Comprehensive Notes on the Respiratory System

Respiratory System

Overview

• The respiratory system is composed of structures involved with ventilation and gas exchange.
• Functions:
  1. Providing a surface area for gas exchange between air and circulating blood.
  2. Moving air to and from the exchange surfaces of the lungs along respiratory passageways.
  3. Protecting respiratory surfaces from dehydration, pathogens, temperature changes, or environmental variations.
  4. Producing sounds for communication.
  5. Facilitating the detection of olfactory stimuli by olfactory receptors in the nasal cavity.

Organization

• Upper Respiratory System: Nose, nasal cavity, paranasal sinuses, and pharynx.
  • Functions to filter, warm, and humidify incoming air to protect the lower respiratory system.
• Lower Respiratory System: Larynx, trachea, bronchi, bronchioles, and alveoli of the lungs.
• Respiratory Tract: Passageways carrying air from outside to the exchange portion (nasal cavity to larger bronchioles).
• Exchange/Respiratory Portion: Smaller bronchioles and alveoli.

Respiratory Mucosa

• The respiratory mucosa lines the conducting portion and removes foreign particles and pathogens.
• Epithelial Types:
  • Nasal cavity and superior pharynx: Pseudostratified ciliated columnar epithelium and goblet cells.
  • Lines the superior lower respiratory system to the larger bronchioles.
  • Inferior pharynx: Stratified squamous epithelium (protection against abrasion).
  • Smaller bronchioles: Ciliated simple cuboidal epithelium.
  • Alveoli: Simple squamous epithelium.

Respiratory Defense System

• The mucosa's epithelium can be damaged by contaminated air.
• Mucus Elevator: Contaminants stick to mucus (goblet cells) and are swept upwards by cilia towards the pharynx to be swallowed and destroyed or expectorated.
• Exposure to noxious stimuli increases mucus production causing congestion.

Upper Respiratory System - Nose and Nasal Cavity

• Air enters through the paired external nares (nostrils) into the nasal cavity.
• Nasal Vestibule: Space lined with epithelium and hairs to prevent entry of large particles.
• Nasal Septum: Divides the cavity into right and left halves (perpendicular plate, vomer, and hyaline cartilage).
• Nasal Conchae: Superior, middle, and inferior conchae project from the lateral walls, lined with olfactory epithelium.
• Meatuses: Air flows through grooves between the conchae (superior, middle, and inferior meatuses) causing air to swirl and:
  • Particles stick to mucosa.
  • Warming and humidification of air.
  • Stimulation of olfactory receptors.
• Hard Palate: Bony floor of the nasal cavity separating it from the oral cavity (maxillary and palatine bones).
• Soft Palate: Fleshy extension marking the boundary between the nasopharynx and the rest of the pharynx.

Pharynx

• Chamber shared by respiratory and digestive systems, extending from internal nares to larynx and esophagus.
• Divisions:
  1. Nasopharynx: Superior portion from internal nares to the soft palate. Contains the pharyngeal tonsil (adenoid) and openings to the auditory tubes.
  2. Oropharynx: Middle portion from the soft palate to the base of the tongue at the hyoid bone. Contains palatine and lingual tonsils.
  3. Laryngopharynx: Inferior portion from the hyoid to the entrance of the larynx and esophagus.

Larynx

• Inhaled air enters the larynx through the glottis.
• Cartilaginous cylinder from C4/C5 to C6.
• Unpaired Cartilages:
  1. Thyroid Cartilage: Largest, hyaline, U-shaped, forming anterior and lateral walls. Reference as Adam’s apple.
  2. Cricoid Cartilage: Complete hyaline ring with expanded posterior.
  3. Epiglottis: Elastic cartilage, shoe-horn shaped, forming a lid over the glottis to block food/liquids during swallowing.
• Paired Cartilages:
  1. Arytenoid Cartilages: Superior to the posterior cricoid.
  2. Corniculate Cartilages: Articulate with the superior arytenoid cartilages; involved with sound production.
  3. Cuneiform Cartilages: Lie within folds between arytenoids and epiglottis.
• Vestibular Folds: Inelastic projections protecting vocal folds.
• Vocal Folds: Elastic, involved in sound production (vocal cords). Air passing through the rima glottidis vibrates the folds.
  • Tense vocal folds: higher pitch.
  • Relaxed vocal folds: lower pitch.
  • Children: Short vocal cords cause a higher pitch.
  • Puberty (males): Larynx enlarges, vocal folds thicken and lengthen, producing lower tones.

