Chapter 16 Respiratory System Pulmonary Ven8la8on Overview of Respira on External respira on encompasses four process Pulmonary ven la on movement of air into and out of the lungs Exchange of oxygen and CO2 between lung air spaces and blood by di usion Transporta on of oxygen and CO2 between lungs and body ssues Exchange of oxygen and CO2 blood and ssues by di usion Anatomy of the Respiratory System Air gets into and out of the lungs by way of the upper airways that lead to the respiratory tract The respiratory tract can be func onally divided into two components a conduc8on zone and a respiratory zone The Conduc ng Zone The upper part of the respiratory tract func ons in conduc ng air from the larynx to the lungs As the trachea enters the thoracic cavity it divides into leH and right bronchi that conduct air to each lung Each secondary bronchus divides into smaller ter ary bronchi which in turn divide into successively smaller resul ng in approximately 8 million tubules some less than 0 5 mm in diameter The Respiratory Zone The site of gas exchange Arrange in such a way to maximize surface area and minimize thickness such that di usion of oxygen and carbon dioxide between air and blood is facilitated The bronchioles terminate in alveolar ducts which lead to alveoli the primary structures where gas exchange occurs The Respiratory Zone The wall of an alveolus consists of primarily of a single layer of epithelial cells called type I alveolar cells Together the capillary and alveolar wall form a barrier called the respiratory membrane 0 2 m thick that separates air from blood Also located in the alveoli are type II alveolar cells and macrophages which engulf foreign par cles inhaled into the lungs Blood Supply to the Lungs The capillary supply of blood to the lungs is so extensive that it is oHen thought of in terms of a sheet of blood surrounding the alveoli LeH resin cast of pulmonary arteries and bronchi Blood Vessels are in red airways in white Right Scanning electron micrograph of capillaries around the alveoli Structures of the Thoracic Cavity Structures of the chest wall are designed to protect the lungs The internal and external intercostals located between the ribs and the diaphragm are the muscles used in breathing Because the chest wall forms a con nuous barrier around the lungs the compartment enclosing the lungs is air ght Forces for Pulmonary Ven la on Ven la on occurs because of the presence of pressure gradients between the alveoli and the outside air or atmosphere Air moves down this gradient from an area of high pressure to an area of low pressure How are these pressure gradients created Boyle s Law The rela onship between pressure and volume follows Boyle s law which states For a given quan ty of any gas in an air ght between container The rela onship he pressure ptressure and is inversely related to P 1 V Mechanics of Breathing The process of inspira on is ini ated by neural s mula on of the inspiratory muscles Contrac on of the diaphragm causes it to aEen and move downward while contrac on of external intercostals causes the ribs to pivot up and out The combined e ect is that this increases the volume in the thoracic cavity Mechanics of Breathing During quiet breathing expira on is a passive process by simply relaxing to allow the elas c chest and lungs to recoil to their res ng posi ons A more forceful expira on can be produced by contrac on of expiratory muscles which produces a greater and more rapid decrease in the volume of the thoracic cavity How do these pressure changes a ect alveolar pressure Forces for Pulmonary Ven la on Atmospheric pressure Patm is the pressure of the outside air around 760 mm Hg at sea level Intra alveolar pressure Palv pressure of air within the alveoli which varies during the phases of ven la on Changes in Alveolar Pressure Before inspira on at rest intra alveolar pressure is 0 mm Hg During inspira on expansion of the lungs causes the Palv to decrease Air ow then increases the gas in the lungs which increases the pressure During expira on the lungs collapse inward causing Palv to increase Air ows out of the lungs down a pressure gradient Forces for Pulmonary Ven la on Intrapleural pressure Pip is the pressure inside the pleural space which contains uid Varies with ven la on Transpulmonary pressure is the di erence between Palv Pip It is a measure of the force across the lungs Pneumothorax At rest the intrapleural pressure is 4mm Hg In order to maintain this pressure the pleural sac must be air ght If it is broken the pressure will equilibrate with atmosphere Without nega ve force of intrapleural pressure drawing the lungs outward the lung collapses due to elas8c recoil while the chest wall expands
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