respiratory

Overview and Major Functions of the Respiratory System

• Primary Function: Respiration, which encompasses two main tasks:

  • Supplying the body with oxygen ($O_2$) required for cellular respiration.

  • Disposing of carbon dioxide ($CO_2$), which is a waste product generated during cellular respiration.

• Scope of Processes: Respiration involves four interdependent processes requiring cooperation between the respiratory and circulatory systems.

• Secondary Functions:

  • Olfaction: Facilitation of the sense of smell.

  • Speech: Production of vocal sounds.

Functional Anatomy of the Respiratory System

• Major Organs:

  • Nose, nasal cavity, and paranasal sinuses.

  • Pharynx.

  • Larynx.

  • Trachea.

  • Bronchi and their subsequent branches.

  • Lungs and alveoli.

• Division of Functional Zones:

  • Respiratory Zone: The actual site of gas exchange. It consists of microscopic structures including respiratory bronchioles, alveolar ducts, and alveoli.

  • Conducting Zone: Includes all other respiratory structures that act as conduits to the gas exchange sites. These structures also work to cleanse, warm, and humidify incoming air.

• Respiratory Muscles: The diaphragm and other muscles (such as intercostals) promote ventilation by changing thoracic volume.

The Nasal Cavity and Paranasal Sinuses

• Nasal Cavity Functions:

  • Provides a dedicated airway for respiration.

  • Moistens and warms air as it enters the body.

  • Filters and cleanses inspired air of foreign matter.

  • Acts as a resonating chamber to enhance speech quality.

  • Houses sensory receptors for olfaction.

• Structural Details:

  • Nasal Vestibule: The portion of the nasal cavity located just superior to the nostrils.

  • Vibrissae: Internal nasal hairs that filter coarse particles from inspired air.

  • Nasal Conchae: Superior, middle, and inferior projections that protrude medially from the lateral walls. Their purpose is to increase mucosal surface area and enhance air turbulence/vortices.

  • Nasal Meatus: Grooves located inferior to each corresponding concha.

• Mucous Membranes:

  • Olfactory Mucosa: Contains the slit-like olfactory receptors.

  • Respiratory Mucosa: Lines the majority of the nasal cavity.

• Paranasal Sinuses:

  • Found within the frontal, sphenoid, ethmoid, and maxillary bones.

  • Functions include lightening the skull, secreting mucus, warming and moistening air, and providing resonance to amplify the voice.

The Pharynx (Throat)

• Description: A muscular tube extending from the base of the skull to the sixth cervical vertebra ($C6$).

• Connection: It links the nasal cavity and mouth to the larynx (superiorly) and the esophagus (inferiorly).

• Composition: Skeletal muscle lined with mucosa.

• Three Distinct Regions:

  1. Nasopharynx: The most superior portion; contains the pharyngeal tonsils and the opening of the pharyngotympanic tube.

  2. Oropharynx: The middle portion; contains the uvula, palatine tonsils, lingual tonsils, and the isthmus of the fauces.

  3. Laryngopharynx: The inferior portion; continuous with the esophagus and larynx.

The Larynx (Voice Box)

• Location: Attaches to the hyoid bone, opens into the laryngopharynx, and is continuous with the trachea inferiorly.

• Primary Functions:

  • Ensures a patent (open) airway.

  • Acts as a switching mechanism to route air and food into their proper channels.

  • Voice production via the housing and vibration of vocal folds.

• Key Anatomical Structures:

  • Epiglottis: Composed of elastic cartilage; it covers the laryngeal inlet during swallowing to prevent food from entering the airway. It is covered in mucosa containing taste buds.

  • Thyroid Cartilage: Large cartilage plate that includes the laryngeal prominence (Adam’s apple).

  • Cricoid Cartilage: Ring-shaped cartilage located inferior to the thyroid cartilage.

  • Vocal Folds (True Vocal Cords): Folds that vibrate as air rushes up from the lungs to produce sound.

  • Glottis: The opening between the vocal folds.

  • Vestibular Folds (False Vocal Cords): Located superior to the true vocal folds. They play no role in sound production but help close the glottis during swallowing.

• Valsalva's Maneuver: An event where vocal folds act as a sphincter.

  • The glottis closes to prevent exhalation.

  • Abdominal muscles contract.

  • Intra-abdominal pressure rises.

  • Purpose: Helps empty the rectum (defecation) or stabilizes the trunk when lifting heavy loads.

The Trachea and Conducting Zone

• Trachea (Windpipe):

  • Extends from the larynx into the mediastinum.

  • Dimensions: Approximately $4\,\text{inches}$ in length.

  • Carina: A spar of cartilage located on the last expanded tracheal cartilage; it marks the point where the trachea branches into the two main bronchi.

