What is glomerular filtration?
Glomerular filtration is the first step in urine formation. You see, in order to clean out the blood, you have to have a way of accessing it. And what we clean out is the plasma (not the cells). So, in glomerular filtration, a lot of the blood plasma spills out into the glomerular capsule.
This picture (kind of like the one I drew in class) illustrates what is happening inside the renal corpuscle. Blood comes to the kidney via the renal artery, branches into the smaller and smaller arteries you learned about in lab, and eventually into the afferent arteriole. The afferent arteriole feeds into the glomerulus, providing the blood for the glomerular capillaries.
As the blood travels through these capillaries, filtration (similar to what we saw with systemic capillaries) causes a lot of the plasma contents to spill out. When they spill out of the glomerular capillaries, though, they end up within the glomerular capsule. The glomerular capsule is continuous with the rest of the renal tubules. The solution that spills out in the renal corpuscle is the solution that we will clean out to make urine.
Why does glomerular filtration occur?
In the systemic capillaries you saw that filtration pressure was a combination of blood pressure pushing the plasma against the walls of the capillaries and osmotic pressure drawing water into the capillaries. In the glomerulus, similar pressures exist. But there are still some important differences.
We can now take all of these pressures and put them into one equation to understand the net pressure on the fluids within the renal corpuscle. When you take the pressures that are pushing plasma components out of the blood and subtract the pressure that are pushing the capsular fluid back into the blood, you get: 60 mm Hg - (32 mm Hg + 18 mm Hg) = 10 mm Hg outward. This is illustrated in the Marieb CD (by A.D.A.M.) as shown below. Please note that she uses slightly different numbers for the pressures than your textbook uses. Because of that, she gets a larger net outward pressure of 17 mm Hg.
Therefore, in the renal corpuscle, the net force on fluids is OUT of the blood, and it is called the filtration pressure.
What barriers are there to the passage of filtrate?
If you take a look at the same membrane that we have been viewing (from my drawing), you will see that there are two layers of cells that fluid (with its contents) has to pass through during filtration. The first layer is the endothelium of the glomerular capillary. You have already learned how materials cross the endothelium in systemic capillaries. Actually, in glomerular capillaries, it is even easier for material to leave... you see, the endothelium doesn't just have some slats, but it also has fenestrae. A fenestra is a hole, and fenestrae is the plural form of the word. The holes are big enough to let small molecules and water out, but not large molecules. You can see the fenestrae in this illustration from your textbook.
The second layer is the visceral layer of the capsule, formed by podocytes. Podocytes are epithelial cells, forming a simple squamous epithelium, but they are rather unusual in appearance. In this illustration, the podocytes are yellow and the endothelium is red. It is clear that the podocytes are cells with many processes. The processes are the pedicels. If you step back and take another look at the picture, you should be able to see that the podocytes all contact their neighbors so that they actually create a sheet around the capillary-- it is just a porous sheet. Can you see that? Maybe if you squint while looking at the figure it will be more obvious. The pores in this porous sheet are called slit pores.
As fluid leaves the blood, it thus passes first through the fenestrae, and then through the slit pores of the visceral layer of the capsule.
What materials are able to pass into the glomerular capsule?
Not everything can pass through. If you imagine that the fenestrae and slit pores have a maximal diameter, then some things can fit through and others can't. You know that birthday party game when you try to throw bean bags through a board with some holes in it? Only certain sizes of bean bags will fit through, and something larger, like a beach ball, will not.
What fits through?
What doesn't fit through?
Just because material can squeeze out of the glomerular capillary and into the capsule doesn't mean it is supposed to leave the body in urine. The next steps for us are to reabsorb some of this material lost through filtration. Specifically, we will want to reabsorb water, some electrolytes, nutrients, and some hormones. We will NOT want to reabsorb our wastes-- so we don't. They are headed out in our urine from here (for the most part).
Glomerular Filtration Rate (GFR)
We need to pass our blood through our kidneys to keep it clean. The speed with which we pass our blood through our kidneys and filter it is proportional to how well we can clean our blood. That means that we have to be able to filter a good deal of blood every minute to keep up with the wastes our bodies make. The speed of filtration is the GFR. Consider how much both kidneys combined can filter in one minute using all their nephrons-- it turns out to be 125 mL per minute. Can you picture 125 mm? It is 1/8 of a liter. That is a lot of filtering!
Over the course of a day, 125 mL/minute translates into 180 liters!!! We only have about 5 liters of blood in our bodies, so those 5 liters get cleaned out 56 times in one day! That is a lot of cleaning!
However, the figure of 125 mL/min is an average number. That can change depending on our situation. You see, the amount of filtrate changes with blood pressure changes. When your blood pressure rises, it rises in the afferent arteriole, too. That causes a greater outward filtration pressure within the renal corpuscle. If, on the other hand, your blood pressure drops, there is a lower outward filtration pressure due to a lower afferent arteriole pressure. You may even be able to imagine a situation where a person has suffered a trauma and has lost a lot of blood (this is shown in the Marieb CD)... if that person loses too much blood, their blood pressure will fall so low that there will be no net outward pressure in the renal corpuscle, so no filtrate will form-- therefore no urine (to remove the wastes) will form; this is dangerous and has to be rectified with intravenous fluids.
The changes in one's blood pressure cannot affect the filtration rate too much or we would run into problems. For example, when you exercise, your blood pressure rises. If we couldn't control the effect of this increased blood pressure on our kidneys, we would have to urinate a lot when we exercised! That would not be good. Therefore, we have a nice regulatory system built in. We can regulate incoming blood pressure by vasoconstricting or vasodilating the afferent arteriole. So, when we start exercising, we have to vasoconstrict the afferent arteriole. This constriction decreases the blood pressure entering the glomerular capillaries. It only decreases it enough to compensate for the increased body blood pressure from exercising.
You learned that a juxtaglomerular apparatus lies at a junction point between the afferent arteriole and the distal convoluted tubule. This apparatus gets involved in the regulation of afferent arteriole vasodilation/vasoconstriction. This is described more in the Marieb CD.
© 2011 STCC Foundation Press