Renal Exam 3

C

clearance (mL/min)

[U]

concentration in urine (mg/mL)

[P]

concentration in plasma (mg/mL)

V (w/ dot above it)

urine flow rate (mL/min)

GFR

glomerular filration rate (120 mL/min)

RPF

renal plasma flow (660 mL/min)

RBF

renal blood flow (1200 mL/min)

[TF]

concentration in tubular fluid (mg/mL)

[TF/P]x

concentration of x in tubular fluid relative to concentration of x in plasma

[TF/P]inulin

concentration of inulin in tubular fluid relative to concentration of insulin in plasma

[TF/P]x/[TF/P]inulin

fraction of the filtered load remaining in tubular fluid, or fractional excretion

How to calculate clearance

Cx=[U]x(Vdot) / [P]x (in mL/min)

or concentration of urine times urine flow rate divided by concentration in plasma

x=any substance

How to calculate clearance ratio

C x / C inulin

also means fractional excretion of x

How to calculate effective renal plasma flow (RPF)

[U]_PAH x V^. / [P]_PAH (in mL/min)

underestimates RPF by 10%; equals C_PAH

How to calculate renal blood flow

RPF / 1-Hct (in mL/min)

1 minus Hct is fraction of blood volume that is plasma

How to calculate Glomerular filtration rate

[U]_inulin x V^. / [P]_inulin (in mL/min)

equals C_inulin

How to calculate filtration fraction

GFR / RPF

How to calculate reabsorption or secretion rate

filtered load - excretion (in mg/min)

if POSITIVE, net reabsorption

if NEGATIVE, net secretion

% of blood flow/cardiac output that the kidney does

25%

Main role of the kidney

regulate volume of fluid in body and composition of that fluid

when things are off, it is on the kidneys to adjust

What vessels go into calculating renal input/output

1 renal artery, 1 renal vein, 1 ureter per kidney

ureters account for what was excreted in urine

3 hormones used in kidney

renin (bp thing), epo (helps RBC production), calciferol (vit D)

What do prostaglandins do

inflammatory mediators

too much inflammation —→ pathology

too little —→ things also go bad

there’s a “sweet spot”

Example of a prostaglandin inhibitor

Ibuprofen / Advil

2 types of nephrons

Superificial and Juxtamedullary

Nephron that is higher up to the cortex (outer aspect of kidney)

Superficial nephron

Main type of nephron (much longer loop of Henle)

Juxtameduallary nephron

Where action of nephrons take place

Loop of Henle

Segments of the nephron, in order

glomerular capillaries and bowman’s space —→ proximal convoluted tubule —→ proximal straight tubule —→ thin descending limb —→ thin ascending limb —→ thick ascending limb —→ macula densa —→ distal convoluted tubule —→ connecting tubule —→ cortical collecting duct —→ outer medullary collecting duct —→ inner medullary collecting duct

Where does urine production occur in the nephron

collecting duct

Term for substance not bound to anything (ex: Na+)

“freely filtered”

Afferent arterioles refer to them going into what

the glomerulus

How much is filtered?

about 25%

How can you excrete more than you absorb?

only possible through reabsorption

Where does reabsorption/secretion occur; also has access to the rest of the nephron

Peritubular capillary

Where does the filtered load go?

Bowman’s space

Describe inulin

• related to GFR

• freely filtered, but not reabsorbed or secreted

  • so if you know what was excreted, you know filtered load bc it is not reabsorbed

• not naturally in body (has to be infused)

Structure involved in filtration part, absorb/secrete along the path

Glomerulus

Filtered out of nephron and then body put it back in

reabsorption

Coming out of plasma and into urine to be secreted by bladder

Secretion

What does knowing if something is reabsorbed or secreted tell us?

the NET / balance of these things

Use creatinine or sometimes BUN to get this value; easier than infusing inulin to get value

GFR

Measured by the clearance of a glomerular marker

GFR

A substance that is freely filtered across the glomerulus capillaries and is neither reabsorbed nor secreted by the renal tubules.

a glomerular marker

The ideal glomerular marker; thus, the clearance of this is the GFR

inulin

Tells us how much was cleared / in vs. out ; how much am I getting rid of vs. what was in the plasma

Clearance (C_x in mL/min)

What is decreased GFR a sign of

that you are losing kidney function

Typical GFR (clearance of inulin)

about 120 mL/min

Basically the water part of your blood; really just maintaining / getting rid of water

plasma

What does “true” renal plasma flow assume?

that we are excreting 100% —→ 10% error

Fick principle for measuring RPF

amout of a substance entering an organ equals the amount leaving it

What is unique about PAH

once it is filtered, it gets secreted

What is typical urine outflow

1 mL/min or about 1.5 L/day

can be changed through hydration and diet

What is the difference btwn effective RPF and “true” RPF

about 10% difference; effective RPF is lower by that amount

Calculated from the measured renal plasma flow and the hematocrit

Renal blood flow

What does hematocrit tell you

viscosity of blood

• women tend to have less hematocrit (38-45%) vs. men (42-48%)

• goal is to be less than half

  • as it gets thicker, we start to have resistance to flow

RPF divided by 1 minus the hematocrit = _____

RBF

The fractional blood volume occupied by RBCs

hematocrit

1 - hematocrit = fractional blood volume occupied by plasma

As you dehydrate, what happens to hematocrit?

it increases

Do afferent or efferent arterioles have more receptors?

afferent

What determines capillary volume?

