The Pharynx

Overview of the Pharynx

The pharynx is a muscular tube that plays a critical role in both the respiratory and digestive systems. It is divided into three main regions:

1. Nasopharynx: This uppermost part of the pharynx is connected to the nasal passages. It serves as a passage for air and is lined with ciliated mucosa that helps trap and expel pathogens.

2. Oropharynx: Located just below the nasopharynx, the oropharynx connects to the mouth. It serves as a pathway for both food and air and contains the tonsils, which are important for immune defense.

3. Laryngopharynx: The lowest section of the pharynx connects the nasopharynx and oropharynx to the larynx and esophagus. It is crucial for directing food to the esophagus and air to the larynx.

Function of the Pharynx

• Warms and Humidifies Air: As air passes through the pharynx, it is warmed and humidified before reaching the lungs, which helps protect delicate respiratory tissues.

• Filters Air: The nasopharynx aids in filtering air through nasal hairs and mucus, capturing dust, allergens, and pathogens, thus safeguarding the respiratory cells.

The Bronchial Tree

Structure of Bronchi

The bronchial tree resembles an inverted tree, branching out from the trachea into the lungs:

• Primary Bronchi: The trachea divides into two primary bronchi (left and right), entering the lungs.

• Secondary Bronchi: Each primary bronchus branches off into secondary bronchi corresponding to each lung lobe, further dividing into tertiary bronchi.

• Smaller Bronchi: As these bronchi continue to branch, they transform into smaller bronchi and bronchioles, showing variations in cartilage support and contributing to the lung's structure.

Branching Pattern

The bronchial tree's branching pattern can be compared to a tree's branches, where the trunk (trachea) splits into larger limbs (primary bronchi) and further divides into numerous small branches (bronchioles). This structure maximizes surface area for gas exchange.

Function of Bronchi

• Trapping Debris: The bronchi has a lining that produces mucus, which traps debris and pathogens.

• Mucus Movement: Cilia, small hair-like structures lining the bronchi, move the mucus upwards toward the mouth and nose, facilitating its expulsion through coughing or swallowing.

The Respiratory Zone

Components

The respiratory zone includes:

• Respiratory Bronchioles: These are the smallest branches of the bronchi that mark the beginning of the respiratory zone.

• Alveoli: Tiny air sacs where the majority of gas exchange occurs. They are surrounded by a network of capillaries to facilitate the exchange of oxygen and carbon dioxide.

  • Significance of Alveoli: 

    • Focus on understanding alveoli is vital since they are the primary site for gas exchange and crucial for respiratory health. They increase surface area, thereby enhancing the efficiency of oxygen uptake and carbon dioxide removal.

Gas Exchange

• Efficiency of Gas Exchange: Less than 5% of gas exchange happens in respiratory bronchioles, but significant exchange takes place in the alveoli, where oxygen enters the blood, and carbon dioxide is expelled.

Bronchial System and Alveoli

The bronchial system can be visualized as a tree, with the alveoli acting as the leaves. This intricate structure is essential for efficient gas exchange in the human body.

Structure and Function of Alveoli

• Alveoli Shape: Alveoli have a bulbous or spherical shape that maximizes surface area for gas exchange, allowing for an increased capacity to absorb oxygen and expel carbon dioxide.

• Cell Composition: Alveoli are composed of squamous epithelial cells which provide a thin membrane necessary for rapid diffusion of gases.

  • Type 1 Alveolar Cells: These are thin, flat cells that constitute the majority of the alveolar surface area and are primarily responsible for gas exchange between the air in the alveoli and the blood in the capillaries.

  • Type 2 Alveolar Cells: These cells produce surfactant, a substance essential for reducing surface tension in the alveoli and aiding lung function by promoting stability during breathing.

  • Type 3 Alveolar cells (Alveolar Macrophages): These immune cells are crucial for maintaining the health of the alveoli by engulfing and digesting foreign particles, including dust and pathogens.

• Alveoli have a vital role in oxygenation of blood as approximately 300 million alveoli are present in human lungs, providing an extensive surface area of around 70 square meters for gas exchange. This large surface area is crucial for meeting the body’s oxygen demands during physical activity and at rest.

• The efficiency of gas exchange is also affected by factors such as age, lung diseases (e.g., emphysema), and environmental conditions (e.g., pollution). Understanding these influences can help in the prevention and management of respiratory disorders.

Specialization for Gas Exchange

• The human lungs have evolved over thousands of years with various adaptations for optimal gas exchange. One key adaptation includes the formation of a thin respiratory membrane that facilitates efficient oxygen and carbon dioxide exchange.

