The exchange of respiratory gases in living organisms
O2 in, CO2 out. Simple description of why O2 required, and CO2 produced
during respiration. Diffusion. Description and explanation of Fick's Law.
Plants - spongy mesophyll - Thin, moist cell walls, large surface area in
direct contact with air spaces, stomata. Lenticels. Roots have large surface
area. Cuticle prevents the diffusion of gases over the surface on the aerial
parts of the plant.
Small animals, eg unicellular amoebae, Platyhelminthes exchange gases
by diffusion alone, because relatively inactive or small volume:surface area.
Cnidaria eg. jellyfish only two layers (diploblastic) are respiratory active,
simple diffusion is sufficient.
Larger & more active animals diffusion not adequate for cells’ needs,
because of distance between gas exchange surface and other cells.
Large animals can obtain sufficient oxygen by having a body with high
SA:volume, or with a blood system. This increases diffusion rate by
maintaining concentration gradient.
Gas exchange surfaces have:
a)large surface area:volume
b)rich blood supply (except insects)
c)thin walls – shorter diffusion distance
d)moist surfaces – dissolving respiratory gases helps them to diffuse more
easily
Examples of organisms:
Annelids - body surface.
Fish – gills. Flow of blood in gill plates is opposite to that of water passing
over gills, therefore countercurrent set up – most oxygenated water next to
least oxygenated blood and vice versa. Continuous flow maintained by
pumping of water over gills by muscles. Ram ventilation.
Humans – lungs. Alveoli – 1 cell thick walls, richly supplied with blood from
capillaries. Blood from pulmonary artery high in CO2 and low in O2, gases
diffuse along gradients from air in alveoli. Breathing mechanism needed to
exchange air in lungs – diaphragm and intercostal muscles. In: diaphragm
down, external muscles contract. Out: diaphragm down, internal muscles
contract.
Insects – tracheal system. NO BLOOD involved in gas transport. Air-filled
tubes called trachae – intucking of epidermis, lined by cuticle, thickened by a
spiral of chitin. Along sides of body are spiracles, which can be opened and
closed. Trachae divide into smaller tracheoles, ends of which are filled with
fluid. Each tracheole ends in an individual cell. In tissues partial pressure O2
lower than atmospheric pp, O2 diffuses along trachae. Most CO2 leaves body
via spiracles. Trachae of large insects often ventilated by abdominal muscles
pumping air.