Renal 1 & 2

Question 1

What are the four main processes that happen in the kidneys?

Answer 1

1. Filtration
2. Reabsorption
3. Secretion
4. Excretion

Question 2

What are the respective approximate osmolarities of the interstitium of the renal cortex and renal medulla?

Answer 2

Cortex is around 300mOsm (same as plasma) and the medulla is 1200mOsm.

Question 3

What percentage of the cardiac output is received by the kidneys?

Answer 3

20%

Question 4

What are the two major types of nephrons? Which one is more common? What are the differences?

Answer 4

Cortical nephrons - 90%. Glomeruli are in the outer cortex, they have short loops of Henle, and efferent arterioles form peritubular capillaries that put absorbed products back into the circulation.
Juxtamedullary nephrons - 10%. Glomeruli are at the border of the cortex and medulla, they have long loops of Henle, and the efferent arterioles form both peritubular capillaries and vasa recta.

Question 5

The distal convoluted tubules come into close contact with the glomeruli in the ________ region. Describe two cell types found in this region and what their functions are.

Answer 5

juxtaglomerular region.

Cells:
Macula densa cells - found inside the DCT, they monitor fluid flow and composition, particularly NaCl conc.

Juxtaglomerular cells are located just outside the afferent and efferent arterioles and these produce renin to control constriction/dilation of afferent and efferent arterioles.

Question 6

What is the basic equation for excretion?

Answer 6

Excretion = Filtration - Reabsorption + Secretion

Question 7

What percentage of plasma entering afferent arteriole is filtered through capillary membrane into Bowman’s space?

Answer 7

20% - important!

Question 8

What is the main reason why proteins can’t get into Bowman’s space?

Answer 8

The basement membrane has a negative charge, and since most proteins at neutral pH have a negative charge, they are repelled.

Question 9

What important protein is found in the diaphragm between the foot processes of podocytes? What happens if people have a gene mutation that codes for this protein?

Answer 9

Nephrin - it intercalates in the diaphragm between the foot processes. If mutated you get nephrotic syndrome –> loss of protein in urine, edema, renal failure eventually.

Question 10

Define glomerular filtration rate (GFR).

Answer 10

The rate at which fluid is filtered through the glomerulus.

Question 11

Define renal plasma flow (RPF).

Answer 11

The rate at which plasma is delivered to the kidneys.

Question 12

GFR = Kf (filtration coefficient) x net filtration pressure

What contributes to the net filtration pressure?

Answer 12

Afferent hydrostatic pressure in the glomerular capillary (Pgc) + oncotic pressure of the filtrate in Bowman’s space (should be zero) - hydrostatic pressure in Bowman’s space - oncotic pressure of the incoming blood.

Question 13

Why is the net pressure for glomerular filtration lower at the efferent arteriole?

Answer 13

As fluid is taken out of the blood and into Bowman’s space, it becomes more concentrated, creating higher oncotic pressure.

Question 14

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

Answer 14

Renal plasma flow subtracts the cellular volume in blood…

RPF = RBF x (1 - Hct)

Question 15

What is a typical value for renal plasma flow?

Answer 15

1250 (25% of cardiac output) x (1 - .45) = 687 ml/min

.45 is a typical value for Hct

Question 16

What is filtration fraction?

Answer 16

The fraction of plasma filtered through the glomerulus - typically 0.2 (20%)…

687 (renal plasma flow) x .2 = 137 ml/min

Question 17

What is clearance?

Answer 17

The VOLUME of plasma per unit time from which a substance has been completely removed and excreted.

C = UV/P

where U is the concentration of the substance in the urine (mg/ml)

V is the urine flow rate (ml/min)

P is the concentration of the substance in the plasma (mg/ml)

Note that UV = the excretion rate of the substance (mg/min)

Question 18

What are the four requirements of a substance as it pertains to the kidneys that are necessary to calculate GFR?

Answer 18

1. Freely filtered.
2. Not reabsorbed or secreted.
3. Not metabolized or produced by the kidney.
4. The substance doesn’t alter GFR.

Question 19

What is most commonly used substance to clinically measure GFR? Explain the basis of this.

Answer 19

Creatinine - produced endogenously in skeletal muscle at a farily constant rate, it is freely filtered and not reabsorbed in the kidneys.

A small amount is secreted by the renal tubule (10%) and this should be corrected for in calculations.

Question 20

Para-aminohippuric acid (PAH) - a small organic anion that must be introduced by infusion - is used to measure ______. Why?

Answer 20

Used to measure RPF because it is freely filtered and very efficiently secreted into the renal tubule, so all the plasma that goes into the kidneys comes out the other side nearly free of PAH.

Question 21

What is the difference between renal plasma flow and effective renal plasma flow?

Answer 21

RPF = ERPF / extraction efficiency

in the case of PAH, only 90% of the PAH that enters the kidney is secreted, so the calculation for RPF is boosted by dividing by .9

Question 22

How is tubular secretion rate calculated?

Answer 22

For a substance that is not reabsorbed:
Secretion rate = excretion rate - filtration rate…

secretion rate = UV - GFR x P (P is plasma conc. of the substance)

Question 23

How is tubular reabsorption rate calculated?

Answer 23

For a substance that is not secreted:
Reabsorption rate = filtration rate - excretion rate

reabsorption rate = GFR x P - UV
(P is plasma conc of the substance)

Question 24

At around ___mg/ml blood glucose concentration, Na/glucose transporters in the kidneys that resorb glucose are overwhelmed, and at levels above this one sees a ______ increase of glucose levels in the urine.

Answer 24

above 3 mg/ml blood glucose levels, transporters are overwhelmed and a linear increase in urine glucose is seen

Question 25

What is clearance ratio (CR)?

Answer 25

Clearance of a substance / GFR

so if CR is 1, that tells you that the substance was not reabsorbed or secreted

if CR > 1, secretion must be occurring

if CR < 1, reabsorption must be occurring

Question 26

How is renal blood flow and GFR regulated physiologically? What endogenous substances do this?

Answer 26

Contraction of dilation of afferent and efferent arterioles.

Norepinephrine will constrict, NO (released by endothelial cells) will dilate

Question 27

How are GFR and RBF affected by the following situations?…

1. Constriction of afferent arteriole (efferent stays the same)
2. Constriction of efferent arteriole (afferent stays the same).
3. Dilation of efferent arteriole (afferent stays the same).
4. Dilation of afferent arteriole (efferent stays the same).

Answer 27

1. Constriction of afferent arteriole (efferent stays the same)
   - both GFR and RBF decrease
2. Constriction of efferent arteriole (afferent stays the same).
   - GFR increases and RBF decreases (its exerting a higher pressure, driving fluid into Bowman’s space)
3. Dilation of efferent arteriole (afferent stays the same).
   - GFR decreases and RBF increases
4. Dilation of afferent arteriole (efferent stays the same).
   - both GFR and RBF increase

Question 28

What are the two intrinsic autoregulatory mechanisms that maintain constant GFR and RBF?

Answer 28

Myogenic mechanism: constriction of smooth muscle of afferent arterioles when they are stretched

Tubuloglomerular feedback: increased tubular flow is sensed by macula densa –> sends a signal (adenosine) –> constriction of afferent arterioles.