Symmetry is a very nice idea, it makes the world seem balanced and equal. And in the human frame is seems reasonable based on what we see on the outside but that is not the whole picture. To be alive requires a whole set of tools to do jobs like getting oxygen in and carbon dioxide out, pumping blood around, digesting and processing food and balancing the chemistry that makes the whole life process happen. The organs to do all this work come in a variety of shapes and sizes and they all have to be packed inside of you in the skull, thorax and abdomen so you can cart them around, using the musculoskeletal system, to wherever you want to be alive .
Now to pack all that in, from an engineering standpoint, is not that hard but most of those organs come as singles, not pairs. You get one liver, one stomach, one heart, etc. (Why are they singles? It’s an evolutionary trade off. By having only one digestive tract, one liver one heart, etc. we save a huge amount of energy and space which makes us more mobile. But we have don’t have spares if we get into trouble.) Even the ones you do get pairs of like kidneys and lungs are not symmetric in size or location. So think of packing a whole bunch of items of different sizes and shapes in a suitcase. You would not expect to open it up and see a nice symmetric arrangement. Inside of us it is no different.
The way that these organs get packed in us is important. All those organs and tissues are attached to other things including your muscular and skeletal system so they stay in the right relationship to other parts. There are suspensory ligaments that connect the heart and lungs to the ribs and spine, others connect the liver to the diaphragm and spine, and still others to hold other organs in place inside your body. And many of those organs have muscles and change shape as they work. They squeeze and shorten and do all sorts of movement inside of you. This means you always have asymmetric pulls and movement going on in you, movement that is mechanically transferred to the joints and muscles. Think of holding a wriggling baby in your arms. Even if you have a good grip in the baby (like ligaments on organs) your muscles and joints have to keep making small movements to balance and control the baby so it stays “in the same place” (your arms) and does not go somewhere else (the floor) but at the same time lets the baby breath and move some.
The upshot of all this is that our bodies have a constant, mechanical, asymmetric pull on them that is created by our visceral systems.
Next time I will write about how this contrasts with the activity of the musculoskeletal system.