So, blood pressure is the pressure that components of blood exert against the walls of the blood vessels. The components of blood being the plasma (which is the fluid part of the blood excluding the cells) and the cells (which is the… the other part of the blood)
Blood pressure is produced as a consequence of the action of the heart and as a consequence of how much blood is contained within the blood vessels. These things can vary – the heart can pump faster and more forcefully, the blood may vary in volume (if you are dehydrated there is less fluid in your body so the blood volume may decrease, if you chop off your arm and start squirting blood everywhere then there is less blood in your body – do NOT try this at home) and the capacity of the blood vessels can change – you don’t suddenly acquire more blood vessels but their size can be altered by muscles in the blood vessel walls.
The blood pressure readings that you are probably familiar with – 120 over 80 is a good reading – is actually 120mmHg when the heart contracts and 80mmHg when the heart relaxes.
But why do we even need blood pressure?
(I’m a bit worried that I’m not being totally accurate in my terminology here – if anybody wants to pick me up and correct me on it, that would be smashing!)
So, having blood pressure means that all of our body gets supplied with blood. Which means that all of our body gets oxygen and nutrients. Without blood pressure bits of us would start to drop off. Our brains would stop working, our organs wouldn’t function. It would all go horribly wrong.
But sometimes blood pressure does go wrong. Sometimes it is too high and sometimes it is too low.
If it is too high, what could happen?
If it is too low, what could happen?
So if blood pressure temporarily goes too high or too low, what does our body do about it?
So, if your blood pressure drops all of a sudden, your heart speeds up and pumps more forcefully. The little blood vessels around your body constrict so that although there is the same amount of blood in your body, it is contained within effectively a smaller space. All of these return your blood pressure to normal.
If your blood pressure rises suddenly then the opposite effects occur.
For longer term control of your blood pressure, other systems come into play which are more involved in regulating the volume of blood in your body. Remember that blood pressure is related to blood volume? Of course you do.
So with a drop in blood pressure, there is constriction of the little blood vessels supplying the kidneys. This, plus the overall drop in blood pressure means that the kidneys are filtering less blood so less water is lost via the kidneys and less sodium is lost via the kidneys. It also causes release of an enzyme called renin from the kidneys.
Now, I am about to go into a little bit of detail about something, purely because it will help later. It is mildly complicated with some long chemically-sounding names, but it isn’t that complicated (says the person who has struggled to cope with this all his life).
I’m going to give it to the chickens to explain…
And it has an effect on part of the brain which leads to thirst and to the production of a hormone called anti-diuretic hormone (ADH). If it helps, diuresis is to do with the production of urine. And so anti-diuresis is to do with producing less urine and as a result keeping more fluid within the body.
It also (is there no end to its effects?) causes the adrenal glands (which are near to the kidneys) to produce the hormone aldosterone.
To sum up this little section – or the bit which is particularly relevant and why I’ve gone on at length. Without the ACE chainsaw that Daisy was wielding, none of these effects would happen because you wouldn’t have any angiotensin 2.
Quickly then and just to try to not leave a loose end dangling. Aldosterone is required for normal sodium reabsorption and for normal potassium elimination (strictly, apparently, secretion) Aldosterone takes hours to have its effect, because the body has to manufacture proteins in order for aldosterone to work.
There are diagrams depicting all of these effects. Here is one of them…
What happens when all these systems get a bit out of control?
And by the way, if I’ve got anything wrong here or if I need to explain anything better or differently (and that is quite likely as my brain is starting to hurt) somebody please tell me so that I can make it right!