Respiratory System Anatomy

Anatomy of the Respiratory System

• The respiratory system's main function is to supply oxygen to body tissues for cellular respiration.
• It also removes the waste product, carbon dioxide, and helps maintain acid-base balance.
• The system is also involved in non-vital functions like sensing odors, speech, and straining.
• Respiration requires four processes, utilizing both the respiratory and circulatory systems.
• Pulmonary ventilation (breathing) moves air into and out of the lungs, providing air to the alveoli for gas exchange.
• Pulmonary gas exchange involves exchanging gases with the external environment within the alveoli.
• Oxygen loads into the blood from the lungs, while carbon dioxide unloads from the blood into the lungs.
• Gas exchange and transport of gases occur as the cardiovascular system delivers oxygen-loaded blood to tissues, where oxygen is released and carbon dioxide is picked up.
• Tissue gas exchange is the actual exchange of gases within the internal environment; the blood "unloads" oxygen and "loads" carbon dioxide.
• The respiratory system is divided into a conducting zone and a respiratory zone.
• The conducting zone includes organs/structures not directly involved in gas exchange.
• Gas exchange occurs in the respiratory zone.
• The conducting zone provides a route for air, removes debris/pathogens, and warms/humidifies air.
• The nasal passages' epithelium is essential for sensing odors, while the bronchial epithelium can metabolize airborne carcinogens.

The Nose and its Adjacent Structures

• The nose is the major entrance/exit for the respiratory system and can be divided into the external nose and the nasal cavity (internal nose).
• The external nose consists of surface and skeletal structures contributing to appearance and functions.
• The apex is the tip of the nose, with nostrils (nares) formed by the alae on either side.
• The nares open into the nasal vestibule, the most anterior part of the nasal cavity.
• The nasal cavity is divided into left and right sections by the nasal septum formed anteriorly by septal cartilage and posteriorly by the ethmoid and vomer bones.
• Each lateral wall of the nasal cavity features the superior, middle, and inferior nasal conchae (bony projections).
• Superior and middle conchae are portions of the ethmoid bone, while the inferior conchae are separate bones.
• Conchae increase the surface area of the nasal cavity and disrupt airflow to filter air.
• Conchae and meatuses conserve water and prevent nasal epithelium dehydration during exhalation.
• The palate forms the nasal cavity floor; the hard palate (bone) is anterior, and the soft palate (muscle tissue) is posterior.
• Air exits through the posterior nasal apertures into the pharynx.
• Cleft palate results from incomplete fusion of the hard palate, causing communication between nasal and oral cavities.
• Paranasal sinuses, air-containing spaces in the bones forming the nasal cavity walls, warm/humidify air.
• Each paranasal sinus (frontal, maxillary, sphenoidal, ethmoidal) is named for its associated bone and lined with mucosa, producing mucus and lightening the skull's weight.
• The nares and anterior nasal cavities are lined with mucous membranes containing sebaceous glands and hair follicles that prevent large debris from entering.
• An olfactory epithelium, used to detect odors, is located deeper in the nasal cavity.
• Conchae, meatuses, and paranasal sinuses are lined by respiratory epithelium comprised of pseudostratified ciliated columnar epithelium plus goblet cells that produce mucus.
• Cilia remove mucus and debris towards the throat to be swallowed.
• Cold air slows cilia movement, causing mucus accumulation and runny nose.
• Moist epithelium warms and humidifies incoming air.
• Capillaries beneath the nasal epithelium warm the air through convection.
• Serous and mucus-producing cells secrete lysozyme and defensins (antibacterial properties).
• Immune cells in the connective tissue provide additional protection.

Pharynx

• The pharynx is a tube formed by skeletal muscle, lined by a mucous membrane continuous with nasal cavities.
• The three major regions of the pharynx are the nasopharynx, oropharynx, and laryngopharynx.
• The nasopharynx is posterior to the nasal conchae, serving only as an airway with pharyngeal tonsils at the top.
• The uvula, a teardrop-shaped structure at the soft palate's apex, moves with the soft palate to close off the nasopharynx during swallowing.
• Pharyngotympanic tubes connect to the middle ear cavities, allowing the inner ear to equalize with atmospheric pressure.
• The oropharynx serves as a passageway for both air and food, bordered by the nasopharynx, oral cavity, and epiglottis.
• It contains palatine and lingual tonsils.
• The laryngopharynx is inferior to the oropharynx and posterior to the larynx, routing air and ingested material until the digestive/respiratory systems diverge.
• The laryngopharynx opens into the larynx (anteriorly) or esophagus (posteriorly).

