Glomerular Filtration and Renal Blood Flow Flashcards
What are some barriers to filtration in the kidneys?
Glomerular capillary endothelium = red blood cell barrier
Basement membrane = plasma protein barrier
Slit processes of podocytes = plasma protein barrier
What must fluid filtered from the glomerulus into the Bowman’s capsule pass through?
The three layers that make up the glomerular membrane
What are the forces that make up the net filtration pressure?
Glomerular capillary blood pressure (BPgc)
Bowman’s capsule oncotic pressure (COPbc)
Bowman’s capsule hydrostatic pressure (HPbc)
Capillary oncotic pressure (COPgc)
How is the net filtration pressure calculated?
(BPgc + COPbc) - (HPbc + COPgc)
What is the oncotic pressure equal to?
The colloid osmotic pressure
What is the balance of hydrostatic forces and osmotic forces?
Staring forces
What is the glomerular filtration rate (GFR)?
Rate at which protein-free plasma is filtered from the glomeruli into the Bowman’s capsule per unit time
How can the glomerular filtration rate be calculated?
GFR = Kf x net filtration pressure
What is Kf?
The filtration coefficient = how holey the glomerular membrane is
What is the normal glomerular filtration rate?
125 ml/min
What is the major determinant of glomerular filtration rate?
Glomerular capillary fluid (blood) pressure (BPgc)
How is GFR regulated extrinsically?
Sympathetic control via baroreceptor reflex
How is GFR regulated intrinsically?
Autoregulation = myogenic mechanism and tubuloglomerular feedback mechanism
How does increased arterial blood pressure affect the GFR?
Increases blood flow into glomerulus = glomerular blood pressure and net filtration both increase, which increases GFR
What happens to the glomerular filtration rate if the glomerular blood pressure (BPgc) falls?
GFR decreases = occurs after constriction of afferent arteriole
Why don’t changes in systemic blood pressure always result in changes in GFR?
Autoregulation prevents short term changes in systemic arterial pressure affecting GFR
If GFR protected form changes in mean arterial blood pressure over a wide range of values?
Yes = as is renal blood flow
What occurs in the myogenic mechanism in the kidneys?
If vascular smooth muscle is stretched (increases arterial pressure), it contracts to constrict the arteriole
What occurs in the tubuloglomerular feedback mechanism?
If GFR rises then more NaCl flows through the tubule leading to constriction of the afferent arteriole
What part of the kidney is involved in the tubuloglomerular feedback mechanism?
The juxtaglomerular apparatus
What senses the NaCl of tubular fluid?
The macula densa
What are some things that may cause a decrease in GFR?
Increased HPbc (i.e due to kidney stone) Increased COPbc (i.e diarrhoea)
What are some things that may cause an increase in GFR?
Decreased COPgc (e.g severely burned patient) Decreased Kf = due to change in surface area available for filtration
What is plasma clearance a measure of?
How effectively the kidneys can clean the blood of a substance
What is the plasma clearance equal to?
The volume of plasma completely cleared of a substance per minute
Is plasma clearance unique to each substance?
Yes
How can plasma clearance be calculated?
Rate of excretion/plasma concentration
Urine concentration x urine volume/plasma concentration
What is the clearance of inulin equal to?
The GFR = measurement of clearance can be used to clinically determine GFR
What are some features of inulin?
Freely filtered at the glomerulus, neither absorbed nor secreted, not metabolised by kidney
How does inulin enter the urine?
By filtration alone
Where can inulin be measured?
Easily measured in urine and blood
What can be used instead of inulin to measure GFR?
Creatine clearance
How are the plasma and urine concentrations of inulin related?
Plasma inulin x GFR = Urine inulin x urine volume
What is the clearance of substances which are filtered, completely reabsorbed and not secreted?
Clearance = 0 (e.g glucose, also applies to substances not filtered and not secreted)
What is the clearance of substances which are filtered, partly reabsorbed and not secreted?
Clearance < GFR (e.g urea, only portion of plasma is cleared)
What is the clearance of substances which are filtered, secreted but nor reabsorbed?
Clearance > GFR (e.g H+, all of filtered plasma is cleared of a substance, as is the peritubular plasma from which it was secreted?
What happens of clearance is less than GFR?
Substance is reabsorbed
What happens if clearance equals GFR?
Substance is neither reabsorbed nor secreted
What happens if clearance is greater than GFR?
Substance is secreted
What is used to calculate renal plasma flow?
Para-amino hippuric acid (PAH) = exogenous organic anion, used clinically to measure RPF (= 650ml/min)
What are some features of PAH?
Freely filtered at the glomerulus, secreted into tubule (not reabsorbed), and completely cleared from plasma
What happens to the PAH that escapes filtration?
Secreted from the peritubular capillaries
What are some example clearance values?
Glucose = 0 PAH = 650 ml/min Inulin = 125 ml/min Creatine = 125 ml/min
What are some benefits of measuring creatine clearance?
Produced at near constant rate, freely filtered and not reabsorbed but is slightly secreted, gives close approximation of GFR
What is the drawback of using inulin clearance as a marker for GFR?
Requires constant infusion to insure constant plasma concentration
What are the features of a good clearance marker?
Non-toxic, inert (doesn’t metabolise) and easy to measure
What are some features of a good GFR marker?
Filtered freely, not secreted or reabsorbed
What are some features of a good RPF marker?
Should be filtered and completely secreted
What is the filtration fraction?
Fraction of plasma flowing through the glomeruli that is filtered into the tubules
How is the filtration fraction calculated?
GFR/Renal plasma flow
How does GFR compare to the plasma volume?
GFR is much greater
How is renal blood flow calculated?
RPF x (1/1-Hct)
What is Hct?
Haematocrit
Does the afferent or the efferent arteriole have a wider diameter?
The afferent arteriole
Where are the pores in the glomerular capillaries located?
In between endothelial cells = exclude red blood cells
What is the basal lamina (basement membrane) made of?
Collagen and glycoproteins = confer net negative charge which acts as barrier to large plasma proteins
