Exchange materials between them and environment

Needs can't be met by body alone

Exchange between cells and exchange surfaces

Exchange materials between environment, exchange surface and different parts of the body

Depends on:

S.A:Vol ratio

How active organism is

low S.A:Vol ratio and active = bigger need for both

Suitable medium to carry materials (normally water based as readily dissolves things and easy to move around)

Transport medium moved around in bulk over large distances

Closed tubular vessel system with transport medium and branching network (distribute to all parts of the organism)

Mechanism for moving transport medium within vessels

Animals rely on muscular contractions or specialised pumping organ

Mechanism to maintain mass flow in one direction

Means of controlling flow to suit changing needs of different parts of the organism

Closed blood system confined in vessels (heart circulates)

Double circulatory system (passes through heart twice for each circuit)

Blood through lungs is low pressure

High pressure to go to heart before circulation so substances delivered quickly

Important for high body temperature and metabolism

Last bit = diffusion into cells

Rapid as large S.A, short diffusion pathway and steep gradient