Respiratory System: Upper and Lower Tracts

• There are two divisions of the respiratory system; the upper and lower respiratory tracts.
• Objectives are to describe the tracts, discuss gas exchange, define lung volumes, and discuss respiration regulation.

Upper Respiratory Tract

• Structures are outside the chest cavity.
• Includes the nose, nasal cavities, pharynx, larynx, and upper trachea all serve as air passages.
• Hair inside nostrils block entry of dust.
• Nasal mucosa is ciliated epithelium with goblet cells
• Nasal mucosa warms & moistens incoming air.
• Mucus traps dust & microorganisms, with cilia sweeping to the pharynx.
• Olfactory receptors respond to vapors in inhaled air.
• Paranasal sinuses open into the nasal cavities.
• Paranasal function is to lighten the skull & provide resonance for voice.
• Pharynx is posterior to nasal & oral cavities.
  • Nasopharynx sits above the soft palate, which blocks it during swallowing and is a passageway for air only.
  • Eustachian tubes from middle ears open into the nasopharynx.
  • Lymph node on the posterior wall is called an Adenoid.
  • Oropharynx is behind the mouth and is a passageway for both air & food.
  • Palatine tonsils are on the lateral walls of the oropharynx.
  • Laryngopharynx is a passageway for both air & food.
  • Laryngopharynx opens anteriorly into the larynx & posteriorly into the esophagus.
• Larynx is the voice box and airway between the pharynx & trachea and consists of 9 cartilages.
  • Thyroid cartilage is largest & most anterior.
  • Epiglottis is the uppermost cartilage and covers the larynx during swallowing.
  • Vocal cords are lateral to the glottis, which is the opening for air.
  • When speaking, vocal cords are pulled across the glottis & vibrated by exhaled air producing sound.
• Trachea: Extends from larynx to primary bronchi.
  • The C-shaped tracheal wall cartilages keep it open.
  • The mucosa being ciliated epithelium with goblet cells so Cilia sweeps mucus, trapped dust, & microorganisms upward to pharynx.

Lower Respiratory Tract

• Structures are within chest cavity.
• Lower trachea, lungs (including bronchial tubes & alveoli), pleural membranes, diaphragm, and intercostal muscles.
• Bronchial Tree extends from trachea to alveoli.
  • Right & left primary bronchi are branches of trachea; one to each lung.
  • Secondary bronchi goes to the lobes of each lung (3 right, 2 left).
  • Bronchioles have no cartilage in their walls.
• Lungs extend from the diaphragm up to level of clavicles.
  • The rib cage protects lungs from mechanical injury.
  • Hilus: indentation on medial side.
  • Site of entry of primary bronchus, pulmonary artery & veins, and bronchial vessels.
• Pleural membranes are serous membranes of the thoracic cavity: Parietal pleura lines the chest wall while Visceral pleura covers lungs.
  • Serous fluid is between layers preventing friction & keeps membranes together during breathing.
• Alveoli: Site of gas exchange with Simple squamous epithelium thinnest permitting diffusion.
  • Surrounded by pulmonary capillaries.
  • Pulmonary arterioles constrict in response to hypoxia of poorly ventilated alveoli which shunts blood to better-ventilated alveoli.
  • Elastic connective tissue is important for normal exhalation.
  • Each alveolus is lined by thin layer of tissue fluid mixed with surfactant that decreases surface tension & permits inflation of alveoli.
  • Macrophages & neutrophils phagocytize foreign material.

Mechanism of Breathing

• Ventilation:
  • Movement of air in & out of lungs includes inhalation & exhalation.
  • Ventilation is regulated respiratory centers in medulla & pons.
• Key respiratory muscles: Diaphragm and external intercostal muscles.
• Pressures:
  • Atmospheric pressure (air pressure) is 760 mm Hg at sea level.
  • Intrapleural pressure inside potential pleural space is always slightly below atmospheric pressure.
  • Intrapulmonic pressure is inside the bronchial tree & alveoli and fluctuates during breathing.
• Inhalation:
  • Medulla triggers motor impulses via phrenic nerves to stimulate the diaphragm.
  • Diaphragm contracts and moves down & flattens.
  • Impulses sent along intercostal nerves to external intercostal muscles.
  • The contraction pulls ribs up & out.
  • Chest cavity expands with parietal pleura with Visceral pleura adhering to parietal pleura & also expands to causes lung expansion.
  • Intrapulmonic pressure decreases enabling air rushes into lungs.
• Exhalation is normally passive.
  • Motor impulses from medulla decreases diaphragm & external intercostals causing them to relax.
  • Chest cavity becomes smaller, compressing lungs.
  • Elastic lung tissue recoils further compressing alveoli which increases intrapulmonic pressure and forces air out of lungs.
• Forced exhalation: Utilize accessory muscles of expiration.
  • Internal intercostals pull ribs down & inward.
  • Abdominal muscles force diaphragm upward.