Larynx/Voice Box

• The larynx is a cartilaginous structure connecting the pharynx to the trachea, regulating the volume of air entering/leaving the lungs.
• The larynx consists of cartilage pieces, including three large ones: thyroid (anterior), epiglottis (superior), and cricoid (inferior) cartilage.
• The thyroid cartilage is the largest and includes the laryngeal prominence or "Adam's apple."
• The cricoid cartilage is a thick ring with a wide posterior region and a thinner anterior region.
• Smaller, paired cartilages (arytenoids, corniculates, cuneiforms) attach to the epiglottis and vocal cords, aiding in speech production.
• A cough reflex is triggered if anything other than air enters the larynx.
• The epiglottis, attached to the thyroid cartilage, is a flexible flap of elastic cartilage covering the trachea opening.
• When "closed," the epiglottis rests on the glottis.
• The glottis includes vestibular folds, true vocal cords, and the space between these folds.
• A vestibular fold (false vocal cord) is a mucous membrane pair.
• A true vocal cord is a white, membranous fold attached by muscle to the thyroid and arytenoid cartilages, which vibrate to produce sound.
• Vocal cord size varies among individuals, causing different pitch ranges in voices.
• Swallowing lifts the pharynx/larynx, causing the epiglottis to swing downward and close the trachea opening, preventing food/beverages from entering.

Trachea

• The trachea extends from the larynx toward the lungs, comprised of 16-20 stacked, C-shaped hyaline cartilage pieces connected by dense connective tissue.
• The cartilage rings support structure and prevent collapse, with the esophagus bordering posteriorly.

Bronchial Tree

• The trachea branches into the right and left main (primary) bronchi at the carina, lined by pseudostratified ciliated columnar epithelium with goblet cells.
• The carina contains specialized nervous tissue that triggers coughing if a foreign body is present.
• Rings of cartilage support the bronchi.
• Main bronchi enter the lungs at the hilum (a concave region) with blood/lymphatic vessels and nerves.
• Inside the lungs, bronchi branch into lobar (secondary) bronchi, then segmental (tertiary) bronchi.
• The bronchial tree is the collective term for these multiple-branched bronchi.
• Bronchi provide a passageway for air and trap debris/pathogens.
• Segmental bronchi branch into bronchioles (about 1 mm diameter), then into terminal bronchioles leading to gas exchange structures.
• There are over 1000 terminal bronchioles per lung.
• Muscular walls in the bronchioles, without cartilage, change tubing size to control airflow.

Respiratory Zone

• The respiratory zone includes structures directly involved in gas exchange.
• It begins where terminal bronchioles join respiratory bronchioles (the smallest type), leading to alveolar ducts that open into alveoli clusters.
• The alveolar duct is a tube of smooth muscle and connective tissue opening into alveoli clusters.
• An alveolus is one of grape-like sacs attached to alveolar ducts.
• An alveolar sac consists of numerous individual alveoli responsible for gas exchange.
• Alveoli (200 µm in diameter) have elastic walls that stretch during air intake, increasing surface area for gas exchange and are connected to neighbors by alveolar pores for equal pressure.
• The alveolar wall is comprised of type I alveolar cells (squamous epithelial cells, 97% of surface area), type II alveolar cells, and alveolar macrophages.
• Type II alveolar cells secrete pulmonary surfactant (phospholipids and proteins) reducing surface tension, and preventing alveolar collapse.
• Alveolar macrophages are phagocytic immune cells removing debris/pathogens.
• The pulmonary artery carries oxygen-poor arterial blood to the alveoli, branching into a pulmonary capillary network.
• At the capillary wall, the respiratory membrane is formed.
• Oxygenated blood drains via pulmonary veins, exiting the lungs through the hilum.
• The respiratory membrane, formed by type 1 alveolar and capillary membranes, is approximately 0.5 µm thick allowing gases to cross for oxygen and carbon dioxide exchange.

