Respiratory System PDF

Chapter 22 The Respiratory System Respiration Involves both the respiratory and the circulatory systems Four processes that supply the body with O2 and dispose of CO2 Respiration Pulmonary ventilation (breathing): movement of air into and out of the lungs External respiration: O2 and CO2 exchange be...

Chapter 22 The Respiratory System Respiration Involves both the respiratory and the circulatory systems Four processes that supply the body with O2 and dispose of CO2 Respiration Pulmonary ventilation (breathing): movement of air into and out of the lungs External respiration: O2 and CO2 exchange between the lungs and the blood Transport: O2 and CO2 in the blood Internal respiration: O2 and CO2 exchange between systemic blood vessels and tissues Respiratory system Circulatory system Respiratory System: Functional Anatomy Major organs – Nose, nasal cavity, and paranasal sinuses – Pharynx – Larynx – Trachea – Bronchi and their branches – Lungs and alveoli Pharynx Larynx Vocal Cords The major respiratory organs in relation to surrounding structures. Trachea Functional Anatomy Respiratory zone: site of gas exchange – Microscopic structures: respiratory bronchioles, alveolar ducts, and alveoli Conducting zone: conduits to gas exchange sites – Includes all other respiratory structures Respiratory muscles: diaphragm and other muscles that promote ventilation The Upper Respiratory Tract Bronchi and Subdivisions Air passages undergo 23 orders of branching Branching pattern called the bronchial (respiratory) tree Conducting Zone Structures Trachea → right and left main (primary) bronchi Each main bronchus enters the hilum of one lung – Right main bronchus is wider, shorter, and more vertical than the left Each main bronchus branches into lobar (secondary) bronchi (three right, two left) – Each lobar bronchus supplies one lobe Conducting Zone Passages Respiratory Zone Respiratory bronchioles, alveolar ducts, alveolar sacs (clusters of alveoli) ~300 million alveoli account for most of the lungs’ volume and are the main site for gas exchange Respiratory Zone Respiratory Membrane ~0.5-m-thick air-blood barrier Where alveolar and capillary walls meet Alveolar walls – Single layer of squamous epithelium (type I cells) Scattered type II cells secrete surfactant and antimicrobial proteins Alveoli Surrounded by fine elastic fibers Contain open pores that – Connect adjacent alveoli – Allow air pressure throughout the lung to be equalized House alveolar macrophages that keep alveolar surfaces sterile Respiratory Membrane Fissures Pleura of the Lungs Pleurae Thin, double-layered serosa Parietal pleura on thoracic wall and superior face of diaphragm Visceral pleura on external lung surface Pleural fluid fills the slitlike pleural cavity – Provides lubrication and surface tension Blood Supply Pulmonary circulation (low pressure, high volume) – Pulmonary arteries deliver systemic venous blood – Pulmonary veins carry oxygenated blood from respiratory zones to the heart Systemic circulation (high pressure, low volume) – Bronchial arteries provide oxygenated blood to lung tissue – Bronchial veins carry most venous blood back to the heart Mechanics of Breathing Pulmonary ventilation consists of two phases 1. Inspiration: gases flow into the lungs 2. Expiration: gases exit the lungs Pressure Relationships in the Thoracic Cavity Atmospheric pressure – Pressure exerted by the air surrounding the body – 760 mm Hg at sea level Respiratory pressures are described relative to 760 – Negative respiratory pressure is less than 760 – Positive respiratory pressure is greater than 760 Pressure Relationships Intrapulmonary Pressure and Intrapleural Pressure Intrapulmonary (intra-alveolar) pressure – Pressure in the alveoli – Fluctuates with breathing Intrapleural pressure: – Pressure in the pleural cavity – Fluctuates with breathing – Always a negative pressure Intrapleural Pressure Negative Intrapleural pressure is caused by opposing forces – Two inward forces promote lung collapse 1. Elastic recoil of lungs decreases lung size 2. Surface tension of alveolar fluid reduces alveolar size – One outward force tends to enlarge the lungs 1. Elasticity of the chest wall pulls the