Renal lecture 2 glomerular filtration, tubular reabsorption/secretion, concept of clearance

Question 1

What are the 3 barriers in glomerular filtration?
What gets filtered by glomerular filtration?

Answer 1

1.-endothelial cells
2.-BM
3.-podocytes
-water and low molecular weight substances

Question 2

Wht cannot get filtered (should not be in urine)

Answer 2

-cells
-proteins (albumins, globulins)
-protein bound substances (1/2 of calcium ion and fatty acid)

Question 3

What does it mean when albumin is in urine?
Why should onyl half of Ca2+ ions not be filtered?

Answer 3

-means filtration barrir is messed up (protein uria)
-1/2 of them are bound to large proteins so they should not get filtered, only 1/2 should be filtered

Question 4

What is RBC in urine and WBC in urine called?
What does this mean damage is in?

Answer 4

alot of RBC in blood=hematuria, which means damage to glomerulus
alot of WBC=piuria

Question 5

what forces favor and oppose filtration?
What is the value of each force in mmhg

Answer 5

favor filtration=glomerular capillary BP (Pgc)=60 mmhg
opposing filtration (pushes against fluid trying to get in:
fluid in bowmans space (Pbs)=15 mmhg
-osmotic foce due to protein in plasma (pi gc)=29mmhg

Question 6

What is the formula for net glomerular filtration pressure?
What is the value?

Answer 6

=Pgc-Pbs-PIgc=16mmhg

Question 7

What is the definition of glomerular filtration rate?

Answer 7

Glomerular filtration rate (GFR): the volume of fluid
filtered from the glomeruli into Bowman’s space per unit
time

Question 8

What is GFR regulated by, what are the 3 things (what pressure, permeability,etc)?

Answer 8

GFR is regulated by: NMS
N-net filtration pressure
M-membrane permeability
S-surface area available for filtration

Question 9

What is the normal GFR value for a 70kg person?
What is plasma volume of this person?
How often is plasma filtered in glomeruli?

Answer 9

-Normal GFR (70 kg person): 180 L/day (125 ml/min)
-3.5L (5% of body weight)
-51 times a day

Question 10

What does decreased GFR cause? What do you dilate and constrict?
What does this cause to glomerular capillary BP?

Answer 10

-constrict the left side of the afferent arteriole
-dilate the efferent arteriole
-causes a decrease in Pgc

Question 11

What does an increase in GFR mean and cause in terms of constriction/dilation?
What does this cause to glomerular capillary BP?

Answer 11

-it means increased filtration
-constrcit efferent arteriole
-dilate afferent arteriole to increase blood flow
-increase in Pgc

Question 12

What is the definition of filtered load?
What is the formula for filtered load?
What is the filtered load of glucose?

Answer 12

Filtered load: total amount of any freely filtered substance
Filtered load = GFR x plasma concentration of the substance
ex.Filtered load of glucose = 180 L/day x 1 g/L = 180 g/day

Question 13

What does it mean when filtered load is greater than amount excreted in urine, and vice versa?

Answer 13

Filtered load > amount excreted in the urine: net reabsorption
Filtered load < amount excreted in the urine: net secretion

Question 14

What is the definition of reabsorption?
What does transcellular vs paracellular mean/

Answer 14

-it is when you go from the tubular luomen to the peritubular capillaries
-transcellular=going through tubular lumen and tubular epithelial cells
-paracellular goes from tubular lumen through tight junction

Question 15

What is the amount of water, sodium, glucose, and urea that is reabsorbed?
What can having too much potassium cause?

Answer 15

water=99%
sodium=99.5%
glucose=100%
urea=44%
-too much K+ can stop your heart

Question 16

What are the first 2 most imporant points about tubular reabsorption (what is large and incomplete)?

Answer 16

1. Filtered loads are enormous, generally greater than the amounts of the substance in the body.
2. Reabsorption of waste products is relatively incomplete (e.g. urea).

Question 17

What aer the last 2 important points about tubular reabsorption?

Answer 17

3. Reabsorption of most useful plasma components (e.g. water, inorganic ions, and organic nutrients) is relatively complete
4. Reabsorption of some substances are not regulated (e.g. glucose, amino acids), while others are highly regulated (water, inorganic ions).

Question 18

Where does reabsorption by diffusion occur?
Where does urea reabsorption occur, and where is it filtered (what part of kidney)

Answer 18

Reabsorption by diffusion: often across the tight junctions connecting the tubular epithelial cells
-e.g. urea reabsorption in the proximal tubule
-Urea is freely filtered at glomerulus

Question 19

Where does water reabsorption occur?
Whre does urea concentration become higher?
where does urea diffuse along the conc. gradient across Tj into?

Answer 19

-In the proximal tubule, water reabsorption occurs.
-Urea concentration in the tubular fluid becomes higher.
-Urea diffuses into the interstitial fluid and peritubular capillaries.

