What forces influence glomerular filtration?
Only a small part, about 1/5 (20%) of the blood that enters the renal glomeruli undergoes the filtration process; the remaining 4/5 reach the peritubular capillary system through the efferent arteriole. If all the blood that enters the glomerulus is filtered, in the efferent arteriola we will find a dehydrated cluster of plasma proteins and blood cells, which could no longer escape from the kidney.
When needed, the kidney has the ability to vary the percentage of plasma volume filtered through the renal glomeruli; this capacity is expressed by the term filtration fraction and depends on this formula:
Filtration fraction (FF) = Glomerular filtration rate (VFG) / fraction of renal plasma flow (FPR)
In the filtration processes, in addition to the anatomical structures analyzed in the previous chapter, also very important forces come into play: some oppose this process, others favor it, let's see them in detail.
- The hydrostatic pressure of the blood flowing in the glomerular capillaries favors filtration, thus the leakage of the liquid from the fenestrated endothelium towards Bowman's capsule; this pressure depends on the acceleration of gravity imposed on the blood by the heart and by the vascular patency, so that the greater the arterial pressure and the greater is the thrust of the blood on the capillary walls, therefore at hydrostatic pressure. The capillary hydrostatic pressure (Pc) is about 55 mmHg.
- The colloid-osmotic pressure (or simply oncotic) is linked to the presence of plasma proteins in the blood; this force opposes the previous one, recalling the liquid towards the inside of the capillaries, in other words it opposes the filtration. As the blood protein concentration increases, the oncotic pressure and the obstruction to filtration increase; vice versa, in a protein-poor blood the oncotic pressure is low and the filtration greater. The colloid-osmotic pressure of the blood flowing in the glomerular capillaries (πp) is about 30 mmHg
- The hydrostatic pressure of the filtrate accumulated in Bowman's capsule also opposes filtration. The liquid that filters from the capillaries must in fact oppose the pressure of that already present in the capsule, which tends to push it backwards.
The hydrostatic pressure (Pb) exerted by the liquid accumulated in Bowman's capsule is about 15 mmHg.
Adding the forces just described shows that the filtration is favored by a net ultrafiltration pressure (Pf) equal to 10 mmHg.
The volume of filtered liquid in the unit of time is called glomerular filtration rate (VFG). As anticipated the average value of the VFG is 120-125 ml / min, equal to about 180 liters a day.
The filtration speed depends on:
- Net ultrafiltration pressure (Pf): resulting from the balance between the hydrostatic and colloid-osmotic forces acting through the filtration barriers.
but also from a second variable, called
- Ultrafiltration coefficient (Kf = permeability x filtering surface), in the kidney 400 times greater than that of the other vascular districts; depends on two components: the filtering surface, or the surface area of the capillaries available for filtration, and the permeability of the interface that separates the capillaries from the Bowman capsule
To fix the concepts expressed in this chapter, we can say that reductions in the glomerular filtration rate may depend on:
- a reduction in the number of functioning glomerular capillaries
- a reduction in the permeability of the functioning glomerular capillaries, for example due to infectious processes that subvert their structure
- an increase in the liquid contained in Bowman's capsule, for example due to the presence of urinary obstructions
- an increase in colloid-osmotic blood pressure
- a reduction in the hydrostatic pressure of the blood flowing in the glomerular capillaries
Among those listed, for the purpose of regulating the glomerular filtration rate, the factors most subject to variations, therefore subjected to physiological control, are the colloid-osmotic pressure and above all the blood pressure in the glomerular capillaries.
Colloid-osmotic pressure and glomerular filtration
Previously, we stressed that the colloid-osmotic pressure inside the glomerular capillaries is approximately 30 mmHg. In reality this value is not constant in all the stretches of the glomerulus, but increases as one moves from the contiguous segments to the afferent arteriola (beginning of the capillaries, 28 mmHg) to those that are collected in the efferent arteriola (end of the capillaries, 32 mmHg). The phenomenon can be easily explained on the basis of the progressive concentration of plasma proteins in the glomerular blood, result of its deprivation of liquids and solutes filtered in the previous tracts of the glomerulus. For this reason, as the filtration rate (VFG) increases, the oncotic pressure of the glomerular blood increases progressively (being deprived of larger quantities of liquids and solutes).
In addition to VFG, the increase in oncotic pressure also depends on how much blood reaches the glomerular capillaries (fraction of renal plasma flow): if little is reached the colloid-osmotic pressure increases to a greater extent, and vice versa.
The colloid-osmotic pressure is therefore influenced by the filtration fraction:
- Filtration fraction (FF) = Glomerular filtration rate (VFG) / fraction of renal plasma flow (FPR)
Under normal conditions, renal blood flow (FER) amounts to about 1200 ml / min (about 21% of cardiac output).
The colloid-osmotic pressure is also influenced by the
- Plasma protein concentration (which increases in case of dehydration and decreases in case of malnutrition or liver problems)
There are many more plasma proteins in the blood arriving at the glomeruli and the greater the colloid-osmotic pressure is in all the segments of the glomerular capillaries.
Arterial pressure and glomerular filtration
We have seen how the hydrostatic pressure, that is the force with which the blood is pushed against the walls of the glomerular capillaries, increases as the arterial pressure increases. This suggests that when the arterial pressure values increase the filtration rate is also raised.
As shown in the graph, if the mean arterial pressure remains within values between 80 and 180 mmHg, the glomerular filtration rate does not change. This important result is obtained firstly by adjusting the fraction of renal plasma flow (FPR), thus correcting the amount of blood passing through the renal arterioles.
- If the resistance of the renal arterioles increases (the arterioles become narrower and less blood passes), the glomerular blood flow decreases
- If the resistance of the renal arterioles decreases (the arterioles dilate allowing more blood to pass), the glomerular blood flow increases
The effect of arteriolar resistance on the glomerular filtration rate depends on where this resistance develops, particularly if the dilation or narrowing of the vessel lumen affects the afferent or efferent arterioles.
- If the resistance of the renal arterioles afferent to the glomerulus increases, less blood flows downstream of the obstruction, therefore the glomerular hydrostatic pressure is reduced and the filtration rate decreases
- If the resistance of the efferent renal arterioles to the glomerulus decreases, upstream of the obstruction the hydrostatic pressure increases and with it the rate of glomerular filtration also increases (it is like partially occluding a rubber tube with a finger, it is observed that upstream of the obstruction the walls of the tube swell due to an increase in the hydrostatic pressure of the water, which pushes the liquid against the walls of the tube).
Summarizing the concept with formulas
| Afferent arterioles resistance | Efferent arterioles resistance |
| ↓ R → ↑ Pc and ↑ VFG (↑ FER) | ↑ R → ↑ Pc and ↑ VFG (↓ FER) |
| ↑ R → ↓ Pc and ↓ VFG (↓ FER) | ↓ R → ↓ Pc and ↓ VFG (ER FER) |
R = arteriole resistance - Pc = capillary hydrostatic pressure - VFG = glomerular filtration rate - FER = renal blood flow | |
To conclude, we emphasize that the increase in VFG due to increased resistance of efferent arterioles is valid only when this increase in resistance is modest. If we compare the efferent arterial resistance to a tap, we note that as we turn off the tap - increasing the flow resistance - the glomerular filtration rate increases. At a certain point, continuing to turn off the tap, the VFG reaches a maximum peak and slowly begins to decrease; this is the consequence of the increase in the colloid-osmotic pressure of the glomerular blood.
