Respiratory System: Processes and Components

Major Processes of the Respiratory System

• Ventilation involves moving air into and out of the lungs
• External respiration is the exchange of oxygen and carbon dioxide between air and alveoli, and alveoli and the blood supply
• Transport involves oxygen and carbon dioxide
• Internal respiration is the exchange of oxygen and carbon dioxide between blood and metabolically active cells
• Other functions of the respiratory system include speech generation, balancing blood pH, and odor detection

Overall Components of the Respiratory System

• The respiratory system is divided into the conducting division and the respiratory division
• The conducting division provides a passageway for air to move, warms and humidifies the air, and cleanses the air
• The respiratory division provides a site of gas exchange between the lungs and the blood
• There is also a separation of the systems into upper and lower divisions via the vocal cords

Structures of the Conducting Division: Mouth

• The mouth functions primarily as an organ of the digestive system
• It acts in parallel with the nose as a conduit for air entry and exit and is especially important when breathing under exertion
• It warms, humidifies, and cleanses air

Structures of the Conducting Division: External Nose Anatomy

• Apex: The tip of the nose
• Nares (nostrils): Openings to the nose
• They contain hair to filter large particles
• Ala nasi: Cartilaginous flap on the lateral side of each nostril
• Dorsum nasi: Length of nose
• Bridge: Connects the root to the rest of the nose
• Root: A region located between the eyebrows

Structures of the Conducting Division: Internal Structures of the Nose

• Nasal septum: Divides the nasal cavity into the left and right sides
• Superior, middle, and inferior conchae: Three pairs of C-shaped bones extending laterally from the nasal septum
• Meatuses: Passageways surrounding the conchae that allow for air to flow

Structures of the Conducting Division: Nasal Sinuses

• The nasal cavity connects to 4 pairs of sinuses that are air-filled spaces
• The sinuses warm and humidify incoming air
• They contribute to the resonance of voices

Structures of the Conducting Division: Pharynx

• The pharynx includes the nasopharynx, oropharynx, and laryngopharynx
• Nasopharynx: A conduit for air only, and adenoids lie in its posterior wall
• Oropharynx: A conduit for digestion and respiration with tonsils at the border
• Laryngopharynx: The shortest conduit for both air and food, opening into the larynx and esophagus

Structures of the Conducting Division: Larynx

• The larynx directs air into the trachea and food into the esophagus
• It contains the vocal cords
• The superior portion is lined with stratified squamous epithelium
• The inferior portion is lined with a mucous membrane for the movement of trapped debris into the pharynx for swallowing

Structures of the Conducting Division: Glottis and Laryngeal Cartilage

• Cartilage prevents the larynx from collapsing
• Large cartilage includes the epiglottis, thyroid cartilage, and cricoid cartilage
• Small cartilage includes the arytenoid, corniculate, and cuneiform cartilage
• The glottis functions as the larynx's vocal apparatus

Structures of the Conducting Division: Trachea

• The trachea extends from the larynx to its division into the left and right bronchi
• It contains hyaline cartilage between fibrous tissue ligaments
• The carina is a ridge of cartilage that senses solid or liquid substances and triggers violent coughing to expel the substances
• Ciliated cells on the interior form the mucociliary escalator

Structures of the Conducting Division: Bronchi Through Terminal Bronchioles

• Primary bronchi, secondary bronchi, tertiary bronchi, primary bronchioles, and terminal bronchioles make up the structure
• Bronchi are supported by cartilage while their interiors contain ciliated mucous cells
• Bronchioles lack cartilage and mucous, but feature smooth muscle and ciliated cells

Structures of the Respiratory Division: Respiratory Bronchioles and Alveolar Sacs

• Respiratory bronchioles have minimal smooth muscle
• Alveolar ducts are short conduits comprised of mainly connective tissue
• Alveolar sacs consist of grape-like clusters of individual alveoli with elastic structures that open from the alveolar ducts

Structures of the Respiratory Division: Alveoli

• Alveoli are structures across which gas exchange occurs
• They have thin walls with a large lumen
• They allow intimate contact between inhaled air and blood in pulmonary capillaries that wrap the alveolar walls
• The alveoli collectively have a surface area of 70 m2

Structures of the Respiratory Division: Alveolar Cells

• Type I alveolar cells: the most common type connected to a thin basement membrane with a pulmonary endothelial cell on the other side, which can be defined as "respiratory membrane."
• Type II alveolar cells: cuboidal cells that make and secrete surfactant which reduces surface tension between water molecules lining inner alveoli surfaces, being essential between at 24-28 weeks and before 37 weeks
• Type III alveolar cells (a.k.a. alveolar macrophages): resident alveolar immune cells that scavenge microorganisms and other particles
• Infant Respiratory Distress Syndrome (IRDS) is a condition related to these cells

Structures of the Respiratory Division: Lungs

• The lungs occupy most of the thoracic cavity and are encased by the pleural membrane
• The right lung has 3 lobes, while the left lung has 2
• Lobes are further divided into bronchopulmonary segments which are then divided into pulmonary lobules