thorax outward Pulmonary Pressures Atelectasis Atelectasis: partial or complete collapse of the lung Causes – Plugged bronchioles → collapse of alveoli – Wound that admits air into pleural cavity (pneumothorax) Pulmonary Ventilation Pulmonary ventilation is inspiration and expiration Mechanical processes that causes volume changes in the thoracic cavity – Volume changes due to pressure changes – Pressure changes cause air flow in and out of the lungs – Pressure equalizes temporarily between inspiration and expiration Inspiration Inspiration is an active process – Inspiratory muscles contract – Thoracic volume increases – Lungs are stretched and intrapulmonary volume increases – Intrapulmonary pressure drops forming a negative pressure – Air flows into the lungs Expiration Quiet expiration is normally a passive process – – – – Inspiratory muscles relax Thoracic cavity volume decreases Elastic lungs recoil Intrapulmonary pressure increases forming a positive pressure – Air flows out of the lungs Note: forced expiration is an active process: it uses abdominal and internal intercostal muscles Pulmonary Ventilation Physical Factors Influencing Pulmonary Ventilation Inspiratory muscles consume energy to overcome three factors that hinder air passage and pulmonary ventilation 1. Airway resistance 2. Alveolar surface tension 3. Lung compliance Airway Resistance As airway resistance rises, breathing movements become more strenuous Severely constricting or obstruction of bronchioles – Can prevent life-sustaining ventilation – Can occur during acute asthma attacks and stop ventilation Epinephrine dilates bronchioles and reduces air resistance Alveolar Surface Tension Surface tension: attraction of liquid molecules to one another at a gas-liquid interface – Surface tension in the alveoli will cause a constriction of the alveoli Surfactant – Detergent-like lipid and protein complex – Reduces surface tension of alveolar fluid and discourages alveolar collapse Lung Compliance A measure of the change in lung volume that occurs with a given change in transpulmonary pressure Normally high due to – Distensibility of the lung tissue – Alveolar surface tension Diminished by – Nonelastic scar tissue (fibrosis) – Reduced production of surfactant – Decreased flexibility of the thoracic cage Respiratory Volumes and Capacities Nonrespiratory Air Movements Most result from reflex action Examples include: cough, sneeze, crying, laughing, hiccups, and yawns Gas Exchanges Between Blood, Lungs, and Tissues External respiration Internal respiration To fully understand the above processes, first consider – Physical properties of gases – Composition of alveolar gas Basic Properties of Gases: Dalton’s Law of Partial Pressures Dalton’s Law: total pressure exerted by a mixture of gases is the sum of the pressures exerted by each gas The partial pressure of each gas is directly proportional to its percentage in the mixture Basic Properties of Gases: Henry’s Law Henry’s Law: when a mixture of gases is in contact with a liquid, each gas will dissolve in the liquid in proportion to its partial pressure At equilibrium, the partial pressures in the two phases will be equal The amount of gas that will dissolve in a liquid also depends upon its solubility – CO2 is 20 times more soluble in water than O2 – Very little N2 dissolves in water External Respiration Exchange of O2 and CO2 across the respiratory membrane Influenced by – Partial pressure gradients and gas solubilities – Ventilation-perfusion coupling – Structural characteristics of the respiratory membrane Partial Pressure Gradients and Gas Solubilities Partial pressure gradient for O2 in the lungs is steep – Venous blood Po2 = 40 mm Hg – Alveolar Po2 = 104 mm Hg O2 partial pressures reach equilibrium of 104 mm Hg in ~0.25 seconds, about 1/3 the time a red blood cell is in a pulmonary capillary Partial Pressure Gradients and Gas Solubilities Partial pressure gradient for CO2 in the lungs is less steep: – Venous blood Pco2 = 45 mm Hg – Alveolar Pco2 = 40 mm Hg CO2 is 20 times more soluble in plasma than oxygen CO2 diffuses in equal amounts with oxygen Ventilation-Perfusion Coupling Ventilation: amount of gas reaching the alveoli Perfusion: blood flow