Autoregulation is a specific form of homeostasis used to describe the tendency of the body to keep blood flow constant when blood pressure varies..
While most systems of the body show some degree of autoregulation, it is most clearly observed in the kidney, the heart, and the brain. Perfusion of these organs (especially the latter two) is essential for life, and through autoregulation the body can divert blood (and thus, oxygen) where it is most needed.
One key component of autoregulation is the absence of central control. The "auto" in autoregulation refers of the abiliity of the organ to maintain a stable flow without the involvement of the autonomic nervous system.
In the kidneys, autoregulation is primarily concerned with maintaining renal blood flow and glomerular filtration rate.
There are two main techniques by which renal autoregulation is maintained:
- Myogenic mechanism: as blood flow increases, the afferent arterioles are stretched, they contract, and subsequently reduce blood flow
- Tubuloglomerular feedback: the macula densa "senses" the blood pressure and causes vasoconstriction
In the heart, the behavior of autoregulation is similar to that in the kidney, but there is no macula densa involved in the circuit. In the case of heart tissue that is in a state of high metabolic activity, autoregulation occurs due to increased metabolic bi-products such as lactic acid, carbon dioxide, and decreased oxygen and causes coronary vessel dilation. The autoregulation of coronary vessels is mediated by the equilibrium of ATP, ADP, AMP, and Adenosine in the myocardial cell. When there is a high amount of oxygen, the equilibrium is shifted towards ATP. When there is a lack of oxygen, it is shifted toward Adenosine. Adenosine causes vasodilation and therefore increases the supply of oxygen.
More so than most other organs, the brain is very sensitive to overperfusion, and cerebral autoregulation plays an important role in maintaining an appropriate blood pressure to that region. More detail is available at Cerebral perfusion pressure.