Things you always wanted to know the answer to [Vol. 5]

I need to clarify a few things before the explanation, otherwise it may not make complete sense.

It's true that respiratory drive is controlled by your blood CO2 levels, not O2 levels (except in a particular group of COPD patients but that's irrelevant at this point).

A short period of breath holding isn't enough to significantly decrease pO2 (partial pressure of oxygen) or SaO2 (arterial oxygen saturation), and a coughing fit wont make a blind bit of difference to either of those. However, oxygen is important.

Brain is dependent exclusively on aerobic respiration, i.e., oxygen utilising energy generation. Unlike pretty much all other tissues, e.g., muscles, it can't utilise anaerobic respiration. Furthermore, it also can't use any substrates other than glucose delivered freshly by blood; it has no emergency reserves or stores. Therefore, cutting off or reducing its blood supply affects consciousness very quickly. This can manifest as dizziness, visual changes, fainting etc.

With those as our starting points, next we need to look at what happens when you cough. During a single cough, there is a sudden contraction of chest wall muscles, intercostal muscles etc., all of which results in a big spike in intrathoracic pressure. It's that sudden, massive rise in intrathoracic pressure that expels whatever the body is trying to get rid of from the lungs; the lungs don't have a way of generating that pressure themselves to that extent. They have some elastic recoil, but for that to work, you'd need to induce a pneumothorax.

Now, under normal circumstances that's just a momentary increase and doesn't have much of an effect. However, if you have a coughing fit where you're coughing continuously for a period, then your intrathoracic pressure will remain elevated as it will not have the chance to return to normal between each cough. The reason why this is important is because return of venous blood to the heart from the rest of the body is dependent on intrathoracic pressure being very low, negative, in fact. Therefore, a prolonged period of increased intrathoracic pressure reduces blood return back to the heart. The first consequence of this is that there's no blood to pump out to the lungs to re-oxygenate. The follow on consequence is that there's no oxygenated blood to pump around the rest of the body, including the brain. The rest of the body can cope, but the brain starts to suffer within seconds if its blood supply is disrupted. The other consequence of preventing blood returning back to the heart to continue circulating round the body is that you get venous congestion which becomes more apparent where it's more visible - face. Venous blood is a much darker colour than arterial blood and venous congestion results in the typical purple/blue appearance as a result.

The effects of a coughing fit are similar to the effects of a Valsalva manoeuvre, which, if done extremely powerfully, can cause loss of consciousness (including in the middle of a med school physiology experiment where one may have been the subject...) where intrathoracic pressure is raised by taking a deep breath, closing the glottis and breathing out against it.

In short, the dizziness and similar effects caused by a coughing fit arise as a result of the changes it induces in intrathoracic pressure and the consequent reduction in blood flow to the brain. The more reduction, the worse the dizziness and purple face, they are directly correlated.

Going back to gears, bicycles often have gear shifters on both handlebars, but they work in opposite directions, I think. Pushing the left shifter forward gives you a lower ratio, while pushing the right one forward gives a higher ratio, and vice versa when pulling them back.

Do Opticians (Boots, specifically) have a policy that absolutely every member of staff must wear glasses even if they don't need glasses, or does it just seem like it?

As 68% of men and 77% of women regularly wear glasses or contacts it's not that weird is it ?

Interesting

I had an eye test in Boots Opticians today, and every member of staff had glasses on. But then I popped into SpecSavers to have a look at their frames and one member of staff wasn't wearing glasses which suggests that it's not policy there.

Do dogs (or any other animal besides human) engage in paedophilia? If so what is the reaction from other animals in the pack (assuming social animals)?

With some bees and wasps the males mate with the females as soon as they emerge from the cell.

Not sure you can cite this as a legal defence though ...

I don't think it is particularly worse. It's always been thus