Respiratory System I: Functional Anatomy (LEC#6)

• Lecture 6 covers Respiratory System I and the functional anatomy of the respiratory system.

Elements of survival

• The average person can survive:
  • 1-2 months without food
  • 3 days without water
  • 2-3 minutes without oxygen

Objectives of the lecture

• Learn the location, structure, and function of the upper respiratory system including the nose, nasal cavity, sinuses, and pharynx.
• Describe the protective mechanisms of the respiratory system.
• Learn the location, structure, and function of the lower respiratory system including the larynx, trachea, and bronchi.
• Describe the features of the bronchial tree.

Respiration

• Respiration involves four processes:
  • Pulmonary ventilation: Movement of air into and out of the lungs (breathing)
  • External respiration: Exchange of oxygen and carbon dioxide between the lungs and blood.
  • Transport: Transport of oxygen and carbon dioxide in the blood.
  • Internal respiration: Exchange of oxygen and carbon dioxide between systemic blood vessels and tissues.
• Respiratory System is comprised of the lungs, Pulmonary Circulation
• Circulatory system is comprised of Systemic Circulation

Functional Anatomy: Major Organs

• Major organs of the respiratory system can be divided into upper and lower respiratory system.
  • Upper respiratory: nose and nasal cavity, paranasal sinuses, and pharynx
  • Lower respiratory: larynx, trachea, bronchi and branches, lungs and alveoli

Lower Respiratory System

• Consists of the larynx, trachea, bronchi with microscopic structures, and lungs.
• Broken into two zones:
  • Conducting zone: Conduits that transport gas to and from gas exchange sites, cleanses, warms, and humidifies air.
  • Respiratory zone: Site of gas exchange.
    • It has all other respiratory structures
    • Has microscopic structures such as respiratory bronchioles, alveolar ducts, and alveoli

Lower Respiratory - Larynx

• The larynx (voice box) extends from the 3rd to 6th cervical vertebra and attaches to the hyoid bone.
• It opens into the laryngopharynx and is continuous with the trachea.
• There are three functions of the larynx:
  • Provides a patent airway.
  • Routes air and food into proper channels.
  • Responsible for voice production
• Includes the vocal folds

Larynx - Epiglottis and Vallecula

• The epiglottis consists of elastic cartilage.
  • Covers the laryngeal inlet during swallowing
  • Covered in taste bud-containing mucosa
• Vallecula: Space between the epiglottis and tongue
  • Important landmark during endotracheal intubation

The Trachea

• The trachea (windpipe) extends from the larynx into the mediastinum.
• It divides into two main bronchi.
• Approximately 4 inches long and ¾ inch in diameter.
• The tracheal wall has three layers:
  • Mucosa: Ciliated pseudostratified epithelium containing goblet cells.
  • Submucosa: Connective tissue with seromucous glands.
  • Adventitia: Outer layer made of connective tissue, containing C-shaped cartilage rings for support.

The Trachea - Trachealis

• Trachealis consists of smooth muscle fibers that connect posterior parts of cartilage rings.

The Trachea - Carina

• The carina is the last tracheal cartilage.
• It expands at the point where the trachea branches into left and right bronchi.
• The right bronchus deviates 20 to 30 degrees from the midline.
  • Compared to the left, it's smaller in diameter and twice as long.
• The left bronchus deviates 45 to 55 degrees from midline.
• Isothermic Saturation Boundary (ISB) is where air is 100% saturated at a temperature of 37°C.
• The mucosa of the carina is highly sensitive.

Bronchi and Subdivisions - Conducting Zone

• Each lobar bronchus branches into segmental (tertiary) bronchi, which divide repeatedly and become smaller.
• Bronchioles are less than 1 mm in diameter.
• Terminal bronchioles are the smallest of all branches and less than 0.5 mm in diameter.
• In the conducting zone, from bronchi to bronchioles, the structures change:
• Support structures change: cartilage rings become irregular plates, and elastic fibers replace cartilage altogether in bronchioles.
• Epithelium type changes: Pseudostratified columnar gives way to cuboidal, and cilia and goblet cells become more sparse.
• The amount of smooth muscle increases, which allows bronchioles to provide substantial resistance to air passage.

Bronchi and Subdivisions - Respiratory Zone

• The respiratory zone begins where terminal bronchioles feed into respiratory bronchioles.
• Respiratory bronchioles lead into alveolar ducts which then lead into alveolar sacs (saccules).
• Alveolar sacs have clusters of alveoli.
  • Most lungs have 300 million alveoli that are the sites of gas exchange.

