Module 6: The Respiratory System Study Notes
Module 6: The Respiratory System
Course Information
Course Code: BIOL 2458
Course Title: Anatomy & Physiology II
Learning Objectives
By the end of this lesson, students will be able to:
Identify the structures of the upper respiratory tract and describe their functions, including the nose, nasal cavity, paranasal sinuses, and pharynx.
Identify the structures of the lower respiratory tract and describe their functions, including the larynx, trachea, bronchi, bronchioles, and alveoli.
Describe the histology of the respiratory tract and how it changes from the nasal cavities through the alveoli, noting the transition from ciliated pseudostratified columnar epithelium to simple squamous epithelium.
Define pulmonary ventilation and describe the muscles involved in inhalation vs. exhalation, differentiating between quiet and forced breathing.
Define the gas pressures involved in pulmonary ventilation, describe how Boyle’s law relates to these pressures during inhalation vs. exhalation, and the difference between an open and closed system, specifically mentioning atmospheric, intrapulmonary, and intrapleural pressures.
List the cells found in alveoli, describe their functions, and contribute to the respiratory membrane, including Type I alveolar cells, Type II alveolar cells, and alveolar macrophages.
Explain the importance of surfactant in preventing alveolar collapse by reducing surface tension within the alveoli.
Define the volumes (TV, IRV, ERV, RV) and capacities (IC, FRC, VC, TLC) of the lungs and identify them on a spirogram, including typical values for each.
Differentiate between the processes of pulmonary and systemic gas exchange, highlighting the partial pressure gradients and environmental differences (lungs vs. tissues).
Describe the major ways that carbon dioxide is transported in the body, including dissolved in plasma, bound to hemoglobin, and as bicarbonate ions.
Identify the directions of CO2 and O2 movement in tissues and in the alveoli, based on their respective partial pressure gradients.
List the respiratory control centers of the body (medulla oblongata, pons) and identify the key functional differences between each structure, detailing the roles of the VRG, DRG, pneumotaxic, and apneustic centers.
Identify the major factors influencing the processes of ventilation and perfusion and explain why these two processes are coupled for efficient gas exchange, including partial pressure of oxygen and carbon dioxide.
Predict how high or low levels of CO2 would influence breathing rate and depth, explaining the role of central and peripheral chemoreceptors in regulating pH and PCO2.
Describe COPD (emphysema, chronic bronchitis) and the effects of smoking on the trachea and lungs, including the impacts on the tissues in both locations, such as destruction of alveolar walls and excessive mucus production.
Anatomy of the Respiratory System
Upper Respiratory Tract
Structures and Components
Cartilaginous Framework: These cartilages provide structural support and flexibility to the external nose.
Lateral nasal cartilages
Septal nasal cartilage
Alar cartilage (major and minor)
The philtrum is an indentation above the upper lip, embryologically marking the fusion of nasal processes.
Nose and Nasal Cavity Functions: The nasal cavity is lined with a rich vascular supply to regulate temperature and humidity.
Warm and moisten incoming air (following nasal conchae), facilitated by highly vascularized mucous membranes.
Trap and filter foreign particles (ciliated pseudostratified epithelium and nasal hair/vibrissae in the vestibule).
Olfaction (olfactory epithelium located superiorly, containing bipolar sensory neurons).
Voice resonation, acting as a chamber to modify vocal sounds.
Bony Framework: These bones form the rigid structure of the nasal cavity and paranasal sinuses.
Frontal bone
Nasal bones
Maxilla (housing the maxillary sinuses)
Ethmoid bone (contributing the perpendicular plate to the nasal septum and superior/middle conchae)
Additional Anatomy:
Dense fibrous connective and adipose tissue forming the external nose's soft parts.
Posterior nasal aperture (choanae) leading into the nasopharynx.
Frontal and sphenoidal sinuses, which are air-filled cavities that lighten the skull and are lined with respiratory epithelium.
