing mechanics

  • Ventilation = breathing; in mammals, referred to as pulmonary ventilation.
  • Modes of breathing:
    • Mammals: negative-pressure breathing (air pulled into lungs).
    • Amphibians (e.g., frog): positive-pressure breathing (air pushed into lungs).
  • Inhalation (inspiration):
    • Rib cage expands; intercostal muscles contract.
    • Diaphragm contracts and moves downward.
    • Lung volume increases; intrapleural pressure becomes more negative (drops).
  • Exhalation (expiration):
    • Rib cage relaxes; muscles relax; diaphragm relaxes and moves upward.
    • Lung volume decreases; intrapleural pressure relaxes toward baseline.
    • Intra-alveolar pressure increases, pushing air out.
  • Key pressure relationships that drive ventilation:
    • Atmospheric pressure: Patm≈760 mmHgP_{\text{atm}} \approx 760\ \text{mmHg}
    • Intra-alveolar (intra-pulmonary) pressure: PalvP_{\text{alv}} (varies with ventilation; ~equal to atmospheric at rest but fluctuates during breathing)
    • Intrapleural pressure: Pip≈756 mmHgP_{\text{ip}} \approx 756\ \text{mmHg}
    • Pressure gradient across the lung wall: the difference between alveolar and intrapleural pressures causes the lungs to be stretched to fill the thoracic cavity (the gradient is about 4 mmHg4\ \text{mmHg}).
    • At rest, P<em>alv=P</em>atmP<em>{\text{alv}} = P</em>{\text{atm}} (nearly), while P<em>ip<P</em>alvP<em>{\text{ip}} < P</em>{\text{alv}} creating a suction that keeps the lungs inflated.
  • During inspiration: the thoracic cavity expands, the intrapleural pressure becomes more negative (e.g., falls from approximately 756 to about 754 mmHg), lung volume increases, and $P{\text{alv}}$ drops below $P{\text{atm}}$, drawing air in.
  • During expiration: relaxation and elastic recoil reduce thoracic volume, $P{\text{alv}}$ rises above $P{\text{atm}}$, and air is expelled.

Airway resistance and autonomic control

  • Airflow is influenced by airway resistance; primary determinant is the radius of the conducting airways.
  • Bronchoconstriction vs bronchodilation:
    • Bronchoconstriction increases airway resistance (smaller radius).
    • Bronchodilation decreases airway resistance (larger radius).
  • Neural control and hormonal influences:
    • Parasympathetic stimulation (acetylcholine) promotes bronchoconstriction (increased resistance).
    • Sympathetic stimulation (epinephrine) promotes bronchodilation (decreased resistance).
  • Dual innervation: most organs are under both sympathetic and parasympathetic control, typically producing opposite effects to allow precise regulation.
  • General state tendencies:
    • Parasympathetic activity dominates in rest/digest conditions.
    • Sympathetic activity dominates in emergency or fight-or-flight conditions.
  • Definitions:
    • Tidal volume: the volume of air inhaled or exhaled with each normal breath.
    • Vital capacity: the maximum tidal inhalation plus exhalation (the maximum amount of air a person can expel from the lungs after a maximum inhalation).
    • Residual volume: the amount of air remaining in the lungs after a maximal exhalation.
  • Functional consequence: the intrapleural suction and the airway radius together regulate how easily air flows during breathing.

Lung volumes, capacities, and exchange surface

  • Tidal volume (TV): volume of air moved per normal breath.
  • Vital capacity (VC): maximum amount of air a person can expel after a maximum inhalation (often used as a measure of respiratory health).
  • Residual volume (RV): air left in the lungs after a maximal exhalation.
  • Mixed air concept: each inhalation mixes fresh air with residual air in the alveoli, ensuring some O2 delivery even when metabolic demand is high.
  • Gas exchange surface: about S≈100 m2S \approx 100\ \text{m}^2 with around N≈3×108N \approx 3\times 10^{8} alveoli, enabling efficient diffusion.
  • Alveolar diffusion: O2 moves from alveolar air to blood, and CO2 moves from blood to alveolar air via diffusion across the alveolar-capillary barrier.

