Large organisms need a mass transport system like circulation to overcome the limitations of diffusion for transporting substances like oxygen and glucose throughout their bodies. For single-celled organisms, diffusion is sufficient but larger organisms have cells that are too far for diffusion to occur quickly enough. Circulation systems address this by using exchange surfaces, vessels, and a transport medium like blood to quickly bring substances from one exchange site to another while maintaining diffusion gradients and ensuring effective cell activity. Circulation can be open or closed and single or double, with double circulation in birds and mammals having advantages like preventing mixing of oxygenated and deoxygenated blood and delivering fully oxygenated blood quickly under pressure to tissues.

2. 1. The principles of
circulation
Chapter 1B

3. Learning objective
O Know why many animals have a heart and
circulation which act as a mass transport
system to overcome the limitations of
diffusion.

4. The need for transport
O In single celled organisms and
microscopic multicellular organisms,
diffusion is sufficient to supply all their
needs.
O For bigger organisms diffusion alone is
not enough.

5. O Single-celled organisms can gain oxygen and glucose
directly from their surroundings and the molecules
can diffuse to all parts of the cell quickly due
to short diffusion distances
O Larger organisms, however, are made up of many
layers of cells, meaning that the time taken for
substances such as glucose and oxygen to diffuse to
every cell in the body would be far too long
O The diffusion distances involved are too great
O Large organisms often have high energy
requirements, so the delivery of reactants for
metabolism needs to happen quickly

6. O To solve this problem their exchange surfaces
are connected to a mass transport system.

7. O Mass transport systems help to
O Bring substances quickly from one exchange site
to another
O Maintain diffusion gradients at exchange sites
and between cells and their fluid surroundings
O Ensure effective cell activity by supplying
reactants and removing waste products

8. Surface area to volume ratio
O Helps us to understand
the importance of this ratio
for diffusion, exchange of
materials.

9. Features of mass transport
systems
O Exchange surfaces to get materials
O A system of vessels
O Substances must move in the right direction
O Moving substances fast enough(by heart, by
gradient)
O Suitable transport medium
O A way of adapting the rate of transport to the needs
of organism

10. Circulatory systems
O Can be:
O 1. Open c.s.
O 2. Closed c.s.

12. Circulations
O Also can be:
O 1. Single circulation – only in fish
O 2. Double circulation – other vertebrates

14. Advantages of double circulation
in birds and mammals
O Ensures that oxygenated and
deoxygenated blood cannot mix.
O Fully oxygenated blood can be delivered
quickly to the body tissues at high
pressure.
O Can control pressure differences between
circulations.