Came across this thread as a result of the link to the site where I wrote the article where I suggested that the cause of MANY of the collapses might have been low Blood pressure. I see that Rojo and Wejo posted a link to it as well, and so I'm obviously grateful...!
But I do recognize that I did not in fact explain the WHY? which was perhaps a little short-sighted of me. So I apologize, but I will say that I didn't want to go into detail for fear of writing what would have been a MONSTER article! But perhaps I can try to explain it now...
The reason the blood pressure drops at all during exercise (not withstanding the heat, which I'll get to in a moment) is because during exercise the body faces TWO challenges.
1. It has to get blood to the muscle where it is needed.
2. It has to get blood to the skin where it is needed for cooling.
Now the problem is that when the blood is redistributed to the skin, it lowers what has been called a number of things, but let's go with Central Blood Volume, because that is what is easiest to conceptualize. Effectively, what is happening is that the body is sending its blood away from the centre of the body, which is where all the sensors for blood pressure are. The result of this, is that the blood pressure DROPS.
HOWEVER, when you are exercising, the legs are pumping blood back to the heart, in effect, helping the circulation and defending that central blood volume. This prevents the drop in blood pressure.
Now, ON A VERY HOT day, the problem is that you need to send even more blood to the skin (because you have to cool off more), and so this means that the "muscle pump" of blood back to the centre of the body must be even more effective, which it sometimes is not.
Perhaps if I can give an example, it makes sense - think of soldiers standing on parade. Or perhaps of a group of children lined up at attention for some kind of assembly. Almost invariably, one of them faints and falls over. Why? Because what is happening is that gravity combined with the heat is forcing the body to send blood away from the centre of the body and to the skin (to cool off) and to the legs (because of gravity). The problem is that this person is standing dead still, so there's no muscle in the legs to "help" that blood back to the centre, and the blood pressure falls. That's why they faint over in a parade!
Now, applying this to exercise, what happens is that you are running along and your muscles are 'pumping' blood back to the heart - this keeps your blood pressure normal. But AS SOON AS YOU STOP, the 'muscle pump' stops working and so all of a sudden, there is a drop in the blood pressure. Hey presto, you faint! That is the reason why MOST PEOPLE COLLAPSE WHEN THEY STOP. If you think about it, how many of you have actually seen someone collapse while they are running? It's incredibly rare! It usually happens when you stop. Therefore, it is the act of STOPPING that causes the collapse, because suddenly that pump that was helping before is gone! I hope this makes sense...?
If I may end off with some speculation about your specific incident, Top 100. I think that what probably happened is that in moving from shade to direct sun, your skin suddenly heated up. We know that the skin responds to heating by 'asking' for more blood - it's called vasodilation. This vasodilation, as I explained above is what causes blood to move away from the centre of the body. The muscle pump, which as this stage is going as hard as it can, cannot keep pace and the new 'distribution' of blood to the skin causes a drop in BP. Your body senses this, and you start displaying the symptoms of a drop in BP - you feel a little ill, can't drink anymore, you can't run etc. That seems to me to be exactly what happened to you...
Now, as for what to do, it may surprise you to hear that DEHYDRATION AND DRINKING ARE NOT THE ANSWER - the drop in BP is in no way causes by dehydration. If you think for a second about the reality of running - the fastest guys drink the least. Maybe 400 to 600ml IN THE WHOLE RACE. They finish the race and have lost perhaps 2 or 3 kg, which is maybe 5 to 7% of their body weight (these are real numbers, incidentally, based on research I've been involved in at marathons and IronMan triathlons). yet they are fine! No low blood pressure, no dehydration, no heat stroke, no death...
that should immediately be a sign that it's not so straightforward. And this is already a MONSTER reply, so I won't go into too much detail on this (perhaps in the future, I will do a post on this on the website - in fact, I definitely will). But let's just say that drinking all you can would only have made you feel even more sick and possibly be fatal.
Instead, the answer is heat adaptation. The FIRST adaptation to heat exposure, which happens in a day or two, is that your blood pressure regulation improves. Suddenly, where it was a problem before, it isn't anymore. In that article quoted earlier and on the Letsrun site, I mentioned the story of the soldiers sent to SE Asia - three days is all it took and they were fine. I won't go into reasons for this, but some heat adaptation would prevent this. This is the reason I wrote that the biggest issue in the race on Sunday was simply that the runners were 'ambushed' by the temperatures. It was very unfortunate to have that kind of day for the race.
But I'm pleased you're recovered, and hopefully no worse for wear! Again, sorry for the mega-long post, but I really should have put it up the first time.
(science of sport web blog co-author)