Quiz #7 Flashcards
What are some of the kidneys functions?
regulate plasma ion concentrations regulate BV and BP regulate blood osmolarity stabilize blood pH removal of waste
What are the 3 distinct regions of the kidney?
renal cortex
renal medulla
renal pelvis
What is the renal cortex?
granular-appearing superficial region
What is the renal medulla?
deep to cortex
composed of cone-shaped medullary pyramids
- broad base of pyramid faces cortex, papilla (tip) faces inwards
- separated by renal columns (inwards extensions of cortical tissue)
- form lobes
What is a lobe?
medullary pyramid and its surrounding cortical tissue
about 8 per kidney
What is the renal pelvis?
funnel-shaped tube continuous with ureter
What are the minor and major calyces?
minor = cup shaped areas that collect urine draining from pyramidal papillae
major = areas that collect urine from minor calyces and empty into renal pelvis
Describe urine flow starting in the renal pyramid.
renal pyramid - minor calyx - major calyx - renal pelvis - ureter
What are nephrons?
the structural and functional units that form urine
2 main parts
- renal corpuscle
- renal tubule
What is the renal corpuscle?
made of the Bowmans capsule and glomerulus
Bowmans capsule
- hollow structure that contains the glomerulus
glomerulus
- ball of specialized capillaries
What is the renal tubule?
proximal tubule
- where most reabsorption occurs
- have microvilli to increase surface area
loop of Henle
- creates dilute urine
- regulates concentration gradient to concentrate urine
- osmotic gradient generated since descending end only permeable to water, ascending end only permeable to salt
distal tubule
- fine regulation of salt and water balance
What are the collecting ducts?
collect filtrate from many nephrons
fine regulation of salt and water balance
What are the 2 types of nephrons?
cortical
- 85%
- almost entirely in cortex
juxtamedullary
- originate near junction between cortex and medulla
- important for production of concentrated urine
- have very long nephron loops
What is filtration?
anything that exits the capillary
What is reabsorption?
anything that goes back in the capillary
How does blood flow enter the kidney?
afferent arteriole brings blood into the glomerulus, efferent arteriole brings blood away from the glomerulus
What capillary bed are cortical nephrons associated with?
peritubular capillaries
What is the vasa recta associated with?
juxtamedullary nephrons
What is the role of the glomerulus?
capillaries specialized for filtration
fed and drained by arteriole
- afferent arteriole enters glomerulus, efferent arteriole leaves and feeds into either peritubular capillaries or vasa recta
Why is blood pressure in the glomerulus high?
afferent arteriole is larger in diameter than efferent, restricting the blood that can leave
arterioles are high resistance vessels
What are peritubular capllaries?
low pressure, porous capillaries adapted for absorption of water and solutes
where reabsorption occurs
arise from efferent arterioles, empty into venules
cling to adjacent renal tubules in cortex
What is the vasa recta?
long, thin-walled vessels parallel to long nephron loops of juxtamedullary nephrons
arise from efferent arterioles
function in formation of concentrated urine
What is a juxtaglomerular complex?
each nephron has one
involves modified portion of :
ascending limb of loop of Henle
afferent (sometimes efferent) arteriole
important in regulating the rate of filtrate formation and blood pressure
What are the 3 cell populations seen in juxtaglomerular complexs?
macula densa
granular cells
extraglomerular mesangial cells
What are macula dense?
tall, closely packed cells of the ascending limb
contain chemoreceptors that sense salt content of filtrate (too much mean glomerular filtration rate is too high, vice versa)
What are granular cells?
enlarged, smooth muscle cells of afferent arteriole
act as mechanoreceptors to sense blood pressure in arteriole
contain secretory granules containing renin
What are extraglomerular mesangial cells?
located between arteriole and tubule cells
interconnected with gap junctions
may pass signals between macula densa and granular cells
What are the 3 processes involved in urine formation and adjustment of blood composition?
glomerular filtration
tubular reabsorption
tubular secretion
What is glomerular filtration?
produces cell and protein free filtrate
movement of blood into the lumen of the nephron (takes place only at renal corpuscle)
walls of glomerular capillaries and Bowmans capsule are modified to allow bulk flow
What is tubular reabsorption?
