Glomerular Filtration

What Gets Filtered?

As whole blood flows along in the fenestrated capillaryCapillaries with pores (fenestrations) that allow rapid exchange of fluids and small solutes (found, small substances like ionsCharged atoms or molecules. of sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. and potassium(K⁺): Major ICF cation; essential for muscle and nerve function. can fit through. They pass through both the fenestrations of the capillary and the filtrationThe process by which fluid moves out of capillaries into surrounding tissues due to hydrostatic pre slits of the podocytesSpecialized cells in the glomerulus that form filtration slits, preventing large molecules from ente. This would allow these cations to enter into the fluid contained in Bowman’s capsule. They also enter into the filtrateThe fluid that is filtered from the blood into the nephron and will eventually become urine. that is formed from filtration. Any substances that are larger than albuminA plasma protein that helps maintain osmotic pressure and transport substances. will remain in the fenestrated capillary and leave through the efferent arterial. 

Your red blood cellsThe basic structural and functional units of life. and white blood cells are too large to be filtered out of your whole blood. This is why red blood cells and white blood cells are abnormal when found in urineThe liquid waste excreted by the kidneys.. They should not be in urine and if they are it indicates a breakdown of this filtration membrane. Anything larger than albumin should remain in your whole blood. However, proteinsLarge molecules made of amino acids with various functions in the body. smaller than albumin are not abnormal to be seen in urine.

Most nonpolar hormones or steroid-based hormones are metabolized in the liverA large organ that produces bile, detoxifies blood, and stores nutrients.. Their waste components are used to make bileA digestive fluid produced by the liver and stored in the gallbladder; it helps emulsify fats for di. However, some polar hormones, such as epinephrineadrenaline): Fight-or-flight hormone from the adrenal medulla., are metabolized in the kidney. This means that they’re removed by the kidney. This also would indicate that these proteins are smaller than albumin. They are capable of fitting through the fenestrations and the podocyte slits.

Therefore small hormones are expected to be present in the urine. Cations and anions will always be filtered into the filtrate by the glomerulusA network of capillaries in the nephron where blood filtration occurs.. However, the second capillary bed makes adjustments. It could remove these cations or anions from the filtrate. This renders them absent in the urine.

Filtration Pressures

We have already discussed filtration when we talked about it in the blood vessels many lecture. Filtration is a process that moves items from a blood capillary into interstitial fluids. Here in the kidney those interstitial fluids are in the capsular spaceThe space between the glomerular capsule and the glomerulus where filtrate collects. of the renal corpuscleThe structure in the nephron that consists of the glomerulus and Bowman’s capsule, where filtratio.  That space caught between the podocytes and the parietal layer of the capsule. 

Blood in the glomerulus exerts a hydrostatic pressureThe force exerted by a fluid, such as the pressure of blood pushing against the walls of capillaries pushing substances through the fenestrations and the filtration slits of the podocytes.  Don’t forget that the podocytes are the extra layer in the renal corpuscle.

On the venule side of a normal capillary bed, colloid osmotic pressureThe pressure exerted by proteins (mainly albumin) in the blood that pulls water into the capillaries of the blood draws fluids from the interstitial fluids. These fluids move back into the capillary bed. The same is still true here in the renal corpuscle. There is colloidA mixture where small particles are dispersed but not dissolved in a liquid. osmotic pressureThe force exerted by water moving across a membrane due to osmosis. or the attractiveness of solutes in the blood.   The colloid osmotic pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. generated by your blood is a force that opposes filtration. The COP of the blood keeps fluid in the glomerulus.  Those colloids attract waterThe universal solvent essential for life..  One more force affects the filtration rate. It discourages filtration along with BCOP or blood colloid osmotic pressure.  The fluid in the capsule actually pushes back on the glomerulus.  Think of the glomerulus like a water ballon submerged in water.  The water inside the ballon exerts force on the balloon wall, but so does the water on the outside.  This pushback is CsHP or the hydrostatic pressure of the

Net Filtration Pressure

Net filtration pressure is a concept we saw before, although you may not have realized it them.. Of course, this was in the section A cut or slice of the body or an organ for study. on blood vessels.  Remember how fluids leave capillary beds via filtration? Not all of the fluid gets reabsorbed on the venule side of the bed.  That excess was absorbed by your lymphatics.  In other parts of the body, the filtration in capillary beds is always greater than reabsorptionThe process of fluid moving back into capillaries from surrounding tissues due to colloid osmotic p. This creates a net pressure. The pressure forces things out of the capillary beds more than into them.  We called this net filtration pressure(NFP): The difference between hydrostatic and colloid osmotic pressures, determining whether fluid and we used hydrostatic pressures and colloid osmotic pressures to calculate it.

