9: Physiology 3 Flashcards
What are the three barriers which make up the glomerular membrane?
Endothelium of glomerular capillaries
Basement membrane
Podocytes of Bowman’s capsule and their slit processes
Glomerular capillaries are more ___ than other capillaries.
permeable
What is the glomerular basement membrane made up of?
Type IV collagen
(Goodpasture’s syndrome is a Type II hypersensitivity reaction where antibodies attack the Type IV collagen of the basement membrane)
What charge does the glomerular basement membrane have?
Negative charge
What’s good about the negativity of the glomerular basement membrane?
Large negatively charged proteins can’t pass through it
Is filtration an active or passive process?
Active process
What is hydrostatic pressure?
Fluid pressure
What is oncotic pressure?
Pressure exerted by negatively charged plasma proteins
The glomerular capillaries have a high, constant ___.
pressure
What is the fluid pressure within Bowman’s capsule called?
Does it push fluid in or out of the glomerular capillaries?
Bowman’s capsule HYDROSTATIC pressure
Pushes fluid INTO capillary
What type of pressure keeps plasma proteins within the capillary?
What causes this?
Capillary oncotic pressure
Negatively charged basement membrane
What is the value of the oncotic pressure of Bowman’s capsule?
Zero
No plasma protein in the capsule
Which forces
a) aid filtration
b) block filtration?
a) Capillary blood pressure
b) Bowman’s capsule hydrostatic pressure, capillary oncotic pressure
What is the name for the total force which aids glomerular filtration?
What value does it have?
Net filtration pressure
10 mmHg
What is GFR?
Glomerular filtration rate
Rate of filtration of protein-free plasma into Bowman’s capulse per unit time
What is the equation for GFR?
GFR = Kf x net filtration pressure
Kf = filtration coefficient, or how permeable the glomerular membrane is
Net filtration pressure was 10 mmHg according to last slide
What is the main determinant of GFR?
Capillary hydrostatic pressure
which is the strongest force pushing plasma out of the capillaries
What is the normal GFR?
120 ml/min
What extrinsically regulates GFR?
Sympathetic nervous system
via Baroreceptor reflex
Increasing glomerular capillary blood pressure also increases ___.
GFR
What increases and decreases capillary blood flow and therefore blood pressure, GFR?
Vasodilation of afferent arteriole increases GFR
Vasoconstriction of afferent arteriole decreases GFR
How do you calculate MABP?
((2 x diastolic) + (systolic)) / 3
How does renal blood flow and therefore GFR react to short term changes in blood pressure?
Not very much
autoregulation to prevent water and electrolyte imbalance
When is autoregulation of renal blood flow and GFR useful?
Exercise - BP increases but you don’t want to lose too much water and electrolytes
Low BP - you still want to filter adequately
What is myogenic regulation of GFR?
Increased blood flow / BP causes walls of arteriole to stretch
Automatic constriction of smooth muscle to reduce blood flow > reduce GFR
Keeps GFR the same during short term hypertension/hypervolaemia
Which part of the distal tubule regulates GFR?
Juxtaglomerular apparatus
If solute concentration in the distal tubules increases, which cells cause smooth muscle constriction in the afferent arteriole to reduce blood flow and therefore GFR?
Macula densa cells detect inc salt
Granular/juxtaglomerular cells cause smooth muscle constriction
This is the short term regulation, juxtaglomerular apparatus also involved in RAAS which is the long term regulation
How do kidney stones affect GFR?
Blockage in collecting system, buildup of filtrate
so Bowman’s hydrostatic pressure increases, stronger force AGAINST plasma movement into capsule from capillary, GFR decreases
How does diarrhoea affect GFR?
Dehydrated
Same amount of plasma protein
Capillary oncotic pressure increases (against movement OUT of capillary into bowman’s)
GFR decreases
What is Kf a measure of?
Filtration coefficient
i.e surface area available for filtration
What is plasma clearance?
How effectively kidneys can clear blood of a substance
What is the unit of plasma clearance?
Volume per MINUTE
What is the equation for plasma clearance?
Plasma clearance = rate of excretion / [X]plasma
Where rate of excretion = [X]urine x urine flow rate
What substance is used to determine the GFR of patients?
Why?
Inulin (not insulin)
It passes straight through glomerular membrane with no trouble, and isn’t reabsorbed, so Plasma clearance = GFR
Inulin needs to be infused into the patient before determining their GFR.
What is naturally produced, freely filtered like inulin and can be used instead?
Creatinine
Under normal circumstances, how much glucose is reabsorbed by the nephrons?
All of it
What is the clearance of glucose in normal patients?
Zero
What does glycosuria mean?
Glucose in the urine, i.e clearance isn’t zero, i.e kidney problem
diabetes, cushing’s, acromegaly…
When may clearance be higher or lower than GFR?
When substance is reabsorbed into capillaries or secreted into tubules
If clearance is less than GFR, the substance is being ___.
reabsorbed
If clearance = GFR, the substance is being ___.
neither reabsorbed nor secreted
If clearance > GFR then the substance is being ___.
secreted into the tubules
What is the clearance of
a) glucose
b) creatinine
c) inulin
d) PAH?
a) 0
b) GFR
c) GFR
d) Equal to renal plasma flow (some reabsorbed and ALL secreted)
What chemicals are used as
a) renal blood flow
b) GFR
markers?
a) PAH
b) Creatinine, inulin
What is filtration fraction?
The proportion of plasma flowing through the glomeruli which is actually filtered
How do you calculate filtration fraction?
Filtration fracture = GFR / Renal blood flow
What percentage of blood is filtered in the
a) glomerulus
b) peritubular capillaries?
a) 20%
b) 80%
How do you know that renal blood flow is 25% of cardiac output?
cuz you can use all of the above to calculate renal blood flow