Question 20

What are the 2 mechanisms of reabsorption?

Answer 20

diffusion and mediated transport

Question 21

where does reabsorption by mediated transport occur?
What does it normally require the participation of?
what is it usually coupled with?

Answer 21

-Reabsorption by mediated transport occurs across tubular
cells (transcellular epithelial transport)
-Requires the participation of transport proteins in the plasma membrane of tubular cells.
-Usually coupled to the reabsorption of sodium

Question 22

Where is the Na, K ATPase in mediated transport?
Where does glucose pass along concentration gradient?
How do amino aciids leave tubular epithelial cells/

Answer 22

-it is on the basal side of the membrane
-on the basolateral side
-via amino acid transporters

Question 23

What is the definiton of transport maximum (Tm)
What is an example of when this can occur, what happens in this example?
What happens to the urine as a result of this example?

Answer 23

When the membrane transport proteins become saturated, the tubule can not reabsorb the substance any more.
- in people with uncontrolled diabetes mellitus, the plasma concentration of glucose can become very high and the filtered load of glucose exceeds the capacity of the tubules to reabsorb glucose (Tm is exceeded).
As a result, glucose appears in the urine (glucosuria).

Question 24

What is the definition of tubular secretion?
What are the 2 mechanims that mediate secretion?

Answer 24

Tubular secretion moves substances from peritubular capillaries into the tubular lumen (opposite of reabsorption)
-diffusion and transcellular mediated transport

Question 25

What are the substances secreted most by the tubules?
What is secretion normally coupled with?

Answer 25

hydrogen ion and potassium.
-Tubular secretion is usually coupled to the reabsorption of sodium (like in reabsorption.

Question 26

what needs to be large in order to excrete wate products adequately?
What filtered loads need to large?

Answer 26

-the GFR must be very large.
-Thus, the filtered volume of water and the filtered loads of all the nonwaste plasma solutes are also very large

Question 27

How much water and solute does the proximal tubule reabsorb?
What is it a major site of secretion for?

Answer 27

-reabsorbs 2/3 of filtered water and solutes
-It is also a major site of secretion for various solutes, except K+ (which gets secreted at a later point)

Question 28

What does Henle’s loop reabsorb?

Answer 28

• reabsorbs relatively large quantities of the major ions (less water)

Question 29

What do the DCT (distal convoluted tubule)/CD (collecting duct) determine, what is it for, and what type of control is exerted here?

Answer 29

-Determines the final amounts excreted in the urine by
adjusting the rates of reabsorption, and, in a few cases,
secretion
-Fine-tuning
-Most homeostatic controls are exerted here

Question 30

What is the definition of the clearance concept?

Answer 30

-Clearance: the volume of plasma from which that substance is completely removed (“cleared”) by the kidneys per unit time.

Question 31

What is the formula for clearance of something?
How do you calculate the mass of S excreted per unit time?

Answer 31

=Mass of S excreted per unit time/Plasma concentration of S (Ps)
Mass of S excreted per unit time
= Urine concentration of S (Us) x Urine volume per unit time (V)

Question 32

What is inulin (not insulin)?
Where is it freely filtered, but what is it not?

Answer 32

-Inulin is a polysaccharide that would be administered intravenously.
-It is freely filtered at glomerulus but is NOT reabsorbed, secreted, or metabolized by the tubule.

Question 33

What is the clearance of inulin equal to?

Answer 33

the clearance of inulin (CIN) is equal to the volume of plasma originally filtered (GFR)
CIN=GFR (its the most accurate marker of GFR)

Question 34

What is the urine volume per day?
What is concentration of inulin in plasma?
What is inulin conc in urine?

Answer 34

Urine volume = 2.4 L/day
inulin in plasma=4mg/L
Inulin concentration in the urine= 300 mg/L

Question 35

What is creatinine?
Where is it freely filtered, and is it reabsorbed?

Answer 35

Creatinine is a waste product produced by muscle.
It is filtered freely at glomerulus and is NOT reabsorbed.

Question 36

Is it secreted at the tubule, is it metabolized?
What is creatinine used as?
What can this replace?

Answer 36

It is secreted at the tubule but the amount is small.
-It is NOT metabolized by the tubule.
Thus, creatinine clearance is used as a clinical marker for GFR
-can be a replacement for inulin since you dont need IV infusion

Question 37

What is the creatinine clearance formula and what are the variables?

Answer 37

Urine volume (V)
Urine concentration of creatinine (Ucr)
Plasma concentration of creatinine (Pcr)
creatinine clearance=UcrV/Pcr=GFR

Question 38

What happens when clearance of a substance is larger than GFR vs when it is smaller than GFR?

Answer 38

-Clearance of a substance > GFR ,It is secreted at the tubule.
-Clearance of a substance < GFR ,It is reabsorbed at the tubule