BIOL 1911 Chapter Notes - Chapter 42.5: Trachea, Thoracic Cavity
29 Apr 2018
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42.5 – Gas exchange occurs across specialized respiratory
surfaces
Partial Pressure Gradients in Gas Exchange
• Gas exchange = uptake of molecular O2 from the environment and discharge of CO2 to the environment
o aka respiratory exchange or respiration
o NOT energy transformations cellular respiration
• Partial pressure = pressure exerted by a particular gas in a mixture of gases
• Determining partial pressures enables us to predict net movement of a gas at an exchange surface
o Net diffusion: always from a region of higher partial pressure to a region of lower partial pressure
Respiratory Media
• Conditions for gas exchange vary considerably depending on whether the respiratory medium – the source of O2 is air or
water
• Breathing air is relatively easy and does not need to be particularly efficient
• W’ O2 content, greater density, and greater viscosity mean that aquatic animals have to expend considerable
energy to carry out gas exchange
o Adaptations have evolved that enable most aquatic animals to be very efficient in gas exchange
o Many adaptations involve the organization of the surfaces dedicated to exchange
Respiratory Surfaces
• R = ’
o Specialization for gas exchange is apparent in the structure on the respiratory surface
• Respiratory surfaces are always moist
o Cells that carry out gas exchange have a plasma that must be in contact with an aqueous solution
• Movement of O2 and CO2 takes place by diffusion across respiratory surfaces
o Rate of diffusion proportional to the surface area across which it occurs and inversely proportional to the
square of the distance through which molecules must move
o i.e. gas exchange is fast when the area for diffusion is large and the path for diffusion is short
o therefore, respiratory surfaces tend to be large and thin
• in some relatively simple organisms, every cell in the body is close enough to the external environment that gases can
diffuse quickly between any cell and the environment
o sponges, cnidarians, flatworms
• , ’
o respiratory surface in these animals is a thin, moist epithelium that constitutes a respiratory organ
• skin serves as a respiratory organ in earthworms and some amphibians
o a dense network of capillaries just below the skin facilitates the exchange of gases between the circulatory
system and the environment
o general body surface lacks sufficient area to exchange gases for the whole organisms in most animals
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