32 Glomerular Filtration and Renal Blood Flow

Question 1

what is glomerular ultrafiltration?

Answer 1

the formation of a fluid with solute concentrations that are similar to those in plasma water

Question 2

what are the exceptions to Glomerular Filtration?

Answer 2

proteins and other high-MW compounds which are present at reduced concentrations

Question 3

what is the glomerular filtrate free of?

Answer 3

formed blood elements like red and white blood cells

Question 4

how great is the filtration that occurs in the glomeruli?

Answer 4

it exceeds that in all the other capillaries of the circulation combined

Question 5

what is the GFR of the two kidneys? Why?

Answer 5

125 ml/min or 180 liters/day inorder to expose the entire ECF to the renal tubule epithelium

Question 6

if there was a sudden increase in plasma level of a toxic material would excretion be delayed?

Answer 6

no

Question 7

T/F, Steady-state plasma levels of waste materials would not reach to high levels?

Answer 7

T

Question 8

why is inulin a good glomerular marker? is this the most reliable method? Practical?

Answer 8

it has the same concentration in the glomerular filtrate as in plasma and is neither reabsorbed nor secreted along the nephron; inulin clearance is the most reliable for measuring GFR but not practical because it has to be administered IV

Question 9

what is the equation for calculating the urine output? can you calculate the GFR from this?

Answer 9

Px x GFR = Ux x V

where urine volume 1 ml/min

Px: [solute] in plasma
GFR: glomerular filtration rate (for a glomerular marker)
Ux: [solute] in urine
V: urine flow

Px x GFR is input into Bowmans space

Ux x V is output into urine;

Yes, so it would be
GFR = UV/P

always calculate the GFR first!!!

Question 10

what do we mean by clearance?

Answer 10

we are clearing x amount of blood from given molecule

Question 11

the problems of intravenous infusion of GFR marker can be avoided using an endogenous substance with inulin like properties called?

Answer 11

creatinine

Question 12

where is the source of plasma creatinine found?

Answer 12

from the normal metabolism of creatine phosphate in muscle; when you record the next value after baseline this indicates the health of the kidneys

Question 13

what does the normal metabolism of creatine phosphate mean for men and women in terms of creatinine clearance in measuring GFR?

Answer 13

higher numbers than females because of muscle mass, so in men metabolism generates creatinine at the rate of 20 to 25 mg/kg body weight per day

and for women, the value is 15 to 20 mg/kg body weight per day

numbers not necessary to memorize

Question 14

T/F, In the steady state, the amount of the creatinine appearing in the urine per day is equal to the metabolic production rate?

Answer 14

T

Question 15

how can we avoid errors in estimating the GFR from the creatinine clearance?

Answer 15

by excluding non-steady state pathologic conditions of creatinine release like hyperthermia or other conditions of muscle wasting or damage or ingestion of meat which contains high creatinine content the night before

Question 16

what is the effect of decreased GFR on plasma creatinine concentration?

Answer 16

if GFR falls by half, then double the creatinine amount inside the plasma

Question 17

1040

the glomerular filtration barrier consists of 3 elements, what are they?

which of the three are covered in negative charges?

Answer 17

• endothelial cells
• the glomerular basement membrane
• epithelial podocytes

the glomerular basement membrane

epithelial podocytes

Question 18

what does the relative concentration of a solute in filtrate depend on?

Answer 18

molecular weight and effective molecular radius

such as water, urea, glucose, and inulin —appear in the filtrate in the same concentration as in plasma.

Question 19

what is the difference between renal blood flow (RBF) and renal plasma flow (RPF)?

Answer 19

renal blood flow has to do with the hematocrit amount, its calculated from RPF/1-hematocrit and is approx 1 liter/min out of the total cardiac output of 5 liters/min and renal plasma flow is around 0.40 with RPF approx 600 ml/min

Question 20

what is the term used to describe what lies between the afferent and efferent arterioles on the glomerular capillary?

Answer 20

filtration equilibrium point and so at low glomerular plasma flow, filtration equilibrium occurs halfway down the capillary leaving the end of the capillary as wasted

Question 21

how does the filtration equilibrium pressure shift toward the efferent arteriole at normal glomerular plasma flow?

Answer 21

1) while progressing along the capillary, ultrafiltration pressure (and hence filtration) remains greater,
2) filtration occur along a greater stretch of the glomerular capillary, thereby increasing the useful surface area for filtration.

Question 22

what can the use of the wasted capillary be?

Answer 22

The end of the capillary that is “wasted” at low plasma flow rates is in fact “in reserve” to contribute at higher rates.

Question 23

what happens to afferent arteriolar resistance on the remnant kidney after a nephrectomy in a kidney donar?

Answer 23

the loss of renal tissue causes a dramatic decrease in afferent arteriolar resistance on the remnant kidney and GFR nearly doubles

Question 24

what is the effect of angiotensin II inhibitors like captopril on patients with hypertension?

Answer 24

efferent arteriolar effect is seen after administration of angiotensin II inhibitors and so administering such agents not only decreases blood pressure but also leads to a significant fall in GFR

Question 25

what are the peritubular capillaries?

Answer 25

they deliver oxygen and nutrients to the epithelial cells and are responsible for taking up from the interstitial space the fluid that the renal tubules absorb

Question 26

relate the glomerular and the peritubular capillaries to the starling forces?

Answer 26

in glomerular capillaries Starling forces favor filtration

in the peritubular capillaries, the Starling forces always favor absorption because glomerular filtration elevates the oncotic pressure of blood entering the peritubular capillary network

Question 27

why is PAH clearance a good measure of RPF? what is the equation?

Answer 27

Because the kidneys almost completely clear blood of PAH in a single pass through the kidneys

PAH excreted in urine= PAH filtered in glomerulus + PAH secreted by tubules

Question 28

what is the effect of the and autoregulation of the renal blood supply and influence of renal nerves and circulating hormones?

Answer 28

they are independent

Question 29

what arteriole does the autoregulatory response occur at?

Answer 29

the afferent arteriole and so the resistance to flow rises with increasing perfusion pressure

Question 30

what is the effect of a wide range of renal arterial pressures on the efferent arteriolar resistance, capillary resistance and venous resistance?

Answer 30

they all change very little

Question 31

what are the two mechanisms that underlie renal auto regulation?

Answer 31

myogenic response of the smooth muscle of the afferent arterioles and tubuloglomerular feedback

Question 32

what is the myogenic response of the smooth muscle of the afferent arterioles?

Answer 32

An increase in vessel diameter in afferent arterioles opens stretch-activated, nonselective cation channels in vascular smooth muscle, depolarizing the cell and leading to an influx of Ca 2+ that stimulates contraction.

Question 33

what is the tubuloglomerular feedback?

Answer 33

The juxtaglomerular apparatus mediates this feedback mechanism. The macula densa cells in the thick ascending limb sense an increase in GFR and, with feedback, translate this to an increase in the tone of the afferent arteriole, and hence a decrease in GFR.

Question 34

what increases the GFR?

Answer 34

Volume Expansion & High-Protein Diet

Question 35

how does volume expansion increase the GFR?

Answer 35

Expansion of the extracellular fluid decreases the sensitivity of the tubuloglomerular feedback loop.

whereas

volume contraction increases the sensitivity of TGF, helping preserve fluid by reducing GFR.

Question 36

how does the high protein diet increase GFR?

Answer 36

A high-protein diet enhances NaCl reabsorption along the thick ascending limb, so that luminal [NaCl] at the macula densa falls.

The flow in the loop of Henle, and thus GFR, must be higher to raise [NaCl] to a particular level at the macula densa.