Gases move by:

1. Gases move by: • Bulk Transport: flow of a gas or of a medium • in which the gas is dissolved • Diffusion: the net movement of a substance • from a region of high concentration to a region • of low concentration

2. concentration gradient Ds Dt DC Dx = -DA Ci amount of s moving per unit time - rate of diffusion C DC Co diffusion coeff. area of barrier Dx x Fick’s Law of Diffusion

3. Ds Dt DC Dx = -DA Co C DC Ci Dx x

4. Ds Dt DC Dx = -DA Co C DC Ci Dx x

5. 6K • PO2 VO2 r = • • VO2 = rate of O2 consumption = 0.001 ml cm-3 min-1 E. Newton Harvey - Limitations of diffusion for the delivery of O2 to tissues r = 0.12 cm = 1.2 mm K = amt. of O2 that will diffuse across 1cm2/min if grad. is 1 atm/cm = 11 x 10-6 cm2 atm-1min-1 r

6. Larger organisms could obtain O2 by diffusion alone if tissues are no more than ~1mm from the surface, e.g. flatworms How can larger organisms augment diffusion in order to get O2 to tissues that are deeper than 1mm from the surface?

7. Mechanisms for augmenting diffusion • Specialized respiratory structures • gills, lungs, tracheae • Circulation of internal fluids • bulk transport • must deliver to within ~1mm (capillaries are only 0.1 to 0.3 mm apart) • Respiratory pigments • increase O2-carrying capacity of fluid

8. Ds Dt DC Dx = -DA Respiratory Structures What common characteristics would you employ if you were designing a respiratory structure for an organism? • Large surface area • Thin • Well ventilated • Well vascularized and perfused

9. Contrasts between Aquatic and Aerial Respiration Aquatic Aerial Viscosity 100x air Density 1000x air Diffusion rate 10,000x H2O O2 content 0-10ml/l >100ml/l CO2 capacity ~air ~water Mode of ventilation continuous tidal O2 consumption of pump (% of total) ~20 ~1-2 % O2 extraction ~80 ~25