selectively returning 99% of substances from filtrate to blood in renal tubule and collecting ducts
What is tubular secretion?
selectively move substances from blood to filtrate in renal tubules and collecting ducts
What are the 4 main processes occurring at the kidneys?
filtration - blood to lumen
secretion - blood to lumen
reabsorption - lumen to blood
excretion - lumen to external environment
What is excretion?
excretion = filtration + secretion - reabsorption
100% volume enters afferent arteriole
20% of volume is filtered
over 19% of fluid is reabsorbed
less than 1% of volume is excreted to external environment
Describe step 1 of urine formation.
glomerular filtration
passive/no energy required
hydrostatic pressure forces fluids and solutes through filtration membrane into glomerular capsule
What is the filtration membrane?
porous membrane between the blood and the interior of the glomerular capsule
allows water and solutes smaller than plasma proteins to pass
3 layers
macromolecules stuck in membrane are engulfed by glomerular mesangial cells
What do plasma proteins do?
remain in blood to maintain colloid osmotic pressure
prevents loss of all water into capsular space
proteins in filtrate indicate membrane issue
What Starling forces in glomerular filtration favour filtration?
glomerular capillary hydrostatic pressure
- 60 mmHg (high due to resistance of efferent arteriole)
Bowmans capsule osmotic pressure
- 0 mmHg (low due to lack of protein in filtrate)
What Starling forces in glomerular filtration oppose filtration?
Bowmans capsule hydrostatic pressure
- 15 mmHg (relatively high due to large volume of filtrate in closed space)
glomerular oncotic pressure
- 29 mmHg (higher than in systemic capillaries due to plasma proteins in smaller volume of plasma)
What is the net filtration pressure?
outward pressure - inwards pressure
Pgc - (Pbc - Pgo)
60 - (15 + 29)
16 mmHg
What is glomerular filtration rate?
volume of filtrate formed per minute by both kidnets
directly proportional to net filtration pressure, surface area available for filtration, and filtration membrane permeability
How can resistance at the efferent or afferent side alter hydrostatic pressure and flow?
afferent side - decreases hydrostatic pressure, decreases GFR
efferent side - increases hydrostatic pressure, increases GFR
How are GFR and systemic blood pressure related?
increased GFR causes increased urine output which lowers blood pressure, vice versa
What things regulate glomerular filtration?
local intrinsic controls try to maintain GFR in kidney
- renal autoregulation
- when MAP is in range of 80-180 mmHg
extrinsic controls try to maintain systemic blood pressure
- nervous system and endocrine mechanisms
What are the 2 types of renal autoregulation?
myogenic mechanism
tubuloglomerular feedback mechanism
What is the myogenic mechanism?
local smooth muscle contracts when stretched
high BP causes muscle to stretch, leading to constriction of afferent arterioles
restricts blood flow entering glomerulus
protects glomeruli from damaging in high BP (decreased BP causes dilation of afferent arterioles)
What is the tubuloglomerular feedback mechanism?
flow-dependent mechanism directed by macula densa cells (responding to filtrates salt concentration)
if GFR increases, filtrate flow rate increases, leads to decreased reabsorption time causing high salt levels in filtrate
feedback mechanism causes constriction of afferent arteriole to lower GFR and allow more time for salt reabsorption (reverse for when GFR is decreased)
How does the SNS act to control GFR?
under normal rest conditions, renal blood vessels are dilated and renal autoregulation mechanisms prevail
under abnormal conditions (low ECF volume/low BP):
- NE released by SNS and E released by adrenal medulla causing systemic vasoconstriction (increasing BP) and constriction of afferent arterioles (decreasing GFR)
What is the renin-angiotensin-aldosterone mechanism?
main mechanism for increasing BP
3 pathways to renin release by granular cells:
- direct stimulation of granular cells by SNS
- stimulation activated by macula densa cells when filtrate salt concentration is low
- reduced stretch of granular cells
Describe the 2nd step of urine formation.
tubular reabsorption
quickly reclaims most tubular contents and returns them to blood (99%+)
almost all organic nutrients reabsorbed, water/ion reabsorption is hormonally regulated
includes active and passive reabsorption
substances can follow 2 routes
Where does most solute reabsorption occur>
proximal convoluted tubule
some in distal convoluted tubule
What are the 2 routes substances can follow in tubular reabsorption?