Gross Anatomy of the Lungs

• Lungs are pyramid-shaped, paired organs connected to the trachea by bronchi and bordered by the diaphragm that are attached to the mediastinum by the root of the lung.
• The right lung is shorter/wider; the left lung is smaller (cardiac notch).
• The apex is the lung's superior region, while the base is near the diaphragm; the costal surface borders the ribs, and the mediastinal surface faces the midline.
• Each lung is composed of lobes separated by fissures.
• The right lung has superior, middle, and inferior lobes separated by oblique and horizontal fissures.
• The left lung has superior and inferior lobes divided by an oblique fissure.
• A bronchopulmonary segment is supplied by its own tertiary bronchus and blood artery and is a division of a lobe.
• Diseases may affect one or more bronchopulmonary segments, which can be surgically removed.
• A pulmonary lobule is a subdivision where bronchi branch into bronchioles, which has one large bronchiole; an interlobular septum (connective tissue wall) separates lobules.
• The lungs are enclosed by pleurae (double-layered serous membrane) that attach to the mediastinum.
• The right and left pleurae enclose right and left lungs respectively, separated by the mediastinum and consist of two layers.
• The visceral pleura is the inner layer superficial to the lungs, lining the fissures.
• The parietal pleura is the outer layer connecting to the thoracic wall, mediastinum, and diaphragm.
• Visceral and parietal pleurae connect at the hilum, with the pleural cavity between them.
• Pleurae produce pleural fluid (lubricates surfaces, reduces friction), maintain lung position against the thoracic wall (surface tension), and create organ divisions (prevents interference/infection spread).

Mechanics of Pulmonary Ventilation

• Pulmonary ventilation (breathing) involves air movement into and out of the lungs.
• The major mechanisms for it are atmospheric, , intrapulmonary and intrapleural pressures.
• Atmospheric pressure (Patm) is the force exerted by gases in the air surrounding any surface (e.g., the body), measured in atm or mm Hg (1 atm = 760 mm Hg at sea level).
• Intrapulmonary pressure (Ppul) is the pressure within the alveoli, which changes during breathing phases and equalizes with atmospheric pressure.
• Intrapleural pressure (Pip) is the pressure within the pleural cavity (between visceral and parietal pleurae) that changes during breathing.
• Intrapleural pressure is lower (negative) than intra-alveolar/atmospheric pressure and remains about -4 mm Hg throughout the breathing cycle.
• Competing forces in the thorax create negative intrapleural pressure.
• The lungs' elasticity and alveolar fluid's surface tension pull the lungs inward, away from the thoracic wall.
• Opposing forces from the pleural fluid and thoracic wall counter this inward tension.
• Transpulmonary pressure (Ppul - Pip) is the difference between intrapulmonary and intrapleural pressure, which keeps airways open and prevents lung collapse (atelectasis), and must always be positive.
• Pulmonary ventilation has two steps that depend on pressure differences: Inspiration and Expiration.
• Inspiration moves air into the lungs, and expiration moves air out.
• A respiratory cycle involves one sequence of inspiration and expiration.
• Normal inspiration uses the diaphragm and external intercostal muscles, or inspiratory muscles.
• The diaphragm contracts and moves inferiorly, enlarging the thoracic cavity and lung space.
• External intercostals contract, moving ribs upward/outward, expanding the rib cage and thoracic cavity volume.
• Pleural fluid adheres, expanding the lungs.
• Increased volume decreases alveolar pressure below atmospheric pressure creating a gradient driving air into the lungs.
• Inspiration and expiration occur due to expansion and contraction of the thoracic cavity, respectively.
• Normal expiration is passive, not requiring energy, where the lungs recoil due to tissue elasticity as inspiratory muscles relax.
• Thoracic cavity and lungs decrease in volume, raising intrapulmonary pressure above atmospheric pressure causing a reverse pressure gradient driving air to leave the lungs.
• Quiet breathing occurs at rest without conscious thought requiring contraction of the diaphragm and external intercostals.
• Forced breathing happens during exercise or singing, needing active manipulation of breathing, so both inspiration and expiration occur due to muscle contractions.
• Accessory muscles contract during forced contraction.
• Neck muscles lift the thoracic wall, increasing lung volume during forced inspiration.
• Abdominal muscles (obliques) contract/force abdominal organs upward against the diaphragm during forced expiration which helps push the diaphragm further into the thorax, pushing more air out.
• Additional muscles (internal intercostals) compress the rib cage reducing the volume of the thoracic cavity.

