When you stand up after lying down gravity very quickly pulls blood towards your feet and away from your head. As a result, blood delivery to the brain decreases momentarily and you may feel dizzy. What type of feedback system would be best to manage these changes in blood pressure? Where should the sensors be located? What would happen if the system failed to work properly? Ageing and medications can adversely affect the function of this homeostatic system.

We need to ensure that we have adequate blood pressure to maintain blood flow to the brain. Standing up reduces the pressure in the blood vessels supplying the brain, so the blood pressure is now too low. We need to get the blood pressure back to normal to maintain adequate blood flow. In homeostatic terms what has happened is that the controlled variable (the blood pressure) has fallen below the setpoint and needs to be returned to the setpoint. The best system to do this would be a negative feedback system. The best place to measure blood pressure would be in the vessels that are delivering the blood to the brain, and important sensors are located in these vessels (but because we want an overall picture of blood pressure in the body senses are located in many additional places in the body). If the system failed to work properly you would experience prolonged dizziness, and possibly even faint. That would increase the risk of falling, which is an important problem in older people. The control of blood pressure is very complex and you will hear more about it in subsequent modules.