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6.4 Gas Exchange Some basic ideas: We have to breathe so that we can exchange the carbon dioxide that our cells produce during cell respiration for the oxygen that our cells need for cell respiration • ventilation – the exchange of air between our lungs and the atmosphere • Gas exchange – the back and forth diffusion of gases – Occurs in our lungs (exchange with bloodstream) – Occurs in capillary beds (exchange with surrounding cells) Why do we need a Ventilation System? • To maintain the concentration gradients of oxygen and carbon dioxide in the alveoli. • The body needs oxygen to make ATP via cell respiration. • The body needs to get rid of carbon dioxide which is a product of cell respiration. • Oxygen needs to diffuse from the alveoli into the blood. Carbon dioxide needs to diffuse from the blood into the alveoli. • To do so there must be a high oxygen concentration and a low carbon dioxide concentration in the alveoli. • A ventilation system makes this possible by getting rid of the carbon dioxide in the alveoli and bringing in more oxygen. Tim & Moby: Respiratory System Anatomy of the Respiratory System 1. Nose/Mouth – brings in/removes gases 2. Pharynx – back of your throat where air and food will be separated so that food goes down esophagus and air down trachea 3. Trachea – transports air to and from lungs 4. Bronchi – the branching airway to each lung 5. Bronchioles – smaller branches in lungs leading to alveoli 6. Alveoli – air sacks where the diffusion of gases takes place Ventilation Process The alveoli • Alveoli are found in clusters at the ends of the smallest bronchioles (think about a bunch of grapes) • Surrounding each cluster of alveoli is a capillary bed • Blood entering the capillary beds is from the pulmonary arteries (CO2-rich/O2-poor) and here the exchange takes place; now the blood (O2-rich/CO2poor) can return to the heart via the pulmonary veins • The gases must each diffuse through two membranes (phospholipid bilayers) – The cell membrane of the alveoli – The cell membrane of the capillary • As long as you continue breathing then you will maintain the proper concentration gradients of O2 and CO2 Summary of Features of Alveoli that allow for Efficient Gas Exchange: • Great numbers increase the surface area for gas exchange. • Wall made up of single layer of cells and so are the walls of the capillaries so diffusion distance is small allowing rapid gas exchange. • Covered by a dense network of capillaries which have low oxygen and high carbon dioxide concentrations. This allows oxygen to diffuse into the blood and carbon dioxide to diffuse out of the blood. • Some cells in the walls secret fluid allowing gases to dissolve. Fluid also prevents the sides of alveoli from sticking together. This process works like Boyle’s Law : P1 x V1 = P2 x V2 Process of Breathing: Volume/Pressure Relationship Inspiration (air in)/Expiration (air out): Cycle: Relaxed state: diaphragm and intercostal muscles relaxed Inspiration: diaphragm contracts (pulling muscle down), intercostal muscles contract elevating chest wall and expanding volume of chest which lowers pressure in lungs therefore pulling in air Expiration: muscles relax, diaphragm resumes dome shape, intercostal muscles allow chest to lower resulting in increase of pressure in chest and expulsion of air Video Clip Review Video