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 (CO 2 -rich/O 2 -poor) and here the exchange takes place; now the blood (O 2 -rich/CO 2 - poor) 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 O 2 and CO 2

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. Summary of Features of Alveoli that allow for Efficient Gas Exchange:

This process works like Boyle’s Law : P 1 x V 1 = P 2 x V 2

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 Process of Breathing: Volume/Pressure Relationship Video Clip

Review Video