So let's put everything that we've learned together and come up with a homeostasis loop for regulating mean arterial pressure. So here we have our controlled variable which is going to be MAP and let's pretend we're having a big drop in MAP so you you fell sleep on the Lee St. railroad tracks got your arm cut off and you lost a lot of blood and your blood pressure's dropping so big emergency situation Well this will be detected by sensors as part of our homeostais loop and the sensors that monitor blood pressure are called baroreceptors and we know that we have baroreceptors in a couple of places we have them in our kidney and we also have them in our aortic arch and carotid artery so that we have sensors close to our heart brain loop because as we know it's really important to maintain adequate blood delivery to our heart and brain which are tissues that can't afford to be without any oxygen for any length of time because they can't replace themselves so these afferent neurons are going to report to an integrating center so these are what we had originally called error signals when we first did a homiostasis loop. They are afferent neurons bringing information into the central nervous system so within the central nervous system then or CNS we've got a couple of places where information from baroeceptors is integrated. One is the HT/PPG and the other is the cardiovascular control center which is in the medulla oblongata so CVCC equals cardiovascular control Center in the medulla oblong which is part of the brainstem. Ok se have integrated that information and then we're going to have to have some efferent output to send messages to our effector tissues which are going to act on cells, or rather cells and tissues that will bring our MAP back up again so before we put in our signals lets put what our effector tissues are so we have vascular circular smooth muscle of our blood vessels and we have the SA node of the heart so we have autorhythmic cells we have our myocardial pumping cells and the other tissue we have, we haven't really talked a lot about this yet but we will is the kidney OK so let's start with the output from the hypothalamic posterior pituitary axis So when low MAP is detected or we could have high plasma osmolarity then the HT/PPG increases secretion of ADH or vasopressin, so do you guys recall where you have ADH listed in your notes and you're probably thinking it's in your list vasoconstrictors and it is, so ADH has receptors on vascular circular smooth muscle and it causes, when it binds with receptors we get increased peripheral vasoconstriction so we're going to be squeezing blood into that heart brain loop. There's also receptors in the kidney for ADH. We'll talk about that in a lot more detail later but we have increased water conservation as the activity of the kidney. We can't at it back to the blood but we can conserve what we have. Okay so now let's look at what is the response of the cardiovascular control center to signals from afferent neurons that blood pressure is low after that is integrated. We have increased sympathetic output so you can recall we've got NE from sympathetic neurons and EPI from the adrenal medulla so where do we have receptors for EPI/NE? That would be adrenergic receptors. Well we know that we have adrenergic receptors on blood vessels so we're going to have vasoconstriction at alpha adrenergic receptors and we know that we've got receptors on the SA node of the heart so that when EPI/NE binds there we get an increase in heart rate and we know we have receptors for EPI/NE on the pumping cells so we'll get an increase in stroke volume and the sympathetic output also influences water conservation by the kidney because it reduces blood flow to the kidneys through alpha-adrenergic receptors so we can implicate the sympathetic nervous system in water conservation as well okay what about there kidneys acting as baroreceptors? If you recall when the kidneys detect low renal blood pressure we have increased secretion of renin and remember that converts that precursor protein that is always circulating in your blood called angiotensinogen, so it's always there just circulating around not doing anything until the kidney detects low-pressure secretes renin, converts it to you angiotensin I. ANG I doesn't do anything at least as far as I know and it is converted by ACE, angiotensin-converting enzyme, to ANG II and this is an active hormone and it's not a steroid hormone it's a non steroid so few guys recall angiotensin II is on your list of vasoconstrictors so it also causes increased peripheral vasoconstriction it also has receptors in the hypothalamus where it also causes increased ADH release and also increased thirst because that would be a great way to restore homeostasis if you've lost some fluid Is to drink and get fluid back the third place that we're going talk about receptors for angiotensin II is in the adrenal cortex and when angiotensin II binds with its receptors in the adrenal cortex this causes increased synthesis and release of aldosterone which is a steroid and aldosterone has receptors in the kidney where it also promotes water conservation The response of the kidney an to low MAP is a slower response compared to the peripheral vasoconstriction and increase in heart rate and stroke volume collectively these are referred to as the baroreceptor reflex and these changes happen almost immediately it takes a little bit of time for the effects of kidney to be realized so the end result of all of this of all of these effectors is to bring MAP back up. Now if you've lost a lot of fluid it might not bring it back up to the original your normal blood pressure but it'll certainly bring it back up from where it was here when you initially had that emergency situation maybe due to blood loss I think was our scenario here what you guys should do is take your notes template of this blank and make sure that you can reproduce this for yourself without looking at your notes and also make sure you can do it for the reverse situation What if map went up? Can you fill out this homeostasis loop for increased MAP? That would be a great way to prepare for your next quiz