 So let's start having a look at the physiology and a bit of anatomy of the cardiovascular system. Now no doubt you've learned a lot about it in medical school, but that might be worthwhile for us just to go through one or two things. In this chapter a very brief and schematic look at the anatomy of the heart. We'll ask the question, what does the cardiovascular system do? And I might use the term heart and cardiovascular system interchangeably. How does it function? What are the mechanisms that regulate pressure? And what are the mechanisms that regulate each of these mechanisms that regulate the pressure? Okay, interesting, let's get going. The anatomy, now the heart has four chambers, as you know, is a schematic of it, the right atrium, the right ventricle, the left atrium and the left ventricle. Now that is certainly the way that we are taught in medical school. It's actually a lot more complex than that. We know that there are oracles on the right atrium and the left atrium, which we don't pay particular attention to in early classes on anatomy, but certainly they play a big role. Unfortunately, on too many occasions, had to put very large bore lines in the right oracle. And at some cases even putting phallus catheters into the right oracle, handing it over to the anethnotus, so that he could do massive and a rapid transfusion through those lines directly into the right side of the heart, trying to in a last ditch attempt to save a patient dying of massive hemorrhage. So certainly there are the oracles there as well, but for our purposes here, right atrium, right ventricle, left atrium, left ventricle, we can see that there are two inputs and two outputs for these systems. So blood will return to the right atrium, and there will be a certain pressure associated with that, a mean pressure of about 5 millimetres of mercury. We've got these all in millimetres of mercury. Now that comes from the superior vina cave and the inferior vina cave. It will go into the right ventricle. The right ventricle contracts during that contraction called sisterly. There will be a low-ish pressure, peak pressure of about 20 millimetres of mercury. When it relaxes, there will be a pressure of about 5 millimetres of mercury. It then pushes that blood out. Now there is very poor oxygen in this blood, rich in carbon dioxide, pushes it out the pulmonary arteries. So those will be the only arteries with oxygen poor blood into the lungs. In that pulmonary artery will get pressures of about 25 during sisterly and 8 millimetres of mercury during diasterly. It then goes through this whole capillary network for gas exchange and all sorts of other things. And then return to the left side of the heart. Now oxygen rich, CO2 removed and that pressure there will be about on the other side of a whole capillary bed and the lungs will be at about 8 millimetres of mercury into the left ventricle. That will contract with peak pressures of about 120 millimetres of mercury that the pressure in the chamber reaches and 8 quite low-ish during diasterly. And then out into the aorta through the body, through its capillary system all the way back and the cycle starts again. And the cycle starts again. So we have two capillary bed systems, the systemic system. The blood pressure 120 over 80 sisterly, diasterly and the pulmonary capillary system. So what happens while the blood goes through arteries, a major artery, the first one they aorta, right brachiosophallic with the brachiosophallic with the carotids, with the subclavians, et cetera from the arch of the aorta. We eventually get into smaller arterials. They are smaller. They contain muscle in the wall that can contract, closing down the diameter. Closing down the diameter of the vessels. And we know if we have a tube, if you have a balloon and you squash that balloon, now that's air, the pressure goes up. You can't really take a liquid and compress it that easily. Minimal, but the pressure will definitely rise through this whole system. Then the capillaries where all the exchange take place. So small, so thin. Under microscope you see these single cells actually move through them. Single cell lines move through them. Now we get little venules past that capillary system. There's a confluence of many of these capillaries into venules eventually into larger and larger veins until we get the inferior vena cava and the superior vena cava at the top. So this whole system of moving blood around. But what does the cardiovascular system interesting, phenomenally interesting, the mechanisms that control it, the struggles that we have in intensive care unit to manage this whole system is so beautiful and so intricate which obviously makes it a quite interesting topic of course in space medicine. But why do we have all of this? Well, we require it for a few things. We require it basically for cell function. Each and every living cell in your body needs gases, it needs oxygen, it needs to get rid of carbon dioxide, it needs nutrients, each and every cell and it has to get rid of byproducts of metabolism and these cells need to be controlled somehow to increase their work, to decrease their work, whatever that specific cell is and for that we also have the blood stream. The mechanisms that control many of the functions in these cells a message needs to be sent to them and the cardiovascular