The Respiratory System

The Respiratory System

Chapter 22: Overview of the Respiratory System

• Respiration can have three meanings:

  • Ventilation of lungs (bringing air in and out)

  • Exchange of gases between air and blood and between blood and tissue fluid

  • Use of oxygen gas in cellular respiration

Functions of the Respiratory System

• Pressure gradients for lymph flow

• Acid-base balance regulation

• Platelet production

• Gas exchange

• Pressure gradients for blood flow

• Filters clots in the bloodstream

• Blood pressure regulation

• Facilitates defecation

• Communication through phonation

Anatomy of the Respiratory System

• Thoracic cavity includes:

  • Right lung

  • Left lung

  • Diaphragm

  • Bronchioles

  • Upper respiratory tract (nose, larynx, pharynx, nasal cavity, trachea)

  • Lower respiratory tract (trachea through lungs)

• Alveoli are involved in gas exchange

• Thoracic cage is the bony structure around the thoracic cavity

Conducting and Respiratory Zones

• Conducting Zone:

  • Extends from nostrils through major bronchioles

  • Includes bronchi and bronchioles

• Respiratory Zone:

  • Contains alveoli and other gas-exchange regions

Nasal Cavity

• The nasal cavity expands posteriorly into a larger chamber with little open space.

• Concha bones: project from lateral walls towards septum, creating turbulence to ensure air contacts mucous membranes.

  • Functions include filtering, warming, and moistening the air.

The Pharynx: A Muscular Tunnel

• Components:

  • Nasopharynx:

  • Lined by pseudostratified columnar epithelium

  • Located posterior to nasal apertures; contains auditory tubes and pharyngeal tonsil

  • Traps large particles (>10 μm)

  • Oropharynx:

  • Lined by stratified squamous epithelium

  • Space between soft palate and epiglottis; contains palatine tonsils

  • Laryngopharynx:

  • Lined by stratified squamous epithelium

  • Extends from epiglottis to cricoid cartilage; esophagus begins here

• Nasopharynx passes only air; oropharynx and laryngopharynx can pass air, food, and drink.

The Larynx

• Primary function: Prevent food/drink from entering the airway.

• Epiglottis: a flap of tissue that guards the superior opening of the larynx; it elevates when swallowing and meets the tongue to cover the airway.

Framework of the Larynx: Nine Cartilages

• Epiglottic cartilage: Most superior, spoon-shaped support in epiglottis.

• Thyroid cartilage: Shield-shaped, contains the largest laryngeal prominence (Adam’s apple).

  • Testosterone stimulates its growth, making it larger in males.

• Cricoid cartilage: Ring-like, connects larynx to trachea.

• Arytenoid cartilages (2): Posterior to thyroid cartilage.

• Corniculate cartilages (2): Attached to arytenoid cartilages like horns.

• Cuneiform cartilages (2): Support soft tissue between arytenoids and epiglottis.

The Larynx: Interior Wall

• Contains two pairs of vocal folds:

  • Superior vestibular folds: Play no role in speech; close the larynx during swallowing.

  • Inferior vocal cords: Produce sound when air passes between them.

• Glottis: The vocal cords and the opening between them.

Phonation in the Larynx

• During breathing:

  • Vocal cords relax; glottis is open.

  • Air forced through glottis vibrates vocal cords, producing sound.

  • Changing tension on vocal cords controls pitch.

• Differences between adult male and female vocal cords:

  • Male cords are typically longer and thicker, producing lower-pitched sound.

• Loudness: Controlled by the force of air passing through the vocal cords.

Trachea – The Windpipe

• Structure:

  • Rigid tube, approximately 12 cm long and 2.5 cm in diameter.

  • Anterior to esophagus; consists of 16 to 20 C-shaped rings of hyaline cartilage to prevent collapse.

  • The opening in C-rings faces posteriorly (toward the esophagus) to allow expansion when swallowing food.

• Muscle spans openings in C-rings, can contract or relax to adjust airflow.

Trachea: Histology and “Escalator”

• Lined by ciliated pseudostratified columnar epithelium with mucus-secreting cells.