reaching the alveoli Ventilation and perfusion must be matched for efficient gas exchange Thickness and Surface Area of the Respiratory Membrane Respiratory membranes – 0.5 m thick – Large total surface area (40 times that of one’s skin) Thicken if lungs become waterlogged and edematous, and gas exchange becomes inadequate Emphysema is a condition where walls of alveoli break down causing a loss of surface area for gas exchange O2 and Hemoglobin Loading and unloading of O2 is facilitated by change in shape of Hb – As O2 binds, Hb affinity for O2 increases – As O2 is released, Hb affinity for O2 decreases Fully (100%) saturated if all four heme groups carry O2 Partially saturated when one to three hemes carry O2 O2 and Hemoglobin Rate of loading and unloading of O2 is regulated by – Po2 – Temperature – Blood pH – Pco2 Influence of Po2 on Hemoglobin Saturation Only 20–25% of bound O2 is unloaded during one systemic circulation If O2 levels in tissues drop: – More oxygen dissociates from hemoglobin and is used by cells – Respiratory rate or cardiac output need not increase Other Factors Influencing Hemoglobin Saturation Increases in temperature, H+, Pco2, and BPG – Modify the structure of hemoglobin and decrease its affinity for O2 – Occur in systemic capillaries – Enhance O2 unloading Factors that Increase Release of O2 by Hemoglobin As cells metabolize glucose – Pco2 and H+ increase in concentration in capillary blood Declining pH weakens the hemoglobin-O2 bond (Bohr effect) – Heat production increases Increasing temperature directly and indirectly decreases Hb affinity for O2 External Respiration Internal Respiration CO2 Transport CO2 is transported in the blood in three forms – 10% dissolved in plasma – 20% bound to globin of hemoglobin (carbaminohemoglobin) – 70% transported as bicarbonate ions (HCO3–) in plasma Carbon Dioxide Transport Medullary Respiratory Centers 1. Dorsal respiratory group (DRG) – Near the root of cranial nerve IX – Integrates input from peripheral stretch and chemoreceptors 2. Ventral respiratory group (VRG) – Rhythm-generating and integrative center – Sets eupnea (12–15 breaths/minute) – Inspiratory neurons excite the inspiratory muscles via the phrenic and intercostal nerves Ventral and Dorsal Respiratory Groups Depth and Rate of Breathing Depth is determined by how actively the respiratory center stimulates the respiratory muscles Rate is determined by how long the inspiratory center is active Both are modified in response to changing body demands Rising CO2 levels are the most powerful respiratory stimulant Depth and Rate of Breathing Hyperventilation: increased depth and rate of breathing that makes the body remove too much CO2 – Causes CO2 levels to decline (hypocapnia) May cause cerebral vasoconstriction and cerebral ischemia Respiratory Adjustments: Exercise Adjustments are geared to both the intensity and duration of exercise Hyperpnea – Increase in ventilation (10 to 20 fold) in response to metabolic needs Homeostatic Imbalances Chronic obstructive pulmonary disease (COPD) – Exemplified by chronic bronchitis and emphysema – Irreversible decrease in the ability to force air out of the lungs – Other common features History of smoking in 80% of patients Dyspnea: labored breathing (“air hunger”) Coughing and frequent pulmonary infections Most victims develop respiratory failure (hypoventilation) accompanied by respiratory acidosis Homeostatic Imbalances Asthma – Characterized by coughing, dyspnea, wheezing, and chest tightness – Active inflammation of the airways precedes bronchospasms – Airway inflammation is an immune response caused by release of interleukins, production of IgE, and recruitment of inflammatory cells – Airways thickened with inflammatory exudate magnify the effect of bronchospasms Homeostatic Imbalances Lung cancer – Leading cause of cancer deaths in North America – 90% of all cases are the result of smoking – The three most common types 1. Squamous cell carcinoma (20–40% of cases) in bronchial epithelium 2. Adenocarcinoma (~40% of cases) originates in peripheral lung areas 3. Small cell carcinoma (~20% of cases) contains lymphocyte-like cells that originate in the primary bronchi and subsequently metastasize