system is that highway to get these messages there. One of at least the highways to get the messages there. So when we just look at single cells, organisms there are only a few cells, they are in touch with the environment. They can exchange gases and nutrients right across the cell membrane between them and the environment. As we get to multicellular system though it's not that easy. You cannot just dump the garbage on the outside of your house and that's the end of the story. You have to come and collect it and take it away. You can't just dump it outside. So in a multicell organism to go buy food to put your garbage out et cetera you need a whole mechanism of moving things in and out. So that's why we have a cardiovascular system. Now how does this whole system function well physically quite easy it works from a concept of pressure differential. We just saw the higher pressures there on the left side as it moves the blood forward. As we go from arterials to capillities the pressure goes down, down, down. It's a higher pressure that drives the system. Now depending on the position of course of the human body if you are in a staining position there's also pressure differential. So all that blood needs to be pushed all the way back up against gravity. So gravity plays an enormous part in this circulation and how the cardiovascular system is put together to drive this pressure differential. When you lie down in a recumbent position things change and when you take gravity away things change dramatically. Pressure plays an enormous part and gravity plays as big a part in this pressure differential as anything else and therefore in space medicine it becomes crucial effect of weightlessness of effectively removing the force of gravity by this circular motion around the planet causes problems for us as far as the maintenance of this pressure differential is concerned. So if the function is driven by pressure what mechanisms regulate this function which is pressure? Well here they are. No particular order and also not the full story. There's volume, there's the heart rate. There's stroke volume, there's viscosity and there's vessel radius. Let's look at the volume. Now the blood consists of many things. Everything dissolved in water and cells as well. Many immune cells, red blood cells, platelets etc. So how do we regulate the volume in a 70kg man? There's about 5 litres of volume. Well we can put water into the tank. We can let water out of the tank and we can even produce. We have a factory to produce water inside the tank. And we know if we take oxygen and we burn a substrate like glucose what are we going to get from this? Well we're going to get CO2 of course that we have to expel but we also produce water. So there is certainly definitely a water production system. So the intake, we can regulate the intake by thirst. If the osmalality goes up in other words your blood becomes salty. There are receptors in your brain which can recognise this. You can secrete anti diuretic hormone. Diuretic meaning excretion of urine. Anti meaning against that. So you're going to hold urine back. You're not going to produce as much urine. So you can hold volume back, fluid back which gives you extra volume. You'll also develop the sensation of thirst which drives you to go get some liquid to drink. We can also manage it by the renin angiotensin aldosterin system which has a huge effect on the cardiovascular system. Let's have a quick look at that. That's always quite interesting. So if you have a low blood volume or low blood pressure or your kidney sends that there's a low glomerulol that's funny. Filtration rate, GFR you're going to produce renin. Renin is an enzyme and it is produced by the juxta glomerulir cells in the kidney. I almost got it right this time. So you produce renin. There is a polypeptide that circulates in your blood which is made in the liver called angiotensin. Angiotensin. The name says what it does. Tenses up the blood vessels. And it cleaves this angiotensin into something active. An active polypeptide called angiotensin 1. Angiotensin 1 which needs to be converted to angiotensin 2 by another enzyme called angiotensin converting enzyme. And that enzyme was initially worked on to produce medication to low blood pressure. So that angiotensin 2 can, well it was ace inhibitor of all places made in the lungs or mostly in the lungs. There's other places too. Well it can vasoconstrict and it vasoconstricts those efferent arterials before it gets to the capillaries. So they constrict and it rises the pressure in the arterial system. In the kidneys though it acts actually the efferent vessels after the glomerulus. So it rises the pressure in the glomerulus so there's more filtration so the kidneys still work. So depending on where the system in the body is you get various areas that get vasoconstriction. But this angiotensin 2 certainly raises your blood pressure, arterial blood pressure. So it gives you vasoconstriction of different beds vasoconstriction but it also gives you the release of aldosterone. Aldosterone. So this aldosterone comes from the adrenal cortex cells in the adrenal cortex and that actually causes a reabsorption of salt specifically sodium and together with that we get water that gets retained or reabsorbed into the body from the renal system so we increase the volume. So