Causes of Orthostatic Intolerance 

To regulate blood pressure the autonomic nervous system has two main tools. It can either change the amount of blood flowing by increasing or decreasing the pumping of the heart, or it can change the resistance to blood flowing in the arteries. This is analogous to increasing the pressure in a garden hose by either opening the faucet or squeezing on the hose to narrow the diameter. 

To accomplish this seemingly simple task, however, a complex and redundant system has evolved. The cardiovascular system has high and low pressure receptors to sense pressure in various areas. The brain has a cardiovascular center to integrate all the information it receives. The brain sends out signals to alter the function of the heart and blood vessels to maintain normal blood pressure. The results are then sensed and the necessary corrections are made. In fact, the system shows continuous slight oscillations as the pressure sensors and brain work together to constantly adjust blood pressure. 

What has happened to this finely tuned system that leads to difficulty with blood pressure control in certain situations, such as after space flight? 

Some possible causes include: 

  • The pressure sensors are not sensing differences in pressure
  • The brain is not integrating the information properly
  • An adequate signal is not being sent to the heart and blood vessels
  • The heart and blood vessels are not responding appropriately

  •  Artery controlling blood flow