Makindo Medical Notes

Anatomy and Physiology of the Thoracic Cavity, Diaphragm, and Lungs

The thoracic cavity is a central compartment of the human body that houses vital organs essential for respiration and circulation, including the heart, lungs, and major blood vessels. Understanding the anatomy and physiology of the thoracic cavity, diaphragm, and lungs is crucial for comprehending how the respiratory system functions and for diagnosing and treating various medical conditions.

Thoracic Cage Anatomy

The thoracic cage, also known as the rib cage, provides a protective framework for the thoracic organs and plays a pivotal role in respiration by facilitating the expansion and contraction of the thoracic cavity.

Sternum (Breastbone)

• Manubrium:
  • The uppermost triangular portion of the sternum.
  • Articulates with the clavicles (collarbones) and the first two pairs of ribs.
  • Features the jugular notch at its superior border.
  • Forms the sternal angle (Angle of Louis) where it joins the body of the sternum, serving as an important anatomical landmark for rib counting and locating the second rib.
• Body (Corpus Sterni):
  • The elongated central part of the sternum.
  • Articulates with the costal cartilages of ribs 2 through 7.
  • Provides attachment points for several muscles involved in respiration and upper limb movement.
• Xiphoid Process:
  • The small, cartilaginous inferior tip of the sternum.
  • Varies in shape and ossifies (turns to bone) with age.
  • Serves as an attachment point for muscles such as the diaphragm and rectus abdominis.

Thorax Anatomy

Rib Cage

The rib cage consists of 12 pairs of ribs connected to the sternum anteriorly and the thoracic vertebrae posteriorly, providing support, flexibility, and protection for the thoracic organs.

Structure of the Rib Cage

• True Ribs (1–7):
  • Directly attach to the sternum via individual costal cartilages.
  • Provide primary structural support and rigidity necessary for protection and respiration.
• False Ribs (8–10):
  • Attach indirectly to the sternum by connecting to the cartilage of the seventh rib.
  • Offer flexibility, allowing the thoracic cage to expand during respiration.
• Floating Ribs (11–12):
  • Do not connect to the sternum and are only attached posteriorly to the vertebrae.
  • Provide protection for the kidneys and posterior abdominal organs.

Pleura and Pleural Cavities

• Visceral Pleura: Thin membrane covering the lungs' surface.
• Parietal Pleura: Lines the thoracic cavity walls, diaphragm, and mediastinum.
• Pleural Cavity: The potential space between the visceral and parietal pleurae containing a small amount of lubricating fluid.
  • Maintains negative pressure essential for lung expansion during inspiration.
  • Allows smooth gliding of lungs against the thoracic wall during respiration.
• Pleural Innervation:
  • Parietal pleura is sensitive to pain and supplied by intercostal and phrenic nerves.
  • Visceral pleura lacks pain fibers and is insensitive to pain.

Physiology of Respiration

Mechanics of Breathing

• Inspiration (Inhalation):
  • An active process involving muscle contraction.
  • Diaphragm: Contracts and moves downward, increasing the vertical dimension of the thoracic cavity.
  • External Intercostal Muscles: Contract to elevate the ribs and expand the thoracic cavity laterally and anteroposteriorly.
  • Accessory Muscles: Sternocleidomastoid, scalenes, and pectoralis minor may assist during deep or labored breathing.
• Expiration (Exhalation):
  • Usually a passive process due to the elastic recoil of the lungs and relaxation of inspiratory muscles.
  • Forced Expiration: Internal intercostal and abdominal muscles contract to expel air more rapidly.
• Pressure Changes:
  • Inspiration decreases intrathoracic pressure, allowing air to flow into the lungs (negative pressure ventilation).
  • Expiration increases intrathoracic pressure, pushing air out of the lungs.

Gas Exchange in the Alveoli

• Alveolar-Capillary Membrane: Thin barrier (~0.4 micrometers) through which gases diffuse.
  • Consists of alveolar epithelium, fused basement membranes, and capillary endothelium.
• Diffusion of Gases:
  • Oxygen: Moves from high partial pressure in the alveoli to lower partial pressure in pulmonary capillary blood.
  • Carbon Dioxide: Moves from higher partial pressure in the blood to lower partial pressure in the alveoli.

Oxygen and Carbon Dioxide Transport

• Oxygen Transport:
  • 98% bound to hemoglobin in red blood cells forming oxyhemoglobin.
  • 2% dissolved in plasma.
• Oxygen-Hemoglobin Dissociation Curve:
  • Sigmoidal curve illustrating the relationship between oxygen saturation and partial pressure.
  • Factors shifting the curve to the right (enhancing oxygen release to tissues): Increased CO₂, acidosis (low pH), increased temperature, elevated 2,3-Bisphosphoglycerate (2,3-BPG) levels.
  • Factors shifting the curve to the left (reducing oxygen release): Decreased CO₂, alkalosis (high pH), decreased temperature.
• Carbon Dioxide Transport:
  • 70% as bicarbonate ions (HCO₃^-) in plasma.
  • 20% bound to hemoglobin forming carbaminohemoglobin.
  • 10% dissolved in plasma.

Control of Respiration

• Respiratory Centers: Located in the medulla oblongata and pons.
  • Medullary rhythmicity area controls the basic rhythm of respiration.
  • Pneumotaxic and apneustic centers in the pons regulate the transition between inhalation and exhalation.
• Chemoreceptors:
  • Central Chemoreceptors: Located in the medulla; respond to changes in CO₂ and pH in cerebrospinal fluid.
  • Peripheral Chemoreceptors: Located in the carotid and aortic bodies; respond to decreases in arterial O₂, increases in CO₂, and pH changes.
• Mechanoreceptors: In the lungs and airways; respond to stretch and irritants, influencing breathing patterns.
• Voluntary Control: The cerebral cortex can modify breathing during activities like speaking, singing, or holding breath.