transcellular route
paracellular route
What is the transcellular route?
soulte enters apical membrane of tubule cells
travels through cytosol
exits basolateral membrane of tubule cells
enters blood through endothelium of peritubular capillaries
What is the paracellular route?
between tubule cells
limited by tight junctions, but these are leaky in the proximal nephron
- water, Ca2+, K+, and some Na+ in proximal convoluted tubule move through this route
move through interstitial fluid and into capillary
What is transport maximum?
rate of transport when carriers are saturated
when carriers are saturated, extra is excreted into urine
What is renal threshold?
if solute in filtrate saturates carrier, some solute is excreted in urine
solute in plasma that causes solute in filtrate to saturate carriers and spill over into urine
transport max/GFR
Describe the 3rd step of urine formation.
tubular secretion
reabsorption in reverse, almost completely in proximal convoluted tubule
selected substances moved from peritubular capillaries through tubule cells and out into filtrate
What is tubular secretion important for?
disposing of substances that are bound to plasma proteins
eliminating undesirable substances that were passively reabsorbed (ex. urea)
ridding the body of excess K+
controlling blood pH by altering levels of H+ or HCO3-
What is micturition?
urination
urine formed in renal tubules drains into renal pelvis and into ureter, which leads to bladder
bladder stores urine until it is excreted
Describe the role of sphincters in micturition.
internal sphincter relaxes under PNS control
external sphincter relaxes under motor/conscious control, but not until after age 2
- before this urination is simply reflexive
Describe the process of micturition.
increased volume of fluid in bladder expands bladder walls and activates stretch receptors
signal travels to spinal cord
decreases SNS and increases PNS to open internal urethral sphincter
decreases somatic motor neuron activity to open external urethral sphincter
micturition
What cells regulate solute and water balance?
cells in late distal tubule and collecting duct
principal cells - water and electrolytes
intercalated cells - acid-base balance
Where does water reabsorption occur?
proximal tubules
- 70% of filtered water is reabsorbed
- not regulated
distal tubules and collecting ducts
- most remaining water is reabsorbed
- regulated by ADH
Describe how water reabsorption follows solute reabsorption.
in proximal tubules
primary solute = sodium
solutes are actively reabsorbed, increasing osmolarity of peritubular fluid and plasma, creating osmotic gradient
water reabsorbed by osmosis
Describe how the medullary osmotic gradient is established.
by countercurrent multiplier dependent on loop of Henle
ascending limb
- impermeable to water
- active transport of Na+, Cl-. K+
- fluid osmolarity decreasesas it ascends
descending loop
- permeable to water
- no transport of Na+, Cl-, K+
- fluids osmolarity increases as it decends
What are countercurrent mechanisms?
fluid flows in opposite directions in 2 adjacent segments of the same tube
multiplier
- interaction of filtrate flow in ascending/descending limbs of nephron loops of juxtamedullary nephrons
exchanger
- blood flow in ascending/descending limbs of vasa recta
What is the role of urea in the medullary osmotic gradient?
nitrogen elimination
generated by liver, extremely water soluble, requires urea transporters (UTA, UTB, UTC)
transport through UTA from filtrate to peritubular fluid contributes approximately 40% of osmolarity of gradient
What is the role of the vasa recta in the medullary osmotic gradient?
anatomical arrangement prevents the diffusion of water and solutes from dissipating the osmotic gradient
descending limb
- water leaves capillaries via osmosis and solutes enter by diffusion
ascending limb
- water moves into plasma and solutes move into interstitial fluid
Where in the nephron does water reabsorption occur?
70% in proximal tubule
- unregulated
20% in distal tubule
- ADH regulated
10% in collecting ducts
- ADH regulated
What does water permeability depend on?
presence of aquaporins
How does ADH relate to aquaporin2?
ADH stimulates insertion of aquaporin2 into the apical membrane
What determines the maximum amount of water reabsorbed?
length of the loop of Henle
What would the GFR response be if BP dropped below 80mmHg?
decrease in GFR
decrease in water filtered
decrease in water excreted
What would the GFR response be if BP moved to over 180mmHg?
increase in GFR
increase in water filtered
increase in water excretion