• Impact of Disease: Conditions such as cystic fibrosis can lead to thickening of this membrane, resulting in significant respiratory distress and decreased oxygen availability to the body.

Importance of Surface Area and Surface Tension

Efficient gas exchange depends on several critical factors:

• A thin respiratory membrane: shortens the distance gases must diffuse.

• Maximized surface area of the alveoli: increases the amount of gas that can be exchanged at any given moment.

• Surface Tension Considerations: The surface area is critically influenced by surface tension at the air-water interface in the alveoli, which can hinder gas exchange significantly. An increase in surface tension can decrease the effectiveness of breathing, leading to less efficient oxygen uptake.

Role of Surfactant

• Surfactant, 

  • produced by type ii alveolar cells

  • role in regulating surface tension within the alveoli. 

    • this regulation prevents alveolar collapse, especially during gas exchange when the lung volume changes during inhalation and exhalation.

Clinical Significance: 

A lack of surfactant can lead to conditions such as neonatal respiratory distress syndrome (NRDS) in infants, which can be life-threatening without timely medical intervention. The administration of artificial surfactant can significantly improve outcomes in affected infants, emphasizing the critical nature of this biomolecule in respiratory health.

Components of the Respiratory System

1. Ventilation

• Ventilation (External Respiration): The process of bringing air into and out of the lungs to facilitate gas exchange (oxygen intake and carbon dioxide removal).

• Distinguishes from cellular respiration, which occurs within the mitochondria of cells, converting glucose and oxygen into energy.

2. Pressure Gradients and Airflow

• Air moves from areas of higher pressure to lower pressure. This fundamental principle drives effective ventilation.

• Boyle's Law: Demonstrates the inverse relationship between pressure and volume, implying that volume changes in the thoracic cavity lead to pressure changes that facilitate air movement into or out of the lungs.

  • Inhalation occurs when intrapulmonary (inside) pressure is lowered below atmospheric pressure.

3. Mechanics of Breathing

• Inspiration (Inhalation): The process of drawing air into the lungs by increasing thoracic cavity volume. This is primarily accomplished by:

  • Diaphragm Contraction: The diaphragm contracts and moves downward, expanding the thoracic cavity.

• Tidal Breathing: Normal, shallow breathing pattern where approximately 500 mL of air is exchanged per breath without utilizing full lung capacity.

• Forced Exhalation: Involves the contraction of abdominal muscles and internal intercostal muscles to forcibly expel air, decreasing thoracic volume and allowing for greater air expulsion than in tidal breathing.

4. Interpleural Pressure Dynamics

• Intrapleural Pressure: The pressure between the parietal pleura (which lines the thoracic cavity) and visceral pleura (which covers the lungs);

  • Must remain subatmospheric (negative pressure) to prevent lung collapse (pneumothorax) and ensure lung inflation.

5. Resistance in the Respiratory System

• Airflow Resistance: Similar to blood flow dynamics, airflow through the respiratory system can be affected by resistance in the airways.

  • Bronchodilation: The airway radius increases, decreasing resistance and enhancing airflow into the alveoli; typically occurs through smooth muscle relaxation.

  • Bronchoconstriction: The airway radius decreases, increasing resistance and reducing airflow, commonly associated with conditions like asthma.

    • Hard to breath because air is constricted

6. Regulation of Bronchioles

• Bronchioles: Contain smooth muscle that adjusts based on autonomic nervous system signals, influencing airflow.

  • Sympathetic Activation: Induces bronchodilation via beta-adrenergic receptors, increasing airflow and ventilation, especially during stress.

    • Asthma: Characterized by excessive bronchoconstriction, leading to difficulty breathing; inhalers (often containing bronchodilators) mitigate this.

  • Parasympathetic Activation: Results in bronchoconstriction, which reduces airflow and can exacerbate breathing difficulties.

Nitric Oxide: A vasodilator that can also promote bronchodilation and improve airflow; used therapeutically in certain respiratory conditions.

7. Lung Volumes and Capacities

• Tidal Volume (TV): 

  • Volume of air exchanged during normal breathing 

  • (approximately 500 mL).

• Inspiratory Reserve Volume (IRV): 

  • The additional volume of air that can be inhaled forcibly after normal inhalation 

  • (approximately 3 liters).

• Expiratory Reserve Volume (ERV): 

  • The additional volume of air that can be exhaled forcibly after normal exhalation 

  • (approximately 2.5 liters).