Histology of the Nasal Cavity
Epithelial Layer: Ciliated pseudostratified columnar epithelium with goblet cells (respiratory epithelium) for airway conditioning and filtering. The cilia move mucus laden with trapped particles towards the pharynx to be swallowed.
Components: Nasal conchae (superior, middle, inferior turbinates) and meatuses that help in increasing turbulence in airflow, allowing more time for warming, moistening, and filtering. The olfactory epithelium is specialized for smell, distinct from the respiratory epithelium.
Pharynx
Sections of the Pharynx: A muscular tube extending from the posterior nasal aperture to the larynx/esophagus.
Nasopharynx: The superior part, posterior to the nasal cavity. It transports only air and is lined with ciliated pseudostratified columnar epithelium. The uvula reflexively blocks the nasopharynx during swallowing to prevent food from entering.
Oropharynx: The middle part, posterior to the oral cavity, extending from the soft palate to the epiglottis. It transports air and food, and is lined with non-keratinized stratified squamous epithelium to resist abrasion. Contains palatine tonsils (lateral walls) and lingual tonsils (base of tongue) as part of the immune system.
Laryngopharynx: The inferior part, extending from the epiglottis to the esophagus (posteriorly) and larynx (anteriorly). It splits into the larynx (for air) and esophagus (for food); lined with non-keratinized stratified squamous epithelium. The epiglottis is a cartilaginous flap that covers the glottis (larynx opening) during swallowing to prevent aspiration.
Larynx
Functionality: Also known as the voice box, it connects the laryngopharynx to the trachea and functions in voice production and preventing food/liquid from entering the airway.
Composed of 9 cartilages (3 paired, 3 unpaired), major ones include:
Thyroid cartilage: Largest, shield-shaped, forming the Adam's apple (laryngeal prominence), more prominent in males.
Cricoid cartilage: Ring-shaped, inferior to the thyroid cartilage, providing a complete ring of cartilage around the airway, and serving as an attachment site for vocal cord muscles.
Epiglottis: Leaf-shaped elastic cartilage, covers the glottis during swallowing.
Arytenoid cartilages (paired): Key for vocal cord movement.
Corniculate cartilages (paired): On top of arytenoids.
Cuneiform cartilages (paired): Embedded in aryepiglottic folds.
Sound Production: Air vibrating through the vocal cords (vocal folds), housed within the larynx. Pitch is determined by the length and tension of the cords (controlled by intrinsic laryngeal muscles); tauter and longer cords produce higher pitch. Loudness depends on the force of air passing over the cords.
Trachea
Transport Function: A rigid yet flexible tube (windpipe) about 10-12 cm long, conducting air from the larynx into the chest, branching into primary bronchi at the carina, typically at the level of T4/T5. The carina is a highly sensitive area, triggering a strong cough reflex if irritated.
Tissue Layers:
Mucosa: Innermost layer, composed of ciliated pseudostratified columnar epithelium with numerous goblet cells, and a lamina propria rich in elastic fibers. This mucociliary escalator traps particles and moves them towards the pharynx.
Submucosa: Dense connective tissue layer, containing seromucous glands that produce mucus and water. It also contains hyaline cartilage c-rings (16-20 incomplete rings open posteriorly), which prevent tracheal collapse, smooth muscle (trachealis muscle), blood vessels, and nerves.
Adventitia: Outermost layer of loose connective tissue, blending with surrounding tissues (e.g., esophagus) and containing nerve endings, blood, and lymph vessels.
Trachealis Muscle: A band of smooth muscle connecting the posterior open ends of the C-shaped cartilage rings. Contraction of this muscle narrows the trachea, aiding in the forceful expulsion of air during coughing (increasing air velocity).
Bronchi & Bronchioles
Primary Bronchi (Main Bronchi): Two branches (left & right) entering the lungs, formed by the bifurcation of the trachea. The left primary bronchus is longer, narrower, and more horizontal due to the presence of the heart, while the right is shorter, wider, and more vertical, making it a more common site for inhaled foreign objects.
