Why do multicellular organisms need transport systems?

Multicellular organisms need internal transport systems like the circulatory system.

This is because, compared to single-celled organisms:

• Multicellular organisms are larger, so the diffusion distance across their surface would be too large.
• Multicellular organisms have a higher metabolic rate.
• Multicellular organisms need to supply nutrients and oxygen rapidly to a larger number of active cells.

So overall, diffusion alone would be too slow.

Mammalian circulatory system structure

Mammals have a closed, double circulatory system.

This means that:

1. It is a closed system - Blood is contained in blood vessels.
2. It is a double system - Blood passes through heart twice per circuit.

The two main divisions of the mammalian circulatory system are the systemic system and the pulmonary system.

Systemic circulatory system:

1. Oxygenated blood is pumped out of the heart via the aorta to most body tissues.
2. Other blood vessels branch from the aorta to deliver blood to body tissues (e.g. the renal artery delivers blood to the kidneys).
3. Veins collect deoxygenated blood from these organs (e.g. the renal vein collects deoxygenated blood from the kidneys).
4. Deoxygenated blood is returned to the heart via the vena cava.

Pulmonary circulatory system:

1. Deoxygenated blood is pumped out of the heart via the pulmonary artery to the lungs.
2. Oxygenated blood is returned to the heart via the pulmonary vein from the lungs.