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Do you really want to know?
The heart pumps blood around the body.
blood goes back the heart and "Frank starling law" states that the more that goes in, should come out. the heart has an intrinsic beat of around 100 bpm. however there are many systems that affect the rate, rhythm, volume and pressure of what come out the heart.
on the arch of the aorta (main blood vessel coming out your heart) and in your neck are sinuses that detect blood pressure. these send siganal to your brain, that response with 2 main systems:
if you have low Blood pressure (BP)
"Sympathetic Nervous System" (SNS) also known as "fight of flight reflex" this releases systemic Adrenaline and noradrenaline (if you're american epinephrine) from your adreanl gland and also releases from nerves, noradrenaline all around your heart and blood vessels. These cause the following:
Increase in heart rate
Increase in heart contractility
Vaso & veno constriction
Increase you RAAS system, (later.)
It tends to divert blood to key areas, muscles, heart and brain, and away from your gut kidneys and skin.
so increases youre blood pressure.
You have a parasympathetic nervous systems (eat and sex system) which in a way the opposite of the SNS.
It only has nerves going to your heart, not blood vessels. the nerve to the heart is called the Vagus nerve (crainial nerve X) this slows the heart, hence why a normal resting heart rate is about 70 mpm, not 100bpm.
If you have low blood pressure and your SNS is compensating, it diverts blood away from your kidneys. Kidneys don't like this so ( or realise that the body is low in water) and release an enzyme called Renin.
(bear with me here)
Angiotensinogen is a protein released from the liver.
Renin, snips a bit of Angiotensinogen and it is then called Angiotensin I. THis doesn;t do much.
an enzyme in yor lungs called ACE ( angiotensin converting enzyme) then converts Angiontensin I to Angiontensin II. This is the most important protein in you cardiovascular system with reguards as it is a very potent vasoconstrictor ( tightenes up blood vessels)
Angiotensin II then stimulates the old adreal gland to relase a steroid hormone called aldosterone. This stimulates the kidney to retina sodium, and due to some osmotic melarky, water.
So this is called the RAAS, REnin-Angiotensin-Aldosterone System. it sucks in water.
There is also a system called the vasopressin or AVP system that comtrols salt levels.but that's what it basically does.
as a side note, if you drink alcohol, it tricks the brain, and makes it relase less Vasopressin, this then makes you kidneys relase more water... hence why drunk people pee lots.
There are many more little systems, endothelin, flavanoids, nitric oxide, oestrogen, glucocorioids, Brain&atrial nauturetic peptides. Also local things like temperature, carbon dioide levels, hypoxia...
hope this helps and it is not too complex
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