College of Health SciencesDepartment of Medical Physiology

Advanced Renal PhysiologyPresentation on

Renal Handling of Urea, Uric acid and Creatinine

By Teketel Eristu2/4/2015 1

Presentation Outline

Objectives

Introduction

Renal Clearance

Renal handling of Urea

Renal Handling of Uric acid

Renal Handling of Creatinine2/4/2015 2

1. Objectives:

At the end of the presentation the learners are expected to:

Explain Renal Clearance

Discuss Renal handling of Urea

Know Renal handling of Uric acid

Understand Renal handling of Creatinine

2/4/2015 3

2. IntroductionRenal Clearance

-Is the rate at which that solute disappears from the

body by excretion or by metabolism.

- is expressed as the volume of plasma passing through

the kidneys that has been totally cleared of that solute

in a given period of time

- For any substance that is freely filtered but neither

reabsorbed nor secreted, its clearance is equal to GFR2/4/2015 4

Cont…• For solute X:

Cx = Ux x V

Px

Volume of urine formed in a given time

Conc. of X in systemic blood plasma

Clearance

Conc. of X in urine

GFR also assessed using principles of clearanceSame equation, GFR is Cx if X has certain required properties (i.e. Cinulin).

GFR = Ux x V

Glomerularfiltration rate

Conc. of X in urine

Volume of urine formed in a given time

PxConc. of X in systemic blood plasma

2/4/2015 5

Urinary Excretion of Solutes

Urinary Excretion of

SoluteFiltered Load

Reabsorptionby Tubules

Secretion by Tubules= - +

Rate at which kidneys excrete solute into urine = rate at which solute disappears from blood plasma.

2/4/2015 6

Clearance helps us determine renal handling:

Once we know a person’s GFR, we can determine how the kidney

handles any solute by measuring the solute’s plasma concentration

and its excretion rate

By comparing the filtered load of the solute with its excretion rate,

we can tell how the nephron handled that substance

.2/4/2015 7

3 Renal Urea HandlingUrea:

is the main nitrogenous waste of the body and is excreted by the

kidneys

is produced primarily in the liver as a bi-product of amino acid

metabolism.

50% of the filtered urea is reabsorbed passively in the PCT

-The distal tubule, cortical collecting ducts, and outer medullary

collecting ducts are impermeable to urea; thus, no urea is reabsorbed

by these segments

ADH increases the Urea permeability of the inner medullary collecting

ducts.2/4/2015 8

Cont….

Plasma urea concentration (BUN) ranges from 8 mg/dL to 30mg/dL and is not

subject to homeostatic regulation. Urea concentration in urine is 1820

mg/dl.

Urea accounts for 86% of N2 in urine ( 5% creatinine; 3%NH4; 6% Others).

2/4/2015 9

Cont..

Urea excretion varies with urine flow rate

At high levels of water reabsorption ( low urine flow rate), there is

greater urea reabsorption and decreased urea excretion.

At low levels of water reabsorption ( high urine flow rate), there is less

urea reabsorption and increased urea excretion.

More urea is reabsorbed at low tubular flow rates than at high tubular

flow rates

Plasma Clearance = Ux * V/ Px 18.5/0.25 =70 ml/min2/4/2015 10

Cont….• Dependence of urea excretion

on urine flow rate.

• High plasma ADH levels reduce

urine flow rate.

• Urea excretion decreases during

antidiuresis because urea

reabsorption from the collecting

duct increases2/4/2015 11

Cont….• During antidiuresis, water

reabsorption from the collecting

duct concentrates urea in the

lumen, creating a diffusion

gradient for urea reabsorption.

• This mechanism accounts for

increased blood urea nitrogen

(BUN) in states of increased ADH.

2/4/2015 12

4. Renal Uric acid handling

uric acid

-is a normal product of purine metabolism

-is created when the body breaks down purine nucleotides

-It forms ions and salts known as urates and acid urates such as

ammonium acid urate

-Plasma urate, the anionic form of uric acid, filters freely into Bowman’s

capsule but is almost totally reabsorbed in the first part of the

proximal tubule2/4/2015 13

Cont….

The middle section of the proximal tubule then secretes about half of

the reabsorbed urate back into the lumen, and the terminal section

of the proximal tubule again reabsorbs some of it. The end result is

net secretion.

Concentration of uric acid 4mg/dL in plasma versus 42 mg/dl in urine.

Plasma clearance = 14 ml/min.2/4/2015 14

5. Renal Creatinine Handling

Creatinine

- is a byproduct of the breakdown of creatine and

phosphocreatine, an energy- storage compound found primarily

in muscles

-Creatinine is neither reabsorbed nor secreted, just freely filtered.

Amount excreted = amount filtered

2/4/2015 15

Cont…

The serum creatinine concentration can vary based on a number of

factors including an animal’s diet, muscle mass, and gender.

2/4/2015 16

Cont…

A constant amount of phosphocreatine is spontaneously,

irreversibly and nonenzymatically converted to creatinine

daily and utilized by the body. This amount is directly

proportional to the individual’s muscle mass. Therefore, a

stable amount of creatinine is presented to the kidneys

daily for excretion. 2/4/2015 17

2/4/2015 18

Cont…

24 hour endogenous creatinine clearance is an estimate of GFR

(1.96/0.11=140ml/min).

The normal plasma creatinine level (0.8-1 mg/dl) in the absence of renal

diseases

Can then be compared to creatinine in urine over 24 hour period to

determine clearance

2/4/2015 19

6. References

1. Pearson International Edition Human Physiology An

Integrated Approach 5th Edition.

DEE UNGLAUB SILVERTON

2. Lecture Note on Advanced renal physiology 2011 Tesfaye T.

(PhD)

3. Different internet sources.

2/4/2015 20

09-12-2010 21