Nature's recipes

In he middle of next month the Mayor of Scarborough and his wife are setting out on a cycle tour of the Borough. It isn't often that local politicians in our area take cycling seriously and, since its been something that I've gently been campaigning about for years, I made sure that I got invited to the organising committee meetings and found myself recruited as one of the guides on this year's inaugural ride.

Now whilst I may use a bike as my everyday means of transport and head off into the North York Moors National Park for the odd hour or two, I'm by no means a well practiced cycle tourist and am not one of those people who happily set out for a 100km + ride every weekend. This means that to make sure that I can appear to cope effortlessly with 320km of very hilly riding over 4 days I've been getting in some training and as a one time physiologist I couldn't help but find out what effect this must be having on my body.

One of nature's not so well hidden secrets is that whilst DNA provides a recipe it doesn't provide a blueprint. A favourite example of this is demonstrated by a simple experiment carried out on young chicks. Attach a contact lens to the chick's eye so that the image that's being formed on its developing retina is thrown out of focus and the eye changes shape to make the image clear again. So there isn't a detailed set of instructions to be followed to the letter rather a general idea that is then adapted to the circumstances.

There are loads of much simpler examples of what is now called Adaptive Self Organisation; A tree trunk will by thicker in the direction of the prevailing wind, an astronauts skeleton gets thinner when subjected to micro gravity and muscles get bigger when you exercise them.

But with muscles there are two different types of training. One of these is simple strength training. Subject a muscle to repeated large loads and the fibres get biggger. The other is endurance training, and that's the sort that began to interest me. It turns out that when you repeatedly use a muscle, even at relatively low loads, there are two major changes. The first is that the numbers of mitichondria (the little intracellular energy processing plants that use glucose to turn ADP into ATP which, in turn, is used by the proteins in the muscle fibres as an energy source to ratchet themselves along each other and generate force) increase. The second is that the number of capillaries that supply blood to the muscle fibres, and consequently oxygen and nutrients, also increase.

Now this increase in capillaries also happens with strength training but, it turns out, in a slightly different way. One of the consequences of strength training is that the muscle fibres, and capillaries, suffer direct damage and that during the repair process the damaged capillaries produce sideways sprouts (that can get through the thin layer of connective tissue that surrounds the capillary) and thereby make sure that the new enlarged muscle fibres get a new enlarged blood supply. With endurance training the effects are subtly different. Rather than burst out sideways, new capillaries are formed when an existing capillary simply cleaves along its length; leaving two parallel capillaries where there was one before and without having to break through the surrounding connective tissue. 

Of course, once you get into thinking about this process more and more questions spring to mind. How did the capillary "know" that it needed to split? Once it "knew" how does it tell the cells that its made of what to do? etc. etc. This process involves a great long chain of biochemical processes, all of them are thoroughly ignorant of the final intent. Suffice it to say that, as a physicist at heart (that is as someone too lazy to remember the names of all the parts and happy to just have some basic idea of what's going on), I was interested to discover that the basic signal telling the cells of the capillary that something needs to be done comes from the shear stress felt by the capillary walls as blood flows past.

Take an old fashioned eraser in your hands (the sort you'd use to rub out pencil marks) push one side one way and the other side the other. That's a shear stress. If you pump more blood down the capillary the speed of the blood flow will increase and so will the shear stress felt by the blood vessel's walls. So new capillaries are formed just where they're needed.

In't nature brilliant...