Notes: Structure and Function of the Respiratory System (Ch.29)

Structural Organization of the Respiratory System

  • Ventilation: The movement of air between the atmosphere and the respiratory portion of the lungs

  • Perfusion: The flow of blood through the lungs

  • Diffusion: The transfer of gases between the air-filled spaces in the lungs and the blood

  • The respiratory system consists of the air passages and the lungs (see Fig. 29.1)

    • Mediastinum
    • Apex

  • Divided into two functional parts:

    • Conducting airways: through which air moves as it passes between the atmosphere and the lungs
    • Includes the mucociliary blanket
    • Respiratory tissues of the lungs: where gas exchange takes place

Structure of the Lungs

  • Soft, spongy, cone-shaped organs located side by side in the chest cavity
  • Separated from each other by the mediastinum and its contents
  • Divided into lobes: three in the right lung, two in the left
  • Apex: upper part of the lung; lies against the top of the thoracic cavity
  • Base: lower part of the lung; lies against the diaphragm

Airways and Tissues

  • Conducting airways include:
    • Nasal passages
    • Mouth and pharynx
    • Larynx
    • Trachea
    • Bronchi
    • Bronchioles
    • Mucociliary blanket
  • Respiratory tissues (gas exchange region) and supporting structures:
    • Alveolar bundle
    • Respiratory membrane

Nasopharyngeal Airways (Structure and Function)

  • Connects the oropharynx with the trachea
  • Glottis and epiglottis located in a strategic position between the upper airways and the lungs
  • Functions:
    • Helping to produce speech
    • Vestibular folds: protecting the lungs from substances other than air
    • Reducing risk of aspiration pneumonia

Ventilation and Gas Exchange

  • Ventilation: The movement of gases into and out of the lungs
  • Inspiration: Air is drawn into the lungs as the respiratory muscles expand the chest cavity
  • Expiration: Air moves out of the lungs as the chest muscles recoil and the chest cavity becomes smaller
  • Ventilation depends on the conducting airways and their structures
    • Nasopharynx and oropharynx
    • Larynx
    • Tracheobronchial tree
  • Conducting airways function to move air out of the lungs but do not participate in gas exchange

Alveolar Structures

  • Type I Alveolar Cells: flat squamous epithelial cells through which gas exchange occurs
  • Type II Alveolar Cells: produce surfactant, a lipoprotein that decreases surface tension and facilitates lung inflation
  • Alveolar Macrophages: immune defense within alveoli

Pulmonary and Bronchial Circulation; Other Components

  • Pulmonary circulation: arises from the pulmonary artery; provides the gas exchange function of the lungs
  • Bronchial circulation: arises from the thoracic aorta; supplies lungs and other lung structures with oxygen, distributes blood to conducting airways, warms and humidifies inspired air
  • Lymphatic circulation, innervation, and pleura also contribute to lung function

Properties of Gases

  • Respiratory pressures and pressures pertaining to gases
  • Atmospheric pressure and partial pressures
  • Humidity and temperature effects on gas behavior

Question 1

  • Which of the following is directly responsible for gas exchange?
    • Trachea
    • Bronchi
    • Bronchial circulation
    • Pulmonary circulation
    • Respiratory membrane
    • Answer: Respiratory membrane

Exchange of Gases Between the Atmosphere and the Lungs

  • Respiratory Pressures (see Fig. 29.11):

    • Intrapulmonary (Alveolar) Pressure: pressure inside the airways and alveoli
    • Intrapleural Pressure: pressure in the pleural cavity
    • Intrathoracic Pressure: pressure in the thoracic cavity
    • Partial Pressure: pressure exerted by a single gas in a mixture

  • Lung Compliance:

    • Definition: C = ΔV/ΔP, the change in lung volume (ΔV) achieved for a given change in respiratory pressure (ΔP)
    • Influencing factors: elastin and collagen fibers, surface tension, and surfactant
    • Surfactant reduces surface tension and enhances lung inflation
    • Conditions related to surfactant disorders: infant respiratory distress syndrome, acute respiratory distress syndrome

  • Airway Resistance:

    • The volume of air moving into and out of the gas exchange portion of the lungs
    • Related to Poiseuille's law
    • Directly related to the pressure difference between the lungs and the atmosphere
    • Inversely related to the resistance encountered as air moves through the airways

  • Lung Volumes (typical definitions):

    • Tidal Volume (TV): amount of air moved during a normal breath
    • Inspiratory Reserve Volume (IRV): maximum amount of air that can be inspired beyond the normal tidal inspiration
    • Expiratory Reserve Volume (ERV): maximum amount of air that can be exhaled beyond the normal tidal expiration
    • Residual Volume (RV): air remaining in the lungs after forced respiration

  • Capacities and relationships:

    • Vital Capacity (VC) = IRV + TV + ERV
    • Inspiratory Capacity (IC) = TV + IRV
    • Functional Residual Capacity (FRC) = RV + ERV
    • Total Lung Capacity (TLC) = TV + IRV + ERV + RV

  • Pulmonary Function Studies (Table 29.1):

    • Maximum Voluntary Ventilation (MVV): volume of air a person can move into and out of the lungs during maximum effort lasting 12–15 seconds
    • Forced Expiratory Vital Capacity (FVC): full inspiration to total lung capacity followed by forceful maximal expiration
    • Forced Expiratory Volume (FEV): expiratory volume achieved in a given time period
    • Forced Inspiratory Vital Flow (FIF): respiratory response during rapid maximal inspiration

  • Question 2

  • Which of the following comprises the vital capacity?

