We looked at our heart, but not at all the piping connected to it. There is rather a lot of it and this picture shows only a very, very small part.

There will be talk about testicles, read at your peril.

Whilst the air pipes in the lungs are optimized for keeping constant pressure, for veins and arteries this is not the case. Perhaps there is less selective pressure for optimisation, or it is not possible to achieve? I do not know. The truth is however that there are a number of sub-optimal divisions and loops and one of them is so peculiar that once you learn about it, you will not forget it.

Professor Kos explained to us, that one such badly optimised division is at least in part responsible for the fact that one testicle is usually lower than the other, and the one that is lower is usually the left one. When you look at the picture at the right side and follow the vena cava inferior down from the heart past venae hepaticae you will come to a cross junction where from it split two venae renalis. And looking further down you will find out that venae testicularis do not both split of from the vena cava inferior symmetrically, but vena testicularis sinistra splits of from venna renalis at a near right angle. That is bad engineering – right angles mean loss of pressure automatically.

However when looking at the left side, where arteries are depicted, both arteriae testicularis split symmetrically and directly from arteria abdominalis at an angle in the direction of the blood flow.

This means that whilst fresh blood supply for both testicles through arteries is about equal, the outflow via veins is not. That means different blood flow rates through the testicles, leading to their different sizes and also different position. Because testicles regulate their temperature (which has to be lower than body temperature) via positioning, and this way the left testicle has to hang lower in order to keep the same temperature as the right one.