Anatomy of the Respiratory System and Fetal Development

Fetal Development of the Respiratory System

• Phase 1: Embryonic Period

  • Occurs during the 26th week after fertilization.

  • Lungs first appear as small buds arising from the esophagus.

  • Develops into both right and left lung buds.

  • Features the appearance of tracheal cartilage.

  • Mainstem and bronchi appear, with two on the left and three on the right.

  • Branches begin to form: $10$ on the right and $9$ on the left.

• Phase 2: Pseudoglandular Period

  • Occurs between the 6th and 16th week of development.

  • Segmental, subsegmental, and all lung segments are established.

  • Goblet cells become present in the trachea and large bronchi.

  • Mucous glands begin to appear.

  • By the 16th week, approximately $20$ generations of airways are present.

• Phase 3: Canalicular Period

  • Occurs between the 17th and 26th week of gestation.

  • Terminal bronchioles begin to proliferate (grow or multiply).

  • Primitive bronchioles begin to appear.

  • The lungs become highly vascularized and lobes become clearly recognizable.

  • Lymphatic vessels begin to appear during week $20$.

• Phase 4: Terminal Sac Period

  • Starts at week $26$ and continues until full term ($40$ weeks).

  • The entire acinus develops.

  • Type I and Type II alveolar cells become visible.

  • Surfactant begins to appear.

  • Capillaries begin to appear by the 24th week, although air-blood interfaces are not yet complete.

• Phase 5: Alveolar Period

  • Starts at $32$ weeks and continues through childhood.

  • Alveolar-capillaries increase to approximately $300 \times 10^{6}$.

  • Between week $26$ and $28$, the air-blood interface becomes sufficient and pulmonary surfactant levels increase enough to support life.

  • By the 34th week, respiratory acini are developed.

Fetal Physiological Statistics and Components

• Lung Mechanics of the Newborn

  • Average lung compliance: $0.005\,L/cm\,H_2O$.

  • Airway resistance: $30\,cm\,H_2O/L/sec$.

• Airway Milestones

  • 16th week of gestation: $20$ generations of bronchial airways.

  • Mature lung: $28$ generations of airways.

  • 26th week of gestation: Air-blood interface and surfactant levels are usually sufficient to sustain life.

• Anatomy of the Placenta and Cord

  • The placenta consists of $15$ to $20$ segments called cotyledons.

  • Deoxygenated blood is carried from the fetus via two umbilical arteries.

  • Oxygenated fetal blood flows out of the chorionic villi and returns to the fetus via one umbilical vein.

• Maternal and Fetal $PO_2$ Differences

  • Three factors responsible for the large difference between maternal and fetal $PO_2$:

    • The placenta is an actively metabolizing organ.

    • The permeability (ease of passage) of the placenta varies by region regarding respiratory gases.

    • Presence of fetal and maternal vascular shunts (blood routing).

Transition at Birth

• Removal of Lung Fluid After Birth

  • Three mechanisms remove fluid within the first $24$ hours of life:

    • $\frac{1}{3}$ of fluid is squeezed out as the infant passes through the birth canal.

    • $\frac{1}{3}$ is absorbed by the pulmonary capillaries.

    • $\frac{1}{3}$ is removed by the lymphatic system.

Clinical Connections and Diagnostic Procedures

• Amniocentesis

  • Procedure used to diagnose fetal abnormalities during early pregnancy.

  • Results evaluate several markers:

    • Fetal lung maturity.

    • Bilirubin level (evaluates Rh incompatibility).

    • Creatinine level (assesses kidney function and maturity).

    • Genetic disorders (cellular examination).

    • Presence of meconium (the first fetal stool, a thick, sticky, greenish-black material).

  • Meconium-stained amniotic fluid indicates fetal asphyxia and potential meconium aspiration.

• Respiratory Distress Syndrome (RDS)

  • Approximately $50\%$ of infants born between $26$ and $28$ weeks' gestation develop RDS.

  • Approximately $25\%$ of infants born between $30$ and $31$ weeks' gestation develop RDS.

  • The condition occurs because infants are born before Type II alveolar cells are mature enough to produce adequate pulmonary surfactant.

Anatomy of the Adult Respiratory System

• The Nose

  • Primary functions: Filter inspired air, humidify inspired air, and condition inspired air.

  • Vibrissae: Hair follicles in the nasal vestibule acting as filters.

  • Turbinates: Responsible for humidification and warming of inspired air.

• The Pharynx

  • Divided into three parts: Nasopharynx, Oropharynx, and Laryngopharynx.

  • Pharyngotympanic (Eustachian) tubes: Located in the lateral walls; connect the nasopharynx to the middle ear to equalize pressure.

