 This poor person met with an accident and as you can see on your screen, he is bleeding profusely. So his body's blood volume is gradually declining as he's losing blood, right? So as the blood volume of the body declines, it is natural for the person to have less blood pressure. And if the pressure of the blood is less, the blood flow to the vital organs of the body will not be enough to keep them functioning and gradually, the person will die. But you know what? The human body will not give up that easily. It is a fighter, you know, it will try its best to maintain the volume and pressure of the blood by initiating a full-blown hormonal response in different parts of the body, which works together to keep the person alive. So in this video, we are going to look at that amazing in-built system of our body. Okay? Let's begin. So in the nephron of the kidneys, we have special cells that could sense the declining pressure of the blood. So as the blood flows through the efferent arteriole, let me label them first. This is the efferent arteriole, efferent arteriole, proximal convoluted tubule and the distal convoluted tubule, okay? Now, as the blood is moving with very less pressure in the efferent arteriole, there are special group of cells that could sense the declining pressure. Now, the next thing that happens when the pressure is extremely low is that the filtrate moves very slow through this proximal convoluted tubule because the pressure is less, so it moves very slowly down the PCT. Now in the PCT, sodium extraction takes place from the filtrate and as the filtrate is moving very slowly, a lot of sodium is extracted in this part more than what it would otherwise do. So the concentration of sodium that reaches the DCT is extremely less now and this change in the sodium iron concentration is sensed by another group of cells that are present in the nephron and both these group of cells are interconnected. They're connected through connective tissues and both of them together we call it the juxtaglomerular apparatus. Where juxtag means nearby and since it is nearby the glomerulus, it is called the juxtaglomerular apparatus. Now, as this apparatus gets activated, it releases a special hormone called renin into the blood. So it is now the renin's job to tell the entire body that, hey, we need to increase the blood pressure and the blood volume, okay? And renin starts the process by converting a hormone secreted by the liver called angiotensinogen into angiotensin1. Well, renin did this conversion with the hope that angiotensin1 will be able to convey that strong message to the whole body. But angiotensin1 was not that powerful, okay? So the lungs came to its rescue. The lungs had special enzymes in them that could convert angiotensin1 to a very powerful hormone called angiotensin2. Now this angiotensin2 is potent enough to go to different body parts and convey this message. Now, before we proceed into what angiotensin2 does, let me tell you that all these conversions from angiotensinogen into angiotensin1 and lung secreting enzyme to convert it into angiotensin2, all of these activities are happening in the blood stream, okay? Because these are hormones and hormones are always secreted in the blood. Okay, now coming back to angiotensin2, the first thing it does is constricts the blood vessels. And when the blood vessels are constricted, the blood pressure will increase, right? So angiotensin2 is an awesome vasoconstrictor. So a very important task is done, right? Increasing of the blood pressure. Now after blood pressure, the next target of angiotensin2 will be to increase the blood volume, right? And for that, angiotensin2 knocks at the door of the adrenal glands that sits over the kidneys, especially the adrenal cortex. And the adrenal cortex immediately jumps into action and secretes a hormone called aldosterone. And what does aldosterone do? It rushes to the nephrons of the kidneys and instructs them to absorb more sodium and water, more than what they would otherwise do, okay? And when more sodium and water is taken into the blood, the blood volume will increase, right? So the second target is met, blood volume will increase. Now as the person is bleeding, the situation is of an emergency. The body takes more than one route to tackle the problem of pressure and volume, okay? So angiotensin also reaches other part of the body to ask for help. So one such part, the most important one, is the brain. It stimulates the thirst center of the brain. So the person will feel like drinking more and more water and the water will be absorbed into the blood and the blood volume will increase, okay? Next in the brain, it stimulates the hypothalamus as well. The hypothalamus instructs the posterior pituitary or the neurohypophysis to secret another potent hormone called anti-diuretic hormone, ADH, or it is also called vasopressin. Now diuresis means excessive production of urine. An anti-diuretic hormone is meant to stop that. And how does it stop? By reabsorbing most of the water back into the body. And it does that in the later part of the nephron. And if more water is absorbed back into the body is put into the blood, the blood volume will increase, right? So this is how the bleeding person maintains his blood volume and blood pressure through a chain of hormonal action in different parts of the body. And this wonderful inbuilt mechanism of the body is called the renin angiotensin mechanism or sometimes the renin angiotensin aldosterone mechanism. Now let's imagine that the bleeding person received medical help. His bleeding has stopped and he is out of danger now. So his body should also calm down, right? It should stop increasing the pressure and volume. Otherwise what would happen? I mean if the person is completely normal, not bleeding. But his pressure and volume is increasing like crazy because the internal mechanism is going on. That could again kill the person, right? So all this hormones at some point needs to calm down. And how is that done? Well, all the hormones, most of the hormones have feedback mechanism. Let's say the ADH increased blood volume. Now as the blood volume will increase, the pressure will increase and we will reach a state where we do not want any more increase in the pressure or volume. At that time, the change in the pressure or volume of the blood is sensed by osmoreceptors which are present in the hypothalamus. And those osmoreceptors will send a signal immediately to the posterior pituitary signaling them not to release ADH anymore into the blood. Now one such regulatory effect is also expressed by the heart, the atrium of the heart. So when the pressure and volume of the blood in the body increases, more blood reaches the atrium of the heart, right? So when more blood enters the atrium, the wall of the atrium stretches. And in response to that stretch, it releases a hormone called the atrial netriuretic factor or ANF. Now what is netriuresis? It means that more and more sodium is sent back into the nephron to excrete out. So more and more sodium will be sent out. That is netriuresis. Now water loves sodium a lot. Water will always follow sodium. So when more water is drained out from the kidneys or excreted out from the human's body, that means the blood volume will decrease and the blood pressure will also decrease. Right? That is why ANF is also called an awesome vasodilator. So this is how different hormones from different parts of the body works in harmony to maintain the blood volume and blood pressure of a person.