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Glomerular Apparatus and Filtration - Anatomy & Physiology (view source)
Revision as of 10:45, 4 September 2008
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==Glomerular Filtration Rate==
The glomerular filtration rate or GFR is the amount of fluid filtered from the capillaries into the Bowmans capsule per unit time. The GFR can be expressed as the following formula:
'''GFR = K<sub>f</sub> x net filtration pressure'''
K<sub>f</sub> = the filtration coefficent
K<sub>f</sub> can furthermore be expressed by the following formula
'''K<sub>f</sub> = membrane permeability x filtration area'''
The GFR is practically proportional to metabolic body mass. Therefore the bigger the animal the greater the GFR. This obviously makes sense.
===Regulation of the GFR===
'''The following formula helps us to understand GFR and how various factors affect it. Whilst reading this article you may find it useful to refer back to it:'''
<big>'''Q = (PA - PE) ÷ R'''</big>
'''Q''' = Flow, '''PA''' = Pressure in afferent arteriole, '''PE''' = Pressure in efferent arteriole, '''R''' = Resistance
There are two major forces opposing GFR. These are the hydrostatic pressure in the Bowmans space and the plasma protein osmotic pressure. These are not under physiological control. The filtration coefficient is also beyond the realms of physiological control. On the other hand the hydrostatic pressure in the capillaries and the renal blood flow are under physiological regulation and adjust filtration according to the bodies needs.
====Regulation of Renal Blood Flow and Capillary Hydrostatic Pressure====
These two factors are determined by the arterial blood pressure coupled with the contraction of both the afferent and efferent arterioles. The total resistance of the afferent and efferent arterioles, which is determined by the contraction of them, determines the renal blood flow and any particular arterial pressure. Therefore it is important that they change with arterial pressure in order to maintain a steady renal blood flow.
=====Constriction of the Afferent and Efferent Arterioles=====
Normally the afferent arteriole is of larger diameter than the efferent. This means there is high resistance as the blood is forced from a wider vessel to a narrower one and this promotes filtration. If the arterial blood pressure remains constant then contracting either vessel reduces blood flow as it increases resistance. However contracting either has opposite effects on the filtration pressure. If you contract the afferent arteriole there will be less of a pressure difference between the afferent and efferent arteriole so there will be reduced filtration pressure. However if you constrict the efferent arteriole you are increasing the pressure difference between the two and filtration pressure increase.
Overall the constriction of the afferent arteriole decreases both blood flow and filtration pressure where as constricting the efferent arteriole decreases blood flow but increases filtration pressure. <sub>(Both of these statements are assuming a constant blood pressure)</sub>. The fact that both can be altered allows independent regulation of both GFR and blood flow.
=====Physiological Regulators of GFR=====
The main systems which regulate renal blood flow and GFR are:
'''[[Autoregulation of GFR - Anatomy and Physiology|Autoregulation]]'''
'''[[Renin-Angiotensin-Aldosterone System (RAAS) - Anatomy & Physiology#Effects of Angiotensin 2 on GFR|Angiotensin 2]]'''
'''[[The Role of the Sympathetic Nervous System on GFR - Anatomy & Physiology|Sympathetic Nervous System]].'''
'''[[The Effects of Nitrous Oxide and Prostaglandins on GFR|Nitrous Oxide and Prostaglandins]]'''
==Renal Clearance==
Renal clearance is the ability of the kidney to remove a compound from the blood. It is intimately linked to the '''glomerular filtration rate''', tubular reabsorption and secretion. Renal clearance is just part of the '''total body clearance''' of a compound. Other methods of clearance include; biliary, pulmonary and salivary excretion. To look at the renal clearance you need to know the amount of plasma that must have been filtered to produce that much of the substance in the urine per given unit of time. This is given by the following formula:
<big> '''Clearance = (Urine Concentration of a Substance x Urine Volume per unit time) / Plasma Concentration of the Substance''' </big>
===Renal Clearance and GFR===
For the clearance to represent the GFR the substance looked at must be filtered freely and from that point on as it travels through the nephron its concentration must not be altered. Therefore it must not be secreted or reabsorbed.
====Determination of GFR from Renal Clearance====
Inulin, Creatinine or Urea can be used.
=====Inulin=====
Inulin is injected into the plasma. It is freely filtered by the glomerulus and therefore its rate of excretion is directly proportional to the rate of filtration of water and solutes.
=====Creatinine=====
Creatinine is also freely filtered by the glomerulus so it represents a close approximation to GFR. You can measure endogenous creatinine. It's production from muscle is constant. In renal failure its secretion is reduced and it is secreted into the gut. You can also measure it by infusing exogenous creatinine. It is difficult as urine collection needs to be carried out over time and blood samples need to be taken.
=====Urea=====
The idea behind this is that you measure factors which would normally be excreted. The same idea is behind the measurement of endogenous creatinine. However this is not ideal as it is altered by non-renal factors and they only change when the renal failure is very advanced as there is a large functional reserve in the kidneys. Urea is also not idea in horses and ruminants as it is used by the digestive micro flora to make microbial protein.