I dunno how much this is true, but I heard that people can learn to control their own heart rate to a relatively accurate degree, if they want to.
But as for the reason why it is easier to control breathing, well, perhaps it's because there are many situations in which it is useful to stop breathing for a while (if you are in a body of water, for example), while there are not as many situations in which slowing the heart rate would be as useful (perhaps when you are bitten by a snake? I'm not really sure...)
EDIT: ninja'd
edited 11th Mar '12 1:50:36 AM by Carciofus
But they seem to know where they are going, the ones who walk away from Omelas.Disclaimer
The act of breathing requires the movement of muscles (mainly the diaphragm
and intercostals - the ones between the ribs
, but also involving coordination with the other muscles in and around the neck - like those in the larynx - and chest), which are in turn connected to the brain. So breathing is primarily a "brain" function
. You have a "breathing centre" in the medulla / pons, which is affected by things like the concentrations of carbon dioxide, hydrogen ions and oxygen in your blood. It's then linked to the autonomic nervous system which then supplies the relevant muscles. You have limited conscious control as well, but it's not a complete override.
The electrical impulses in your heart, though, mainly originate from your sino-atrial node,
which depolarises automatically to give you a regular heartbeat. In addition, heart muscle is capable of spontaneously depolarising as well,
which means that even if your SA node should fail, you have reserves.
Plenty of reserves.
The autonomic nervous system influences the heartbeat and cardiac output through the SA and AV nodes, but it's not responsible for actually causing the heart to beat. And there is little to no direct conscious control of the heart rate, as far as I'm aware, although stress responses and the emotional state are closely linked.
So that's why you can't stop your heartbeat voluntarily, but you can stop breathing - for a while. Note that you can't suffocate yourself though breath-holding (please don't try this at home, kids) as your autonomic centres will eventually override your conscious control - either because the urge to breathe is too great, or you pass out and the conscious inhibition is taken away.
EDIT: Of course, that's just the "how". If you're looking for the "why"... well, can't help you there.
edited 11th Mar '12 4:49:38 AM by Pyrite
Not a substitute for a formal medical consultation.As just mentioned, there's redundancy that overrides conscious control. You can through biofeedback influence the heart in its contractions but the redundancy prevents such influence from becoming an on/off switch for the heart.
In shorter terms you can somewhat control your heart rate but you cannot make it stop consciously.
Carc: The only thing I can think of in that area is meditation and people slowing their heart rate, but that drops whether or not they think about dropping it it seems. Breathing reaches a very slow pace too. To the point where you have to train yourself to not freak out because you think you aren't breathing...
You can control your body temperature in that you can raise it with meditation though?
If someone wants to accuse us of eating coconut shells, then that's their business. We know what we're doing. - Achaan Chah

Yep, this is random: If the medulla oblongata controls things like the respiratory and cardiac systems, and those systems are supposedly involuntary, why is it that you can regulate your breathing, but not your heart rate?
And why can you stop breathing for several minutes, but not make your heart so much as skip a beat?
(PS. You are now breathing manually)
Leave your dignity at the door.