Bell Jar Lung Model

• The bell jar represents the chest wall.
• The balloons represent the lungs, which are both elastic structures.
• The pressure inside the balloons represents Ppul.
• The stopper in the top represents the nose.
• The tubing the represents the airways.
• The latex sheet represents the diaphragm.
• The space represents the pleural cavity (place Pip would be measured).

Assessing Ventilation

• Lung volume measurement is useful for determining lung function and diagnosing pulmonary disease.
• A spirometer is used to measure respiratory volumes.
• Types of respiratory volumes:
  • Tidal volume (TV): The volume of air moving in/out during a resting respiratory cycle, which usually is 500 mL.
  • Inspiratory reserve volume (IRV): The volume of air able to be inspired after fully filling with tidal air. Its adult female average is 1900 mL, while the average for adult males is 3100 mL.
  • Expiratory reserve volume (ERV): The volume of air able to be expired following tidal volume after passive expiration. Its adult female average is 700 mL, while the average for adult males is 1200 mL.
  • Residual volume (RV): The air amount in the passageways/alveoli not able to be forcibly exhaled from the lungs is not able to be found when using a spirometer. Its adult female average is 1100 mL, while the average for adult males is 1200 mL.
• Averages for RV is able to be figured out through the equation: RV = VC x (factor)
  • Ages 16-34 factor: 0.250
  • Ages 35-49 factor: 0.305
  • Ages 50-69 factor: 0.455
• Types of respiratory capacities:
  • Vital capacity (VC): The air volume able to be forcefully exhaled while taking one breath following a maximal inspiration. The equation for it is TV + ERV + IRV, where the adult female average is 3100 mL, and the average for adult males is 4800 mL.
  • Inspiratory capacity (IC): The volume available taking a deep inhalation when expiring tidal air where IC - VC - ERV, its average value for adult females is 2400 mL, and the average for adult males being 3600 mL.
  • Functional residual capacity (FRC): The air amount left in the lungs when a normal tidal expiration happens is written as FRC = ERV + RV (avg. adult female 1800 mL; avg. adult male 2400 mL)
  • Total lung capacity (TLC): The total of every lung volume depicting lungs with air when a maximal effort to inspire takes place. Its equation is TLC = TV + IRV + ERV + RV (avg adult female value: 4200 mL; avg adult male value: 6000 mL)
• Other rates to note besides these, are respiratory rate (RR), breaths per minute; and minute ventilation rate (MVR), the exchanged air amount between the lungs and the environment done in 1 min. MVR = TV x RR

Pulmonary Function Tests

• A forceful test done for pulmonary function involves timed vital capacity exhalation (following maximal inhalation). About 80% of the capacity is exhaled when doing this is called FEV1 (forced expiratory volume within 1 sec).
• Tests for this may be used in clinic to find the differences between restrictive and obstructive lung disorders.
• Total lung capacity is smaller due to structural/functional changes when restrictive orders are involved (like tuberculosis and fibrosis), and inflammation, inflation and deflation are diminished. FEV1 will remain at or be more than 80% of VC when lung expansion is limited and ERV. IRV, TLC and VC diminish.
• Obstructive disorders (asthma and COPD with bronchitis and emphysema) occurs when airflow into the lungs are obstructed, but airway resistance is higher. Airways shrink due to asthma and chronic bronchitis. Loss of elasticity and worsening of the alveoli occurs when emphysema impacts. Bronchioles usually collapse forcing air volumes into the lungs reducing ELV with FEV1. Lungs hyperinflation rises RV with TLC and a barrel chest to occur. FEV1 is less than 80% of VC, when speaking restrictively, but is greater than 80% of it for restrictive disorders.