• The Larynx

  • Primary functions: Passageway for air between the trachea and pharynx, protection against aspiration of solids and liquids, and sound generation for speech.

  • Glottis: The space between true vocal cords; the narrowest part of the adult airway.

  • Valsalva's Maneuver: Undifferentiated movement of laryngeal walls and vocal cords (true and false) to the midline. The glottis closes tightly during physical work (lifting, pushing, coughing) to prevent air from escaping.

Airway Structure and the Tracheobronchial Tree

• Airway Zones

  • Conducting Zone: Airway generations $0$ through $19$; spans from the trachea to the terminal bronchioles.

  • Respiratory Zone: Airway generations $20$ through $28$; the site of gas exchange.

  • Airway Types: Cartilaginous and Non-cartilaginous.

• Tracheobronchial Tree Layers

  • Epithelial lining.

  • Lamina propria.

  • Cartilaginous layer.

• Mucociliary Escalator

  • Mechanism: Cilia move in an upward wavelike motion through the sol layer, striking the inner gel layer approximately $1500$ times per minute. This pushes the mucus and foreign particles toward the larynx at a rate of $2\,cm/min$. A cough eventually moves material into the oropharynx.

  • Factors slowing the escalator: Cigarette smoke (cilia paralysis), dehydration, positive pressure ventilation (dry air, e.g., CPAP), endotracheal suctioning, high oxygen concentrations, atmospheric pollutants, and general anesthetics.

Immunology and Alveolar Cells

• Types of Immunity

  • Cellular Immunity: Sensitized lymphocytes responsible for tissue rejection and destroying infected or abnormal cells.

  • Humoral Immunity: Circulating antibodies involved in allergic reactions (e.g., allergic asthma) against environmental antigens like pollen, dander, and feathers.

• Mast Cell - Antibody Response

  • Mast cells are located in the lamina propria throughout smooth muscle bundles.

  • Activation releases substances that alter bronchial airway diameter.

  • Released substances: Histamine, leukotrienes, heparin, platelet-activating factor (PAF), and eosinophilic chemotactic factor of anaphylaxis (ECF-A).

• Pneumocytes

  • Type I (Squamous) Pneumocytes: Form $95\%$ of the alveolar epithelium surface area.

  • Type II (Granular) Pneumocytes: Form $5\%$ of the surface area; source of pulmonary surfactant which decreases surface tension.

Pulmonary Vascular and Lymphatic Systems

• Vascular Structures

  • Arteries: Stiff vessels for high-pressure systemic transport.

  • Arterioles: Resistance vessels that regulate blood distribution; supply nutrients to respiratory bronchioles, alveolar ducts, and alveoli.

  • Capillary: Site of gas exchange; endothelium is selective for water, electrolytes, and sugars.

  • Venules: Continuous with capillaries; empty into veins.

  • Veins: Capacitance vessels that hold large blood volumes at low pressure; four large pulmonary veins empty into the left atrium.

• Lymphatic System

  • Vessels remove excess fluid and protein leaking from pulmonary capillaries.

  • Distribution: More frequent in lower lobes than upper lobes; more frequent in the left than the right.

Neurological Control and Gross Anatomy

• Autonomic Nervous System (ANS)

  • Sympathetic Division: Fight or flight; increases heart rate and blood pressure, relaxes bronchial smooth muscle, decreases GI activity.

  • Parasympathetic Division: Decreases heart rate and blood pressure, constricts bronchial smooth muscle, increases GI activity.

• Lung Landmarks

  • Apices: Tops of the lungs at the level of the first rib.

  • Base: Bottoms of the lungs; anteriorly at the 6th rib/xiphoid process; posteriorly at the 11th rib.

  • Hilum: Center of the mediastinal border where bronchi, blood vessels, lymph vessels, and nerves enter.

• Pleural Conditions

  • Pleural Cavity: Potential space between visceral and parietal pleurae.

  • Pneumothorax: Air in the pleural space.

  • Hemothorax: Blood in the pleural space.

  • Empyema: Pus in the pleural space.

Musculature of Ventilation

• Thoracic Cage

  • Sternum: Comprised of the manubrium, body, and xiphoid process.

  • Diaphragm: Major muscle of ventilation; innervated by the phrenic nerves.

• Accessory Muscles of Inhalation

  • Scalene muscle.

  • Sternocleidomastoid muscle.

  • Pectoralis major muscles.

  • Trapezius muscles.

  • External intercostal muscles.

• Accessory Muscles of Exhalation

  • Internal intercostal muscles (located beneath external intercostals; not in the abdomen).