• Vital Capacity (VC): 

  • Total volume of air that can be moved in and out of the lungs, 

  • calculated as TV + IRV + ERV.

• Residual Volume (RV): 

  • The volume of air remaining in the lungs after maximum exhalation (approximately 1 liter), 

  • essential for maintaining alveolar stability.

9. Dead Space in the Respiratory System

• Anatomical Dead Space: Volume of air that does not reach the alveoli and thus does not participate in gas exchange; includes the trachea and bronchi.

  • An increase in dead space often correlates with increased residual volume and a decrease in vital capacity.

  • Age-related changes can impact compliance and capacity within the conducting zones, compromising efficiency.

• Physiological Dead Space: Refers to dead space in the respiratory zone due to conditions such as emphysema, which adversely affect alveolar volume and gas exchange.

10. Assessing Alveolar Ventilation

• Alveolar Ventilation: Evaluated by measuring carbon dioxide (CO2) levels in exhaled air, as CO2 presence indicates effective gas exchange.

• Measuring oxygen may be misleading due to environmental factors; hence CO2 is the more reliable metric for assessing ventilation efficiency and gas exchange processes.

Hemoglobin Oxygen Dissociation Curve

• X-Axis: The partial pressure of oxygen (pO2) represents the concentration of oxygen dissolved in the plasma, measured in mmHg, which is critical for understanding oxygen transport and release in the body.

Barometric Pressure at Sea Level:

• Total pressure: 760 mmHg, which is the standard atmospheric pressure at sea level.

• Partial pressures: 60 mmHg for oxygen (O2) and 600 mmHg for nitrogen (N2).

• Atmospheric Composition: The atmosphere consists of approximately 21% oxygen and 79% nitrogen, which plays a vital role in determining the behavior of gases in the respiratory system.

Henry's Law and Gas Solubility

• Henry's Law: This law states that the concentration of a gas in a liquid is proportional to its partial pressure above the liquid, indicating how solubility affects gas transport in blood.

• Impact of Environmental Pressure: Variations in barometric pressure significantly influence how gases are transported in both respiratory and circulatory systems.

  • High Altitude: Lower barometric pressure results in less dissolved oxygen in blood, increasing the risk of hypoxia (insufficient oxygen) and altitude sickness.

  • Low Altitude: Higher barometric pressure increases oxygen solubility in blood, which could lead to oxygen toxicity, especially in sensitive individuals or under certain conditions.

Scuba Diving Considerations

• Pure Oxygen Tanks: Utilization of pure oxygen tanks can lead to oxygen toxicity, which leads to the generation of free radicals that may have harmful effects on the central nervous system.

• Air Tank Mixture: A mixture of oxygen and nitrogen is typically used in diving tanks for enhanced safety, reducing the risk of nitrogen narcosis and other complications.

• Nitrogen Accumulation: Prolonged exposure at depth can result in nitrogen narcosis, which impairs cognitive function and motor skills.

• Helium Tanks: Professional divers may use helium mixtures in their tanks to minimize inert gas narcosis, allowing for safer deep dives.

• Decompression: Ascending to the surface requires gradual pressure change to avoid gas bubbles forming in the blood and tissues, a condition known as decompression sickness or 'the bends'.

Gas Exchange and Transport

• Gas Exchange Requirement: Effective gas exchange necessitates adequate ventilation (air reaching alveoli) and perfusion (blood flowing to alveoli). Both are crucial for maintaining oxygen delivery to tissues.

  • Normal Conditions: Optimal ventilation-perfusion matching occurs with a V/Q ratio of approximately 1, which is essential for effective gas exchange.

  • Clinical Assessments: The V/Q ratio is utilized to diagnose respiratory conditions and complications, such as pulmonary shunt or dead space ventilation, which can impair oxygenation.

Regulation of Respiration

• Brainstem Centers: The brainstem contains centers that regulate rhythmic breathing patterns through chemoreceptor input, adjusting ventilation based on changing blood gas levels.

• Pre-Botzinger Complex: Contains pacemaker neurons critical for generating respiratory rhythm, ensuring regular and sustainable breathing.

• Peripheral Chemoreceptors: Located in the aorta and carotid arteries, these receptors monitor oxygen and carbon dioxide levels in the blood, affecting respiratory drive.

• Central Chemoreceptors: Found within the brain (particularly the medulla oblongata), these receptors primarily respond to changes in carbon dioxide levels, influencing respiration rate to maintain homeostasis.

Importance of Carbon Dioxide

• Higher Solubility: Carbon dioxide is significantly more soluble in blood than oxygen, thus acting as a primary regulator of blood pH and influencing respiratory patterns.