a fantastic system and there's actually micro little renin angiotensin systems throughout different areas of the body but Sydney the kidneys and then from the lungs the ACE inhibitor from the liver your angiotensin all played a major part. So antidiuretic hormone will have this same kind of effect antidiuresis wented water out retaining salt and water that comes straight from the brain. So there's all these mechanisms that can work to increase the volume. Not as fast, it's not as rapid so someone ex-sanguinates we do put up large ball lines and we give them volume under restricted conditions these days we don't just believe in filling a human being up with as much fluid as we can until such time as we have control over bleeding we do hold back things a bit these days Ok, so very interesting as far as this is concerned we can not only control volume but we can also control the heart rate if the heart beats faster more blood gets pushed through actually to some extent also increases the pressure and that's mostly under the autonomic nervous system there's a sympathetic sympathetic and parasympathetic pathways you know these well sympathetic pathways they are going to increase heart rate and under restful conditions the parasympathetic tone is predominant and you have a resting heart rate for very fit individuals fit astronauts they go up quite a low heart rate for unfit individuals a bit faster my resting heart rate you can use this to boast a bit there's about 48 beats per minute because I am an avat cyclist and jogger so I try to keep massively fit so there we go we can regulate it through the sympathetic system now this usually works on baroreceptors baro meaning pressure and a receptor little cells with a very specific function that can measure pressure or at least in some four measure pressure we get those in the aortic bodies so in the arch of the aorta in the arch of the aorta which gives off the right back is cephalic which goes up into the carotid right side to the right subclavian gives off the left common carotid left subclavian then goes down as the descending aorta in that arch we get the baro receptor the aortic body but we also get the carotid bodies carotid bodies now they sit at almost at the bifurcation of the common carotid as it goes into the internal and external carotids thinking back of it I have a special connection with the carotid bodies I had a carotid body tumor case in my final exams my final specialist surgery exams I had a patient with a carotid body tumor in any way they can measure pressure if the pressure drops it stimulates sympathetic outflow and you get an increase in heart rate if I were to give you adrenaline it will stimulate the sympathetic pathways and we will increase the heart rate we needn't only increase the heart rate though we can also increase the contractility of the heart the power that it generates the contractility and that's cool well if this increases this that's a chronotropic effect chronotropic that's anotropic you also get dromotropic but let's look at the anotropic so the regulation of that stroke volume I can make that heart contract better and if it does my stroke volume that is how much blood gets pushed out of the ventricles by each contraction okay if I just do that if I have heart rate and I multiply it so many beats per minute and I multiply it by stroke volume I get the cardiac output so how many liters per minute comes out because this is in per minutes this is a milliliters so many milliliters per minute now we see here preload contractility and afterload preload I am supplying blood to the heart that is the preload the more volume there is to give to that the better now this comes from the venous system which we already saw was a very low pressure system so it can't really it can't play a major role by contracting like the arterial system and we do think in astronauts that loss under the loss of gravity plays an enormous part in the preload side of things and we'll get to that a bit later so preload plays an enormous part contractility then the better contracts and the sympathetic or giving a patient adrenaline nor adrenaline or as the North Americans would know it's epinephrine nor epinephrine I can increase the contractility as well and then afterload we looked at the arterial blood pressure as those arterials contract under the angiotensin 2 we get increased afterload so this heart has to pump against a higher pressure so that all plays a part in stroke volume the last two things very quick mention I won't say much about this we are already at 20 minutes it's the viscosity and that means how much it flows there's a difference in viscosity between water and honey Sydney proteins and fluid can play a role in viscosity and that can alter the pressure as well and then lastly vessel radius which we've talked about before track the arterials and I can increase atleast the systemic arterial pressure so that's an overview of the cardiovascular system and a few specifics that I mentioned that we'll see we'll mention again as we come to the effects of space travel on these various systems brief overview you've learnt a lot about this in medical school and should note in quite a bit of detail specifically studying physiology from this knowledge here we'll move on to the effects that space travel has on the cardiovascular system