Circulatory System Physics of Blood Flow in Vessels Part One Losses of Pressure

next we need to think about velocity and the diameter of a single vessel so think about a garden hose if we put some volume of water in at this end say we put one liter in per second fluids are relatively incompressible so if we put one liter in per second that means one liter per second has to come out the other end one liter per second in one liter per second out what if there's a constriction in part of this tube if we're still putting in one liter per second we must still be having one liter per second leave on the other side let's zoom in here for a second and say that here's our leader each of these is a quarter of a leader moving this amount in each second over time this water is here the same four quarters of a liter 250 MLS each and again they move that far in a second what happens when they get to the constriction when they get to the constriction they cannot all go through side by side the only way to get through is to line up instead of moving this distance in one second now they have to move four times the distance as these come out they now can fit and so now they go more slowly this distance in one second so if they're moving this distance in one second here they have to be moving four times faster when they go through the narrower constriction if we apply that to an animal pumping blood out of the heart through arteries it'll increase speed with lower cross-sectional area but it will also increase resistance with lower cross-sectional area a cost of higher speed means more force stronger contractions blood moves through the circulatory system in response to pressure generated by the ventricles pressure is the force of random molecular motion pushing molecules against each other and bouncing them off the walls of a vessel but a lot of the energy of fluid moving through a tube is momentum a portion of the random elec euler motion being directed in which there's a net movement in one direction and that momentum tends to keep fluid moving forward fluids actually moved from areas of greatest total energy to areas of lower total energy that doesn't matter when there is no particular directed movement but in it one of the consequences is that it matters that vessels actually branch off from each other at slight angles because a branch like this will bring a lot even distribution applied between the two branches than a branch like this in which momentum will put a greater tendency to move fluid in this direction than this direction even if the vessel diameters are the same in physiology there's another important element of this same point which is at an area where there's a sudden ballooning of the tissue so imagine an aneurysm where the vessel wall has weakened and thinned and as a result it's bulging out because it's not able to withstand the pressure as well that's an aneurysm physics that I just talked about has an effect on the tendency of aneurysms to get worse fluid entering the area of an aneurysm has laminar flow in the center of the vessel slower as it gets closer to the boundaries and of course turbulent flow along the boundary layer as it enters because of the opening out flue slows add has to spread to fill this space and so for Tex currents like so fluid going through an aneurysm has some of its energy of momentum or inertia converted to R and a molecular motion because of this turbulent flow and the turbulent flow also puts additional pressure some Luud hits against the vessel walls in an aneurysm at random molecular motion is increased heat and increased pressure so in an area that is already weakened also has the most pressure making it even more likely to burst now let's think about the circulatory system as a whole in one sense it's equivalent to along to a garden hose but it starts as a single opening day aorta and then immediately starts to branch off other arteries each element contributes resistance you can think about each individual location contributing its own share to the resistance we can add all of those by looking at what's called the total cross sectional area the total cross sectional area would be all of these different parts and we can add all of them up from every single location among the arteries to get the total cross sectional area of the arteries and we can do the same for capillaries each capillary is tiny but you have miles of them and so the total cross sectional area of the capillaries when you add them all up is much higher than the total cross sectional area of the arteries the veins are somewhere in between again we can total up all of the veins from venules to large veins and again we get a total cross sectional area if we plot that to get a sense of what is the resistance in arteries capillaries and veins on the y-axis we have total cross sectional area what we'd find is that arteries are relatively low in cross-sectional area as we go to arterioles they start to go up by the time we get to capillaries it's high and then for veins we go back down to an intermediate level because the total cross sectional area in arteries is narrow you can think of them as a constricted area and garden hose and so blood moving through the arteries is fast select the narrow part of a river or a stream the total cross sectional area of the capillaries is very high and so blood moving through the capillaries is slow that creates high time a large amount of time for exchange in the veins the rate of movement is intermediate and that's not a problem in veins they're not delivering essential materials fast to any tissues they're simply taking wastes and carbon dioxide to excrete that intermediate rate of transfer is fine the skeletal muscle pump and the relatively low pressures of blood coming out of capillaries is sufficient to move blood from the veins back to the heart among vertebrates selection appears to have favored fast movement through the arteries slow movement through the capillaries and slow movement through the veins pressure also changes with distance from the heart pressure in millimeters of mercury from zero to about 150 and we'll look at arteries arterioles capillaries venules and veins depending on the extent to which are exercising pressure and the arteries may be as low as a hundred or so and could be above a hundred and fifty let's start with somewhere around 120 into your arm pressure is going to decrease with distance because the energy of pressure entering is continually being lost part is simply collisions of molecules from the blood with the walls of the vessel which transmits some energy to molecules on the outside of the blood vessel energy is being lost as energy of vibration and of heat that decreases pressure with distance in addition that pressure is traveling as a pulse in which the elastin molecules in the artery walls are being stretched and then as that pulse passes those elastin molecules go back to their original shape that elastic recoil recovers some of the pressure loss but some of that energy has lost as heat it's lost as heat partly from displacing the molecules and the tissue on the outside of the blood vessel and partly it's lost because the elastins are not a hundred percent efficient you're losing some energy is heat so the pressure may start at somewhere around 120 it declines through arterioles by the time it gets to capillaries it's down in the range of 30 or lower the clients further through the capillaries and is down near five to ten sometimes even lower than that by the time it gets to the venules and the veins that pressure is enough to move blood back but the skeletal muscle pumped is adjusting that muscle movements and pressure will raise that up and down so in particular when you're exercising skeletal muscles are working hard and that can produce significant raises and pressure in the venules and veins returning blood to the heart

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