    • IRV + ERV
    • TV + ERV
    • TV + IRV + ERV
    • TV + IRV – residual volume
    • Answer: IRV + ERV + TV (i.e., TV + IRV + ERV)

Exchange and Transport of Gases

  • Ventilation, Perfusion, Diffusion:

    • Ventilation: flow of gases into and out of the alveoli
    • Perfusion: flow of blood in the adjacent pulmonary capillaries
    • Diffusion: transfer of gases between the alveoli and pulmonary capillaries

  • Types of Air Movement in the Lung:

    • Bulk flow: occurs in the conducting airways; driven by pressure differences between the mouth and airways in the lung
    • Diffusion: movement of gases in the alveoli and across the alveolar-capillary membrane

  • Types of Dead Space:

    • Anatomic Dead Space: contained in the conducting airways
    • Alveolar Dead Space: contained in the respiratory portion of the lung
    • Physiologic Dead Space: sum of anatomic dead space and alveolar dead space

  • Types of Shunts:

    • Anatomic Shunt: blood moves from the venous to the arterial side without moving through the lungs
    • Physiologic Shunt: mismatch of ventilation and perfusion in the lung, leading to insufficient ventilation to oxygenate the blood in alveolar capillaries

  • Matching Ventilation and Perfusion:

    • Essential for gas exchange between alveolar air and blood in pulmonary capillaries
    • Two interfering factors: dead air space and shunt
    • Blood oxygen level reflects mixing of blood from alveolar dead space and physiologic shunting areas as it moves into the pulmonary veins

  • Factors Affecting Alveolar–Capillary Gas Exchange:

    • Surface area available for diffusion
    • Thickness of the alveolar–capillary membrane
    • Partial pressure of alveolar gases
    • Solubility and molecular weight of the gas

  • Oxygen and Carbon Dioxide Transport:

    • Arterial PO2 normally above 80 mmHg80\,\mathrm{mmHg}
    • In chemical combination with hemoglobin: about 98%–99% as oxyhemoglobin (Oxyhemoglobin)
    • Remaining O2 is dissolved in blood
    • PCO2 range: P<em>CO</em>2=35 to 45 mmHgP<em>{CO</em>2} = 35\text{ to }45\,\mathrm{mmHg}
    • Dissolved as CO2 (about 10%)
    • Bound to hemoglobin (carbaminohemoglobin, about 30%)
    • As bicarbonate (HCO3−, about 60%)
    • Acid–base balance is influenced by the amount of dissolved CO2 and the bicarbonate level in the blood

Control of Breathing

  • Respiratory center (see Fig. 29.23):

    • Pacemaker center
    • Pneumotaxic center
    • Apneustic center
    • Phrenic nerve

  • Automatic Regulation of Ventilation:

    • Controlled by input from two types of sensors/receptors:
    • Chemoreceptors: monitor blood levels of oxygen and carbon dioxide; adjust ventilation to meet metabolic needs
    • Lung receptors: monitor breathing patterns and lung function

  • Voluntary Regulation of Ventilation:

    • Integrates breathing with voluntary acts such as speaking, blowing, and singing
    • Initiated by the motor and premotor cortex; can temporarily suspend automatic breathing

  • Cough Reflex:

    • Neurally mediated reflex that protects the lungs
    • Triggered by accumulation of secretions or entry of irritants/destructive substances

  • Mechanisms Involved in Dyspnea:

    • Stimulation of lung receptors
    • Increased sensitivity to ventilation changes perceived via CNS mechanisms
    • Reduced ventilatory capacity or breathing reserve
    • Stimulation of neural receptors in muscle fibers of intercostals and diaphragm and in receptors in skeletal joints
    • Associated conditions include primary lung diseases, heart disease, and neuromuscular disorders

  • Question 3

  • Which of the following describes your breathing pattern after running to class?

    • Cheyne–Stokes
    • Normal
    • Dyspnea
    • Eupnoea
    • Hypoxemia
    • Answer: Dyspnea (shortness of breath) or rapid, increased breathing effort is expected after exertion

Summary Connections and Relevance

  • The respiratory system integrates structure and function across two main zones: conducting airways (air transport, humidification, filtration) and respiratory tissues (gas exchange surface)
  • Gas exchange relies on a thin Alveolar–Capillary membrane and adequate matching of ventilation to perfusion
  • Hemodynamics of the lungs (pulmonary vs bronchial circulation) support gas exchange, temperature/humidification, and defense via immune components (alveolar macrophages)
  • Lung mechanics (compliance, resistance) and alveolar surface tension (surfactant) govern breathing efficiency
  • Gas transport involves both chemical binding to hemoglobin and physical dissolution; CO2 and bicarbonate balance acid–base status
  • Control of breathing integrates automatic homeostatic regulation with voluntary control, enabling speech and protective reflexes like coughing