renal chemistry 2

7/27/2019 Renal Chemistry 2

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Renal Function test


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Normal functions of the kidney

1. To maintain the constancy of the extra-cellular fluid by:

I. Excreting dietary surpluses and metabolic end-products e.g. urea, creatinine,

urate, H+

II. Retaining necessary substances, either by not letting them be filtered (e.g.

proteins) or by reabsorbing them in the tubules (e.g. glucose, amino-acids, HCO3-)

2. To act as an endocrine gland

I. Erythropoietin

II. Renin

III. 1-alpha-hydroxylation of Vitamin D (to make 1:25 di-hydroxycholecalciferol,

calcitriol)

Uses 20-25% of cardiac out put to filter 180L/day = 125ml/min


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Copyright 2009, John Wiley & Sons, Inc.

Overview of kidney functions

Regulation of blood ionic composition

Regulation of blood pH

Regulation of blood volume

Regulation of blood pressure

Maintenance of blood osmolarity

Production of hormones (calcitrol and erythropoitin)

Regulation of blood glucose level

Excretion of wastes from metabolic reactions and foreignsubstances (drugs or toxins)

Each kidney has about 1 million nephrons


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Structures and functions of a nephron

Renal corpuscle Renal tubule and collecting duct

Peritubular capillaries

Urine(contains

excretedsubstances)

Blood(containsreabsorbed

substances)

Fluid inrenal tubule

Afferentarteriole

Filtration from blood

plasma into nephron

Efferentarteriole

Glomerularcapsule

1



Urine(contains

excretedsubstances)


substances)

Tubular reabsorptionfrom fluid into blood


Afferentarteriole


plasma into nephron

Efferentarteriole

Glomerularcapsule

1

2



Urine(contains

excretedsubstances)


substances)

Tubular secretionfrom blood into fluid

Tubular reabsorptionfrom fluid into blood


Afferentarteriole


plasma into nephron

Efferentarteriole

Glomerularcapsule

1

2 3


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Glomerular filtration

Glomerular filtratefluid that enters capsular space Daily volume 150-180 litersmore than 99% returned to blood

plasma via tubular reabsorption

Filtration membraneendothelial cells of glomerular

capillaries and podocytes encircling capillaries Permits filtration of water and small solutes

Prevents filtration of most plasma proteins, blood cells andplatelets

3 barriers to crossglomerular endothelial cells fenestrations,basal lamina between endothelium and podocytes and pedicelsof podocytes create filtration slits

Volume of fluid filtered is large because of large surface area,thin and porous membrane, and high glomerular capillary bloodpressure


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Filtration slitPedicel of podocyte

Fenestration (pore) of

glomerular endothelial cell

Basal lamina

Lumen of glomerulus

(b) Filtration membrane

TEM 78,000x

(a) Details of filtration membrane

Filtration slit

Pedicel

Fenestration (pore) of glomerular

endothelial cell: prevents filtration of

blood cells but allows all components

of blood plasma to pass through

Podocyte of visceral

layer of glomerular

(Bowmans) capsule

1




Basal lamina

Lumen of glomerulus


TEM 78,000x


Filtration slit

Pedicel





Basal lamina of glomerulus:

prevents filtration of larger proteins


layer of glomerular

(Bowmans) capsule

1

2




Basal lamina

Lumen of glomerulus


TEM 78,000x


Filtration slit

Pedicel





Basal lamina of glomerulus:

prevents filtration of larger proteins

Slit membrane between pedicels:

prevents filtration of medium-sized

proteins


layer of glomerular

(Bowmans) capsule

1

2

3


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Net filtration pressure

Net filtration pressure (NFP) is the total pressure that

promotes filtration

NFP = GBHPCHPBCOP

Glomerular blood hydrostatic pressure forcing water and solutesthrough filtration slits

Capsular hydrostatic pressure is the hydrostatic pressure exerted

against the filtration membrane by fluid already in the capsular

space and represents back pressure

Blood colloid osmotic pressure due to presence of proteins inblood plasma and also opposes filtration

Why oliguria developed in shocked patients?