Regulation of ventilation and respiratory control center

  • Chemoreceptors:
    • Located in the aorta and carotid arteries; monitor concentrations of O2 and CO2 in blood and influence breathing.
  • Central control: the brainstem medulla oblongata contains the primary neural circuitry for breathing regulation.
  • Mechanism:
    • The medulla senses levels of CO2 and O2 and pH in cerebrospinal fluid.
    • Signals from the medulla adjust the rate and depth of breathing by affecting the rib muscles and the diaphragm.
    • The overall goal is to match ventilation to metabolic demand, maintaining normal blood gas levels.
  • Automatic control: breathing is largely automatic and does not require conscious effort for routine maintenance.

Respiratory disorders and clinical implications

  • Obstructive lung diseases: difficulty with exhaling due to increased airway resistance.
    • Asthma: bronchoconstriction driven by smooth muscle tightening; commonly treated with adrenergic agonists to dilate airways; parasympathetic activity can worsen constriction.
    • Bronchitis: inflammation of the bronchi/trachea with mucus production; coughing is a hallmark.
    • Emphysema: destruction of alveolar walls; often caused by cigarette smoking; contributes to reduced surface area and airflow limitation.
    • COPD (chronic obstructive pulmonary disease): combination of emphysema and chronic bronchitis.
    • Obstructive sleep apnea: relaxation of soft tissues in the throat during sleep leading to temporary airway obstruction.
  • Restrictive lung diseases: reduced ability of lungs to expand, leading to reduced lung volumes.
    • Examples: pulmonary fibrosis, pneumonia, pulmonary edema.
  • Therapeutic considerations:
    • Adrenergic agonists (e.g., epinephrine) can be used to relieve bronchoconstriction in acute asthma or prophylactically; Epipen autoinjector is used for severe asthma attacks or anaphylaxis.
  • Additional notes:
    • The respiratory system’s defense mechanisms include mucus production and mucociliary clearance to trap and remove inhaled irritants and pathogens.
    • The balance of CO2 and O2 affects blood pH and overall acid-base homeostasis.
  • Practical relevance: smoking is a major risk factor for emphysema and COPD; management of asthma often relies on controlling airway inflammation and using bronchodilators; sleep apnea has significant health consequences and may require behavioral or medical interventions.

Quick recap and connections to broader physiology

  • The respiratory system and circulatory system are tightly integrated to ensure efficient gas exchange and energy production through cellular respiration.
  • Ventilation is driven by pressure gradients created by chest wall and diaphragm movements; pressures of the atmosphere, alveoli, and pleural space govern air movement.
  • The alveolar surface area and capillary network maximize diffusion while minimizing transport distances.
  • Protective, regulatory, and metabolic roles of respiration include defense against pathogens, acid-base balance, and adaptation to metabolic demands during exercise or stress.

Key numbers and quick references

  • Atmospheric pressure: Patm≈760 mmHgP_{\text{atm}} \approx 760\ \text{mmHg} at sea level.
  • Intrapleural pressure: Pip≈756 mmHgP_{\text{ip}} \approx 756\ \text{mmHg}.
  • Pressure gradient driving lung expansion: roughly ΔP<em>gradient=P</em>alv−Pip≈4 mmHg\Delta P<em>{\text{gradient}} = P</em>{\text{alv}} - P_{\text{ip}} \approx 4\ \text{mmHg} during typical breathing cycles.
  • Alveolar surface area: S≈100 m2S \approx 100\ \text{m}^2.
  • Number of alveoli: N≈3×108N \approx 3\times 10^{8}.
  • Alveolar-diffusion distance and diameter ratio: alveolus diameter is described as about 600×600\times larger than the intervening air-blood diffusion distance.
  • Respiratory surface capacity in humans is substantial enough to meet metabolic demands during rest and activity.