Glomerular Filtration Rate and Renal Blood Flow

Question 1

How do you calculate the amount of filtrate excreted in the urine?

Answer 1

amount filtered - amount reabsorbed + amount secreted = amount excreted in urine

Question 2

Compare and contrast the pressures in the afferent, efferent resistances as well as the pressure in the peritubular capillaries.

Answer 2

afferent arterial resistance is less than arteriolar resistance in the systemic arterioles

efferent arteriolar resistance is in series with the afferent arterioles

pressures in the peritubular capillaries is less than a systemic capillary

Question 3

What is the primary driving force of glumerular filtration?

Answer 3

the initial high hydrostatic pressure in the glomerular capillaries drives filtration

this pressure is balanced out by the increased oncotic pressures as filtrate moves through

Question 4

glomerular filtration rate in humans

Answer 4

125 mL/min

represents 20% of renal plasma flow (RPF)

Question 5

filtration fraction (FF)

Answer 5

GFR/RPF

refers to the value of the GFR in relation to renal plasma flow, not renal blood flow

Question 6

What are the key differences between glomerular filtration and systemic capillary filtration?

Answer 6

glomerular capillary permeability x surface area is 100x greater than most systemic capillaries

glomerular capillary hydrostatic pressures is 2 tiems greater than in systemic capillaries

Question 7

Starling forces favoring glomerular filtration

Answer 7

glomerular capillary hydrostatic pressure

Bowman’s space oncotic pressure

Question 8

Starlin forces impeding glomerular filtration

Answer 8

glomerular capillary oncotic pressure

Bowman’s space hydrostatic pressure

Question 9

What are some of the forces driving net absorptive pressure along the peritubular capillaries?

Answer 9

17 mmHg of pressure in the beginning of the capillaries

12 mmHg pressure at the end

constant pressure to reabsorb filtrate

Question 10

What is the equation that describes renal blood flow (RBF)

Answer 10

RBF = (P_{renal artery} - P_{renal vein}) / R_{renal vasculature}

RBF = 125mL blood/min or 4mL blood/min/gm tissue

Question 11

Describe the vascularization of the renal cortex, outer medulla, and inner medulla

Answer 11

renal cortex = high vascularization (90% RBF)

outer medulla = low vascularization (8% RBF)

inner medulla = very low vascularization (2% RBF)

Question 12

anatomical distribution of renal capillary beds

Answer 12

ratio of cortical nephrons to juxtamedullary nephrons is 7:1

glomerular and peritubular capillaries are confined to the cortex

only vasa recta capillaries dive deep into the renal medulla

blood-flow distribution to various regions is not fixed, but can vary

Question 13

How does oncotic pressure in the capillaries change thorughout the filtration process?

Answer 13

increases in glomerular capillaries

decreases in peritubular capillaries

Question 14

In terms of pressures and resistances, what is RBF proportional to?

Answer 14

mean deltaP/(R_aff+R_eff) or 1/(R_aff+R_eff)

Question 15

In terms of pressures and resistances, what is GFR proportional to?

Answer 15

deltaP_GC/R_GC or R_eff/R_aff

Question 16

How does constricting the afferent arteriole affect GFR?

Answer 16

decreases due to the fall in both capillary pressure and renal plasma flow

Question 17

How does constriction of the efferent arteriole affect GFR?

Answer 17

GFR initially increases because of rising capillary pressure

after a point, GFR begins to decrease because of falling RPF

Question 18

What is the normal range for renal perfusion pressure?

Answer 18

80-180 mmHg

Question 19

What are the two theories for renal autoregulation?

Answer 19

myogenic theory

tubuloglomerular theory

Question 20

myogenic theory

Answer 20

smooth muscle of the afferent arteriole contracts or relaxes during changes in transmural pressure

this change in resistance keeps blood flow and GFR relatively constant

Question 21

tubuloglomerular theory

Answer 21

each individual nephron controls its own filtration rate through a sensor located at the macula densa portion of the distal tubule

arterial pressure induced increases in GFR increases solute delivery rate to the macula densa

signal is generated by the macula densa cells that results in afferent arteriolar constriction

conversely, decreases in solute load in the macula densa leads to increases in GFR

Question 22

action of adenosine in the kidney

Answer 22

vasoconstrictor

possible signal from the macula densa

Question 23

intrinsic factors affecting blood flow regulation

Answer 23

nitric oxide

endothelin

prostaglandins

adenosine

kinins

dopamine

Question 24

extrinsic factors affecting blood flow regulation

Answer 24

sympathetic innervation of afferent and efferent arterioles

blood borne substances (angiotensin II, ANP, ADH, ATP, glucocorticoids)

Question 25

factors decreasing RBF and GFR

Answer 25

decreased central blood volume (e.g. hemorrhage)

increased circulating renin and angiotensin (e.g. dehydration)

Question 26

RPF

Answer 26

renal plasma flow = (RBF x (1-Hct))

Question 27

equations involving load

Answer 27

load = mg/min = concentration x flow

kidney input load = Pa_x x RPFa

kidney output load = (PV_x x RPFv) + (U_x x V)

Question 28

What are the three ways that filtered substances may be handled?

Answer 28

filtered and secreted

filtered and partially reabsorbed

filtered and reabsorbed

Question 29

clearance

Answer 29

volume of plasma from which the kidney completely removes (clears) a particular substance per unit time

Cx = [U_x x V_x]/P_x

important for determining how efficient they kidneys clear a certain substance per unit time

Question 30

characteristics of an ideal substance to measure clearance

Answer 30

intert

freely filtered

not secreted

not reabsorbed

not synthesized or broken down by the tubules

ex. inulin

Question 31

What are the parameters for using clearance to determine net renal handling of substances?

Answer 31

if C_Q < GFR, substance Q is filtered and reabsorbed

if C_R = GFR, substance R is only filtered

if C_S > GFR, substance S is filtered and secreted

Question 32

Why is GFR so important to know?

Answer 32

single best estimate of functioning renal mass

serial measurements allow the tracking of the course of the disease

GFR measurement is useful for knowing the appropriate dosage of drugs to use to attain therapeutic levels

GFR varies directly with renal function

Question 33

clearance of creatinine

Answer 33

creatinine is an end-product of skeletal-muscle metabolism

GFR = clearance of creatinine (C_CR)

decreased functional glomeruli decreases GFR and C_CR

decreased C_CR can be estimated by an increased P_CR

Question 34

What is the relationship between P_CR and blood-urea-nitrogen (BUN)?

Answer 34

P_CR x 10 ~ BUN

P_CR is the more accurate estimator of GFR than BUN