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NET FILTRATION PRESSURE (NFP)

=GBHP CHP BCOP

= 55 mmHg 15 mmHg 30 mmHg

= 10 mmHg

GLOMERULAR BLOOD

HYDROSTATIC PRESSURE

(GBHP) = 55 mmHg

Capsular

space

Glomerular

(Bowman's)

capsule

Efferent

arteriole

Afferent arteriole

1

Proximal convoluted tubule


=GBHP CHP BCOP


= 10 mmHg

CAPSULAR HYDROSTATIC

PRESSURE (CHP) = 15 mmHg

GLOMERULAR BLOOD


(GBHP) = 55 mmHg

Capsular

space

Glomerular

(Bowman's)

capsule

Efferent

arteriole

Afferent arteriole

12



=GBHP CHP BCOP


= 10 mmHg

BLOOD COLLOIDOSMOTIC PRESSURE

(BCOP) = 30 mmHg

CAPSULAR HYDROSTATIC

PRESSURE (CHP) = 15 mmHg

GLOMERULAR BLOOD


(GBHP) = 55 mmHg

Capsular

space

Glomerular

(Bowman's)

capsule

Efferent

arteriole

Afferent arteriole

12

3



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Tubular reabsorption and tubular secretion

Reabsorptionreturn of most of the filteredwater and many solutes to the bloodstream About 99% of filtered water reabsorbed

Proximal convoluted tubule cells make largestcontribution

Both active and passive processes

Secretiontransfer of material from blood

into tubular fluid Helps control blood pH

Helps eliminate substances from the body


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Reabsorption routes and transport mechanisms

Reabsorption routes Paracellular reabsorption

Between adjacent tubule cells Tight junction do not completely seal off interstitial fluid from tubule

fluid Passive

Transcellular reabsorptionthrough an individual cell

Transport mechanisms Reabsorption of Na+ especially important Primary active transport

Sodium-potassium pumps in basolateral membrane only

Secondary active transport Symporters, antiporters

Transport maximum (Tm) Upper limit to how fast it can work

Obligatory vs. facultative water reabsorption


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Reabsorption routes: paracellular reabsorption

and transcellular reabsorption


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Reabsorption and secretion in proximal convoluted

tubule (PCT)

Reabsorb what you need 80% water Na and water

- K+ : 95% absorbed usually

- phosphate : active reabsorption which is inhibited by PTH

- HCO3- : mostly absorbed

- glucose and amino acid absorption is normally nearly complete

Fanconi syndrome: is loss (inherited or acquired) of

proximal tubular functions

characterised by glycosuria, amino aciduria, and

phosphaturia.

May also have acidosis and polyuria.


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Reabsorption and secretion in the proximal

convoluted tubule


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Reabsorption in the loop of Henle

Its counter-current multiplier effect (creating either dilute orconcentrated urine)

Key features are an active NaCl pump in the thick ascending limb

water impermeability of the whole of the ascending limb.

At end of the loop of Henle (hypotonic)

About the osmolality of body fluids (~ 120-150 mosmoles/l, ascompared with 250-300 mosmoles/l in plasma).


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Na+K+-2Cl- symporter in the thick ascending

limb of the loop of Henle


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Reabsorption and secretion in the late distal

convoluted tubule and collecting duct

Reabsorption on the early distal convoluted tubule Na+-Cl- symporters reabsorb Na+ and Cl-

Major site where parathyroid hormone stimulates reabsorption of Ca+ depending onbodys needs

Reabsorption and secretion in the late distal convolutedtubule and collecting duct 90-95% of filtered solutes and fluid have been returned by now

Aldosterone ( Renin-angiotensin system) causes sodium to be exchanged forK+ and/or H+.

Conn's syndrome, Addisons disease, and RTA type I all affect DCT function.


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Hormonal regulation of tubular reabsorption and

secretion

Angiotensin II - when blood volume and blood pressure

decrease

Decreases GFR, enhances reabsorption of Na+, Cl- and water in PCT

Aldosterone - when blood volume and blood pressure decrease

Stimulates principal cells in collecting duct to reabsorb more Na+and Cl- and secrete more K+

Parathyroid hormone

Stimulates cells in DCT to reabsorb more Ca2+


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Regulation of facultative water reabsorption by

ADH

Antidiuretic hormone (ADH orvasopressin)

Increases water permeabilityof cells by insertingaquaporin-2 in last part ofDCT and collecting duct

Atrial natriuretic peptide(ANP)

Large increase in bloodvolume promotes release ofANP

Decreases blood volume andpressure by inhibitingreabsorption of Na+ andwater in DCT and collectingduct, suppress secretion ofADH and aldosterone


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Diabetes insipidus

Pituitary form, where no ADH is synthesized due to damage to the

pituitary.