Structures of the Respiratory Division: Lung Pleura

• Each lung is surrounded by a pleural membrane with two layers: visceral and parietal
• The visceral pleura tightly covers each lung
• The parietal pleura lines the inner wall of the thoracic cavity
• Pleural cavity: Small space between the layers containing pleural fluid which is secreted by mesothelial cells
• Fluid lubricates and provides a barrier

Pulmonary Blood Flow in the Lungs

• Deoxygenated blood flows in the pulmonary trunk, pulmonary arteries, lobar arteries, and capillary beds surrounding alveoli
• Oxygenated blood flows in venules, small veins, and pulmonary veins

Zones of Blood Flow in the Lungs

• The lungs have zones based on pressure differences caused by gravity
• Zone 1 pressure: Capillary pressure is less than alveolar pressure, causing them close to atmospheric pressure and help prevent flow
• Zone 2 pressure: Capillary pressure during systole is greater than alveolar pressure, and capillary pressure during diastole is less than alveolar pressure, causing collapse
• Zone 3 pressure: Capillary pressure is greater than alveolar pressure, causing continuous flow and adequate gas exchange, and ventilation needs to match perfusion (V/Q matching)

Pressure Differentials and Gradients

• Pressure differential: The difference in pressure between any two spaces occupied by gas or fluid, regardless of gas movement
• Pressure gradient between the atmosphere and alveoli: e.g., atmosphere pressure is 760 mmHg while the alveoli pressure is 758 mmHg, resulting in a 2mmHg pressure differential, and pressure flows from the atmosphere into alveoli
• Pressure gradient between the alveolar and pleural spaces: e.g., alveolar pressure is 760 mmHg while pleural pressure is 754 mmHg, resulting in a 6mmHg pressure differential with no pathway between alveoli and pleural space

Determinants of Gas Pressure

• Amount of gas particles: If the number of gas particles in a given volume increases, pressure increases
• Temperature: If temperature increases while all other factors remain the same, pressure increases
• Volume of space: If the volume increases while all other factors remain the same, pressure decreases

Determinants of Flow

• F = ΔP / R where F is the flow rate, ΔP is the pressure differential, and R is the resistance to flow

Respiratory Pressures

• Pressure differential between intrapulmonary and intrapleural pressures represents the force that tends to collapse the lungs

Process of Ventilation: Resting Breathing

• Just prior to inspiration, atmospheric pressure equals intrapulmonary pressure
• Lungs start to expand, so intrapulmonary pressure decreases
• Air moves from the atmosphere to the lungs and pressures return to equilibrium
• During expiration, thoracic volume declines and intrapulmonary pressure is greater than atmospheric pressure
• Air moves out of the lungs until the pressures equilibrate again

Non-Breathing Air Movements

• Coughing and sneezing function as a respiratory system effort trying to clear irritants from the airways
• Yawning results from the deepest possible breath
• Hiccups are spasms of the diaphragm that cause rapid bursts of air through the vocal cords
• Laughing/crying are emotional states that drive the ventilatory pattern
• Valsalva maneuver involves attempting to exhale against a closed airway

Gas Diffusion and Partial Pressures

• Equilibrium values for partial pressure are not the average of initial partial pressure values
• Larger partial pressure changes result from a greater percentage change in gas volumes
• With a higher solubility coefficient, a greater volume of gas is transferred into or out of the liquid
• Higher solubility coefficients are associated with a higher rate of diffusion
• The amount of gas in liquid is expressed as the equivalent volume transferred from the gas mixture

Gas Exchange in the Lungs (External Respiration) and Tissues (Internal Respiration)

• Diffusion is based on the gas' partial pressures
• In the lungs, oxygen moves from the air to the blood and carbon dioxide moves from the blood to the air
• In the tissues, oxygen moves from the blood to the tissue and carbon dioxide moves from the tissue to the blood

Transport of Oxygen in Blood by Hemoglobin

• Hemoglobin transports more than 98% of the blood's oxygen
• Each red blood cell (RBC) contains 250-300 million hemoglobin molecules
• Each red blood cell can carry up to 1.2 billion oxygen molecules

Carbon Dioxide Transport in Blood

• Haldane effect: Carbon dioxide more readily binds to unoxygenated hemoglobin
• 7% remains dissolved in plasma (as CO2)
• 23% binds to Hb, forming carbaminohemoglobin, Hb + CO2
• 70% converted to H2CO3 by carbonic anhydrase which dissociates into H+ and HCO3- resulting in 70% conversion to bicarbonate

Overall Control of Ventilation

• Sensors consist of Chemoreceptors that detect chemicals in blood (e.g., PaCO2, PaO2)
• Integrators consist of the respiratory center in the brainstem (neurons in the medulla oblongata and pons)
• Effectors consist of the diaphragm and intercostal muscles that control breathing

Brainstem Regions Involved in Control of Ventilation

• The respiratory center contains pattern-generating neurons with intrinsic depolarizing capacities
• Dorsal respiratory group: