How is Blood pressure regulated?

What is renin angiotensin aldosterone system?This system is something that is involved in long term regulation of blood pressure that is from week to months .Lets first talk about renin.Renin is an enzyme that is produced from kidney more specifically modified smooth muscle cells of afferent arteriole.Afferent arteriole is a blood vessel.It has smooth muscle cells.Some of the smooth muscle cells are modified to produce and store renin.They are called JG cells JUXTA-GLOMERULAR CELLS.

Once blood pressure decreases it is sensed by  this JG cells and there happens chemical changes in pro-renin and renin is formed and released into circulation .Once renin is into circulation it converts angiotensinogen produced at liver  into angiotensin I.Now This angiotensin I reaches lung via circulation since it is in our blood circulation.There  it is converted to angiotensin II via ACE angiotensin converting enzyme .Remember this conversion of angiotensin I to angiotensin II occurs at lungs.Now this angiotensin II is responsible for raising blood pressure .How?Number one mechanism it is a potent vasoconstrictor. let us look here if we have a blood vessel with this diameter and a blood vessel with  this constricted diameter .It is thinkable that this blood vessel due to its small diameter will have more pressure whereas this due to its increased diameter will have less pressure .This principle is of fluid dynamics  you can think of a pipe with water with more diameter and less diameter.less diameter will obviously have more pressure .in this way angiotensin II is a potent vasoconstrictor and constricts arterioles and raise blood pressure .Second mechanism is via renal retention of salt and water .How salt and water retention will raise blood pressure .sodium when absorbed into circulation will withdraw water passively and increase liquid  or increased water or fluid will obviously increase the blood pressure .we have a pipe and we have less water and we have more water  .now think which one will have more pressure .the one with more water with a less diameter .So this mechanism play a role in raising blood pressure but how this angiotensin II helps  in retention of salt and water let us see that here .angiotensin ii stimulates adrenal cortex to secrete aldosterone .now this aldosterone will act on our nephron ,DCT and CD and will absorb sodium from lumen into the blood which will withdraw water and this will increase blood pressure .now exact mechanism of aldosterone acting on nephron, its receptor and how there will be absorption of sodium and water.This is a separate topic .we will talk about that in next video.But for now just remember that angiotensin II stimulates adrenal cortex to secrete aldosterone that is responsible for absorption of sodium and water that will increase blood pressure .Now there is a third mechanism by which this angiotensin II will raise blood pressure .this is again something related to vasoconstricting effect of angiotensin II let us look at this diagram if this afferent arteriole is constricted not only afferent arteriole all the arteriole of the kidney nephron if constricted there will be slow passage of blood .this will allow to rapid reabsorption of fluid from tubules due to slow flow of blood and this will raise blood pressure .in this way RAAS renin angiotenisn aldosterone system helps in maintenance of arterial Blood pressure over long term.