Blood will Flow!
We all know what a reflex action is, right? Something that "just happens" when the doc taps his thingummywhotsit on your knee (the doc calls this knee-jerk a Patellar reflex)? Or when some people see and smell prime rib steak garnished with all the extras (Pavlov's conditioned reflex)?
Well, that's how the heart works, too, see. Reflexes are to help us survive, aren't they, and we're not going to get very far in this world if the heart ain't beating.
Now, can you imagine having to think about your pulse, to actually will it for it to happen? No, neither can I. That's why the beating of your heart is called an unconditional or innate reflex. There all that many of them and, since we have them from birth, no prior experience is needed for them come into play.
The heart's reflex is a visceral one, involving as it does smooth muscles - as opposed to a somatic reflex involving say, the skeletally-attached, striated muscles. It's comprised of a receptor (here, the sino-atrial node, SA node or sinus), visceral afferent(sensory) neurones, 2 visceral efferent (motor) neurone types (pre- & post-ganglionic) and, at last, the effector, in this case called the atrio-ventricular or AV node.
The SA node is at the upper right area of the right atrium, at the top of the heart, It electrically prompts the cardiac muscles (auricles) of the upper 2 chambers (one from the lungs with oxygenated blood, one from the body with depleted of oxygen) to contract in a co-ordinated wave, the systolic. The jolt is received by the AV node; this, in its turn, sends out prompts to relax the auricles and contract the ventricles - the lower 2 chambers, one to the lungs, the other to the remainder of the body. Their rhythmic contraction is the diastolic.
All reflexes, innate or conditioned, involve the spinal cord as the the central pathway between the afferent and efferent neurones and, therefore, the reflex. These, together, form the reflex arc. The signals eventually get to the brain but long after action has been taken.
Other methods of affecting the heart's action may or may not be involve the reflex (autonomic) system. These include:
i) Decreasing Heart Beat (Brachycardia): substances such as noradrenalin decreases heartbeat rate by constricting blood vessels, thereby increasing blood pressure; this, in turn, causes the brain to slow the heart down. Decreasing acidity in blood pH. Decreasing CO2 concentrations. Increasing O2 concentrations Fear and/or grief.
ii)Increasing Heart Beat (Tachycardia): increasing concentration of adrenalin. Decreasing baroreceptor stimuli. Increasing acidity in blood pH. Increasing CO2 concentrations. Decreasing O2 concentrations. Excitement or pain. Fever causing temperature rise in SA.