Nephrogenic form, where renal tubular cells do not respond to normal

levels of ADH.

Both forms give rise to polyuria with dilute urine. Syndrome of inappropriate secretion of ADH (SIADH):

low output of inappropriately concentrated urine in the presence of

hypervolaemia and dilutional hyponatraemia


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Formation of dilute urine

Osmolarity of interstitial fluid ofrenal medulla becomes greater,more water is reabsorbed fromtubular fluid so fluid becomemore concentrated

Water cannot leave in thickportion of ascending limb butsolutes leave making fluid moredilute than blood plasma

Additional solutes but not muchwater leaves in DCT

Low ADH makes late DCT andcollecting duct have low waterpermeability


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Formation of concentrated urine

Urine can be up to 4 times more concentratedthan blood plasma

Ability of ADH depends on presence of osmoticgradient in interstitial fluid of renal medulla

3 major solutes contributeNa+, Cl-, and urea 2 main factors build and maintain gradient

Differences in solute and water permeability in differentsections of loop of Henle and collecting ducts

Countercurrent flow of fluid though descending andascending loop of Henle and blood through ascendingand descending limbs of vasa recta


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Summary of filtration, reabsorption, and

secretion in the nephron and collecting duct


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EVALUATION OF RENAL FUNCTION

GLOMERULAR FUNCTION TESTS:

SERUM CREATININE: Filtered at the glomerulus and nosignificant reabsorption or secretion in the tubules

Derived from creatine phosphate in muscle.

Serum levels are related to muscle mass ,dietary meat intake.

CREATININE CLEARANCE (Cr Cl):

Cr Cl provides a measure of the glomerular filtration rate (GFR).

It is calculated as follows:

Cr Cl. = (Urine Creatinine conc. x volume) / (Plasma

Creatinine conc.)

Normal Cr Cl. is about 120 ml/min.


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If the normal Creatinine clearance is 120 ml/min what would

you suggest are likely values in patients with loss of:

(i) 50 %

(ii) 90% of renal functional mass?

What sort of changes might you expect to see in plasma urea

and/or creatinine?


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Evaluation of kidney function

BLOOD UREA: Is a useful measure of decreased filtration. About 30-40 %

is normally reabsorbed in tubules.

Levels are affected by:

High protein intake, catabolic states, post-surgery and trauma, and gastro-

intestinal haemorrhage

GFR:

calculated using the 4-variables (i.e. serum creatinine concentration, age,

gender and ethnic origin).


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CAUSES OF ABNORMAL SERUM UREA TO

CREATININE RATIO

Increased Decreased

High protein intake Low protein intake

G.I. haemorrhage Dialysis (urea crosses

Hypercatabolic state Severe liver disease Dehydration

Urinary Stasis

Muscle wasting or amputation


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TUBULAR FUNCTION TESTS

Urinary Na+ concentration. Normally low relative to serum concentration

unless on a dietary high salt intake.

Concentration tests (usually after Pitressin), and Dilution tests, after a water

load. Ratio of osmolality (or urea) in urine relative to that in plasma is a simple

practical measure.

Acidification tests, after administration of NH4Cl ( NH3 + H+ + Cl- ).

Seldom done except in differentiation of type I and II renal tubular

acidoses MISCELLANEOUS TESTS:

Microscopy : look for casts, cells, or crystals


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Protein : :> 2.5 g/day: Nephrotic syndrome

Bence-Jones Protein : Myeloma

2-microglobulin, small protein, filtered then

absorbed by tubules, which is a sensitive test oftubular function (but also in some malignancies and

inflammatory conditions).

Mild increase can sometimes be normal (orthostatic,pregnancy).


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A 4 year old child presents with facial edema a few

weeks after a flu-like illness?

What single test will be most informative?


