4_HST110 Glomerular Filtration 2017 Flashcards
What substances in the blood are filtered by the kidneys?
- Ions (e.g. Na+, K+)
- Organic molecules (e.g. glucose, amino acids)
- Small peptides (e.g. insulin, antidiuretic hormone)
What substances are NOT filtered by the kidneys?
Cellular elements (e.g. red blood cells, white blood cells) and most proteins
What is the criteria for a substance to be freely filtered? (e.g. most low molecular weight substances)
Concentration in the filtrate is the same as its concentration in the blood plasma
What is the filtration fraction?
The proportion of the renal plasma flow that is filtered by the glomerulus
Filtration fraction = GFR / RPF ~20%
What is the filtered load?
Amount of a substance that is filtered per unit time
Filtered load of substance X = GFR * [X]plasma
Substances are filtered based on what two criteria in the glomerular filtration barrier?
Size and charge
What are the molecular weight cutoff values for easily filtered, less readily filtered, and not filtered substances?
Easily: 7 kDa
Less Readily: 7-70 kDa
Not Likely Filtered: > 70 kDa
For large macromolecules: For any given molecular radius, (X) charged molecules are less readily filtered and (Y) charged molecules are more readily filtered
X = negatively Y = positively
What are the normal ranges for GFR?
Males: 90-140 mL/min
Females: 80-125 mL/min
Avg: 125 mL/min (180 L/day)
GFR represents the sum of filtration rates of (X) in the kidney and can be used as an index of (Y)
X = all nephrons Y = kidney function
What are the determinants of GFR?
- permeability of the capillaries
- capillary surface area
- net filtration pressure (NFP) acting across the capillaries
What is the equation for filtration rate?
Filtration rate = permeability x SA x NFP
Capillary surface area is difficult to estimate, so we define a parameter called the (X) as the product of the hydraulic permeability and surface area
X = filtration coefficient (Kf)
NFP is the algebraic sum of (X) and (Y)
X = hydrostatic pressures Y = oncotic pressures
Hydrostatic pressure tends to promote movement of fluid (X) of vessels. Osmotic pressure opposes this, promoting movement of fluid (Y)
X = out Y = in
What is the equation for NFP?
NFP = (P_GC – π_GC) – (P_BS – π_BS)
or
NFP = (P_GC + π_BS) – (P_BS + π_GC)
What is the equation for GFR? Given oncotic pressure in Bowman’s space is essentially 0
GFR = Kf x [P_GC – (P_BS + π_GC)]
In normal individuals, GFR is generally regulated by changes in (X)
X = P_GC
Net filtration pressure (X) from the beginning of the glomerular capillaries to the end due to the increase in oncotic pressure in the Glomerular Capillary as water leaves
X = decreases
What is the equation for Renal Bloow Flow? (RBF)
RBF = 0.20 to 0.25 * Cardiac Output = 1.1 L/min
What is the equation for renal plasma flow?
RPF = (1 – Hct) x RBF
subtract blood cell components to get plasma
What 3 factors regulate GFR and RBF?
Regulation of glomerular capillary hydrostatic pressure (PGC)
Autoregulation
Myogenic response
Tubuloglomerular feedback
Renin-Angiotensin-Aldosterone system (in states of decreased renal perfusion)
Regulation of Glomerular Hydrostatic Pressure (PGC) depends on what 3 things?
Systemic arterial blood pressure (i.e. overall renal perfusion)
Afferent arteriolar resistance
Efferent arteriolar resistance
What are the effects of dilation or constriction of afferent or efferent arterioles on RBF?
Dilation of EITHER: Increased RBF
Constriction of EITHER: Decreased RBF
What are the effects of dilation or constriction of afferent or efferent arterioles on P_GC and GFR?
However, because the afferent arteriole controls glomerular inflow and the efferent arteriole controls glomerular outflow, changes in the arterioles have opposite effects on glomerular pressure:
Afferent arteriole: inflow
Dilation - increased PGC & GFR
Constriction - decreased PGC & GFR
Efferent arteriole: outflow
Dilation - decreased PGC & GFR
Constriction - increased PGC & GFR
Like most organs, the kidneys can regulate their blood flow by adjusting vascular resistance to changes in arterial blood pressure. What is this called?
Autoregulation
What is the typical autoregulation range for RBF and GFR?
90-180 mmHg
What are the 2 mechanisms of autoregulation?
Pressure sensitive -> Myogenic response
Chemo sensitive -> Tubuloglomerular feedback
What is tubuloglomerular feedback and where does it take place?
“Crosstalk” between the tubular system and the glomerulus
Takes place at the juxtaglomerular apparatus
What are the signal, sensor, and effector of the tubuloglomerular feedback mechanism?
Signal: NaCl delivery to the distal tubule
Sensor: macula densa
Effector: vascular smooth muscle cells within the afferent arteriole
Follow the process of the tubuloglomerular feedback mechanism starting at increased GFR
Increased GFR - Increased tubular fluid flow rate - increased NaCl delivery to Macula Densa - increased vasoconstrictor signals from macula densa (adenosine) - increased afferent arteriolar constriction = decreased RBF and GFR
What is the effect of RAAS on RBF & GFR (in states of decreased renal perfusion)?
Angiotensin II causes constriction of BOTH afferent and efferent arterioles, but the effect is greater on the EFFERENT arteriole