• Mucociliary escalator: Mucus traps inhaled particles, and cilia move mucus to the pharynx to be swallowed.

• Middle tracheal layer: Contains lymphatic nodules, mucous and serous glands, and tracheal cartilages.

• Adventitia: Outermost layer; fibrous connective tissue that blends into surrounding tissues.

Tracheotomy

• A procedure to create a temporary opening in the trachea to allow airflow; if prolonged, may lead to problems such as:

  • Inhaled air bypassing nasal cavity, drying out mucous membranes, increasing infection risk.

Lungs: Gross Anatomy

• Base of the right and left lung rests on the diaphragm; the apex of each lung projects just above the clavicle.

• Right lung:

  • Shorter than the left due to the liver's position.

  • Has three lobes: superior, middle, and inferior, separated by horizontal and oblique fissures.

• Left lung:

  • Taller and narrower due to the heart's position.

  • Has two lobes: superior and inferior, separated by a single oblique fissure.

The Bronchial Tree: Bronchi

• Primary bronchi:

  • Right bronchus is wider, shorter, and straighter compared to left, making it more likely for inhaled foreign objects to lodge there.

• Secondary (lobar) bronchi:

  • Three in the right, two in the left lung.

• Tertiary (segmental) bronchi:

  • Ten in the right and variable (8-10) in the left.

• As bronchi branch smaller, ciliated pseudostratified columnar epithelium becomes shorter and smooth muscle increases.

The Bronchial Tree: Structure

• Primary bronchi:

  • Supported by C-shaped cartilage rings.

• Secondary (lobar) bronchi:

  • Supported by cartilage plates.

• Tertiary (segmental) bronchi:

  • Supported by smaller cartilage plates.

The Bronchial Tree: Bronchioles

• Bronchioles:

  • No cartilage, but with increasing amounts of smooth muscle; supplies multiple terminal bronchioles.

  • Epithelium consists of ciliated cuboidal cells.

• Pulmonary lobule: Ventilated by one bronchiole; contains terminal bronchioles that supply several alveolar ducts.

• Alveolar ducts: Lined by simple squamous epithelium; lead to alveolar sacs where gas exchange occurs.

The Alveoli

• Cells of the Alveoli:

  • Type I (Squamous) alveolar cells: Form 90% of the alveolar surface area; very thin to facilitate rapid diffusion.

  • Type II (Great) alveolar cells: Repair epithelium when damaged; secrete surfactant to reduce surface tension.

  • Alveolar macrophages (dust cells): Clean the lungs by ingesting debris and pathogens.

Respiratory Membrane

• A thin barrier between alveolar air and blood, measuring approximately 0.2 - 0.6 micrometers. Must prevent fluid accumulation to maintain efficacy in gas exchange.

Preventing Fluid Accumulation in Alveoli

• Alveoli are kept dry by:

  • Low blood pressure in pulmonary capillaries (approximately 10 mm Hg compared to 30 mm Hg in systemic capillaries).

  • The osmotic reabsorption of water prevents excess fluid accumulation in alveoli.

Pleura

• Pleura: A two-layered serous membrane surrounding the lungs, comprising:

  • Visceral pleura: Covers the lung surface.

  • Parietal pleura: Lines the rib cage and Diaphragm; pleural fluid resides between these layers to reduce friction and assist in lung inflation.

Boyle's Law – Pressure-Volume Relationship

• States that at a constant temperature, the pressure of a given quantity of gas is inversely proportional to its volume.

Muscle Action and Thoracic Cavity Size

• Contractions of muscles (diaphragm and intercostals) change the size of the thoracic cavity:

  • Diaphragm accounts for 2/3 of air flow, intercostal muscles account for 1/3 during quiet respiration.

  • Quiet respiration: Reflexive and automatic breathing pattern.

  • Forced respiration: Deep, rapid breathing requiring accessory muscles.

Ventilation: Inspiration & Expiration

• Inspiration: Thoracic cavity expands, decreasing pressure and facilitating airflow into the lungs.

