Re: Why do blood vessels, tree branches,and lightning flow out in the same way?

The lightning is a bit of a coincidence, but not the blood vessels and the trees. As for lightning, its shape is due to the dielectric breakdown of the air. It's still a very complex and poorly-understood problem, the formation and exact physics of such large discharges. But basicly, the current in a lightning strike (which you see in an instant, but is actually about 30 rapid discharges in the blink of an eye) finds the paths through the air that are the most conductive (these are somewhat random) and follows them. Search old issues of National Geographic from the late 80's and early 90's, they had a fantastic section on lightning.

The tree branches and blood vessels, however, have evolved into their most effective forms. Blood vessels, for example, must exchange nutrients and oxygen with the surrounding cells. Our lives depend on it. This happens through the membranes at the surface of the cells and blood vessels. In order to reach all the cells and to maximize the surface area through which they can exchange food and oxygen, blood vessels must branch. However, to make the central pumping station more efficient (viscous drag forces are higher in small tubes like capillaries), our heart pumps blood in and out through large arteries. The successive subdivision of blood vessels is also the structure that delivers blood to and from the cells fastest, which also reduces the energy it takes to pump blood. They could further increase their surface area by being flat-shaped, but this would reduce their strength. Blood vessels are ridiculously strong for soft structures, to withstand the pressure the heart puts on them. You can see how they always take the most sensible shape. Imagine having one insanely long capillary running to each cell in turn! It just wouldn't work, the first cells would take all the oxygen and the rest would just die. Our lungs are similarly branched, ending in tiny sacs which all together maximize their surface area so that we can exchange oxygen and CO2 with maximum efficiency.

Trees use a branched structure to deliver nutrients as well. They must also maximize the surface area they can deliver nutrients to and from, because they need the maximum area possible to be exposed to sunlight. That's why they end in flat leaves. It also minimizes the distance nutrients and water have to travel between the roots and individual leaves while at the same time making each leaf approximately the same distance up the structure.

You find that a lot in nature, these branched structures. They minimize distance, and maximize carrying capacity. At the same time, the cells of most biological entities exist in a careful balance of nutrients in and waste products out. This type of structure makes sure that they're all a roughly equal distance from the central processing organs, so that they all get the same amount of food and dispense with waste with the same efficiency.