Pulmonary System Overview

Overview of the Pulmonary System

• A description of the upper and lower respiratory tracts needs defining
• A discussion of gas exchange and transport should be made
• Lung volumes and capacities need to be defined
• The regulation of respiration needs to be talked about, specifically by the nervous system and chemical mediation

Divisions of the Respiratory System

• The pulmonary system consists of the upper and lower respiratory tracts and the respiratory muscles like the diaphragm and intercostal muscles
• The upper respiratory tract includes structures outside the chest cavity, such as the nose, nasal cavities, pharynx, larynx, and upper trachea
• The lower respiratory tract includes structures within the chest cavity, like the lower trachea, lungs (bronchial tubes and alveoli), pleural membranes, diaphragm, and intercostal muscles

Functions of the Upper Respiratory Tract

• The nose has hairs in the nostrils to block dust
• The nasal cavities contain nasal mucosa, which is ciliated epithelium with goblet cells
• The nasal meatus warms and moistens incoming air
• Mucus in the nose traps dust and microorganisms, and cilia sweep them toward the pharynx
• Olfactory receptors in the nose respond to vapors in inhaled air
• Paranasal sinuses open into nasal cavities and lighten the skull and provide resonance for voice
• The pharynx is posterior to nasal and oral cavities
• The nasopharynx is above the soft palate, which blocks it during swallowing, and serves as a passageway for air only
• Eustachian tubes from middle ears open in the nasopharynx
• Adenoid is a a lymph nodule on the posterior wall
• The oropharynx is behind the mouth and serves as a passageway for both air and food; it contains palatine tonsils on the lateral walls
• The laryngopharynx serves as a passageway for both air and food
• The laryngopharynx opens anteriorly into the larynx and posteriorly into the esophagus
• The larynx is the voice box and airway between the pharynx and trachea
• The larynx consists of 9 cartilages
• The thyroid cartilage is the largest and most anterior
• The epiglottis is the uppermost cartilage that covers the larynx during swallowing
• Vocal cords are lateral to the glottis, which is the opening for air
• When speaking, the vocal cords are pulled across the glottis and vibrated by exhaled air to produce sound
• The trachea extends from the larynx to the primary bronchi
• C-shaped cartilages in the tracheal wall keep the trachea open
• The trachea's mucosa is ciliated epithelium with goblet cells
• The cilia sweeps mucus, trapped dust, and microorganisms upward to the pharynx

Functions of the Lower Respiratory Tract

• The bronchial tree extends from the trachea to the alveoli
• The bronchial tree consists of the right and left primary bronchi, which are branches of the trachea that go to each lung, and secondary bronchi
• Secondary bronchi go the the lopes of each lung, with 3 on the right and 2 on the left
• Bronchioles have no cartilage in their walls
• Lungs extend from the diaphragm up to the level of the clavicles
• The rib cage protects the lungs from mechanical injury
• Hilus is an indentation on the medial side of each lung, which serves as the site of entry for the primary bronchus, pulmonary artery and veins, and bronchial vessels
• Pleural membranes are serous membranes of the thoracic cavity
• The parietal pleura lines the chest wall
• The visceral pleura covers the lungs
• Serous fluid between the pleural layers prevents friction and keeps the membranes together during breathing
• Alveoli are the site of gas exchange
• They consist of simple squamous epithelium, whose thinness permits diffusion
• Alveoli are surrounded by pulmonary capillaries
• The pulmonary arterioles constrict in response to hypoxia of poorly ventilated alveoli, which shunts blood to better-ventilated alveoli
• Elastic connective tissue between alveoli is important for normal exhalation
• Each alveolus is lined by a thin layer of tissue fluid mixed with surfactant, which decreases surface tension and permits inflation of alveoli
• Macrophages and neutrophils phagocytize foreign material in the alveoli

Mechanism of Breathing

• Ventilation involves the movement of air in and out of lungs
• Ventilation consists of inhalation and exhalation and is regulated by, respiratory centers in the medulla and pons
• Diaphragm and external intercostal muscles are key respiratory muscles
• At sea level, atmospheric pressure, or air pressure, is 760 mm Hg
• Intrapleural pressure within the potential pleural space is always slightly below atmospheric pressure ("negative")
• Intrapulmonic pressure within the bronchial tree and alveoli fluctuates during breathing
• Inhalation: The medulla triggers motor impulses via phrenic nerves to stimulate the diaphragm, which contracts and moves down and flattens
• Impulses are sent along intercostal nerves to external intercostal muscles, whose contraction pulls ribs up and ou
• The chest cavity expands with parietal pleura
• The visceral pleura adheres to parietal pleura and also expands
• All of this causes lung expansion
• Intrapulmonic pressure decreases, causing air rushes into lungs
• Exhalation is normally passive
• Motor impulses from medulla decrease, which makes the diaphragm and external intercostals relax
• The chest cavity becomes smaller, which compresses the lungs
• Elastic lung tissue recoils further, compressing alveoli
• Because intrapulmonic pressure increases, air is forced out of lungs
• Forced exhalation utilizes accessory muscles of expiration
• Internal intercostals pull ribs down and inward
• Abdominal muscles force diaphragm upward