  • Structure: Consists of mucosa, submucosa (containing seromucous glands), hyaline cartilage rings, and adventitia. The trachealis muscle is located posteriorly against the esophagus.

• Main Bronchi:

  • The trachea divides into the Right and Left Main (Primary) Bronchi.

  • Each enters the hilum of its respective lung.

  • Right Main Bronchus Characteristics: Wider, shorter, and more vertical than the left bronchus.

• Branching Hierarchy:

  1. Lobar (Secondary) Bronchi: Three on the right (supplying three lobes), two on the left (supplying two lobes).

  2. Segmental (Tertiary) Bronchi: Branch further into smaller tubes.

  3. Bronchioles: Tubes less than $1\,\text{mm}$ in diameter.

  4. Terminal Bronchioles: The smallest tubes in the conducting zone, less than $0.5\,\text{mm}$ in diameter.

The Respiratory Zone and Respiratory Membrane

• Beginning of Respiratory Zone: Starts where terminal bronchioles feed into respiratory bronchioles.

• Path of Airflow: Respiratory bronchioles $\\rightarrow$ alveolar ducts $\\rightarrow$ alveolar sacs.

• Alveoli:

  • Alveolar sacs contain clusters of alveoli.

  • There are approximately $300\,\text{million}$ alveoli in the lungs, which account for the majority of lung volume.

  • These are the primary sites of gas exchange.

• The Respiratory Membrane (Air-Blood Barrier):

  • Thickness: Approximately $0.5[\\mu]m$ ($0.5\,\mu \text{m}$).

  • Composition: The fused basement membranes of alveolar and capillary walls.

  • Mechanism: Gas exchange occurs via simple diffusion.

• Alveolar Wall Cells:

  • Type I Alveolar Cells: A single layer of squamous epithelium.

  • Type II Alveolar Cells: Scattered cuboidal cells that secrete surfactant (a lipid molecule) and antimicrobial proteins.

  • Alveolar Macrophages ("Dust Cells"): Provide protection by engulfing bacteria, carbon particles, and debris.

  • Alveolar Pores: Connect neighboring air sacs to equalize air pressure throughout the lung.

Gross Anatomy and Coverings of the Lungs

• Anatomical Features:

  • Apex: Superior tip, located deep to the clavicle.

  • Base: Inferior surface that rests on the diaphragm.

  • Hilum: Found on the mediastinal surface; the site for entry/exit of blood vessels, bronchi, lymphatic vessels, and nerves.

  • Left Lung: Smaller than the right; includes the cardiac notch (a concavity to accommodate the heart) and is divided into superior and inferior lobes by an oblique fissure.

  • Right Lung: Larger; divided into superior, middle, and inferior lobes.

• Blood Supply:

  1. Pulmonary Circulation (Low Pressure, High Volume):

     • Pulmonary arteries deliver systemic venous blood for oxygenation.

     • Pulmonary veins carry oxygenated blood from respiratory zones back to the heart.

  2. Bronchial Circulation (High Pressure, Low Volume):

     • Bronchial arteries arise from the aorta to provide oxygenated blood to the lung tissue (except the alveoli).

     • Bronchial veins anastomose with pulmonary veins; most venous blood returns via pulmonary veins.

• The Pleurae:

  • Pulmonary (Visceral) Pleura: Covers the external lung surface.

  • Parietal Pleura: Lines the walls of the thoracic cavity.

  • Pleural Fluid: Fills the pleural cavity between layers to allow gliding and reduce friction during breathing.

  • Pleural Space: A potential space between the two layers.

Mechanics of Breathing (Pulmonary Ventilation)

• Principle: Mechanical process dependent on volume changes in the thoracic cavity.

• Sequence: Volume changes $\\rightarrow$ pressure changes $\\rightarrow$ flow of gases to equalize pressure.

• Phases:

  1. Inspiration (Inhalation): Air flows into the lungs.

  2. Expiration (Exhalation): Air leaves the lungs.

• Intrapleural Pressure: Normal pressure within the pleural space is always negative. This negative pressure is essential to prevent lung collapse.

• Clinical Conditions:

  • Atelectasis: A collapsed lung.

  • Pneumothorax: The presence of air in the intrapleural space.

Respiratory Volumes and Capacities

• Measurement: Respiratory capacities are measured using a spirometer.

• Standard Respiratory Volumes:

  • Tidal Volume (TV): Normal breathing moves about $500\,ml$ with each breath.

  • Inspiratory Reserve Volume (IRV): Amount of air forcibly taken in over tidal volume ($\approx 3,100\,ml$).

  • Expiratory Reserve Volume (ERV): Amount of air forcibly exhaled after a tidal expiration ($\approx 1,200\,ml$).

