Capillary Exchange and Bulk Flow (Nursing)

00:00 So now let's look at the speed or velocity of blood flow through our capillaries.

00:07 The velocity of flow is going to change as blood travels through the systemic circulation.

00:13 Our velocity is fastest in the aorta and slowest in the capillaries.

00:19 It then increases again once it goes into the veins.

00:23 The speed is inversely related to the total cross sectional areas.

00:29 Capillaries have very large cross sectional areas and therefore have the slowest flow.

00:37 Slow capillary flow is important because it allows for adequate time for the exchange between our blood and the tissue surrounding them.

00:49 Vasomotion is the intermittent flow of blood through our capillaries.

00:54 And it happens due to the on and off, opening and closing of our precapillary sphincters.

01:02 Recall that once blood is in the capillaries, molecules in the blood are going to move across the endothelium and either diffuse directly through the endothelial membranes, such as with lipid-soluble molecules pass through the intercellular clefts, such as water soluble solutes, pass through the fenestrations or small holes, such as water-soluble solutes or by way of active transport via pinocytotic vesicles or caveolae which are going to move larger molecules such as proteins.

01:41 So now let's look at how molecules flow in and out of the capillaries into the interstitial fluid.

01:50 This usually happens by diffusion.

01:52 As molecules move down their concentration gradient.

01:58 Fluid is going to be forced out of those intercellular clefts of the capillaries at the arterial end of the capillary bed.

02:07 And most of this fluid will return to the blood at the venous end of the capillary bed.

02:13 Fluid movement is extremely important in determining the relative fluid volumes in the blood as well as the interstitial space.

02:22 Bulk fluid flow across our capillary walls caused continuous mixing of fluid between our plasma and our interstitial fluid and maintains our interstitial environment.

02:35 The direction and the amount of fluid flow, it's going to depend on two opposing forces.

02:42 We have our Hydrostatic Pressures and our Colloid Osmotic Pressures.

02:47 Let's take a closer look at each of these pressures.

02:51 Starting with hydrostatic pressure, this is the force that is exerted by a fluid pressing up against the wall.

02:59 There are two types of hydrostatic pressure.

03:03 Inside the capillary, we have the Capillary Hydrostatic Pressure.

03:08 This is the blood pressure that tends to force fluid through the capillary walls.

03:14 It is greater at the arterial end of the capillary bed than it is at the venule end of the capillary bed.

03:22 The second type of hydrostatic pressure is interstitial fluid hydrostatic pressure.

03:28 This is the pressure that is pushing back up against the vessel from the outside.

03:34 This is usually assumed to be zero because also in our interstitium we have our lymphatic vessels, which are going to further drain the interstitial fluid out of the interstitium.

03:48 The next type of pressure is our Colloid Osmotic Pressure.

03:53 There are two types of colloid osmotic pressure.

03:56 First, we have the capillary colloid osmotic pressure which is going to be the sucking pressure that is created because the plasma proteins inside of the capillaries are not able to move out.

04:09 This causes water to be pulled into the capillaries.

04:14 Our capillary colloid osmotic pressure is usually about 26 mm Hg.

04:21 The second colloid osmotic pressure is the Interstitial Fluid Colloid Osmotic Pressure.

04:28 This pressure however is inconsequential, because our interstitial fluid has such a low protein content.

04:35 So, this osmotic pressure is only around 1mm Hg.

04:42 So, we measure these together as our Net Filtration Pressure, which is going to comprise all the forces that are acting on the capillary bed.

04:54 The net filtration pressure is the addition of our hydrostatic pressure in the capillary to the osmotic pressure in the interstitial fluid minus the hydrostatic pressure in the interstitial fluid added to the osmotic pressure in the capillary.

05:14 The net fluid flow out of an arterial end is referred to as our filtration.

05:21 A net fluid flow at the venous end is referred to as reabsorption.

05:26 More fluid leaves at the arterial end than returns at the venous end.

05:33 But the excess interstitial fluid is going to be returned to the blood by the lymphatic system.

05:43 So, to summarize, there are two kinds of pressure driving bulk flow of fluid through our body.

05:51 We have hydrostatic pressure, which is due to fluid pressing up against a boundary such as a capillary wall.

06:00 Hydrostatic pressure is sometimes referred to as the pushing pressure because it pushes fluid across a boundary.

06:07 And in our blood vessels this is going to be due to our blood pressure.

06:13 The osmotic pressure is due to nondiffusible solutes that cannot cross boundaries.

06:20 This leads to fluid coming into that boundary or being pulled in.

06:25 So our osmotic pressure is sometimes referred to as our pulling pressure.

06:30 And our blood vessels, our osmotic pressure is due to the presence of our plasma proteins which cannot diffuse out.