Filtration and Clearance

Question 1

glomerular filtration

Answer 1

• the process by which plasma is filtered across the glomerular caps to form a protein free ultrafiltrate in bowmans space
• differences in oncotic and hydrostatic pressure across the glomerular capillaries drive the net efflux of a plasma ultrafiltrate
• GFR is approx 125 ml/min or 180 L/day
• more than 10 fold of the extracellular volume and equivalent to filtering entire ECF evert 2 hours
• serves the purpose of maintaining ECF volume and solute composition within narrow limits by rapidly responding to correct changes in ECF volume and solute comp

Question 2

Kf

Answer 2

-filtration coefficient of the glomerular capillary and is the product of the cap hydraulic conductivity and the SA available for filtration

Question 3

filtration rate

Answer 3

• driven by starling forces
• = Kf [(Pgc-Pbs)-(pigc-pibs)]
• when DF favoring and opposing glomerular filtration become equivalent toward the efferent end of the glomerular cap, filtration stops
• hydrostatic pressure of GC starts bigger and then oncotic pressure of GC smaller (out then in)
• Pgc and pi BS favor filtration first
• then Pbs and pi GC favor stopping

Question 4

Pcg-Pbs

Answer 4

-difference in hydrostatic pressure inside the glomerular cap and bowmans space

Question 5

pi gs- pi bs

Answer 5

• difference in oncotic pressure inside the glomerular cap and bowmans space
• presence of proteins

Question 6

Pgc

Answer 6

• 45-50 mm Hg at beginning of cap and decreases 3 to 41-47 at the end
• occurs despite a decrease of plasma volume and is due to post cap efferent ateriole constriction

Question 7

Pi gc

Answer 7

• 25 mm Hg at the beginning of cap and increases to 35 mmHg at the end
• due to plasma filtration and concentration of plasma protein

Question 8

Pbs

Answer 8

-10 mm

Question 9

pi bs

Answer 9

• 0

- increases in nephrotic syndrom due to filtration of plasma protein

Question 10

glomerular barrier to filtration

Answer 10

• endothelial cells of glomerular caps restrict passage of cellular elements into bowmans space
• cap basement membrane restricts filtration of solutes larger than 1 kDa
• an anionic charge favors filtration of cations and restricts filtration of anionic proteins
• podocytes of the visceral epithelial layer of bowmans cap have foot processes covering the glomerula caps
• contiguous foot processes are separated by filtration slits where anionic charge further restricts filtration of anionic proteins, but not smaller organic and inorganic anions
• glycoproteins with neg charge also cover podocytes, filtration slits and slit diaphragms favor filtration of small cationic solutes

Question 11

glomerulus and Bowman’s capsule

Answer 11

• permselectivity of the glomerular barrier is determined by the size and charge of the solute
• water and solutes with a diameter of less than 4 nm (effective molecular radius of less than 2 nm) are freely filtered

Question 12

size dependence of solute perselectivity at glomerular barrier

Answer 12

• water, salt glucose, inulin are freely filtered, same concentration in plasma and filtrate
• as radius increases, filtration decreases

Question 13

dependence of filterability on charge

Answer 13

• neg charge on BM and foot impedes passage of neg solutes (proteins)
• allows positive
• removing negative charge from barrier allows neg ions to pass
• happens with increased filtration of plasma proteins in nephrotic serum nephritis

Question 14

renal hemodynamics

Answer 14

• CO is 5-6 L/min, 7200-8640 L/24 hrs
• renal blood flow (RBF)- 1-1.2, 1440-1728
• RPF-600-720 ml, 860-1040 L/24 hrs
• GFR 125 ml/ min, 180 L/24 hrs (constant)
• urine output 1ml/min to 10 ml/min
• 20% of CO perfuses kidney, CO is 7% of body weight

Question 15

RPF

Answer 15

• 55% of RBF
• 20% of RPF is filtered at glomeruli
• fraction of RPF filtered at glomeruli is filtration fraction

Question 16

FF

Answer 16

GFR/RPF

-0.2

Question 17

amt filtered

Answer 17

• 180 L of plasma per day, more than 10x ECF vol

- constant vol and solute comp

Question 18

GFR

Answer 18

• increases with increasing RPF
• FF decreases with increasing RPF
• if increase RPF, GFR increases and FF decreases
• max GFR at 600 ml/min
• as plasma increases, more SA is filtering
• normal 125 ml/min at plasma flow of 600 ml/min means 20% FF

Question 19

afferent arteriolar constriction

Answer 19

• decrease Pgc

- RPF decreases and GFR decreases

Question 20

efferent arteriolar constriction

Answer 20

• increase Pgc
• RPF decreases
• GFR increases

Question 21

afferent and efferent arteriolar constriction

Answer 21

• RPF decreases a lot

- RPF dependence of GFR is a function of SA, which is more or less maximal at normal or above normal rate of RPF

Question 22

increase plasma protein

Answer 22

• increase pi gc

- GFR decreases

Question 23

decreased plasma proteins

Answer 23

-GFR increases

Question 24

obstruct ureter

Answer 24

• increases Pbs

- GFR decreases

Question 25

fluid reabsorption

Answer 25

• post glomerular and peritubular caps
• from interstitial space into peritubular caps
• peritubular cap oncotic pressure difference driving fluid absorption exceeds peritubular cap hydrostatic pressure difference.

