Transportation in Animals

That list of different methods looks like four unrelated facts to learn. It is not. All four are answers to a single question, and once you have the question the whole topic arranges itself in order.

The body of a unicellular animal is made of one cell only. In an animal such as an amoeba, the food and oxygen it needs pass into the body through the cell membrane, and unwanted substances pass out through the same membrane. Once inside, those substances are moved about by the cytoplasm itself.

There is no transport organ because there is nothing to transport anything across. Every part of an amoeba is already at the surface. Food is at the surface, oxygen is at the surface, waste is at the surface. This is the simplest possible answer to the distance question, and it is only available to something extremely small.

A sponge has many cells, so its cells cannot all be at the surface. It solves the problem with a trick that is worth admiring. Instead of delivering supplies to its cells, it lets the outside world flow through itself. Water carrying oxygen and food enters through the many small holes in its body, which are called ostia, passes through channels inside, and leaves again.

Look at what that achieves. A sponge does not shorten the journey by building roads. It abolishes the journey by making sure that no cell is ever more than a moment away from open water. The sea is not outside the sponge, it is running through the middle of it.

Simple multicellular animals such as hydra and jellyfish have special organs of a kind for transport. Their bodies have pores and a mouth-like opening through which food, water and oxygen enter, and a canal or tissue inside carries materials about and pushes useless material back out of the body.

Now look at the picture of the hydra again and notice its shape, because the shape is the answer. A hydra is a hollow bag. Its body wall is only two thin layers of cells thick, an outer layer and an inner layer. So every single cell in a hydra is touching either the water outside the animal or the water in the hollow space inside it. Nothing is buried.

That is the third trick, and it is the same trick a flatworm uses. If you refuse to get thick, you never need blood. It works beautifully and it has a hard limit, because an animal that must stay two cells thick can never build a leg, a jaw or a brain.

A developed animal such as a human being is solid all the way through and many cell layers thick. It cannot put every cell at the surface, it cannot let river water run through the middle of itself, and it cannot stay two cells thin. Every trick has run out, so it has to do the expensive thing and build real plumbing.

In humans and other developed animals a whole system has developed for transport, and the chief one is the blood circulatory system. It is made of three things: blood, blood vessels and the heart. Oxygen and nutrients reach every cell through the blood vessels. The cells absorb the nutrients they need by diffusion, and send the substances they cannot use back into the blood, which carries them to the excretory organs.

Most students picture blood carrying oxygen right into a cell and dropping it off, like a postman handing over a letter. That is not what happens, and the truth is more interesting.

Notice the word the textbook uses. Cells absorb what they need by diffusion. Diffusion simply means that a substance spreads from where there is a lot of it to where there is little of it, all by itself. It needs no pump and no energy. And it has one very important property: it is quick over tiny distances and hopelessly slow over large ones. Diffusion will cross the width of a cell in an instant and would take years to cross a leg.

So here is what the blood system is actually for. It does not deliver oxygen into a cell. It delivers oxygen to within a fraction of a millimetre of every cell, and then stops, and diffusion does the last tiny step by itself. The whole job of the heart, the vessels and the blood is to shorten the distance until diffusion can finish it.

Once you see that, the whole ladder in this topic becomes one idea instead of four. Diffusion is the only way anything ever finally enters a cell, in every animal that has ever lived. An amoeba, a sponge and a hydra have all arranged their bodies so that diffusion alone is enough. We could not, so we built a delivery network to carry things most of the way, and left diffusion to do the last little bit.