• Expiration: Mainly passive, caused by elastic recoil of the thoracic cage, increasing pressure and expelling air from the lungs.

Pressure Changes During Ventilation

• Three types of pressure during ventilation:

  • Atmospheric Pressure: Pull of gravity on air; ~760 mm Hg at sea level.

  • Intrapulmonary Pressure: Pressure within alveoli; rises and falls during breathing, equilibrating with atmospheric pressure.

  • Intrapleural Pressure: Pressure in pleural cavity; remains lower than intrapulmonary pressure (~4 mm Hg less) to prevent lung collapse.

Importance of Intrapleural Pressure

• Intrapleural pressure creates a suction effect that combats lung collapse. If intrapleural pressure increases to atmospheric levels, the lungs collapse.

Brainstem Respiratory Centers

• Breathing is controlled by three pairs of respiratory centers in the medulla and pons:

  • Medullary Respiratory Center:

  • Ventral Respiratory Group (VRG): Primary generator of respiratory rhythm.

  • Dorsal Respiratory Group (DRG): Modifies the rate and depth of breathing from external influences.

  • Pontine Respiratory Group (PRG): Adjusts rhythm based on special circumstances (sleep, exercise, vocalization).

Central and Peripheral Input to Respiratory Centers

• Central Chemoreceptors: Respond to pH changes in cerebrospinal fluid, reflecting CO2 levels in blood. Regulate respiration to maintain stable pH.

• Peripheral Chemoreceptors: Located in carotid and aortic bodies; respond to O2, CO2, and blood pH.

• Stretch Receptors: Located in bronchi, bronchioles, and visceral pleura; respond to lung inflation, triggering protective reflexes when overinflated.

Airflow Regulation in the Respiratory System

• Airflow is governed by principles of pressure, resistance, and radius:

  • Flow (F) is influenced by pressure difference (ΔP) and resistance (R): $F \propto \Delta P / R$ and $F \propto 1/R$

• Factors influencing pulmonary ventilation include:

  • Diameter of bronchioles (bronchodilation increases airflow; bronchoconstriction decreases airflow).

Pulmonary Compliance and Surface Tension

• Pulmonary Compliance: Refers to the lungs' ability to expand; reduced by replacement of elastic fibers with collagen in lung damage.

• Surface Tension: Water molecules at a gas/water boundary attract each other, potentially collapsing small alveoli; surfactant helps decrease surface tension.

Alveolar Ventilation

• Anatomic Dead Space: 150 mL of air occupies the conducting zone where no gas exchange occurs.

• Physiologic Dead Space: The sum of anatomic dead space and alveolar dead space from pulmonary diseases.

• Alveolar Ventilation Rate (AVR) is critical for assessing gas exchange efficiency

  • $AVR = \text{Air that ventilates alveoli} \times \text{Respiratory Rate}$

Spirometry and Respiratory Volumes

• A spirometer measures various variables of breathing (rate, depth, speed of expiration).

• Respiratory Volumes:

  • Tidal Volume (TV): Volume of air inhaled/exhaled per cycle (~500 mL).

  • Inspiratory Reserve Volume (IRV): Volume of air that can be inhaled over tidal volume, roughly 3000 mL.

  • Expiratory Reserve Volume (ERV): Volume exhaled beyond tidal volume, roughly 1200 mL.

  • Residual Volume (RV): Volume remaining in lungs after max expiration, about 1200 mL.

Lung Capacities

• Vital Capacity (VC): Total amount of air that can be inhaled and exhaled, calculated as $VC = ERV + TV + IRV \approx 4700 mL$.

• Inspiratory Capacity (IC): Maximum air inhaled after normal expiration, calculated as $IC = TV + IRV \approx 3500 mL$.

• Functional Residual Capacity (FRC): Air remaining post normal expiration, calculated as $FRC = RV + ERV \approx 2500 mL$.

• Total Lung Capacity (TLC): Maximum lung volume, calculated as $TLC = RV + VC \approx 6000 mL$.

Spirometry Measurements

• Forced Expiratory Volume (FEV): Percentage of VC exhaled in a specific time interval (75%-85% in 1 second for healthy adults).

