1Lecture 17The Respiratory SystemThe Evolution of Lungs Most of the primitive phyla of organisms obtain oxygen by directdiffusion from seawater Aquatic animals possess special respiratory organs called gills Terrestrial arthropods use a network of air ducts called trachea Terrestrial vertebrates use respiratory organs called lungs Amphibians on land are able to respire through moist skin However, the main respiration route is the lung A sac with a convoluted internal membrane Reptiles are more active so they need more oxygen But they cannot respire through skin Instead, their lungs contain many more small chambers, greatly increasing the surface area Mammals have an even greater oxygen demand because they maintain a constant body temperatureThey increase the lung surface area even more Alveoli: Small chambers in interior of lung Bronchioles: Short passageways connecting clusters of alveoliRespiration in Terrestrial Vertebrates2Respiratory System Conducting zone  Provides rigid conduits for air to reach the sites of gas exchange Includes all other respiratory structures (e.g., nose, nasal cavity, pharynx, trachea) Respiratory zone Site of gas exchange  Consists of bronchioles, alveolar ducts, and alveoli Respiratory muscles diaphragm and other muscles that promote ventilationThe Pathway of Air Air normally enters through the nostrils It passes to the larynx (voice box) and then the trachea And then through the bronchus to the lungs A pair of lungs hang free in the thoracic cavity An air tube called abronchus connects each lung to a trachea Lungs contain millions of alveoli Sites of gas exchange between air and blood The thoracic cavity is bounded on the bottom by a thick layer of muscle called the diaphragmVocal Cords Composed of elastic fibers that form mucosal folds called true vocal cords The medial opening between them is the glottis They vibrate to produce sound as air rushes up from the lungs False vocal cords Mucosal folds superior to the true vocal cords Have no part in sound production3The Pleurae Thin, double-layered membrane  Parietal pleura Covers the thoracic wall and top of the diaphragm Continues around heart and between lungs Visceral, or pulmonary, pleura Covers the external lung surface Divides the thoracic cavity into three chambers The central mediastinum Two lateral compartments, each containing a lung How the Lungs Work Two forces act to pull the lungs away from the thoracic wall, promoting lung collapse Elasticity of lungs causes them to assume smallest possible size Surface tension of alveolar fluid draws alveoli to their smallest possible size Opposing force – elasticity of the chest wall pulls the thorax outward to enlarge the lungs Transpulmonary pressure keeps the airways open Transpulmonary pressure = difference between the intrapulmonary and intrapleural pressures How breathing works Breathing – Active pumping of air in and out of lungs During inhalation Diaphragm contracts and flattens Chest cavity expands downwards and outwards This creates negative pressure in lungs and air rushes in During exhalation Diaphragm relaxes Volume of chest cavity decreases  Pressure in lungs increases and air is forced out4The Lungs & DiaphragmDiaphragmNote how it is dome shaped rather than flat LungsThe Mechanics of Breathing In a human, a typical breath at rest moves about 0.5 liters of air called the tidal volume When each breath is completed, the lung still contains a volume of air (~ 1.2 liters) called the residual volume Each inhalation adds from 500 milliliters (resting) to 3,000 milliliters (exercising) of additional air Each exhalation removes approximately the same volume as inhalation addedOverview of Respiratory Gas ExchangePlayRespiratory Gas Exchange5Hemoglobin Oxygen moves within the circulatory system carried piggyback on the protein hemoglobin Hemoglobin contains iron, which combines with oxygen in a reversible way Hemoglobin bind O2within red blood cells (RBCs) This causes more to diffuse in from blood plasma In the lungs, most hemoglobin molecules carry a full load of O2 As cells metabolize glucose, carbon dioxide is released into the blood causing: Increases in PCO2and H+concentration in capillary blood Declining blood pH (acidosis) weakens the hemoglobin-oxygen bondTransport and Exchange of Oxygen Nitric oxide (NO) is a vasodilator that plays a role in blood pressure regulation Hemoglobin is a vasoconstrictor and a nitric oxide scavenger (heme destroys NO) However, as oxygen binds to hemoglobin: Nitric oxide binds to an amino acid on hemoglobin  Bound nitric oxide is protected from degradation by hemoglobin’siron The nitric oxide is released as oxygen is unloaded, causing vasodilation As deoxygenated hemoglobin picks up carbon dioxide, it also binds nitric oxide and carries these gases to the lungs for unloadingHemoglobin-Nitric Oxide Partnership6 Carbon dioxide is transported in the blood in three forms Dissolved in plasma – 7 to 10%  Chemically bound to hemoglobin – 20% is carried in RBCs as carbaminohemoglobin Bicarbonate ion in plasma – 70% is transported as bicarbonate (HCO3–)  Carbon dioxide diffuses into RBCs and combines with water to form carbonic acid (H2CO3), which quickly dissociates into hydrogen ions and bicarbonate ions In RBCs, carbonic anhydrase reversibly catalyzes the conversion of carbon dioxide and water to carbonic acidTransport and Exchange of Carbon DioxideBicarbonate ionHCO3–Hydrogen ionH+↔↔↔↔+ H2OWater↔↔↔↔Carbonic acidCarbon dioxide+H2CO3CO2Transport and Exchange of Carbon Dioxide At the tissues: Bicarbonate quickly diffuses from RBCs into the plasma The chloride shift – to counterbalance the outrush of negative bicarbonate ions from the RBCs, chloride ions (Cl–) move from the plasma into the erythrocytesTransport and Exchange of Carbon Dioxide At the lungs, these processes are reversed Bicarbonate ions move into the RBCs and bind with hydrogen ions to form carbonic acid Carbonic acid is then split by carbonic anhydrase to release carbon dioxide and water Carbon dioxide then diffuses from the blood into the alveoli7 The carbonic acid–bicarbonate buffer system resists blood pH changes If hydrogen ion