Pulmonary Volumes and Capacities

• Lung capacity varies with size and age of an individual; generally, lung capacities are smaller as people age
• Taller people have larger lungs
• Lung capacity diminishes with age due to loss of tissue elasticity and decreased efficiency of respiratory muscles
• Tidal volume (TV): The amount of air inhaled and exhaled in normal quiet breathing, TV = ~ 500 mL
• Minute respiratory volume (MRV): The amount of air inhaled and exhaled in 1 minute. MRV is calculated by multiplying TV by the number of respirations per minute
• Avg RR is 12 to 20 per minute
• Shallow breathing is associated with smaller TV
• As a result, it requires RR to achieve the necessary MRV
• Inspiratory reserve volume (IRV) is the volume that can be inhaled beyond TV; Normal IR is 2000 - 3000 mL
• Expiratory reserve volume (ERV) is the volume that can be exhaled beyond TV; Normal ER is 1000 - 1500 mL
• Residual volume (RV) is the amount of air left in lungs after max forceful exhalation; Avg range is 1000 - 1500 mL
• RV ensures there is some air in lungs at all times, which maintains continuous exchange of gases
• Inspiratory capacity is TV + IRV
• Inspiratory capacity is defined as the amount of air that can be inhaled beginning from tidal exhalation
• Functional residual capacity (FRC) is RV + ERV
• FRC is defined as the Amount of air remaining in lungs following tidal exhalation
• Vital capacity (VC) is the amount of air in lungs under volitional control; VC is TV + IRV + ERV; Average VC is 3500 - 5000 mL
• Total lung capacity (TLC) is TV + IRV + ERV + RV

Process of Gas Exchange

• Exchange occurs in lungs and within body tissues
• Air breathed in is about ~ 21% O2 & 0.04% CO2
• Air exhaled is ~ 16% O2 & 4.5% CO2
• Some oxygen is retained within the body, and CO2 produced by cells is exhaled
• External respiration is the exchange of gases between air in alveoli and blood in pulmonary capillaries
• Internal respiration is the exchange of gases between blood in systemic capillaries and interstitial fluid
• Diffusion of gases reflects concentration of a gas in a particular site, known as partial pressure
• Partial pressure is the % of gas in mixture × total pressure; for example, the partial pressure of O2 in the atmosphere is calculated with, 21% × 760 mm Hg = 160 mm Hg (PO2)
• Air in alveoli has high PO2 & low PCO2, the blood in pulmonary capillaries has low PO2 & high PCO2; therefore, O2 diffuses from air in alveoli to blood, and CO2 diffuses from blood to air in alveoli
• By the time blood returns to the heart it has high PO2 & low PCO2
• In internal respiration, arterial blood in systemic capillaries has high PO2 & low PCO2, while body cells & tissue fluid have low PO2 & high PCO2
• Therefore O2 diffuses from blood to interstitial fluid, & CO2 diffuses from interstitial fluid to blood
• This means blood of systemic veins returning to heart has low PO2 & high PCO2

Transport Of Gases In The Blood

• Some O2 is dissolved in blood plasma (~ 1.5% of total O2 transported)
• Most O2 is transported in blood bonded to hemoglobin in RBCs
• The Oxygen-hemoglobin bond forms in lungs
• A relatively unstable bond allows O2 to readily dissociate when passing through tissues with low PO2
• The lower the PO2 in a tissue, the more O2 is released which ensures adequate oxygenation of active tissues
• A high PCO2 (a lower pH) and high tissue temperature also increase oxygen release
• Some CO2 is dissolved in blood plasma, and some is bound to hemoglobin (carbaminohemoglobin) which accounts for about ~ 20% total CO2 transport
• Most CO2 is transported in plasma in the form of bicarbonate ions (HCO3–)
• CO2 enters RBCs, where carbonic anhydrase catalyzes the reaction of CO2 and H2O to form carbonic acid: CO2 + H2O → H2CO3
• Carbonic acid then dissociates: H2CO3 → H+ + HCO3–
• In the red blood cells, bicarbonate ions diffuse out of RBCs into the plasma, leaving H+ in RBCs
• Hemoglobin acts as a buffer to prevent acidosis from the H+
• Cl- ions shift from plasma into RBCs to maintain ionic equilibrium
• Reactions are reversed when blood reaches the lungs; this means CO2 reforms and diffuses into alveoli

Nervous System Regulation of Respiration

• The medulla contains inspiration & expiration centers
• The inspiration center (dorsal respiratory group) automatically generates impulse in rhythmic spurts, which travel to respiratory muscles causing contraction & subsequent lung expansion
• Receptors in lung tissue detect stretching & send impulses to medulla to depress inspiration center
• These receptor mediated responses make up the Hering-Breuer inflation reflex, which prevents overinflation of the lungs
• The expiration center (ventral respiratory group) is Stimulated by the inspiration center when forceful exhalations are needed, and it generates impulses to internal intercostal & abdominal muscles
• The pons, which is superior to the medulla, contains an apneustic center, which prolongs inhalation, and pneumotaxic center, which helps bring about exhalation
• The apneustic and pneumotaxic centers both work with the inspiration center to produce normal breathing rhythm
• The hypothalamus influences changes in breathing in emotional situations, and the cerebral cortex permits voluntary changes in breathing
• Reflex centers in medulla cause coughing and sneezing which remove irritants from upper respiratory tract

Chemical Regulation of Respiration

• Chemoreceptors detect changes in blood gases and pH that are located, located in carotid and aortic bodies, as well as medulla
• Chemoreceptors in medulla detect increased blood CO2 levels, which triggers increased respiration to exhale more CO2; therefore, CO2 is the major regulator of normal respiration
• Excess CO2 decreases the pH of body fluids, because CO2 + H2O → H2CO3 → H+ + HCO3–
• Excess H+ ions lower pH and can lead to acidosis
• Having abnormally elevated CO2 leads to hypercapnia
• Oxygen becomes a major regulator of respiration when central chemoreceptors are desensitized to CO2
• The state of being desensitized to CO2 occurs when the patient has Severe, chronic pulmonary disease
• When blood O2 is too low (hypoxemia) it is detected by chemoreceptors in carotid and aortic bodies
• This detection of hypoxemia causes sensory impulses travel to medulla, which increases RR or depth (or both) to bring more air into lungs