Trachea

• Flexible tube inferior to the larynx (C6) ending at the mediastinum (T5) where it splits into primary bronchi.
• Contains 15-20 C-shaped tracheal cartilages to prevent collapse.
• Incomplete portion of rings posterior to accommodate esophageal expansion during swallowing.

Primary Bronchi

• Arise at the carina (internal ridge).
• Have C-shaped cartilaginous rings.
• The right primary bronchus is larger, descends at a steeper angle, and is where most foreign objects lodge.
• Primary bronchi travels to the hilum.
• The root of the lung contains vessels, lymphatics and primary enter the hilum.

Lobes of the Lungs

• Located in individual pleural cavities.
• Apex extends to the first ribs, bases sit on the diaphragm.
• Right Lung: Three lobes (superior, middle, inferior) separated by horizontal and oblique fissures.
• Left Lung: Two lobes (superior and inferior) separated by the oblique fissure.
• The right lung is broader, the left lung is longer.
• Costal Surface: Convex, following the rib cage.
• Mediastinal Surface: Irregular, contains the hilum.
• Cardiac Notch: Indentation on the left lung for the heart.

Bronchi

• Primary bronchi branch to form the bronchial tree.
• Each primary bronchus divides into secondary (lobar) bronchi, which supplies an individual lobe.
  • Right lung: 3 secondary bronchi.
  • Left lung: 2 secondary bronchi.
• Secondary bronchi divide into tertiary (segmental) bronchi.
• The walls of the bronchi contain less cartilage and more smooth muscle.
• Smaller bronchi are at risk for spasm and constriction (bronchitis).

Bronchioles

• Each tertiary bronchus branches into multiple bronchioles that form the terminal bronchioles.
• Bronchiole walls lack cartilage and are composed of smooth muscle to regulate air distribution and control resistance.
• Autonomic Nervous System (ANS) Regulation:
  • Sympathetic Activation: Relaxation of smooth muscle, bronchodilation.
  • Parasympathetic Activation: Constriction of smooth muscle, bronchoconstriction.
• Bronchoconstriction also results from allergic reactions (anaphylaxis) due to histamine release or asthma (paroxysmal spasms of smooth muscle).

Pulmonary Lobules

• Each lobe is divided into smaller compartments called pulmonary lobules by fine partitions called interlobular septa.
• Each lobule is supplied by a terminal bronchiole.
• Terminal bronchiole branches into respiratory bronchioles which supply each alveolus or multiple alveoli along alveolar ducts.
• These ducts end at alveolar sacs (common chambers connected to individual alveoli).

Alveoli

• Each lung contains about 150 million alveoli.
• A network of capillaries and elastic fibers surrounds each alveolus.
• Epithelium:
  • Pneumocytes Type I: Simple squamous cells.
  • Alveolar Macrophages (Dust Cells): Patrol the epithelium, phagocytizing particulate matter.
  • Pneumocytes Type II: Scattered among squamous cells, produce surfactant (oily secretion of lipids and proteins).
    • Surfactant coats alveoli to decrease surface tension and prevent collapse upon exhalation (respiratory distress syndrome).

Gas Exchange

• Occurs across the respiratory membrane of the alveoli.
• Components of the Respiratory Membrane:
  1. Squamous cells of the alveolus.
  2. Endothelial cells of adjacent capillaries.
  3. Fused basal laminae between alveolar and endothelial cells.
• Short distance (0.1 to 0.5 µm) allows rapid diffusion of O2 and CO2.
• In pneumonia, the lung lobules become infected and inflamed, forming liquid in the alveoli, severely compromising gas exchange.

Blood Supply to the Lungs

• Pulmonary arteries enter the hilum and branch with the bronchi as they approach the lobules.
• Each lobule receives an arteriole and venule.
• Arterioles branch into capillaries that surround each alveolus.
• Capillaries converge to form pulmonary venules.

Pulmonary Embolism

• Lungs are prone to blockage by blood clots (pulmonary emboli).
• Alveoli collapse causing decreased gas exchange and hypoxia.
• Negative feedback causes right ventricle to work harder, possible congestive heart failure (CHF).

Pleural Cavities and Membranes

• Each lung occupies a pleural cavity lined by the pleura (serous membrane).
• Layers:
  1. Parietal Pleura: Covers the inner surface of the thoracic wall, diaphragm, and mediastinum.
  2. Visceral Pleura: Covers the outer surface of the lungs, extending into the fissures.
• Pleural cavity: A potential space between the two layers filled with pleural fluid.
• Deficient pleural fluid results in pain and inflammation, called pleurisy, causing difficulty in breathing.

Respiration (Pulmonary Ventilation)

• Physical movement of air into and out of the respiratory tract.
• Airflow direction depends on the relationship between atmospheric pressure and intrapulmonary pressure.
• Air flows from higher pressure to lower pressure.
• A single respiratory cycle includes inspiration (inhalation) and expiration (exhalation).
• At the start of each breath, pressures inside and outside are the same.
• During inhalation, the diaphragm contracts and moves inferiorly: increase of volume and decreased pressure.
• Boyle’s Law: P = frac{1}{V}
• Thoracic cavity enlargement lowers intrapulmonary pressure; air rushes in until pressures equalize.
• Diaphragm relaxes and rises superiorly: volume decreases, intrapulmonary pressure increases, air pushed out.
• Tidal Volume: Amount of air moved during a single respiratory cycle.

Pneumothorax

• Chest wall injury (bullet, stabbing, rib fracture) penetrates the pleural cavity, and air fills the cavity causing lung collapse (atelectasis).
• Requires immediate sealing of the wound and air removal from the pleural cavity to avoid hypoxia and congestive heart failure (CHF).

Respiratory Muscles

• Primary: Diaphragm (75% of air movement) and external intercostal muscles (25%).
• Accessory (during exercise/stress): Internal intercostals, SCMs, serratus anterior, pectoralis minor, scalenes, and abdominal muscles.

Modes of Breathing

• Quiet Breathing (Eupnea):
  1. Diaphragmatic (deep) breathing: Diaphragm contraction.
  2. Costal (shallow) breathing: Rib cage shape change due to external costal muscle contraction/relaxation.
• Forced Breathing (Hyperpnea): Combination of primary and accessory muscle use.

Respiration Parameters

• Respiratory Rate: Breaths/minute.
  • Normal adult: 12-18 breaths/minute.
  • Children: 18-20 breaths/minute.
• Respiratory Minute Volume (V_E): Air moved into/out of the respiratory tract per minute.
  • Formula: VE = f shortmid (breaths/minute) shortmid * VT (tidal volume).
  • Normal V_E: About 6 liters/minute.

Spirometry

• Measurement of air capacity in the lungs useful in diagnosing problems with pulmonary ventilation.
• Measurements
  1. Resting Tidal Volume (V_T): Air moved during a single respiratory cycle at rest. Approximately 500mL for both males and females.
  2. Expiratory Reserve Volume (ERV): Amount of air voluntarily expelled after a quiet respiratory cycle.
     • Males: 1000mL.
     • Females: 700mL.
  3. Residual Volume: Air remaining in the lungs after maximal exhalation.
     • Males: 1200mL.
     • Females: 1100mL.
  4. Inspiratory Reserve Volume (IRV): Amount of air taken into the lungs above the tidal volume.
     • Males: 3300mL.
     • Females: 1900mL.
  5. Inspiratory Capacity: Air drawn into the lungs after a quiet respiratory cycle.
     • Inspiratory capacity = Tidal volume + IRV.
  6. Functional Residual Capacity (FRC): Air remaining in the lungs after a quiet respiratory cycle.
     • FRC = ERV + Residual volume.
  7. Vital Capacity: Maximum air moved in the lungs in a single respiratory cycle.
     • Vital capacity = ERV + Tidal volume + IRV.
     • Males: 4800mL.
     • Females: 3400mL.
  8. Total Lung Capacity: Total volume of the lungs.
     • Total lung capacity = Vital capacity + Residual volume.
     • Males: 6000mL.
     • Females: 4200mL.

Gas Exchange Principles

• Air is a gas mixture: Nitrogen (78.6%), Oxygen (20.9%), Water vapor and Carbon dioxide (0.5%).
• Atmospheric pressure (760 mm Hg): Combined effects of all molecular collisions.
• Dalton’s Law: Each gas contributes to the total pressure in proportion to its abundance.
• Partial pressure: Pressure contributed by a single gas in a mixture.

Atmospheric pressure (760 mm Hg) = P{N2} + P{O2} + P{H2O} + P{CO2}

• Henry’s Law: The amount of gas in solution is directly proportional to the partial pressure of that gas.
  • Gas under pressure contacts a liquid, pressure forces the gas molecules into the solution.
  • If the partial pressure goes up, more gas molecules go into solution, and vice versa.
  • When the pressures reach equilibrium the movement of gases into solution and out of solution are the same (soda can).

Gas Exchange Dynamics

• Blood arriving in pulmonary arteries has: lower PO2 and higher PCO2 than alveolar air.
• Diffusion between alveolar gas and capillaries increases PO2 of blood and lowers PCO2.
• Blood entering pulmonary venules reaches equilibrium: PO2 ≈ 100 mmHg, PCO2 ≈ 40 mmHg.
• Entering the left atrium and reaching peripheral tissues, partial pressures drop: PO2 is 95 mmHg and PCO2 is 40 mmHg.
• In interstitial fluid: PO2 is 40 mmHg and PCO2 is 45 mmHg.
• After gas exchange, venous blood partial pressures become PO2 = 40 mmHg and PCO2 = 45 mmHg

Oxygen and Carbon Dioxide Transport

• Oxygen Transport
  • Hemoglobin in RBCs binds oxygen, forming oxyhemoglobin.
  • Each RBC can carry over a billion oxygen molecules.
  • In capillaries, lower tissue oxygen concentration causes oxygen to dissociate from hemoglobin and be used by the body.
• Carbon Dioxide Transport
  • CO2, a product of aerobic respiration, attaches to hemoglobin forming carbaminohemoglobin, for delivery to the lungs.
  • Carbon monoxide competes with oxygen for Hb binding sites. Stronger CO affinity leads to oxygen deprivation and potential death.

Respiratory Centers of the Brain

Medulla Oblongata

• Dorsal Respiratory Group (DRG): Neurons innervating the diaphragm and external intercostal muscles (quiet breathing).
• Ventral Respiratory Group (VRG): Neurons innervating accessory muscles for forced breathing.

Pons

• Apneustic Centers: Regulate respiration rate and depth in response to sensory stimuli.
• Pneumotaxic Centers: Inhibit apneustic centers and promote exhalation.

Respiratory Reflexes

• Sensory Information Received By Respiratory Centers:
  1. Chemoreceptors: sensitive to PCO2, pH, or PO2 in blood or CSF.
  2. Baroreceptors: in aortic arch or carotid sinuses; sensitive to blood pressure changes.
  3. Stretch receptors: Respond to lung volume changes.
  4. Irritant receptors: Detect physical or chemical stimuli in the nasal cavity, larynx, or bronchial tree.
  5. Other sensations: Includes pain, temperature change, and abnormal visceral sensations.

Respiration and Aging

• Efficiency declines due to:
  • Deterioration of elastic tissue, decreasing vital capacity.
  • Arthritis in rib cage joints and calcification of costal cartilages.
  • Emphysema (common in people over 50 due to smoke or irritants).
• Impact of Smoking

Chronic Obstructive Pulmonary Disease (COPD)

• Combination of chronic bronchitis and emphysema.