Now, we're going to start talking about external regulation and we're going to begin with the neural mechanism. Though our stimulus here is going to be increased sympathetic nervous system activity. And it's basically going to override renal autoregulation. So when we are in some kind of fight or flight situation, the sympathetic nervous system will basically totally take over the body in that moment. And so sympathetic activation is going to trigger the release of norepinephrine. And norepinephrine is going to constrict any blood vessels in non-essential organs, and that includes our afferent and efferent arteriole. So when both the afferent and efferent arterioles are constricted, we end up with a decrease in glomerular filtration rate.
If you look at our image here, if we have constriction in our afferent arteriole, imagine basically that we have turned the sink down. Right? Flow is restricted, though not very much blood is getting into that glomerulus. And if we have constriction of our efferent arteriole, kind of like our drain is clogged. So not much blood can get in, but not much blood can get out either. Basically, this entire process is just sort of stunted at the moment and can't really do very much. And the purpose of this is to help the body minimize fluid loss. We don't want to be prioritizing making urine if we're in some kind of emergency situation. Right? This is why we typically don't need to urinate during a fight or flight, situation. And this will also help to preserve blood volume and blood pressure at more vital organs. So, it has a very important evolutionary function.
Alright. So that is our first external regulation mechanism, and I'll see you guys in our next video. Bye bye.