• Breathing Response: Small fluctuations in CO2 levels result in rapid adjustments to ventilation, ensuring acid-base balance is maintained.

• Holding Breath: Breath-holding leads to an increase in carbon dioxide concentration, sparking panic due to CO2 buildup, not simply the lack of oxygen. Exhaling helps reduce this stress.

pH Homeostasis and Buffer Systems

• Carbonic Acid Formation: Accumulation of carbon dioxide leads to carbonic acid formation, impacting blood pH and necessitating regulation via respiratory activity.

• Respiratory Controls: Hyperventilation can cause respiratory alkalosis through excessive loss of CO2, while hypoventilation can lead to respiratory acidosis due to CO2 retention.

Conditions Affecting Gas Exchange

• Ventilation-Perfusion Coupling: Achieving a delicate balance between ventilation and perfusion is vital for optimal gas exchange functionality across the respiratory system.

• Disturbances: Any factors affecting this balance, such as obstructive or restrictive lung diseases, can lead to significant respiratory complications, impacting overall health and oxygen transport.

Congenital Hypoventilation Syndrome

• FOX2B Transcription Factor: This transcription factor is essential for the regulation of ventilatory response to hypoxia. Its absence or mutation can disrupt normal respiratory patterns, especially during sleep, leading to inadequate breathing and oxygenation during rest.

Functions of the Respiratory System

1. Gas Exchange: The primary function is to facilitate the exchange of oxygen and carbon dioxide between the alveoli and blood.

2. Ventilation: Involves bringing air into and out of the lungs to allow for gas exchange, which differentiates it from cellular respiration that occurs in cells.

3. Regulation of Blood pH: By controlling CO2 levels in the blood, the respiratory system helps maintain acid-base balance.

4. Protection: The respiratory system filters and humidifies air, trapping pathogens and debris, thus safeguarding the lungs.

5. Sound Production: The larynx allows for vocalization during breathing, which is a part of communication.

6. Olfactory Function: The nasal cavity contains receptors for smell, contributing to the sense of olfaction.

Boundaries of the Thoracic Cavity

• The thoracic cavity is bounded superiorly by the first rib and the clavicle, laterally by the ribs, posteriorly by the vertebral column, and anteriorly by the sternum. Inferiorly, it is limited by the diaphragm, a muscle that plays a critical role in ventilation.

Pleural Membranes

• The pleurae are double-layered membranes surrounding the lungs.

  • Visceral Pleura: Covers the lung surface.

  • Parietal Pleura: Lines the thoracic cavity. Between these two layers is the pleural cavity, which contains a small amount of pleural fluid that reduces friction during respiration and helps maintain negative pressure.

Respiratory Pressures

1. Alveolar Pressure (Intrapulmonary Pressure): The pressure within the alveoli, which fluctuates during breathing but generally equals atmospheric pressure at rest.

2. Intrapleural Pressure: The pressure within the pleural cavity, normally subatmospheric (negative pressure), which helps keep the lungs inflated and prevents collapse.

3. Transpulmonary Pressure: The difference between the intrapulmonary pressure and intrapleural pressure; it measures the elastic recoil of the lungs (Transpulmonary Pressure = Alveolar Pressure - Intrapleural Pressure).

Mechanics of Ventilation

• Inspiration: The process of drawing air into the lungs, primarily achieved by:

  • Diaphragm Contraction: The diaphragm contracts and moves downward, increasing thoracic volume.

  • Accessory Muscles (like the intercostals): May assist in deeper inhalation.

• Expiration: The process of expelling air from the lungs, involving:

  • Passive Process: During normal breathing, expiration is passive (elastic recoil of lungs).

  • Forced Exhalation: Involves muscle contraction, such as abdominal muscles (internal intercostals) to actively push air out.

Muscles Involved in Ventilation

• Diaphragm: Main muscle for inspiration, separating thoracic and abdominal cavities.

• External Intercostal Muscles: Assist in lifting the rib cage during inhalation.

• Internal Intercostal Muscles: Assist in forced expiration by pulling ribs down.

• Accessory Muscles: Include muscles in the neck and chest (e.g., sternocleidomastoid and scalene) that help during intense breathing efforts.

Boyle's Law and Its Application to Ventilation

• Boyle's Law states that at a constant temperature, the pressure of a gas varies inversely with its volume (P1V1 = P2V2). During inhalation, as thoracic volume increases (due to diaphragm contraction), intrapulmonary pressure decreases, allowing air to flow in. Conversely, during exhalation, as thoracic volume decreases, pressure increases, pushing air out.

