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.

A diagram illustrating the circulatory system in the human body

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?

Substance transportedFromTo
OxygenThe lungsAll the body's cells
Carbon dioxideAll the body's cellsLungs
GlucoseDigestive systemThe liver, then all the body's cells
UreaLiver cellsKidneys

Other substances transported in the blood include:

Oxygen is carried in red blood cells. Dissolved substances are carried in the plasma.