Bronchi and Subdivisions - Respiratory Membrane

• Respiratory membrane: Blood-air barrier consisting of alveolar and capillary walls along with their fused basement membranes.
  • Very thin (~0.5 μm) allows gas exchange across the membrane by simple diffusion.
• Alveolar walls consist of:
  • Single layer of squamous epithelium (type I alveolar cells).
  • Scattered cuboidal type II alveolar cells secrete surfactant (and antimicrobial proteins).

Respiratory Membrane - Alveolar Wall

• Major cell types found in alveoli consist of:
  • Single layer of squamous epithelium (type I alveolar cells).
  • Scattered cuboidal type II alveolar cells secrete surfactant and antimicrobial proteins.
  • Alveolar macrophages keep alveolar surfaces sterile, with 2 million dead macrophages/hour carried by cilia to the throat and swallowed.
• Other features of alveoli:
  • Surrounded by fine elastic fibers and pulmonary capillaries.
  • Alveolar pores connect adjacent alveoli, equalize air pressure throughout the lung, and provide alternate routes in case of blockages.

Gross Anatomy of the Lungs - Blood and Nerve Supply

• Lungs are perfused by two circulations:
  • Pulmonary circulation: Pulmonary arteries deliver systemic venous blood from the heart to lungs for oxygenation.
    • Branches feed into pulmonary capillary networks where gas exchange occurs.
    • Pulmonary veins carry oxygenated blood from respiratory zones back to the heart.
      • Low-pressure, high-volume system.
    • Lung capillary endothelium contains many enzymes that act on different substances in blood, like angiotensin-converting enzyme (blood pressure).
  • Bronchial circulation: Bronchial arteries provide oxygenated blood to lung tissue
    • These arise from the aorta and enter lungs at hilum.
    • It supplies all lung tissue except alveoli.
    • Bronchial veins anastomose with pulmonary veins, which carry most venous blood back to the heart.

Respiratory Physiology

• Pulmonary ventilation consists of two phases:
  • Inspiration: Gases flow into lungs.
  • Expiration: Gases exit lungs.

Pressure Relationships in the Thoracic Cavity

• Atmospheric Pressure (Patm): the pressure exerted by the air surrounding the body.
  • At sea level 760 mm Hg equals one atmosphere.
• Respiratory pressures are described relative to atmospheric pressure.
  • A respiratory pressure of -5 mm Hg means the pressure is lower than Patm by 5 mm Hg.

Pressure Relationships in Thoracic Cavity: Intrapulmonary Pressure

• Intrapulmonary Pressure (Ppul): Pressure in the alveoli also called intra-alveolar pressure.
  • Fluctuates with breathing, but always eventually equalizes with Patm

Pressure Relationships in Thoracic Cavity: Intrapleural Pressure

• Intrapleural Pressure (Pip): Pressure in the pleural cavity
  • Fluctuates with breathing and is always a negative pressure (less than Patm and Ppul).
  • Usually 4 mm Hg less than Ppul
• A minimum amount of fluid level must be maintained, with excess fluid pumped out by the lymphatic system.
• If fluid accumulates, positive Pip pressure develops, leading to lung collapse.

Pressure Relationships in Thoracic Cavity: Intrapleural Pressure

• Two inward forces promote lung collapse:
  1. Lungs' natural tendency to recoil (because of elasticity, lungs always try to assume the smallest size).
  2. Surface tension of the alveolar fluid: Surface tension pulls on alveoli to try to reduce alveolar size.
• One outward force promotes lung expansion: Elasticity of the chest wall pulls the thorax outward or like a compressed spring.
• One outward force tends to enlarge lungs:
  • Elasticity of chest wall pulls thorax outward 'like a compressed spring'
• Negative Pip is affected by opposing forces, maintained by strong adhesive force between parietal and visceral pleurae
  • Chest wall pulls outward into the intrapleural space.
  • It causes forces created by recoil and surface tension.
  • Lungs pull inward that tends to separate the visceral and parietal pleurae.

Pressure Relationships in Thoracic Cavity - Transpulmonary Pressure

• Transpulmonary pressure is the difference between intrapulmonary and intrapleural pressures (Ppul - Pip) and keeps air spaces in the lungs open.
• If this is compromised (pneumothorax), transpulmonary pressure will fail.
• In order for lungs to not fail and remain inflated the negative Pip must be maintained.

Changes in Pip and Ppul During Inspiration and Expiration

• During Inspiration
  • Pressure inside lung decreases as lung volume increases
  • Pleural cavity pressure becomes more negative as chest wall expands
• During Expiration
  • Pressure inside lung increases
  • Pleural cavity pressure returns to initial value