OK... so material is supposed to be able to get from the blood to the tissues, and vice versa.  What material?  Think back to the list of what is in the blood and might get out (or in):

We'll talk more about gas exchange during the respiratory unit, nutrient exchange during the digestive unit, and waste exchange during the urinary system unit.  But we'll hit on most of these things here, too.

    There are three basic ways for things to cross the capillary wall:

Let's take these one at a time...

Filtration

    Most of the items listed above are dissolved in the blood plasma.   Therefore, if we could get fluid out of the capillary, we could get these dissolved materials out, too.  As you saw on the capillary structure web page, capillaries can leak out material at the junctions between endothelial cells.  These spaces are rather small, so they can act together like a giant capillary strainer:  keeping larger things (RBCs) inside, while letting the fluid (plasma) out.  This is like how a pasta colander keeps pasta in and lets starchy water drip out.  The process of straining the fluid out is referred to as filtration.  Now the only major questions about stuff getting across capillaries are:

Let's answer these two questions...

Why and in what direction does fluid escape from the capillary?

    The capillaries are porous enough that the fluid within them (plasma) is in contact with the fluid outside of them (extracellular fluid, a.k.a., interstitial fluid).  Because these fluids can exchange (but must do so through tiny spaces), whether they will do so or not depends on the pressures on the fluids.

    There are two main pressures on the fluids.  One is osmotic pressure (shown here using blue arrows for the water).  The other is the blood pressure on the capillary walls (shown here using red arrows for blood).

    Osmotic pressure exists whenever two solutions with differing solute concentrations interact.  Water always leaves the hypotonic solution, pushing its way into the hypertonic solution.  This "pushing" is the osmotic pressure.   It turns out that the blood is hypertonic to the extracellular fluid.  So water is always trying to push its way into the blood vessel due to osmotic pressure.   The pressure due to osmosis remains constant across the entire capillary.

    The blood pressure, or hydrostatic pressure of the blood (a.k.a., outward hydrostatic pressure) is an outward pressure.  That's because the blood is pushing on the walls of the capillary as it runs through the capillary from arteriole to venule ends.  This pressure, however, is not constant throughout the capillary.   You see, there is more pressure on the arteriole side because it is closer to the heart (which is applying the pressure in the first place).  So it starts off at the arteriole side with a higher pressure than at the venule side.

    Taken together, you will see that the hydrostatic pressure of the blood pushing out is greater at the arteriole end than the osmotic pressure pushing in.   That means that the net pressure on all fluids at the arteriole side is outward.   Yet, at the venule side, the hydrostatic pressure of the blood pushing out is less than the osmotic pressure pushing in.  That means that the net pressure on all fluids at the venule side is inward.  Your book quantifies these pressures in a diagram of capillaries of the systemic circuit, which is shown in the systemic capillary exchange page.  But the overall scheme is that more fluid ends up being filtered out of the capillary than into it.

Diffusion

    Some things can diffuse across the capillary.  Whether a blood item can diffuse or not depends on its chemical nature.  Those items in the blood that are hydrophobic (or nonpolar or lipid-soluble) have no problem diffusing.  This is because hydrophobic materials have no problem crossing right through the endothelial cells themselves.  They do so because they do not see the membranes of the endothelial cells as barriers-- remember, membranes are based in lipids, so other lipid-soluble material can pass right through them.

    The hydrophobic materials that need to pass through the capillary are the gases (oxygen and carbon dioxide) and steroid hormones.

    The majority of the other substances have to pass through the endothelial slits (the gaps between endothelial cells).  Only smaller materials can do this, like nutrients.  Some of the way they pass is due to filtration pressure, although they can also pass by diffusion.

Diapedesis

    You already learned about this... white blood cells, which are too big for filtration or diffusion, squeeze out of the endothelial slits in capillaries.   The leukocytes have to crawl and squeeze to sneak through this tiny little gap.

© 2006 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.