  • Residual Volume (RV): Air remaining in lungs after expiration ($\approx 1,200\,ml$). It keeps alveoli open and allows continuous gas exchange.

• Calculated Capacities:

  • Vital Capacity (VC): Total amount of exchangeable air ($VC = TV + IRV + ERV$). Approximately $4,800\,ml$ in men and $3,100\,ml$ in women.

  • Total Lung Capacity: Approximately $6,000\,ml$ (sum of all volumes).

  • Dead Space Volume: Air that remains in the conducting zone and never reaches alveoli ($\approx 150\,ml$).

  • Functional Volume: Air that actually reaches the respiratory zone ($\approx 350\,ml$).

• Influencing Factors: Size, sex, age, and physical condition.

Nonrespiratory Air Movements

• Caused by reflexes or voluntary actions:

  • Cough and Sneeze: Clears lungs/nasal passages of debris.

  • Crying: Emotionally induced mechanism.

  • Laughing: Similar mechanism to crying.

  • Hiccup: Sudden inspirations.

  • Yawn: Very deep inspiration.

Gas Exchange Mechanisms

External Respiration (Pulmonary Gas Exchange)

• Oxygen ($O_2$) loads into the pulmonary blood from alveoli.

• Carbon dioxide ($CO_2$) unloads into the alveoli from the blood.

• Chemical Equations in Phagocyte/Plasma:

  • $Hb + O_2 \rightarrow HbO_2$ (Oxyhemoglobin formation).

  • For $CO_2$ release: $HCO_3^- + H^+ \rightarrow H_2CO_3 \rightarrow CO_2 + H_2O$.

Internal Respiration (Systemic Capillary Gas Exchange)

• Gas exchange between blood and body cells.

• Unloading: Oxygen diffuses from blood into tissue cells.

• Loading: Carbon dioxide diffuses from tissue cells into blood.

• Chemical Equations:

  • $CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$ (Bicarbonate ion formation).

  • $HbO_2 \rightarrow Hb + O_2$.

Regulation of Respiration

• Neural Regulation:

  • Phrenic and Intercostal Nerves: Transmit activity of respiratory muscles to/from the brain.

  • Medulla Oblongata: Contains the Ventral Respiratory Group (VRG). It acts as a pacemaker to set the basic rhythm of breathing.

  • Pons: Smooths out the respiratory rate.

• Respiratory Rates:

  • Eupnea: Normal rate, $12$ to $15$ respirations per minute.

  • Hyperpnea: Increased rate, often due to extra oxygen demand.

• Non-Neural Factors:

  • Physical: Temperature increase, exercise, talking, coughing.

  • Conscious Control (Volition): Breath-holding or forced breathing.

  • Emotional: Fear, anger, or excitement.

• Chemical Factors:

  • CO2 Levels: The most important stimulus for breathing. Increased $CO_2$ (resulting in lower/acidic blood pH) acts directly on the medulla to increase rate/depth.

  • Oxygen Levels: Monitored by chemoreceptors in the aorta and common carotid artery. Oxygen becomes the primary stimulus only for those with chronic high $CO_2$ due to disease.

• Ventilation Variations:

  • Hyperventilation: Rapid breathing due to rising $CO_2$ (acidosis) to blow off $CO_2$. Can lead to dizziness or alkalosis and apnea.

  • Hypoventilation: Extremely slow/shallow breathing when blood is alkaline (alkalosis) to allow $CO_2$ to accumulate.

Respiratory Disorders

Chronic Obstructive Pulmonary Disease (COPD)

• Common features: History of smoking, dyspnea (labored breathing), coughing, frequent infections, hypoxia, and respiratory acidosis.

• Chronic Bronchitis: Severe inflammation of lower respiratory mucosa. Excess mucus impairs ventilation. Patients are "blue bloaters" due to cyanosis/hypoxia.

• Emphysema: Permanent enlargement and destruction of alveoli. Lungs lose elasticity and become fibrotic. Leads to overinflation and a "barrel chest." Sufferers are often called "pink puffers."

Lung Cancer

• Highly aggressive and metastasizes rapidly; responsible for 1/3 of U.S. cancer deaths.

• Types:

  1. Squamous cell carcinoma.

  2. Adenocarcinoma.

  3. Small cell carcinoma.

Developmental Aspects

• Infant Respiratory Distress Syndrome (IRDS): Occurs in premature infants (before $28{-}30$ weeks) where surfactant production is inadequate to keep lungs inflated.

• Sudden Infant Death Syndrome (SIDS): Healthy infants stop breathing in sleep. May involve neural control centers or heart rhythm abnormalities. Recent research suggests a genetic component.