This is a work in progress... please bear with me, come back and watch it grow? Send constructive suggestions?

What you see below might be seen as an outline for what will eventually be spread across multiple pages.

We speak elsewhere of getting chemicals into and out of organisms, and of using them, and the creation of new chemicals.

Circulation is the challenge of moving the chemicals around inside organisms. When you eat an apple, it goes first to your stomach. It is still, biologically speaking, "outside" your body. Within your stomach, and the rest of your alimentary tract, the apple changes, and chemicals from it enter your bloodstream. Later, those chemicals are used in your liver, in your muscles, etc. The chemicals are moved around your system by your circulatory system.

The biggest things that have ever lived... the trees on the west coast of the USA, and in Australia, have leaves hundreds of feet above the forest floor. Those leaves need more water than they can absorb from falling rain. The trees have circulatory systems to supply the leaves with water from the ground. The problem of getting water to the upper levels of the tree is probably the challenge that prevents these trees from being even larger.

For a more modest, but none- the- less interesting (I hope) example of circulation in plants, try my essay on a houseplant I have which grew 3m (c.10 foot) roots.

As I will say again and again: Slice and dice. By that I mean continually think about the grid that is formed if you put the Challenges of Life on the columns and the different sorts of animals on the rows. We've already briefly looked at the circulation column for "animals" and for "plants". What about very tiny plants and animals? There are many problems which arise if you are very tiny... but the circulation challenge becomes almost moot... Chemicals can move from one end to the other of a microscopic organism by mere diffusion. But maybe that's too much of a good thing? How do you keep things where you need them, if they can move too easily? Ah, the quandaries that arise!

Vertebrates all have some form of heart and closed network of blood vessels ("vascular system") for circulation... but that familiar design is by no means the only option.

In vertebrates, the circulation of oxygen (and removal of carbon dioxide) is accomplished by the blood. In insects, not only is the "vascular system" reduced to not much more than a tube, open at both ends, but it isn't responsible for getting oxygen to the organism's tissues.

In insects, oxygen is brought to the tissues by a network of tiny tubes, open to the outside air. A straightforward system... but the down-side is that it increases the insect's problems with drying out.

The insect "blood system" can be explained as follows:

Think of the insect as a "bag" filled with tissues and liquid, with a tube running from one end to the other. The tube has muscles in it which contract in a series of waves, "squeezing" liquids from one end to the other (by "peristalsis"). This keeps the liquids in the "bag" "mixed", so that no region becomes stagnant, with useful chemicals depleted and waste chemicals accumulating.

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