Glomerular filtration rate (GFR): Lecture Notes

 

 

image

Glomerular filtration rate (GFR):

 

Before starting discussion, remember that basic renal functions are (Glomerular filtration, tubular reabsorption and tubular secretion) these issues will be discussed in concern

 

GFR: is the rate of forming a filtrate from passing plasma through Glomerular capillary tuft, it is about 125 ml/min, and in order filtration to occur, there must be a force that enforce the water and solutes to pass through Glomerular membrane openings, this force is called the net filtration force

* Remember that forces that act on GFR are (Glomerular capillary pressure, plasma colloid osmotic pressure and Bowman’s capsule hydrostatic pressure) and the net of these forces is the net filtration pressure, which normally favor filtration

Q. Is the GFR is constant or variable? And how to be regulated?

A. GFR depend on the three forces mentioned above, but, the two forces (plasma colloid pressure and the bowman’s capsule hydrostatic pressure) are constant in normal conditions, so, only the Glomerular capillary pressure will affect the GFR, if increased it will increase the GFR and vise versa

* Glomerular capillary pressure depends on amount of blood flow to glomerulus, which depend on the diameter of the afferent arteriole (caliber), when the diameter of the afferent arteriole increase (dilation), the blood flow will increase, and by this increasing the Glomerular capillary pressure, so that filtration will increase (GFR)

Q. What are the factors that regulate GFR?

A/ 1.sympathetic system: sympathetic stimulation to afferent arteriole make vasoconstriction (narrowing of blood vessel) and by this reducing blood flow to glomerular capillary which lead to decrease GFR, this is in addition to effect of sympathetic system on systemic blood vessels to make vasoconstriction and elevate blood pressure

Example: if blood pressure decreased for some reason, baroreceptors will detect this, and will stimulate sympathetic system to make generalized vasoconstriction on blood vessels including afferent arteriole and by this will reduce blood flow to glomerulus and reduce GFR, reduced GFR will lead to reduction in urine volume, in other word conserving water and salts in blood instead of excretion in urine, by this increasing blood volume and that’s means increasing blood pressure, notice that how regulation of GFR is related to control of blood volume and as result blood pressure

Note: notice that when blood volume increase it will increase blood pressure, and blood volume is increased when there’s more water and salts (mainly sodium Na+) are conserved in body

 

2. Auto regulation of afferent arteriole: when systemic blood pressure increase it will dilate afferent arteriole, this dilation will stimulate the smooth muscles of afferent arteriole to contract, as to say when they contract they will narrow, and by this they are opposing the effect of dilation to keep the diameter of afferent arteriole constant i.e. to keep blood flow to glomerulus constant, this is called auto regulation and vise versa when blood pressure decrease, smooth muscle of afferent arteriole will relax

Calculation of GFR: before talking how to measure GFR, there is important concept should be clear, which is needed to calculate GFR, that is Renal clearance for a substance

 

Renal clearance: when there is substance in the plasma that should be excreted by kidneys, this occurs gradually each time blood pass through kidneys from a single heart pump, some amount of this substance will be cleared completely from a certain volume of plasma, as to say the kidneys are gradually clearingthe plasma from this substance until the whole amount of this substance that is present in the plasma is excreted in urine by several cycles of blood entering the kidneys, and this clearance occur in a rate which could be different from one substance to other, and this clearance occur by glomerular filtration of this substance, and may be in addition, by tubular secretion of this substance in to urine. So;

 

Clearance of a substance is the rate of plasma volumes that are completely cleared of this substance per minute. (So … the unit here is ml/minute)

 

Q. How to use renal clearance for a substance to measure GFR?

A. We can measure the GFR by measuring the rate of clearance of certain substances, which have following feature “should be neither secreted nor reabsorbed from renal tubules” like inulin, which is a substance that is injected, not toxic, and not metabolized by the body, and like creatinine, which is a waste product formed by the body and excreted by kidneys through glomerular filtration only (this substance is better than inulin in measuring GFR)

GFR = clearance of x {if x is only filtered}

 

Law of the clearance (for a substance x)

Clearance of x (Crx) = (concentration of x in Urine X volume of urine per minute) / Plasma concentration of x

Crx = Ux X V / Px

Note: remember that the substances that its clearance is used to measure GFR Inulin and creatinine

Renal plasma flow:

As mentioned before that amount of filtrate is about 20% of plasma passing through glomerulus, as to say 1/5 of plasma, so, its logical that plasma flow is 5 times the filtrate flow (GFR)

 

 

Q. Can we measure renal plasma flow?

A. Yes, by measuring the clearance of substance that has the feature should be filtered by glomerulus and secreted by tubules so that they are completely cleared from the plasma per one passage of plasma through kidneys “, as to say, that the plasma passing through the kidney is completely cleared of this substance for a single passage

 

* The important example of such substance is [Para Amino Hipporic acid (PAH)]

How to measure the renal blood flow

As you know that blood consist of cells and plasma, plasma constitute about 55%, and the cells constitute about 45%, so, when talking about blood flow that’s means the flow of these two components, and by knowing the flow rate of plasma (as mentioned above) we can conclude renal blood flow rate, example for simplicity we propose that plasma constitute 50% of whole blood, and cells is 50% of whole blood, so the blood flow rate is double that of renal plasma flow rate, if plasma flow rate is 625 ml/min , so the blood flow rate is 625 X 2 = 1250ml/min

Tubular reabsorption

Selective transfer of important substances that are present in tubules back to the peritubular blood

* By glomerular filtration all plasma constituents including waste products, nutrients, excess materials, and electrolytes, except plasma proteins , all these substances are filtered , but, the important substances that the body need , will be selectively reabsorbed

* Tubules have high reabsorptive capacity for some substances e.g. water reabsorption is about 99% of filtered , and sugar reabsorption is about 100% of the filtered in normal conditions, but, for waste products, the tubules have low reabsorptive capacity so they are excreted in urine, while for excess electrolytes they are also excreted in urine to maintain normal composition of plasma and ECF

* Each part of tubular system has its characteristic features which enable them to perform certain functions, example, for proximal convoluted tubule cells have extensive brush border in the luminal surface (brush border is membrane folds that increase surface area), which enable them for wide reabsorptive capacities for different substances, while in loop of Henle these brush borders are diminished significantly, in distal convoluted tubule cells there are little of them, in collecting duct they are absent

* For any substance to be reabsorbed from renal tubule to blood should pass through following:

1) Luminar membrane of the tubular cells

2) cytoplasm of the tubular cells

3) basolateral membrane of tubular cells (the side of tubular cell that face the capillary in the opposite side of lumen)

4) interstitial fluid between tubule’s cells and capillary

5) capillary wall cell

Related posts:

Posted in Physiology, Urology

FaceBook Page

(function(i,s,o,g,r,a,m){i[\'GoogleAnalyticsObject\']=r;i[r]=i[r]||function(){ (i[r].q=i[r].q||[]).push(arguments)},i[r].l=1*new Date();a=s.createElement(o), m=s.getElementsByTagName(o)[0];a.async=1;a.src=g;m.parentNode.insertBefore(a,m) })(window,document,\'script\',\'https://www.google-analytics.com/analytics.js\',\'ga\'); ga(\'create\', \'UA-69237529-7\', \'auto\'); ga(\'send\', \'pageview\');