Neural or immediate response
and With a reduction in blood volume, a neural or
immediate response occurs within minutes. The right
atrial and left atrial pressures fall, activating low
pressure receptors in the atria and walls of the
pulmonary arteries, great veins and ventricles. With
further intravascular blood loss, the reduction in venous
return causes a decrease in cardiac output and blood
pressure, activating high pressure stretch receptors in the
aortic arch and carotid sinus. Severe hypotension (e.g.
MAP of 50 mmHg or less) activates chemoreceptor
receptors of the carotid and aortic bodies; and at a MAP
of 40 mmHg or less, a central nervous system ischaemic
response occurs. These signals are transmitted to the
vasomotor centre in the medulla and pons, which sends
efferent impulses via the sympathetic and vagus nerves
to increase the heart rate, myocardial contractility and
peripheral arteriolar and venous tone. The baroreceptor
mechanisms act for a few hours only because continued
stimulation leads to adaptation, causing the
baroreceptors to reset to a new value in less than 2
days.