Fenestration & Endothelial Transport

00:01 Endothelial transport is much different than epithelial transport.

00:08 It also though has tight junctions.

00:12 The tight junctions though can be very tight such as in the blood-brain barrier when there’s no movement of fluid, or in capillaries they sometimes have fenestrations, pores, or sometimes even clefts that will allow things to travel through.

00:33 So, endothelial transport is going to rely less on specific transporters than epithelial transport does.

00:43 You’re going to utilize things like pressure and osmolality to move various solutes and solvents around.

00:53 It is primarily a variable associated with the flux of the substance.

01:00 So you will filter certain things, hydrostatic pressure is highly involved, and osmotic and oncotic pressures.

01:10 Remember, osmotic pressures have to do with the ion differences and oncotic pressures have to do with protein differences to draw fluid.

01:23 This is an example of how pressure can move fluid out of a capillary bed.

01:30 The higher the amount of pressure, the more fluid travels through.

01:36 Other examples determine about how much the fenestrations are in terms of their width.

01:43 Some will allow more fluid to travel out and some will allow less.

01:47 The other issue that we need to think about with endothelium versus epithelium is that we sometimes turn the membranes a little bit differently.

01:58 The terminology used for the inside surface is the luminal surface rather than the apical membrane.

02:07 In terms of the outside is termed the basal surface rather than the basolateral membrane.

02:15 But if you keep those linked together, you’ll be better off and able to think about the differences between epithelial and endothelial surfaces.

02:26 Let’s look at how fenestrations can be regulated because normally you think of a pore either being open or being closed, but you can modulate this in certain tissues.

02:38 The lymphatic is a great example of this.

02:42 So you can have some constriction and have the pores closed.

02:47 And then, after constricting these, you can open them up.

02:54 This then will allow fluid to transport between the lymphatic circulation.

03:01 Then when you have smooth muscle constriction, you close them up.

03:06 Good examples of how you can modulate these fenestrations.

03:11 Now, many tissues you don’t have the modulatory ability because either they are open or closed, but you do have some regulation of fenestration widths.

03:22 Let’s summarize now the different ways you’re going to move a solution, a substance, or a gas across the endothelial wall.

03:35 The first thing you could do is use something called pinocytosis, which is the actual pinching off of a small vesicle that contains a solute and solvent.

03:47 It moves from the luminal surface to the basal surface and then releases it out.

03:55 Another way you could get fluid through is by fenestrations, either fluid travels through or sometimes solutes travel along with the fluid.

04:07 The fenestration width will be dependent upon what molecules can make it through.

04:12 Water will always be able to make it through, but sometimes larger substances like big proteins, maybe like albumin, have a harder time in moving through these fenestration slits so they get stuck on one side or the other.

04:27 A primary variable is the pressure at which is in the hydrostatic, which is inside the vessel to push fluid through.

04:39 And that helps with the bulk transport driven by the pressure change.

04:46 You also have diffusion that is capable of moving a solute through these fenestration slits.

04:53 So this is based upon a concentration gradient.

04:57 The bigger the concentration gradient, the more solute is allowed to travel through.

05:03 Other items, such as gases or other things that are very soluble might be able to make it through the endothelial cell all on its own, without a fenestration slit, without pinching off the particular portion of the membrane and having it travel through.

05:21 These substances are usually more lipophilic in nature and therefore can travel through the membrane on their own.