Wheel

Today, I was reading my daughter's social studies text book & Name of the chapter was 'Conquering distance'. The Chapter was mainly about various forms of transportation form ancient times to present day. I was specially attracted one one sentence - 'Turning point in the history of transportation came when the WHEEL was invented'. It is so true. All the human inventions have wheel and wheel is integral part of human life.
But as a student of Medicine I was baffled to realise that nowhere in human body, either macro or microscopic, wheel is used by the nature. There is lot of transportation going on along the length and breadth - vascular or neuronal, across many membranes - lungs or kidneys, along cell membrane and many things - oxygen or glucose to name a few. Wheel, which we think as most efficient form of transportation is not at all used, atleast as far as our current knowledge of anatomy and physiology takes, by the nature. Does nature has a better and efficient form of transport which we are missing ?. Or we bettered nature in efficiency in transportation?. Or are we missing woods for trees ?.