• hydrostatic water pressure

• osmotic pressure

How do we increase filtration?

cause blood to build up pressure

Examples of renal vasoconstrictors

sympathetic nerves (catecholamines; alpha 1), angiotensin II, endothelin

Examples of renal vasodilators

PGE₂ / PGI₂ (prostaglandins; very potent), nitric oxide (NO), bradykinin, dopamine, atrial natriuretic peptide

When discussing renal vasoconstrictors and vasodilators, is it always straight forward?

No

What does constriction of afferent arteriole do to RPF, GFR, and filtration fraction?

RPF: decrease

GFR: decrease

FF: N.C.

What does constriction of efferent arteriole do for RPF, GFR, and filtration fraction?

RPF: decrease

GFR: increase

FF: increase

What does constriction of efferent arteriole do in glomerulus?

builds up the flow there

Why does filtration fraction increase when constricting efferent arterioles?

because you are manipulating pressure in glomerular capillary

What does increased plasma protein concentration do to RPF, GFR, and filtration fraction?

RPF: N.C.

GFR: decrease

FF: decrease

How to calculate filtration fraction?

GFR / RPF

What does an increase in plasma protein concentration mean is happening?

relating to oncotic pressure; will draw water

Does plasma freely go in and out?

No

Why does FF decrease when plasma protein concentration increases?

less force for fluid to leave capillary; less filtered

What happens to RPF, GFR, and FF in decreased plasma protein concentration?

RPF: N.C.

GFR: incease

FF: increase

Why does FF increase in decreased plasma protein concentration?

oppositve effect from increased; favors filtration to normalize plasma protein concentration

What happens to RPF, GFR, and FF in constriction of the ureter?

RPF: N.C.

GFR: decrease

FF: decrease

What does the ureter influence

flow out; increases pressure in Bowman’s space

Plotting renal blood flow (GFR) over P_a of renal artery, when does flow stay the same? What word describes this process?

between 80 and 200 mm Hg

steady increase until then (starting at 80)

“Myogenic regulation”: artery increases tone to maintain pressure (arteriole maintains tone as flow increases or decreases)

What are the dangers of HTN in the kidney?

alter blood flow/BP and kidney failure

How does increased protein diet affect kidney?

does not have evidence related to kidney failure, but DOES increase workload on your kidneys because they must filter out extra waste products, nitrogen, and metabolic acids

Tubuloglomerular feedback is what type of feedback?

Negative; sensed in tubule, feeding back to glomerulus

Steps of tubuloglomerular feedback

1. increase RBF and GFR

2. increase delivery of Na+ and Cl- to juxtaglomerular apparatus (sensed by macula densa - spot in nephron)

3. release of vasoactive substance (e.g., adenosine) from macula densa

4. increase resistance of afferent arteriole (to decrease blood flow —→ which leads to #5)

5. decrease RBF and GFR

Senses [Na+]; Na is freely filtered; as it goes up or down nephron feedback to glomerulus goes up or down

Macula densa

Fraction of RPF that is filtered across the glomerular capillaries

Filtraction fraction (GFR / RPF)

Typical value for filtration fraction of normal kidneys and what does it mean?

0.2 or 20%

means that approximately 20% of renal plasma flow entering the kidneys through the renal arteries is filtered across the glomerular capillaries

If FF is 20%, what happens to remaining 80% of RPF?

leaves the glomerular capillaries through the efferent arterioles and becomes the peritubular capillary blood flow

• still has opportunity to be absorbed or secreted elsewhere in nephron

Refering to something not bound to plasma proteins?

"freely filtered”

The fraction (or percent) of the filtered load that is excreted in the urine

fractional excretion

found by dividing excretion rate (U_x x V^.) by filtered load (GFR x P_x)

What does it mean if filtration fraction is more than 1 or 100%? What does it mean if it is less than 100%?

>100% means more was excreted than filtered; tells you it had to be secreted

<100% means that it was reabsorbed

If, for example, Substance B is 30% bound to plasma proteins, what does that mean?

not freely filtered; only 70% freely filtered

Because the excretion rate of Substance B is much less than the filtered load, Substance B must have been _____.

reabsorbed

calculated from the difference betwee filtered load and excretion rate (positive #)

What is the normal amount of glucose in the urine?

None; 0

Essential to regulating blood glucose

kidney

#1 cause of death

heart disease

Type 1 diabetes mellitus

“insulin-dependent”

insulin stops liver from producing glucose

Important symptoms / exam findings from David case study of Glucosuria: Diabetes Mellitus

Symptoms

• constantly thirsty

• frequent urination

• big appetite, but losing weight

Exam Findings:

• weight went down from last check up

• low BP (especially when standing)

• fasting plasma glucose about 3x higher than normal

• plasma Na+ slightly high

• urine glucose and ketones (normal, none)

• increased urine Na+

Glucose excretion

the sum / balance of filtration, absorption, and secretion

2 Roles of Kidney

fluid balance and composition of body fluids

Why don’t people with diabetes just excrete the excess glucose?

There’s a max transport rate (glucose has to have a transporter), rest of glucose stays in tubular fluid