Secondary Bronchi (Lobar Bronchi): Branch off the primary bronchi, entering each lobe of the lungs. There are 3 in the right lung (superior, middle, inferior) and 2 in the left lung (superior, inferior) because the left lung only has two lobes.
Tertiary Bronchi (Segmental Bronchi): Branch off the secondary bronchi, supplying bronchopulmonary segments (functionally isolated units of the lung).
Transition to Bronchioles: Characterized by the progressive loss of cartilage (disappearing entirely in bronchioles less than 1 mm in diameter) and a relative increase in smooth muscle. This smooth muscle allows for significant changes in airway diameter (bronchoconstriction/bronchodilation), regulating airflow.
Terminal and Respiratory Bronchioles:
Terminal bronchioles mark the end of the conducting zone (pulmonary ventilation). They are the smallest bronchioles, primarily for air transport.
Respiratory bronchioles are the first part of the respiratory zone, characterized by the presence of sparse alveoli budding from their walls, marking the beginning of alveolar ventilation (gas exchange).
Alveoli
Definition: Tiny, thin-walled epithelial sacs (approximately 300−500300−500 million per lung) that are the primary sites for gas exchange between air and blood, providing an enormous surface area (estimated 1400 sq ft1400sqft or 50−100 m250−100m2). They are surrounded by a dense capillary network.
Types of Cells Found in Alveoli: These cells form the delicate respiratory membrane (or alveolar-capillary membrane).
Type I Alveolar Cells (Pneumocytes I): Simple squamous epithelium that forms the vast majority (90−95%90−95%) of the alveolar wall. Their thinness (0.5 um0.5um) facilitates rapid gas exchange by diffusion.
Type II Alveolar Cells (Pneumocytes II): Scattered cuboidal epithelial cells ( 5%5%) that secrete surfactant, a lipoprotein substance. Surfactant reduces the surface tension of the alveolar fluid, preventing the collapse of alveoli, especially during exhalation.
Alveolar Macrophages (Dust Cells): Phagocytic cells that wander the alveolar surface, engulfing dust particles, pathogens, and cellular debris, protecting the respiratory zone from infection and inflammation.
Pulmonary Ventilation
Definition
Pulmonary Ventilation: The overall process of moving air into and out of the lungs (bulk flow), also known as breathing. It involves rhythmic cycles of inspiration and expiration.
Inspiration (Inhalation): The active process of bringing air into the lungs. It primarily involves the contraction of the diaphragm and external intercostal muscles.
Expiration (Exhalation): The process of pushing air out of the lungs. During quiet breathing, it is a largely passive process due to the elastic recoil of the lungs and relaxation of inspiratory muscles. Forced expiration is an active process.
Air Pressure Dynamics
Air movement from high pressure to low pressure, following pressure gradients. The pleural membranes (visceral and parietal pleura) and pleural fluid are crucial for transmitting thoracic cavity movements to the lungs.
During Inspiration: The thoracic cavity enlarges, increasing lung volume. According to Boyle's law, this decreases intrapulmonary pressure below atmospheric pressure, causing air to rush in.
During Expiration: The thoracic cavity contracts (or inspiratory muscles relax), decreasing lung volume. This increases intrapulmonary pressure above atmospheric pressure, forcing air out.
Pressures of Ventilation
Atmospheric Pressure (PatmPatm): Pressure exerted by the air surrounding the body. It changes with altitude and weather but is considered 760 mmHg760mmHg at sea level for physiological calculations.
Intrapulmonary Pressure (PpulPpul or Alveolar Pressure): Pressure within the alveoli of the lungs. It fluctuates with breathing but always eventually equalizes with atmospheric pressure.
Intrapleural Pressure (PipPip): Pressure within the pleural cavity (the space between the visceral and parietal pleura). It is always negative relative to intrapulmonary pressure (typically −4 mmHg−4mmHg at rest) due to opposing forces of lung recoil and chest wall expansion, which helps keep the lungs inflated.
