What are the causes of people collapsing when running?

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JohnnyO Aug 2011 9:29am, 2 Aug 2011 9,923 posts	0 JohnnyO Did somebody ask for a physiologist?
agatha Aug 2011 9:30am, 2 Aug 2011 90 posts	0 agatha Which I should add - I 'know' that muscles demand more oxygen when working harder, I 'know' that the heart moves blood round but somehow hadn't quite connected up the reasoning. I worry myself sometimes....
JohnnyO Aug 2011 9:39am, 2 Aug 2011 9,925 posts	0 JohnnyO The collapse at the end of a race is. as Stumpy suggests, do to reduced cardiac output from lack of venous return to the heart. Your arteries and veins are maximally dilated and the sudden stop leads to a decreased venous return. This combined with the low resistance offered by your dilated arteries means that your blood flow to the brain is reduced after a few beats and you black out. The heat business is a different problem. You're movements rely on enzymatic processes. Enzymes only work efficiently within a fairly narrow temperature range and if you overheat they will decrease efficiency and you will slow. Luckily you have a variety of mechanisms that allow you to keep your temperature down, but these can be overwhelmed by being too hot.
JohnnyO Aug 2011 9:42am, 2 Aug 2011 9,926 posts	0 JohnnyO The amount of oxygen in your bloodstream will rarely drop during exercise, unless you have a significant respiratory condition. Normal lungs are capable of oxygenating a vast amount of blood, far more than you need at rest and in excess of what most people can achieve during exercise. Lack of oxygen is the cause of most immediate post race collapses, but it is related primarily to reduction in blood flow, rather than that blood having reduced oxygen content.
agatha Aug 2011 9:50am, 2 Aug 2011 91 posts	0 agatha brilliant! Thanks JohnnyO.
Springer Aug 2011 3:02pm, 2 Aug 2011 132 posts	0 Springer I collapsed during a race a few years ago, terrifying experience. It was Heat stroke and part of the problem it caused was very high CK levels, a mild form of Rhabdomialysis, I believe. Running a few weeks later was awful felt like I was wading through treacle, I think this was because my muscles had essentially turned to fluid. i wouldn't recommend it! en.wikipedia.org
Alibob Aug 2011 11:36am, 11 Aug 2011 293 posts	0 Alibob I haven't collapsed (thankgod) however, if I stop running I can feel sick and big black lines creep up my vision. I'm ok but have to stop and put my head between my legs. This happens most often if I run in the morning, which I avoid doing. However, it happens to me a fair bit. I can be running 10-20 mins, stop at traffic lights and that's it. It puts me off so I walk home. I try not to run by myself just in case. Years ago I was told that I have low blood pressure and when it happened recently I went to the doctors and got it checked. Nothing! I have no idea why it happens
Stumpy Aug 2011 11:41am, 11 Aug 2011 13,045 posts	0 Stumpy alibob - see my post previous page and JOhnnyO's elaboration of it on this page. keep your legs moving and don't stop suddenly.
Dvorak Dec 2018 7:17pm, 9 Dec 2018 16,982 posts	0 Dvorak From observation: heat, especially unexpected heat and failure to adjust pace for this; overexertion; underfueling; running hard when insufficiently recovered from illness; ; undetected (often cardiac) conditions (the events which generally make the news ). Up here in the coolish North, I reckon the first the most common, with the second coming into play mainly for marathons or longer.
Nellers Dec 2018 6:20am, 10 Dec 2018 33,500 posts	0 Nellers I've never "blacked out" or collapsed during or immediately after a run but I have come close. Usually it's when I finish a race and have to stop so the timing chip can be removed. The worst one was when I had to bend down and take it off my shoe myself. On standing up again I had tunnel vision and the world was spinning in a way it's definitely not supposed to! All of that I put down to exactly the same reasons explained well by Stumpy and JohnnyO.