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Respiratory System

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Western University
Physiology 1021
Sarah Mc Lean

MODULE 10 The Respiratory SystemThe functions of the respiratory system include the following The transport of oxygen from the air into the bloodThe removal of carbon dioxide from the blood into the airThe control of blood acidity pHTemperature regulationForming a line of defense to airborne particlesAnatomyThe lungs are located in the thoracic cavity and are surrounded by the rib cage and the diaphragm The airways consist of the nasal cavity and the mouth which join together at the pharynx The pharynx leads into the larynx or voice box which then becomes the trachea The trachea divides into two main bronchi left and right which continually divide into smaller and smaller bronchioles These bronchioles continually divide and eventually end in the alveoli which are the site of gas exchange in the lung Blood Vessels and Blood FlowThe pulmonary artery which delivers deoxygenated blood to the lungs branches extensively to form a dense network of capillaries around each alveolus The structure of the capillaries and blood flow characteristics maximize gas exchange These characteristics include thin endothelial walls large total crosssectional area and a very low blood velocity Hence in the capillaries oxygen diffuses into the blood while carbon dioxide diffuses out From the capillaries the oxygenrich blood flows back to the left side of the heart through the pulmonary vein Histological Structure of an AlveolusThere are roughly 300 million alveoli in a healthy human lung each with a diameter of roughly 03 mm 1100 of an inch The walls of the alveoli are one cell thick and are composed of alveolar epithelial cells type I cells Type II cells secrete a fluid called surfactant that lines the alveoli Large numbers of capillaries surround the alveoli in close proximity The region between the alveolar space and the capillary lumen is the respiratory membrane This membrane which can have a thickness as narrow as 03 microns is where gas exchange takes place between air and blood Cells of the immune system including macrophages and lymphocytes protect the body from airborne particles that make their way into the alveoli Fibers of elastin and collagen are present in the walls of the alveoli around blood vessel and bronchi Pressures of the LungsIntrapleural PressureIn order to understand inspiration inhaling and expiration exhaling we must understand the environment of the lungs There are two thin pleural membranesone lines and sticks to the ribs the parietal pleura while the other surrounds and sticks to the lungs the viceral pleura These two layers of membrane form the intrapleural space which contains a very small amount of pleural fluid roughly 1015ml This fluid reduces friction between the two pleural membranes during breathing Due to their nature and attached muscle the ribs tend to spring outward while the lungs due to the presence of elastin tend to recoil and collapse Alveolar and Atmospheric PressureThe pressure inside the lungs is called the alveolar pressure or intrapulmonary pressure and the pressure in the intrapleural space is called the intrapleural pressure The atmospheric pressure outside the body is 760 mmHg at sea level Between breaths the alveolar and atmospheric pressures are the same at 760 mmHg 0 difference while the pressure in the intrapleural space is roughly 756 mmHg a difference of 4 mmHg The chest wall and lungs moving in opposite directions cause this lower intrapleural pressure Transpulmonary PressureThe transpulmonary pressure which is the difference between the alveolar and intrapleural pressures can be calculated using equation 1The transpulmonary pressure is important because this difference in pressure across the alveoli and intrapleural space holds the lungs open We can calculate the transpulmonary pressure using the values below and equation 1 aboveIn a healthy set of lungs the transpulmonary pressure is positive outward and keeps the lungs and alveoli open
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