When a person stands up, blood will pool in the legs (particularly in the elderly due to varicose and dilated veins) causing a drop in venous return to the heart which in turn causes blood pressure to fall. This fall in blood pressure is detected by baroreceptors in the carotid sinus and arch of the aorta. Baroreceptors are mechanical stress receptors which generate electrical impulses when stretched. As blood pressure falls (on standing up) the baroreceptors are stretched less and their rate of impulse firing decreases. The impulses are carried to by the vagus nerve to the nucleus of the solitary tract NTS in the brainstem. This in turn causes sympathetic tone to increase and parasympathetic tone to decrease. The sympathetic activity causes vasoconstriction of peripheral blood vessels and an increase in heart rate causing blood pressure to increase. In the elderly there is a loss of the baroreceptor reflex. Other factors can affect the sensitivity and efficacy of the baroreceptor reflex. Sherwood’s physiology
In healthy patients, blood pressure often increases on standing.
Signs and symptoms
- Tricyclic antidepressants
- Addisons disease
- Diabetes (autonomic neuropathy)
- Parkinson’s disease
- Long periods of immobility
- Large meals
- Sitting with crossed legs
Medication should be reviewed, particularly in elderly patients who are more likely to be on many drugs and have impaired vasovagal response.