The circulatory system
The need for transport systems
In single-celled organisms such as protists, and small multicellular organisms, essential molecules will move to where they're needed by diffusion.
Once an organism, eg a very early embryo, gets beyond a certain size, it needs a transport system to distribute essential molecules efficiently.
Features of transport systems
Common features of transport systems:
- they are made up of tubes or 'vessels' that carry materials from one part of the organism to another
- they make close contact with cells, such as those of exchange surfaces
What substances are transported in the blood?
|Oxygen||The lungs||All the body's cells|
|Carbon dioxide||All the body's cells||Lungs|
|Glucose||Digestive system||The liver, then all the body's cells|
Other substances transported in the blood include:
Oxygen is carried in red blood cells. Dissolved substances are carried in the plasma.