Pulmonary Circulation and Perfusion

• Dual Blood Supply:
  • Pulmonary Circulation: Deoxygenated blood from the right ventricle is pumped through the pulmonary arteries to the lungs for oxygenation.
  • Bronchial Circulation: Oxygenated blood from the systemic circulation (bronchial arteries) supplies the lung tissues themselves.
• Pulmonary Vessels:
  • Low-pressure, low-resistance system compared to systemic circulation.
  • Pulmonary arteries have pressures around 25/10 mmHg.
• Hypoxic Pulmonary Vasoconstriction:
  • In response to low alveolar oxygen levels, pulmonary arterioles constrict to divert blood to better-ventilated areas, optimizing gas exchange.

Ventilation-Perfusion Relationship

• Ventilation (V): The flow of air into and out of the alveoli.
• Perfusion (Q): The flow of blood through pulmonary capillaries.
• V/Q Ratio: The ratio of ventilation to perfusion; ideal average is approximately 0.8.
  • High V/Q Ratio: Adequate ventilation but poor perfusion (e.g., pulmonary embolism).
  • Low V/Q Ratio: Poor ventilation but adequate perfusion (e.g., airway obstruction, pneumonia).
• Shunting: Occurs when blood passes through the lungs without being oxygenated due to non-ventilated alveoli.
• Dead Space: Areas where ventilation exceeds perfusion; no gas exchange occurs due to lack of blood flow.
• Alveolar-Arterial Oxygen Gradient (A–a Gradient): The difference between alveolar oxygen (A) and arterial oxygen (a); an increased gradient indicates V/Q mismatch.

Respiratory Volumes and Capacities

• Tidal Volume (TV): The volume of air inhaled or exhaled during normal breathing (~500 mL in adults).
• Inspiratory Reserve Volume (IRV): The maximum volume of air that can be inhaled beyond a normal inspiration (~3100 mL).
• Expiratory Reserve Volume (ERV): The maximum volume of air that can be exhaled beyond a normal expiration (~1200 mL).
• Residual Volume (RV): The volume of air remaining in the lungs after a maximal exhalation (~1200 mL).
• Vital Capacity (VC): The total volume of air that can be exhaled after maximal inhalation (VC = TV + IRV + ERV).
• Total Lung Capacity (TLC): The total volume of the lungs at maximal inflation (TLC = VC + RV).
• Functional Residual Capacity (FRC): The volume of air remaining in the lungs after a normal expiration (FRC = ERV + RV).
• Inspiratory Capacity (IC): The maximum volume of air that can be inhaled after a normal expiration (IC = TV + IRV).

Spirometry

Spirometry is a common pulmonary function test that measures lung volumes and airflow to assess respiratory health.

• Forced Vital Capacity (FVC):
  • The total volume of air that can be forcefully exhaled after maximal inhalation.
  • Provides information about lung capacity and restrictive lung diseases.
• Forced Expiratory Volume in 1 Second (FEV₁):
  • The volume of air forcefully exhaled in the first second of the FVC maneuver.
  • Indicates the degree of airway obstruction.
• FEV₁/FVC Ratio:
  • A key indicator in diagnosing obstructive and restrictive lung diseases.
  • Obstructive Diseases (e.g., COPD, Asthma): Reduced FEV₁, normal or reduced FVC, reduced FEV₁/FVC ratio (<70%).
  • Restrictive Diseases (e.g., Pulmonary Fibrosis): Reduced FEV₁ and FVC, normal or increased FEV₁/FVC ratio.
• Peak Expiratory Flow Rate (PEFR): The maximum speed of expiration, useful in monitoring asthma.

Clinical Considerations

• Pulmonary Conditions:
  • Asthma: Characterized by airway inflammation and hyperreactivity, leading to obstruction.
  • Chronic Obstructive Pulmonary Disease (COPD): Includes emphysema and chronic bronchitis; results in airflow limitation.
  • Pneumonia: Infection causing inflammation of the alveoli, leading to consolidation and impaired gas exchange.
  • Pulmonary Embolism: Blockage of a pulmonary artery by a blood clot, reducing perfusion.
  • Pneumothorax: Air in the pleural space causing lung collapse.
• Diagnostic Procedures:
  • Chest X-Ray: Imaging to visualize lung structures and identify abnormalities.
  • Computed Tomography (CT) Scan: Provides detailed images of the lungs and thoracic structures.
  • Bronchoscopy: Endoscopic examination of the airways for diagnostic and therapeutic purposes.
• Therapeutic Interventions:
  • Oxygen Therapy: Supplemental oxygen to improve hypoxemia.
  • Mechanical Ventilation: Supports or replaces spontaneous breathing in patients with respiratory failure.
  • Medications: Bronchodilators, anti-inflammatory agents, and antibiotics as appropriate.

Conclusion

A comprehensive understanding of the anatomy and physiology of the thoracic cavity, diaphragm, and lungs is fundamental for healthcare professionals. It enables accurate assessment of respiratory function, guides clinical interventions, and facilitates effective management of pulmonary diseases. Knowledge of the structural components and their physiological roles is essential for diagnosing conditions, planning treatments, and improving patient outcomes.