Overview of Renal Structure and Function II Flashcards
Cpah=
UF * Upah/Ppah
Cpah is a good exogenous measure of:
RPF
What parameter measures the fraction of RPF that becomes filtrate?
How do you calculate it?
Filtration fraction
FF = GFR/RPF
Normal GFR =
Normal PRF =
So normal FF =
GFR = 120 mL/min RPF = 600 mL/min FF = 0.2 = 20%
FF is normally constant but can change when:
Volume status changes
When you think autoregulation, think:
Augmentation of reabsorption in PT/augmentation of delivery to distal nephron
Under what condition is autoregulation initiated?
Give an example.
Hypovolemia
Hemorrhage
What is activated when a pt is hypovolemic?
RAAS
Describe the pathway leading to the activation of RAAS.
Decreased CO –> Decreased arterial perfusion pressure –> Decreased Cl- sensed in macula densa –> RAAS
Why must RAAS be activated during a hypovolemic state?
To increase FF so GFR will be maintained
(FF = GFR/RPF; RPF is decreased so FF must be increased)
During autoregulation, the afferent arteriole is vaso___, while the efferent arteriole is vaso___.
Explain how this happens.
Vasodilated
Vasoconstricted
The efferent arteriole has many ATII receptors, while the afferent has few. Furthermore, ATII produces vasodilatory PGs that act on the afferent arteriole, as well as the myogenic response of the afferent arteriole vessel wall.
What 3 factors lead to vasodilation of the afferent arteriole during autoregulation?
- Few ATII receptors
- Vasodilatory PGs produced by ATII
- Myogenic responses of vessel wall
Autoregulation means more plasma becomes filtrate, and more filtrate is reabsorbed in the PT. What mechanism allows this to occur?
ATII causes vasoconstriction of the efferent arteriole, which becomes the peritubular capillaries. Hydrostatic pressure is reduced and oncotic pressure is increased in the peritubular capillaries, which favors reabsorption.
What causes the hydrostatic pressure of the peritubular capillaries to be lowered during autoregulation?
More plasma becomes filtrate
What causes the oncotic pressure in the peritubular capillaries to increase during autoregulation?
The same amount of protein is filtered at the glomerulus (if no glomerular damage present) but less volume reaches the peritubular capillaries
A pt on ibuprofen experiences a common side effect of GI bleeds. During a routine physical, her creatinine is noticed to be 90 mL/min.
What could explain this finding?
GI bleed –> hypovolemia –> autoregulation inhibited by NSAID because ATII can’t produce vasodilatory PGs –> less vasodilation of afferent arteriole –> decreased perfusion pressure + low GFR –> decreased creatinine clearance
What other classes of Rx besides NSAIDs may interfere with the efficiency of autoregulation?
ACE inhibitors (end in -pril)
Angiotensin receptor blockers (ARBs, end in -artan)
Direct renin inhibitor (aliskiren, pepstatin)
What happens to filtration fraction in volume expansion?
Decreases
(RPF increases, so FF must decrease to maintain normal GFR)
How does the kidney reduce FF during volume expansion?
Shut off renin/AngII
(just exactly the opposite of hypovolemia)
Where does bulk reabsorption occur? Fine tuning of solute and water reabsorption?
PT
Distal nephron
What is the most important element in controlling overall reabsorption, thus maintaining homeostasis?
Delivery to the distal nephron
What cells do the reabsorbing in a nephron?
Renal epithelial cells
How do renal epithelial cells increase their surface area for max absorptive capacity?
Microvilli and villi
What important component of renal epithelial cells would ONLY be found on the basolateral membrane?
Na+/K+-ATPase
(for Na+ reabsorption into peritubular capillaries; would be counterproductive on apical membrane)
Two functions of renal epithelial cell tight junctions
Cause polarity of cells
Separate unique apical transporters from basolateral membrane
Low or high capacity vs. resistance:
Proximal nephron?
Distal nephron?
Proximal = high capacity, low resistance
Distal = low capacity, high resistance
What effect does low capacity and high resistance of proximal nephron have?
Allows bulk reabsorption without creating steep gradients so the fluid exchange is isoosmotic
What effect does low capacity and high resistance of the distal nephron have?
Maintenance of steep gradient between urine and plasma for fine tuning
(T/F): Paracellular gradients are independent of the renal epithelial cell gradients.
False - paracellular transport follows transcellular pattern for each nephron segment
What influences paracellular transport?
Claudin proteins composing the tight junctions
Name 6 transport mechanisms in the nephron
Simple diffusion
Facilitated diffusion
Active transport
Coupled transport
Counter transport
Channels
3 things capable of simple diffusion?
Urea
CO2
Any lipid soluble thang
Which substance uses facilitated transport in nephron?
Glucose via glucose transports (GLUT)
What is the main O2-requiring process in the kidney?
Na/K-ATPase for sodium reabsorption
Name 3 ATPases found in the nephron
Na/K-ATPase
H-ATPase
Ca-ATPase
Coupled transport is always linked to the movement of which solute?
Na+
Name some substances that are cotransported wih Na
Glucose, amino acids, organic acids (lactic acid, ketoacid), phosphate, urate
Why is giving glucose with saline a good idea?
Glucose is coupled to Na+ transport; thus, it will help sodium transport
What is the purpose of coupled transport?
To avoid using energy in more than one step
(T/F): Sodium movement across renal epithelial cells into peritubular capillaries always requires energy.
True - can go through Na+ channels on apical membrane, which doesn’t require energy, but must use energy to cross Na/K-ATPase on basolateral membrane.
(Remember: Na/K-ATPase is biggest O2 consumer in nephron)
Name two antiporters in the nephron
Na/H
Cl/HCO3
What is the purpose of channels?
Name three found in the nephron.
Reabsorb when diffusion is not enough
Aquaporins, ion channels (Na, K, Cl), urea
Major difference between transports vs. channels
Channels are non-saturable
Transporters obey Michaelis-Menton, thus are saturable
What is the extent of the proximal nephron?
From PCT to end of TALH/beginning of macula densa
___% of total filtrate has been reabsorbed by the end of the proximal nephron
90%
(remember: proximal nephron built for bulk reabsorption)
Why is 90% of filtrate reabsorbed in the proximal nephron?
The distal nephron is geared toward fine tuning of filtrate; any excess solute delivered to the distal nephron will be lost in the urine
In which part of the kidney is the PCT? PST?
PCT = entirely in cortex
PST = down to corticomedullary junction
What makes up the peritubular capillaries?
CORTICAL efferent arterioles
(remember: juxtamedullary nephron efferent arterioles form the vasa recta)
If reabsorption in PT is isotonic, what is happening to the filtrate?
The composition changes, not the osmolarity
55% of __ and __ reabsorbed in PT
90% of ___ reabsorbed
100% of __ and ___ reabsorbed
55% NaCl and H2O
90% NaHCO3
100% glucose and amino acids
Glucose normally makes up ~___% of solute in thin ascending limb
0%
(normally 100% reabsorbed in proximal tubule
Name 12 things reabsorbed in PT
NaCl, H20, NaHCO3, glucose, amino acids, K+, urea, PO4, urate, lactate, pyruvate, ketoacids
If Na+ reabsorption in the PT was somehow inhibited, you would expect to also see reduced absorption of:
PO4, urate, pyruvate, lactate, ketoanions
(They are coupled to Na transporters)