Simple and complex organisms
The single-celled Amoeba lives in water. It exchanges nutrients and wastes directly through its outer cell membrane to the surrounding fresh water. These substances move by diffusion, from areas of high concentration to areas of lower concentration.

The Amoeba's area of cell membrane is great enough to allow quick entrance of oxygen and small nutrients to maintain life, and quick removal of wastes to avoid poisoning.

As larger, multicellular organisms evolved many cells on the inside of the organisms were no longer in direct contact with the environment (unlike the Amoeba). Diffusion alone was not sufficient to move the substances quickly enough through to the internal cells. Transport systems were needed!

Bigger organisms also require more complex systems for movement (skeleton and muscles), communication (nerves and hormones), and food breakdown (digestive). A system is a co-ordinated group of organs and tissues with a common function to solve a particular problem.

Why have transport systems?
Transport systems in living things bring every cell in the body in contact with the external environment.

Any method of transporting substances directly from the outside would aid survival.
Insects have tiny tubes (spiracles) which open through their hard covering (exoskeleton) and carry air via a system of smaller tubes directly to the body cells.

However, endothermic (warm-blooded) organisms need an even more efficient method to maintain energy supply.

Internal lungs provide an enormous area of alveoli (air sacs) for exchange of gases with the external environment. Capillaries surround these air sacs, and gases are then transported around the body by the blood vessels.

Similarly food can be brought inside the body and digested by the digestive system but the simple food units still need to get to all individual cells. Again, the circulatory system is close to the area of absorption and carries out this role.

Wastes produced in the cells need to be carried via the blood system to the kidneys (excretory system) for removal.

Exchange: Getting in and out
The following factors affect how quickly the substances pass across the cell membrane.

• The bigger the surface area (S.A.) compared to volume for exchange the better! All exchange areas are adapted for maximum surface area, e.g. the millions of airsacs in the lung and the enormous capillary network.
  
  Similarly the walls of the small intestine are highly folded into villi to increase surface area.
  
  The placenta too has an enormous network of capillaries, some belonging to the mother's circulatory system and others belonging to the baby. They lie close together to aid exchange between the two systems.
  
  
• The faster the blood moves at the exchange surface the better. This keeps the concentrations high on one side and low on the other speeding up exchange via diffusion. Good blood pressure is essential. Heart problems, for example, often result in death through kidney failure because wastes cannot be removed efficiently enough.
  
  
• Molecules must be small enough to diffuse easily across the capillary walls, which is why food must be digested to simple units before transportation. Large blood cells and proteins cannot pass through the cell membrane via diffusion.

The capillary walls are only one cell thick to allow for easy exchange of nutrients and wastes.

All internal body cells are surrounded by extra-cellular fluid (ECF), rather like the watery world of the Amoeba. This means, for example, that simple sugars will pass out of the capillary (where their concentration is high) into the extra-cellular fluid, then into the cells where the sugars are continually used up for energy so that he concentration inside the cell is low. Similarly, wastes leave the cell.