Glomerular Filtration Flashcards
What percentage of cardiac output do the kidneys receive?
22%
What is cardiac output in l/min?
5 l/min
What is renal blood flow in ml/min?
1,100 ml/min
Haematocrit makes up what percentage of renal blood flow?
40%
How many ml/min is renal plasma flow?
660 ml/min
What percentage of renal plasma flow passes through the filtration barrier?
20% (132ml)
How many l/day of filtrate is formed?
180 l/day
The ultrafiltrate does not contain which 2 things?
Proteins and blood cells
What are the 3 layers of the glomerular filtration barrier?
Glomerular capillary endothelium
Basement membrane
Epithelial cells (podocytes)
Which substances pass through the glomerular capillary endothelium? What is not filtered through this layer?
Water, solutes, and plasma proteins are all filtered across.
Blood cells and platelets are not as they are too big.
What is the basement membrane made up of? Give 4 examples.
Extracellular matrix proteins and negatively charged glycoproteins.
Type IV collagen
Laminin
Fibronectin
Entactin
Describe the charge of plasma proteins and what this means for the filtration of them.
Plasma proteins have a net negative charge, and they will be repelled and restricted from filtration by the negative charge of the basement membrane.
Describe the arrangement of the primary processes of the podocytes and their function.
Wrap around the glomerular capillaries to strengthen them against high pressures.
Describe the arrangement of the secondary processes of the podocytes.
Interdigitate to form filtration slits
The slit diaphragms which bridge the filtration slits are made of which protein?
Nephrin
Define glomerular filtration rate (GFR).
The volume of filtrate formed by all the nephrons in both kidneys per unit time
What is the equation of GFR (the 2 factors which determine GFR)?
GFR = Kf x NFP
(glomerular capillary filtration coefficient x net filtration pressure)
What is the filtration coefficient?
A measure of the permeability and surface area of the glomerular capillary wall
Which 2 factors contribute to the filtration coefficient?
Hydraulic conductivity (permeability) of the filtration barrier per unit of surface area
Total surface area available for filtration
The filtration coefficient for glomerular capillaries is greater than that of systemic capillaries because of the combination of what?
Higher total surface area
Higher intrinsic water permeability of the barrier due to holes in the basement membrane and fenestrated endothelium
Which forces are involved in net filtration pressure?
Starling forces / pressure - 2 hydrostatic pressures and 2 oncotic pressures
What are hydrostatic pressures?
Forces that push water and solutes out of the compartment they are in
What are colloid osmotic (oncotic) pressures?
Forces holding water and solutes in the compartment they are in
Most physiological regulation of GFR occurs due to changes in which factor?
Glomerular hydrostatic pressure
What 3 things is glomerular hydrostatic pressure dependent on?
Systemic arterial pressure
Afferent arteriole resistance
Efferent arteriole resistance
What effect does angiotensin II have on glomerular hydrostatic pressure and how?
Increases glomerular HP by preferentially constricting the EA
What effect do prostaglandins and atrial natriuretic peptide (ANP) have on glomerular hydrostatic pressure and how?
Increases glomerular HP by dilating the AA
What effect do noradrenaline, adenosine, and endothelin have on glomerular hydrostatic pressure and how?
Reduces glomerular HP by constricting AA
What are the 2 mechanisms of autoregulation of GFR?
Myogenic response
Tubuloglomerular feedback
Define the myogenic response.
The inherent ability of smooth muscle cells in afferent arterioles to respond to changes in vessel diameter by contracting or relaxing
Tubuloglomerular feedback works by linking what?
Changes in NaCl in the tubule lumen to the control of its own afferent arteriole resistance in the same nephron
In tubuloglomerular feedback, which cells sense a change in the concentration of NaCl and where are they located?
Macula densa cells in the early part of the DCT
How are Na+ and Cl- transported into the macula densa cells?
NKCC transporter
Once inside the macula densa cells, how do increased levels of Cl- lead to constriction of the AA and subsequent decrease in GFR?
The Cl- depolarizes the basolateral membrane of the macula densa cells, which opens Ca2+ channels and causes increased intracellular Ca2+ concentration. This increased intracellular Ca2+ releases adenosine from macula densa cells. The adenosine acts locally, by a paracrine mechanism, to cause constriction of the nearby AA.
Which 3 tests are routinely used to assess renal function?
Estimated GFR (eGFR)
Serum creatinine
Serum Urea
eGFR is relatively inaccurate for what? But is useful for what?
As a point measure of GFR.
But useful for monitoring trends.
How would an amputation affect eGFR and why?
Increase eGFR due to lower creatinine levels in the blood
How would the consumption of meat affect eGFR and why?
Decreased eGFR due to higher creatinine levels in the blood
Most calculations which are used to calculate eGFR consider which 3 factors?
Serum creatinine, age, and sex
What is the name of the equation recommended by NICE to calculate eGFR?
CKD-EPI
Newer tests used to calculate eGFR measure which substance?
Serum Cystatin C
Give a limitation of using the CKD-EPI equation to calculate eGFR.
Does not include a measure of body size (amputations, malnutrition etc).
eGFR cannot be used for drug dose calculation for highly toxic drugs such as chemotherapy drugs. In these circumstances, where a more accurate eGFR is required, what test should be used?
Inulin infusion