• Peak Flow: Maximum speed of expiration measured with a handheld meter.

• Minute Respiratory Volume (MRV): Amount inhaled per minute:

  • $MRV = TV \times \text{Respiratory Rate} = 500 mL/breath \times 12 breaths/min = 6000 mL/min$

• Maximum Voluntary Ventilation (MVV): During heavy exercise, may reach 125-170 L/min (125,000-170,000 mL/min).

Respiratory Disorders: Restrictive vs. Obstructive Disorders

Restrictive Disorders

• Decrease pulmonary compliance and reduce inspiratory volume, e.g.:

  • Tuberculosis (with pulmonary fibrosis)

• Characteristics: Smaller vital capacity.

Obstructive Disorders

• Involve airway narrowing or blockage significantly affecting airflow, e.g.:

  • Asthma, characterized by bronchoconstriction and increased mucus.

  • Chronic Obstructive Pulmonary Diseases (COPD) (often linked to smoking):

  • Chronic Bronchitis: Inflammation, excess mucus, cilia paralysis.

  • Emphysema: Breakdown of alveolar walls, reducing gas exchange efficiency.

Gas Exchange Mechanisms

• Pulmonary Ventilation: Introduces fresh air and removes CO2; critical before gas exchange.

• Gas Diffusion Laws:

  • Dalton’s Law: Total pressure equals the sum of partial pressures.

  • Henry’s Law: Gas solubility in liquids is proportional to partial pressure and solubility.

Pulmonary (Alveolar) Gas Exchange

• Oxygen from alveolar air diffuses into blood; CO2 diffuses from blood to alveolar air.

• Rate of diffusion depends on concentration gradients of O2 (PO2) and CO2 (PCO2):

  • Alveolar PO2 = 104 mm Hg vs. Blood PO2 = 40 mm Hg; rapid diffusion occurs.

  • CO2 is about 20X more soluble than O2, facilitating its rapid diffusion.

Factors Affecting Efficiency of Gas Exchange

• Surface area of the respiratory membrane and membrane thickness.

• Ventilation-Perfusion Coupling: Optimizes match between air flow (ventilation) and blood flow (perfusion).

Oxygen Transport

• 98.5% of oxygen is transported bound to hemoglobin; 1.5% is dissolved in plasma.

• Hemoglobin structure:

  • Four protein subunits, each with a heme group capable of binding one O2.

• Oxygen saturation of hemoglobin is impacted by physical conditions (e.g., pH, temperature); demonstrated through the oxyhemoglobin dissociation curve.

Carbon Dioxide Transport

• CO2 transport mechanisms:

  • 7% dissolves in plasma, 23% binds to hemoglobin, and 70% is transformed into bicarbonate via carbonic anhydrase enzymes.

Systemic Gas Exchange

• CO2 loading occurs in tissues; O2 unloading happens in systemic capillaries; direct effect of temperature, acidity, and CO2 concentration on hemoglobin affinity for O2.

Alveolar Gas Exchange

• CO2 unloading occurs as O2 loading takes place in the pulmonary capillaries, maintaining the transport system effectively.

pH Regulation

• Significant concept: pH inversely affects hydrogen ion concentration. Changes in ventilation patterns lead to fluctuations in CO2 levels, reflecting on blood pH, leading to acidosis or alkalosis.

• Acidosis: Blood pH < 7.35; Alkalosis: Blood pH > 7.45 ▪ Hypoventilation results in too much CO2 retention, leading to acidosis, while hyperventilation leads to rapid expulsion causing alkalosis.

Summary of Respiratory Disorders

• Restrictive Disorders: Limits lung inflation, adversely affecting vital capacities.

• Obstructive Disorders: Narrows airways, leading to difficulty in breathing. Specific examples include asthma and COPD.

Concluding Remarks

• The respiratory system is a complex assembly of structures and functions vital for gas exchange, regulation of body pH, and effective ventilation. Understanding the underlying mechanisms and factors influencing performance can lead to better management and treatment strategies for respiratory conditions.