HP always stands for hydrostatic pressure or the pushing of water from one place to another.  Hydrostatic pressure is like the pressure that is exerted against the membrane of a water balloon.  GHP is the hydrostatic pressure of blood in the glomerulus.  That blood is pushing against the fenestrated capillary, forcing fluids from the glomerulus to the capsular space.  That is the one and only force encouraging filtration.  That hydrostatic pressure is largely determined by your blood pressure.  If you like in hypertension, your blood is pushing too much on this membrane.  If you live in hypotension, your blood isn’t pushing against the membrane enough. We know that during surgery for our patient in the case study, his blood pressure went dangerously low. This happened during surgery.  His GHP, the one and only force that encourages filtration, went down.  How low?  Does it matter?  If it goes down, NFP goes down.

CsHP is the hydrostatic pressure of the filtrate in the capsular space.  Just like the blood in the glomerulus has a hydrostatic pressure, so does the capsular space.  This is the force of the filtrate pushing back against the filtration membrane.  This is always lower than GHP because filtrate and urine are always flowing outward.  The renal basin creates a pressure difference in the kidney that actually sucks fluid through nephrons.  If you have a kidney stone in your ureterThe tube that carries urine from the kidney to the bladder., your renal basin becomes clogged with urine wanting to get out.  That clogging pushes back all the way to the capsular spaces in all your renal corpuscle of that kidney.  The hydrostatic pressure of the capsule goes sky high, reducing NFP.

The last force that determines filtration in the glomerulus is the colloid osmotic pressure of the blood.  Recall that colloid osmotic pressure is the attractiveness of solutes.  BCOP specifically refers to the attractiveness of the solutes in blood. It describes their ability to keep fluid in the glomerulus. BCOP prevents fluid from filtering into the capsular space.  This value of BCOP varies with your osmolarityA measure of solute concentration in fluid; affects fluid movement between compartments..  How salty are you?  Are you horribly dehydrated?  Your BCOP is high.  Did you just eat salty crackers?  Did you just pee out a whole bunch of diluted urine?  Your BCOP is high.  If it goes up, NFP goes down.

The significance of all these pressures and calculating NFP lies in the relationship to GFR or glomerulus filtration rate. This is something many of us have seen on a urinalysis.

Glomerular Filtration Rate

The glomerular filtration rate measures the formation of filtrate in the capsular spaces. It considers all your nephrons in one minute.  NFP and GFR are proportionally related to each other.  This means that they rise and fall together.  This makes sense in the big picture.  NFP is the force pushing my blood through the filtration membranes of my nephrons.  This is the force pushing the filtrate in my capsular spaces into all my PCTs.  In our case study, our patient’s GFR dropped horrendously low in surgery.  All his little glomeruli deflated for a moment.  His GFR dropped to essentially nothing. 

More than just the NFP or the ability to create pressure on the membrane determines GFR.  The total surface area for the filtration membrane makes a difference.  For our patient, how many of his glomeruli were able to reinflate and start filtering again?  Not all of em, I can tell you that. 

The last factor determining GFR is the permeability of the membrane.  How healthy are your podocytes?  Are the fenestrations in your glomeruli blown wide open?  Think of a strainer that you might use for say macaroni and cheese.  If the holes in the strainer increased, some of your luscious macaroni might slip through the holes.  Not good.  You need the right size.

In our patient in the case study, his membranes are shot.  Albumin can slip through the fenestrated capillary. Other proteins also pass through the filtration slits of the podocytes.  He is peeing out all his proteins, and we know that from his urinalysis.  We also know that because he seems to have edemaExcess fluid in interstitial spaces..  His blood is not reclaiming fluid filtered out in his capillary beds.  He has hypoproteinemia.  These are more signsObjective clinical findings observable by a provider (e.g., edema, fever). indicate this.