Ppc=20 (constriction of preceding efferent arteriole), pi pc= 35 (filtration of plasma concentrates plasma proteins)
Pis= 6-10, pi is = 4-8

(Ppc-pipc)-(Pis-piis) drives reabsorption or filtration
- neg 17 means net reabsorption into peritubular cap (lies on basolateral side (blood))

Question 26

path through kidney

Answer 26

-tubular fluid in lumen to tubular cell to interstitial fluid to pertibular capillary

Question 27

Glomerular Filtration Rate

Answer 27

• measurement can provide index of the number of functioning nephrons in kidney disease
• renal failure begins when GFR decreases to below 20 ml/min or a loss of function of 85% of nephrons

Question 28

renal clearance

Answer 28

• virtual volume of plasma from which a solute is completely removed from the plasma by the kidney per unit of time
• ml/min

Question 29

renal clearance can be used to measure GFR when

Answer 29

the renal handling of the solute is:

• freely filtered at the glomerulus
• not reabsorbed in any segment of the nephron
• not secreted in any segment of the nephron
• not synthesized by the kidney

Question 30

math

Answer 30

-for solute with correct properties, the amount of solute filtered/time with be equivalent to the amount of solute excreted/time in the urine
-filtration=excretion
-amt filtered=plasma concentration of solute x rate of plasma filtration
= Ps(plasma concentration) x GFR
= urine concentration x rate of urine flow
= Us (urine concentration) x V
GFR=UsxV/Ps

Question 31

solutes used to measure GFR

Answer 31

• inulin is exogenous- given by IV infusion to maintain constant plasma and measured accurately in plasma and urine
• creatinine is endogenous-in the absence of strenuous exercise or disease a constant amy of creatinine/time diffuses from skeletal muscle to plasma
• when production from muscle is constant, increased plasma concentration means decreased clearance from the plasma, means decreased GFT and acute renal failure
• if plasma creatinine is increased, clearance is decreased
• normal is 1 mg/ 100 ml

Question 32

clearance

Answer 32

C= Us x V/ Ps

• if a solute is freely filtered and 50% of the filtered solute was reabsorbed, then amt excreted would be 50% of the amt of solute filtered as indicated by decreased Us
• kidneys only clearing 62.5 ml of plasma/min of solute
• if 99-99.5% of sodium is reabsorbed, the clearance would be 1ml/min or less

Question 33

clearance ratios

Answer 33

• clearance of anything may be more or less than creatinine/inulin
• renal handling of a solute may be only reabsorbing, only secretion, or both, in same or different segments of nephron
• when both occur, difference in solute reabsorption and secretion determines net reabsorption or secretion
• solute clearance less than creatinine indicates the solute is not freely filtered or if it is, net reabsorption has occurred
• clearance greater than creatinine indicates that net secretion has occurred
• less than 1 equals reabsorption

Question 34

fractional excretion of water

Answer 34

• fraction of the glomerular filtrate not reabsorbed from the tubular fluid along the nephron and therefore appearing in urine
• ratio of urine flow rate to GFR

=V/GFR where GFR=C=U xV / P

=V/C(in)= (VxP)/ (UxV)= Pin/Uin

• plasma to urin inulin concentration ratio
• inulins concentration in urine arises directly from the amt of water reabsorption occurring in the nephron
• if inulin is 100x more concentrated in urine–> Pin/Uin= 1/100= 0.01- 1% of filtered water being eliminated (99% reabsorbed)
• can also use creatinine

Question 35

fractional excretion of solute

Answer 35

• for any solute filtered, it is the fraction of filtered solute that appears in uring
• estimated as ratio of clearance to GFR
• creatinine clearance used to estimate GFR, fraction of filtered solute is easily determined by measurement of solute and creatinine concentration in plasma and urine

=(Usx Pcreat)/(Ucreat xPs)

Question 36

water and salt

Answer 36

• when in balance, 1 % of filtered water and Na appears in the uring
• in neg water balance- fraction of water is less than 1, Na still one
• over hydrated- water to 5, Na still 1

Question 37

fractional reabsorption

Answer 37

• fraction of water or solute that is reabsorbed and doesn’t appear in urine
• 1- FE
• in water and Na balance, 99% of filtered Na and water are reabsorbed

Question 38

autoregulation of blood flow

Answer 38

• renal blood flow maintained at constant
• 1.2 L/min
• across wide range of MAPs
• maintains GFR constant
• renal vascular resistance increases and decreases with increased and decreased MAP
• RBF=BP/R
• autoreg due to:
• myogenic response of renal vasculature to pressure changes
• tubuloglomerular feedback at the macula densa cells sensing an increase or decrease in GFR and causing increased or decreased resistance of the afferent arteriole, returning GFR to normal
• intrinsic to kidney and occurs in absence of autonomic innervation to kidney