human anatomy and physiology here we are in episode two on the urinary system and fluid electrolyte and acid base balance in this particular episode we will talk about fluid movement in the preceding lecture episode we highlighted how our body has various compartments and uh how water content and um various elements um tend to move between these compartments so uh INF fluid uh movement will be highlighting in particular a concept known as hydrostatic pressure hydrostatic pressure is essentially the force exerted by a fluid against a wall and it causes the movement of fluid between compartments if we take the example of blood and on this screen you're looking at the capillary exchange pressure is exerted by Blood against the walls of the blood vessels by the pumping action of the heart in the capillaries as you can see on this screen hydrostatic pressure is higher than the opposing coloid osmotic pressure in blood this is a constant pressure primarily produced by the circulating albumin at the arterial end of the capillary this pressure will force the plasma and nutrients out of the capillar reason into the surrounding tissues this results in that process of filtration that you see once again on the left hand side of the screen in uh the arterial end now fluid and the cellular waste in the tissues enter the capillaries at the venu end where the hydrostatic pressure tends to be much less than the osmotic pressure in the vessel filtration pressure will squeeze fluid from the plasma in the blood to the interstitial fluid surrounding the tissue cells the Surplus fluid in the interstitial space that is not returned directly back to the capillaries is drained from the tissues by the lymphatic system and then re-enters the vascular system at the subclavian veins now this concept of uh capillary exchange and filtration uh at the arterial end versus the Venus end is very important when we think about the hydrostatic pressure and how it governs the movement of water in the nephrons or those functional units of the kidneys this process that we're looking at is crucial to ensure the proper filtering of the blood to form urine as the hydrostatic pressure in the kidneys increases the amount of water leaving the capillaries also increases and more urine filtrate is formed if the hydrostatic pressure however in the kidneys drops too low as can happen in the case of dehydration the functions of the kidney become impaired and less nitrogenous waste will be removed from the bloodstream so that's just one clinical example of why um fluid movement is so important once again highlighting um how hydrostatic pressure plays a more significant role at the arterial end as it exceeds the blood uh colloidal uh osmotic pressure and thereby allows for the process of filtration this is in contrast to the uh Venus or the venal end where you have an a net negative filtration pressure and reabsorption occurs and that once again is because the hydrostatic pressure is actually less than the blood colloidal osmotic pressure and that's what we're looking at here on uh the screen now on this next slide we're looking at uh the process of facilitated diffusion and so this is a way once again in which we can have movement between uh the compartments in the body in this particular example we're looking at a facilitated diffusion with glucose molecules GL glucose molecules can use facilitated diffusion to move down a concentration gradient and that's owing to these carrier protein channels that we see embedded within the plasma membrane notably more broadly speaking passive transport of a molecule or an ion will depend on its ability to pass through the membrane as well as the existence of a concentration gradient that allows the molecules to diffuse from an area of higher concentration to an area of lower concentration typically speaking some molecules like gases lipids and water itself which can use water channels in the membrane known as as Aqua porin can actually make their way fairly easily through the cell membrane however other molecules such as glucose do not and therefore rely on this process of facilitated diffusion now let's look at the two types of pressure which we've highlighted in the preceding lecture slides we've mentioned the concept of hydrostatic pressure and we've mentioned osmotic pressure so let's break this down a little bit further simply put hydrostatic pressure is going to be that force or pressure which pushes fluid out of the container we saw this highlighted when we looked at the arterial end of the uh capillary exchange slide earlier in this lecture episode and in contrast colloid or colloidal osmotic pressure is the force or pressure which retains fluid within a container like a sponge I will give one uh analogy and that is of a bucket so you you can imagine you can see a bucket on the top right of your screen if there is a hole in the bucket that has no sponges all the water will leave if there is a hole in the bucket that has one sponge most of the water will leave if there is a hole in the bucket that has 10 sponges most of the water stays when we're talking about the human body in this case those sponges are actually large proteins which cannot leave the contain container one such example is albumin the force that The Bu the force that the sponges have to keep the water in the bucket is therefore referred to as osmotic pressure now we have one last um term uh to Define and that is that net filtration pressure so on the earlier lecture slides when we were talking about the capillary exchange and we talked about filtration that occurs um at the arterial end of the capillary where you have very high hydrostatic pressure and a relatively uh low colloidal osmotic pressure uh that net filtration pressure once again is going to be positive and so once again it's that sum or the balance of the osmotic and the hydrostatic pressures at the opposite end when we were talking about the Venus end of the capillaries um we were mentioning that it's actually h a negative filtration pressure uh because the osmotic pressure is greater than the hydrostatic pressure and so um in the case of the kidneys uh that are responsible for regulating water recovery and blood pressure um it is crucial that you actually have uh a net filtration uh pressure that allows for the the um uh that allows uh uh uh urine to be sorry blood to be filtered and urine to be produced and therefore you can see here when we're looking um at the diagram on the screen here you actually have a positive outward pressure uh which is going to allow once again for that process of filtration to occur with a higher hydrostatic pressure