Blood meridian

Not a throwback to the novel by Cormac McCarthy, rather to May just past when I was thinking about mixing, in the context of mixing bread dough. How well were all the ingredients mixed together by the process of successive rollings out and rollings up? See reference 1.

More recently, I have been thinking about how the body manages to do the same trick with blood. If one were able – or perhaps inclined – to take blood samples from all over the body, would they all be the same? How much would they vary if one did this through time? Does the very idea of the composition of blood break down once one leaves the world of arteries and veins?

Maybe at some point in the not too distant future there will be some scanning contraption that could do this sort of sampling. In the meantime, I conduct a modest thought experiment.

I see two aspects to the business of making the blood a uniform quantity.

Firstly, one has the regulation service of the kidneys. I imagine that the kidneys regulate all kinds of quantities in the blood, adjusting the levels of all kinds of components, with the general idea being that the concentrations of all these components are managed within fairly close limits if we want to stay alive and well. Mammals being a bit fussy on this point. So to the extent that all the blood passes through a kidney from time to time, all the blood is the same.

Secondly, one has the mixing service of the heart. A pump which is pushing the blood out into a branching, circulatory system. My first thought is that the mixing arises from the random way in which any particular bit of blood is pushed out into those branches. Any given millilitre of blood in the heart at time T is going to wind up spread out over hundreds, if not thousands, of capillaries, spread all over the body by time T+1. Then another good dose of random is introduced by the business of the blood travelling through the ether which lies between the arterial and one of the many nearby venous capillaries. With the millilitre we started out with ending up widely spread out through the cavities of the heart when it gets back there. Then there is the stirring action of the heart itself. So a process not so unlike the Rollings & Rollings of dough mentioned above – and with the round trip taking about a minute, not unlike dough time. But with the difference that it goes on all the time; the heart does not pack up after the ten minutes or so I give to the first knead.

At least, I suppose that the movement of blood into branches is random. While it may actually be that the cross-sectional organisation of the blood in a large vessel is largely preserved when that vessel splits into two small vessels. And preserved again on the return journey. Think of those aerial pictures of the confluence of two rather different rivers. Or of the veins in the bark of some trees – on which point see reference 3 and neighbouring posts.

However, I leave all that aside to deduce that the composition of the blood is fairly uniform around the body. That said though, there are always going to be local disturbances, perhaps only affecting a small part of the body for a small period of time. And there may well be global disturbances over longer cycles, perhaps geared to the daily rotation of the earth or the monthly orbits of the moon. More sinister, global movements or trends may be the result of disease, with the disease leaving chemical traces in the blood. Free-loading traces which are neither useful in the way of, say, sugar or oxygen nor harmful in themselves, but which are, nevertheless, information for those with the power to see.

With the question for me being what effect all these disturbances might have on the neurons in the brain. A difference between composition of the blood at point A in the brain and that of standard blood? A difference between the blood at point A in the brain and that at point B? A difference between the blood at point A in the brain at time T and that at the same point at time T+1? A local sugar deficit, for example, would depress the activity of the (energy hungry) neurons in that locality. All kinds of interesting possibilities.

Next up, the mixing of the lymph, which a quick peek at wikipedia suggests is a bit trickier than the mixing of the blood, at least from the present point of view.

PS: I have seen some work which suggests that successive drops of blood taken with the sort of pricking device a diabetic might use might be rather different – different enough to be a problem – although I have not looked closely enough to be sure that proper account has been taken of the possibility that what it is to be measured has been changed by the business of measurement, a possibility well known to particle physicists and possibly in this context called clotting. So it may either be that the mixing is not so good – or rather that I am straying into the land where a little knowledge is dangerous. See reference 2.

Reference 1: http://psmv3.blogspot.co.uk/2016/05/mixing.html.

Reference 2: Drop-to-Drop Variation in the Cellular Components of Fingerprick Blood Implications for Point-of-Care Diagnostic Development - Meaghan M. Bond, Rebecca R. Richards-Kortum PhD – 2015.

Reference 3: http://psmv2.blogspot.co.uk/2014/09/botanic-problem-3.html.