Pulmonary Volumes

• Lung capacity varies with size & age and diminishes as we age due to loss of tissue elasticity and decreased efficiency of respiratory muscles.
• Tidal volume (TV) the amount of air inhaled & exhaled in normal quiet breathing is ~ 500 mL.
• Minute respiratory volume (MRV): Amount of air inhaled & exhaled in 1 minute equal to TV x number of respirations per minute.
  • Average respiratory rate is 12 to 20 per minute: 500 mL x 12 breaths/min = 6000 mL.
  • Shallow breathing associated with smaller TV requires RR to achieve necessary MRV. - Inspiratory reserve volume (IRV): Volume that can be inhaled beyond 2000 - 3000 mL normal IR.
  • Expiratory reserve volume (ERV): Volume that can be exhaled beyond TV is 1000 - 1500 mL normal ER.
  • Residual volume (RV) the amount of air left in lungs after max forceful exhalation is 1000 - 1500 mL.
  • RV ensures there is some air in lungs and maintains continuous exchange of gases.
• Inspiratory capacity is equal to TV + IRV, this is the amount of air that can be inhaled beginning from tidal exhalation.
  • Functional residual capacity (FRC): RV + ERV which is amount of air remaining in lungs following tidal exhalation.
  • Vital capacity (VC) is the amount of air in lungs under volitional control, TV + IRV + ERV. Average VC is 3500 - 5000 mL.
  • Total lung capacity (TLC): TV + IRV + ERV + RV.

Gas Exchange

• Gas Exchange in lungs & within body tissues.
• Air when breathed in holds ~ 21% O₂ & 0.04% CO₂ and when exhaled holds ~ 16% O₂ & 4.5% CO₂ so some oxygen is retained within the body & CO2 is produced by cells that are exhaled.
  • External respiration is Exchange of gases between air in alveoli & blood in pulmonary capillaries.
  • Internal respiration is the exchange of gases between blood in systemic capillaries & interstitial fluid.
• Diffusion of gasses is the partial pressure reflecting concentration of a gas in a particular site.
  • Partial pressure is equal to % of gas in mixture x total pressure so for example: O₂ in the atmosphere is 21% × 760 mm Hg = 160 mm Hg (PO₂).
• External respiration is present when air in alveoli has high PO₂ & low PCO₂ while Blood in pulmonary capillaries (contains blood from body) has low PO₂ & high PCO₂.
  • O₂ diffuses from air in alveoli to blood and CO₂ diffuses from blood to air in alveoli.
  • Blood that returns to heart now has high PO₂ & low PCO₂.
• Internal respiration is present when arterial blood in systemic capillaries has high PO₂ & low PCO2 and Body cells & tissue fluid have low PO₂ & high PCO2.
  • O₂ diffuses from blood to interstitial fluid and CO₂ diffuses from interstitial fluid to blood.
  • Blood of systemic veins returning to heart has low PO₂ & high PCO₂.

Transport of Gasses in the Blood

• Some O₂ is dissolved in blood plasma (~ 1.5% of total O₂ transported) but most is transported in blood bonded to hemoglobin in RBCs.
• Oxygen-hemoglobin bonds form in the lungs and bonds are relatively unstable bonds to allow O₂ to readily dissociate when passing through tissues with low PO₂.
  • The lower the PO₂ in a tissue, the more O₂ is released ensuring adequate oxygenation of active tissues.
  • A high PCO₂ (a lower pH) & high tissue temperature also increase oxygen release.
• Some CO₂ is dissolved in blood plasma & some is bound to hemoglobin(carbaminohemoglobin) accounting for ~ 20% total CO2 transport.
  • Most CO₂ which, is transported in plasma in form of bicarbonate ions (HCO3-): CO₂ enters RBCs and carbonic anhydrase catalyzes the rxn of CO2 & H₂O to form carbonic acid: CO2 + H2O → H₂CO₃.
  • Bicarbonate ions diffuse out of RBCs into plasma leaving H+ in RBCs where Hemoglobin acts as buffer to prevent acidosis from the H+.
  • Cl- ions shift from plasma into RBCs maintains ionic equilibrium and Rxns are reversed when blood reaches lungs.
  • CO₂ reforms & diffuses into alveoli.

Nervous System Regulation of Respiration

• The medulla contains inspiration & expiration centers where the Inspiration center automatically generates impulse in rhythmic spurts.
  • Impulses travel to respiratory muscles causing contraction & subsequent lung expansion.
  • Receptors in lung tissue detect stretching & send impulses to medulla to depress inspiration center to prevent overinflation of the lungs called the Hering-Breuer inflation reflex.
  • Expiration center is stimulated by inspiration center when forceful exhalations are needed and Generates impulses to internal intercostal & abdominal muscles.
• The pons is a key factor:
  • Apneustic center prolongs inhalation.
  • Pneumotaxic center helps bring about exhalation and both work with inspiration center to produce normal breathing rhythm.
  • The hypothalamus influences changes in breathing in emotional situations, the cerebral cortex permits voluntary changes in breathing and Reflex centers in medulla.
  • Coughing & sneezing to remove irritants from the upper respiratory tract.

Chemical Regulation of Respiration

• Chemoreceptors: Detect changes in blood gases & location.
  • Located in carotid & aortic bodies, as well as medulla.
  • Chemoreceptors in medulla detect increased blood CO2 levels triggering increased respiration to exhale more CO₂.
  • The key is that CO₂ is the major regulator of normal respiration:
    • Excess CO₂ decreases the pH of body fluids.
    • Hypercapnia is abnormally elevated CO2.
• Oxygen becomes a major regulator of respiration when the central chemoreceptors are desensitized to CO2 as seen in severe, chronic pulmonary disease.
  • Decreased blood O₂ (hypoxemia) detected by chemoreceptors in carotid & aortic bodies.
  • Medulla increases RR or depth (or both) to bring more air into lungs.

Explore the respiratory system's upper and lower tracts, including structures and functions. Learn about gas exchange, lung volumes, and the regulation of respiration. Discover the roles of the nose, pharynx, and trachea in air passage.