Boyle's Law
Statement: In a closed system and at a constant temperature, the pressure of a gas is inversely proportional to its volume. This relationship is fundamental to pulmonary ventilation.
As volume increases, pressure decreases, and vice versa. Mathematically expressed as: P1V1=P2V2P1V1=P2V2 where PP is pressure and VV is volume at two different states.
Muscles of Respiration
Inspiration Muscles:
Quiet Inspiration:
Diaphragm: The primary muscle, contracts and flattens, increasing the vertical dimension of the thoracic cavity (responsible for about 75%75% of air movement).
External intercostals: Contract and pull the ribs up and out, increasing the anteroposterior and lateral dimensions of the thoracic cavity.
Forced/Deep Inspiration (Accessory Muscles): Activated during strenuous activity or respiratory distress to further increase thoracic volume.
Sternocleidomastoid (elevates sternum)
Scalenes (elevate first two ribs)
Pectoralis minor (elevates ribs 3-5)
Serratus anterior
Expiration Muscles:
Quiet Expiration: Primarily a passive process.
Diaphragm relaxes (moves upward).
External intercostals relax (ribs move down and in).
Elastic recoil of lung tissue and thoracic cage helps expel air.
Active/Forced Expiration: Engaged during exercise, coughing, or when airway resistance is high.
Internal intercostals: Contract and pull the ribs down and inward, further decreasing thoracic volume.
Abdominals (rectus abdominis, external and internal obliques, transversus abdominis): Contract forcefully, pushing abdominal organs against the diaphragm, driving it higher and further decreasing thoracic volume.
Quadratus lumborum
Lung Volumes and Capacities
Definitions
Tidal Volume (TV): The volume of air inhaled or exhaled during a single normal, quiet breath at rest (approx. 500 mL500mL).
Inspiratory Reserve Volume (IRV): The maximum volume of air that can be forcibly inhaled after a normal tidal inspiration (approx. 2100−3200 mL2100−3200mL).
Expiratory Reserve Volume (ERV): The maximum volume of air that can be forcibly exhaled after a normal tidal expiration (approx. 1000−1200 mL1000−1200mL).
Residual Volume (RV): The volume of air remaining in the lungs after a maximal forceful exhalation. This air cannot be voluntarily exhaled and helps keep alveoli open and prevent lung collapse (approx. 1200 mL1200mL).
Inspiratory Capacity (IC): The total amount of air that can be inhaled after a normal tidal expiration (IC = TV + IRV; approx. 2600−3700 mL2600−3700mL).
Functional Residual Capacity (FRC): The volume of air remaining in the lungs after a normal tidal expiration (FRC = ERV + RV; approx. 2200−2400 mL2200−2400mL).
Vital Capacity (VC): The maximum amount of air that can be exhaled after a maximal forceful inhalation (VC = TV + IRV + ERV; approx. 3700−4800 mL3700−4800mL).
Total Lung Capacity (TLC): The total volume of air in the lungs after a maximal forceful inhalation (TLC = VC + RV or TLC = TV + IRV + ERV + RV; approx. 4900−6000 mL4900−6000mL).
Reading a Spirometer
A spirogram is a graph illustrating changes in lung volumes and capacities over time. Measurements are taken while breathing normally, deep breathing, and forceful exhalation, providing diagnostic information about respiratory health and disease.
Gas Exchange Mechanisms
External Respiration
Involves gas exchange between the blood (pulmonary capillaries) and the external environment (alveoli). It is driven by partial pressure gradients of O2 and CO2.
Pulmonary Ventilation: The bulk flow of air into and out of the lungs (conducting zone).
Alveolar Ventilation: The diffusion of gases (O2 into blood, CO2 out of blood) across the thin alveolar-capillary membranes in the respiratory zone. High Po2 (alveoli) > Low Po2 (blood); High Pco2 (blood) > Low Pco2 (alveoli).
Internal Respiration
Exchange of gases between the systemic capillary blood and body tissues at the cellular level. This process is focused on the systemic circulation where oxygen is delivered to cells and carbon dioxide is picked up.
Driven by partial pressure gradients: High Po2 (blood) > Low Po2 (tissues); High Pco2 (tissues) > Low Pco2 (blood).
Factors Influencing Gas Exchange
Pressure Gradients: The difference in partial pressures of gases (e.g., Po2, Pco2) between the alveoli and blood, or blood and tissues, is the primary driving force for gas movement. Gases diffuse from an area of higher partial pressure to an area of lower partial pressure.
Solubility: The ease with which gases dissolve in plasma and alveolar fluid. CO2 is about 20 times more soluble in plasma than O2, meaning a smaller partial pressure gradient is required for CO2 exchange compared to O2.
Surface Area: The large total surface area of the respiratory membrane (e.g., 50−100 m250−100m2) maximizes the efficiency of gas exchange. Diseases like emphysema reduce this area, impairing exchange.
Membrane Thickness/Distance: The respiratory membrane is extremely thin (0.5 um0.5um), minimizing the diffusion distance. Thickening due to edema or fibrosis significantly impedes gas exchange.
Perfusion (Blood Flow): Refers to the blood supply to the alveoli. For efficient gas exchange, ventilation (airflow) must be matched with perfusion (blood flow) – ventilation-perfusion (V/Q) coupling. Areas with good ventilation need good perfusion, and vice-versa. Local regulatory mechanisms adjust bronchiole or arteriole diameter to optimize the V/Q ratio.
Hemoglobin Affinity: Factors that alter hemoglobin's affinity for oxygen (e.g., pH, temperature, Pco2, 2,3-BPG levels) influence O2 loading and unloading (e.g., Bohr effect, Haldane effect).
Disorders Impacting Gas Exchange
COPD (Chronic Obstructive Pulmonary Disease): A group of progressive lung diseases (including emphysema and chronic bronchitis) that cause airflow obstruction and breathing-related problems. Characterized by irreversible airflow limitation and chronic inflammation.
Emphysema: Involves the irreversible destruction of alveolar walls, leading to enlargement of air spaces, loss of elastic recoil, and reduced surface area for gas exchange.
Chronic Bronchitis: Defined by a chronic productive cough for at least 3 months in 2 consecutive years, resulting from hypertrophy of mucus glands, excessive mucus production, and chronic inflammation of the bronchi.
Asthma: A chronic inflammatory disease of the airways characterized by recurrent episodes of wheezing, breathlessness, chest tightness, and coughing. These symptoms are often triggered by allergens or irritants and are associated with widespread, but variable, airflow obstruction that is often reversible, often alleviated with bronchodilators.
Gas Transport Mechanisms
Oxygen Transport
Oxygen is transported in two main ways:
1.5%1.5% dissolved directly in plasma.
98.5%98.5% primarily transported by red blood cells (erythrocytes) via hemoglobin (Hb). Each hemoglobin molecule (composed of four heme groups, each with an iron atom) can reversibly bind up to four molecules of O2. A single red blood cell can contain about 250250 million hemoglobin molecules, capable of transporting approximately one billion molecules of O2.
Factors Affecting Hemoglobin's O2 Affinity:
Po2: Higher Po2 (e.g., in lungs) leads to greater O2 binding.
Temperature: Increased temperature (e.g., during exercise) decreases Hb's affinity for O2, promoting unloading to tissues.
pH (Bohr Effect): Decreased pH (more acidic, e.g., due to increased CO2 or lactic acid) decreases Hb's affinity for O2, enhancing O2 release where it's needed most.
Pco2: Increased Pco2 directly decreases Hb's affinity for O2.
2,3-Bisphosphoglycerate (2,3-BPG): A by-product of glycolysis in RBCs. Higher levels decrease O2 affinity, helping unload O2 at tissues.
CO2 Transport
CO2 is transported in the blood in three major forms:
7% dissolved in plasma: CO2 is more soluble than O2, so a small amount directly dissolves in plasma and is transported to the lungs.