Physical Factors Affecting Ventilation

• Resistance: Constriction of airways (bronchoconstriction) increases resistance, while dilation (bronchodilation) decreases it.

• Lung Compliance: Refers to the ease of lung expansion; impaired compliance (e.g., in fibrosis) diminishes ventilation.

• Elastic Recoil: The lungs' ability to return to their resting volume affects expiration efficiency.

Buffering pH in the Respiratory System

• The respiratory system buffers pH by regulating CO2 levels. Increased CO2 leads to the formation of carbonic acid, lowering pH (acidosis). Conversely, decreased CO2 results in alkalosis. By adjusting ventilation rate, the body can dial down or increase CO2 expulsion to stabilize blood pH levels.

Compliance

• Compliance refers to the ease with which the lungs expand. High compliance means the lungs inflate easily, while low compliance indicates stiffness, making it difficult for the lungs to expand. Factors affecting compliance include lung structure, surface tension, and the presence of surfactant.

Role of Surfactant in Compliance

• Surfactant is a substance produced by type II alveolar cells that reduces surface tension in alveoli, preventing collapse. This reduction in surface tension is crucial for maintaining compliance, enabling the alveoli to expand easily during inflation and ensuring efficient gas exchange without increased energy expenditure during breathing.

Pathologies of the Respiratory System

1. Pneumothorax: A condition where air enters the pleural cavity, resulting in lung collapse. It can occur spontaneously or due to trauma. The presence of air in the pleural space disrupts the negative pressure necessary for lung inflation.

2. Emphysema: A progressive disease characterized by the destruction of alveoli, leading to reduced surface area for gas exchange. It often results from chronic smoking or long-term exposure to irritants.

3. Respiratory Distress Syndrome (RDS): Typically affects premature infants due to inadequate surfactant production, leading to alveolar collapse and impaired gas exchange. Adults can also experience a similar condition (Acute Respiratory Distress Syndrome - ARDS).

4. Congenital Hypoventilation Syndrome: A genetic disorder that impairs the ability to respond appropriately to low oxygen levels, especially during sleep. It leads to hypoventilation and can cause significant respiratory issues.

Partial Pressure

• Partial Pressure: The pressure exerted by a specific gas in a mixture of gases. It is an important concept for understanding gas exchange in the lungs.

  • Atmosphere: 60 mmHg for oxygen (O2) and 600 mmHg for carbon dioxide (CO2).

  • Body: The partial pressures in arterial blood are approximately 95 mmHg for O2 and 40 mmHg for CO2.

Coupling of Ventilation and Perfusion

• Ventilation (airflow reaching alveoli) and perfusion (blood flow to alveoli) are closely coupled to optimize gas exchange. An ideal ventilation-perfusion ratio occurs when the amount of air reaching the alveoli matches the blood flow in the capillaries, ensuring efficient oxygen absorption and carbon dioxide removal.

Transport of O2 and CO2 in the Blood

• Oxygen Transport: About 98.5% of oxygen is transported bound to hemoglobin in red blood cells, while a small amount (approximately 1.5%) is dissolved in plasma.

• Carbon Dioxide Transport: CO2 is transported in three forms: about 70% as bicarbonate ions (HCO3-) in plasma, around 20-23% bound to hemoglobin, and roughly 7-10% dissolved in plasma.

Respiratory Control Centers

• The respiratory control centers located in the brain stem (medulla oblongata and pons) regulate the rhythm and rate of breathing.

  • Medullary Centers: The rhythm generator (Pre-Bötzinger complex) initiates breathing, while the pneumotaxic center modifies the rhythm and depth of breathing during activities such as speaking or exercising.

Effects of Hypoxia, Increased CO2, and Increased H+ Ions on Respiration

• Hypoxia: Low oxygen levels stimulate increased respiratory rate to enhance oxygen uptake.

• Increased CO2: Elevated carbon dioxide levels lead to respiratory acidosis, triggering an increase in ventilation to expel excess CO2.

• Increased H+ Ions: Higher acidity (lower pH) in the blood similarly stimulates an increase in respiratory rate to balance pH levels through CO2 removal.

Regulation of Breathing by Brain Stem Respiratory Centers

• Breathing is regulated by central chemoreceptors that respond to CO2 levels in the cerebrospinal fluid, and peripheral chemoreceptors that respond to oxygen and CO2 levels in the blood. Adjustments in ventilation are made based on these inputs to maintain homeostatic balance in gas exchange and blood pH.