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It looks like when you take a cold shower or bath your veins get smaller and when you take hot shower your veins get wider. But is that right and do you need to breath more in both cases because you got less oxygen? If so how does it work in both cases?

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There is an adaptation to being submerged in cold water in mammals called the Mammalian diving reflex. Have you ever noticed when you jump into cold water, or enter a really cold shower you take a big breath? While not exactly correlated with the dive reflex, it shows that there is a neurological reflex of entering cold water. Here is a brief overview of what can happen when you submerge yourself in cold water:

  1. Lower heart rate - aka Bradycardia, slowing down the rate of blood pumped will decrease the oxygen requirement of red blood cells, which in turn allow organs like the brain to use oxygen.
  2. Smaller veins - aka Vasoconstriction, the blood flow in your fingers and toes is reduced, allowing more blood to be used in the more important organs. Eventually the vasoconstriction will apply to hands and feet.
  3. Blood shift - this adaptation allows the submerged animal to dive without having their thorax to collapse due to increased pressure. Not necessarily attributing to breathing patterns though, so I left the boring stuff out and paraphrased.

In hot water, the opposite happens with respect to #'s 1 and 2. Increased heart rate, like when you go running, tends to increase breathing frequency. Vasodilation allows blood to flow more easily and is primarily for heat exchange. Have the warmer blood in your fingers exchange heat to the environment will cool it for the return to systemic circulation. Surface area is a large factor with heat exchange. Hope this provided some insight.

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  • $\begingroup$ Do you perhaps know what chemical proces causes the drop of heart rate in cold water. And is it true that when you take of hot shower you veins are getting bigger and because of that the bloodpressure drops so you get less oxygen and need to breath more? $\endgroup$ – Marijn Feb 11 '16 at 8:44
  • $\begingroup$ The chemical process is just nerve signals that lower the heart rate. I'm fairly certain it is parasympathetic regulation including the vagus nerve. When your "veins" get bigger, and blood pressure drops, the heart compensates by increasing the pumping. So technically you may need to breath more due to the increased heart rate. You do not get less oxygen though. $\endgroup$ – MG_MD Feb 11 '16 at 15:57

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