23% attached to hemoglobin (or other plasma proteins): CO2 binds to the amino groups of hemoglobin (forming carbaminohemoglobin) at sites different from O2 binding. This binding is favored when O2 levels are low (Haldane effect).
70% converted into bicarbonate (HCO3-) and hydrogen ions (H+): This is the most significant mechanism. In red blood cells, the enzyme carbonic anhydrase rapidly catalyzes the reaction of CO2 with water to form carbonic acid (H2CO3H2CO3), which then dissociates into bicarbonate ions (HCO3−HCO3−) and hydrogen ions (H+H+). CO2+H2O⇌H2CO3⇌H++HCO3−CO2+H2O⇌H2CO3⇌H++HCO3−
The HCO3−HCO3− then moves out into the plasma in exchange for chloride ions (the chloride shift), acting as an important blood buffer.
The H+H+ ions are buffered by binding to hemoglobin, preventing significant changes in blood pH.
Respiratory Control Centers
Regulation of Respiration
Breathing is an involuntary rhythmic process, primarily controlled by neural centers located in the medulla oblongata and pons of the brainstem, which receive input from chemoreceptors, mechanoreceptors, and other brain areas.
Medullary Respiratory Center: Sets the basic respiratory rhythm.
Ventral Respiratory Group (VRG): Contains inspiratory and expiratory neurons. When active, its inspiratory neurons send signals to the diaphragm and external intercostals for forced breathing (both inspiration and expiration). When inactive, forced expiration muscles relax during quiet breathing.
Dorsal Respiratory Group (DRG): Primarily inspiratory. It receives input from peripheral chemoreceptors and stretch receptors and sends signals to stimulate the diaphragm and external intercostals, setting the basic rhythm for quiet inspiration.
Pontine Respiratory Center (Pons): (formerly pneumotaxic and apneustic centers) Coordinates the transition between inspiration and expiration, smoothing the respiratory pattern.
Pneumotaxic center: Inhibits inspiration, leading to shorter, shallower breaths.
Apneustic center: Promotes inspiration, leading to prolonged, deep breaths (can be overridden by pneumotaxic center).
Influence of Breathing Rate and Depth
The rate and depth of breathing are continuously adjusted to maintain homeostatic levels of O2, CO2, and pH levels in the blood.
Increased signals from chemoreceptors (e.g., due to high CO2 or low pH) lead to an increased breathing rate (tachypnea).
Increased signal strength (e.g., stronger stimulation of inspiratory muscles) leads to deeper breaths (hyperpnea).
Voluntary control from the cerebral cortex can temporarily override these centers (e.g., holding breath).
pH and CO2 Monitoring
Chemoreceptors play a crucial role in monitoring blood gas levels and providing feedback for respiratory regulation:
Central Chemoreceptors: Located in the medulla oblongata, these are highly sensitive to changes in the pH of the cerebrospinal fluid (CSF), which is primarily influenced by blood Pco2. An increase in Pco2 leads to increased H+ in CSF, stimulating these receptors to increase breathing rate and depth to expel more CO2.
Peripheral Chemoreceptors: Located in the carotid bodies (at the bifurcation of the common carotid arteries) and aortic bodies (in the aortic arch). These are mainly sensitive to severe drops in blood Po2 (below 60 mmHg60mmHg), but also respond to increases in Pco2 and decreases in blood pH (H+H+ ions). They send signals to the DRG, influencing respiratory rate and depth.
High levels of CO2 (hypercapnia) or low pH (acidosis) are potent stimuli for increasing breathing rate and depth, while low O2 (hypoxemia) also stimulates breathing, particularly below a certain threshold.
Conclusion
The respiratory system's complex architecture supports its critical functions of gas exchange, sound production, regulation of blood pH, and defense against pathogens. Understanding the intricate anatomy, physiology, and neural regulation of the respiratory system is essential for recognizing various respiratory disorders, their impacts on overall health, and the mechanisms by which the body maintains vital gas exchange.