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34. 56 year old female. Symptoms : tiredness, weakness, developing over a long

period. Several years previously she had developed backache due to lumbar disc

prolapse, and had habitually consumed large quantities of analgesic tablets.

serum: Na+ 140 (N 135-145) K+ 5.5 (N 3.5-5.5) Cl- 100 (N 97-107) Bicarbonate

16 (N 22-26) urea 33 mM (N 1.7-6.7) creatinine 900 M (N 75-115) calcium 1.9mM (N 2.1-2.6) albumin 40 g/l (N 30-50) inorganic phos. 4.2 mM (N 0.8-1.4) urate

0.57 mM (N 0.12-0.5)

urine: Na+ 50 mmol/l K+ 30 mmol/l urea 120 mmol/l creat 4.0 mmol/l (4000

moles/l) osmol. 330 mosm/kg urine output: 3 litres/24h

The findings were similar 2 months previously at an outpatient clinic visit.

i. Calculate the creatinine clearance.

ii. The patient has a normal 24h urinary output of urea and creatinine, but markedly

elevated plasma values. Explain this apparent paradox.


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iii. Comment on the plasma bicarbonate concentration.

iv. Comment on the plasma urate and phosphate.

v. Suggest a cause for the hypocalcemia.

vi. Calculate the daily sodium output. What could happen if her sodium

intake fell substantially below this value?

vii. Comment on the plasma K+. Is it important to monitor this regularly?


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What would you expect the urinary Na+ concentration to be in:

(i) A patient who has had a severe haemorrhage (low plasma volume)

(ii) A patient who has impaired tubular function.

16. What is the value of measuring proteins in the urine?

17. What is the Fanconi syndrome, and what anatomical part of the kidney

is affected?


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A mother donates one of her kidneys for transplant to one of her children

who has a severe kidney disease. How will her own renal function be

affected?

What sort of changes might you expect to see in plasma urea and/or

creatinine


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RENAL DISORDERS


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NEPHROTIC SYNDROME

Characterized by increased permeability of the glomerulus to proteins, with

proteinuria of greater than 2.5 g/day, & oedema (why?), hypoproteinaemia,

and increased serum lipids.

Selectivity index:which is the ratio of the clearance of a high M.W. protein such as IgG anda low MW protein such as albumin:

UIgG/PIgG

Selectivity Index = ------------- x 100 This index will increase as the disease progresses.

Ualb/Palb


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in 2 globulins (2 macroglobulin - too large to be filtered easily, and increased as part of

an attempt by the liver to compensate for the protein loss by increasing overall synthesis of

serum proteins)

There is also hypercholesterolaemia and in other serum lipids (cause of elevated

globulins above)


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Clinical and biochemical features

CAUSES FEATURES MECHANISM

Minimal change GN proteinuria Glomerular change

Membranous GN Oedema Low plasma albumin

2ndry hyperaldosteronism

SLE Thrombotic tendency Hyperfibrinogenemia

Low antithrombin III

Diabetic nephropathy

Other forms of GN Hyperlipidemia Increased apolipoprotein

synthesis

some of causes respond to steroid therapy.


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ACUTE RENAL FAILURE (ARF)

Definition: Urine output less than 450 ml/day (in adult) with a rising blood

urea (N = 1.7 - 6.7 mmoles/l)

The blood urea typically rises by 5 mmoles/l/day, but in surgical, trauma, or

gastro-intestinal bleeding it can rise by up to 15 mmoles/l/day.

ARF has three different types :

1. Pre-renal failure (defect before the kidney)

2. Intra-renal failure (defect in the kidney e.g. acute tubular necrosis,

glomerulonephritis)

3. Post-renal failure (defect after the kidney, e.g. prostatic enlargement,

urolithiasis).

Pre-renal Intra-renal Post-renal


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Hypovolaemia

Acute tubular necrosis

(ischaemic or toxic)

Bilateral ureteric obstruction

Decreased cardiac output Acute glomerulonephritis Urethral obstruction

Renovascular obstruction Interstitial nephritis

Intrarenal vasoconstriction

(e.g. sepsis)

Tubular obstruction (e.g. uric

acid crystals)


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Why urgency to differentiates

1.Pre-renal failure if not rapidly treated can

progress to the much more serious intra-renal

failure (acute tubular necrosis).

2. Some aspects of the treatment of intra-renal

failure are the opposite of those for pre-renal

failure.


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Diagnostic strategy

Assess tubular function in a situation where glomerular

malfunction is predominating

Tubular function will be defective in Intra-renal failure but

normal (for a while) in pre-renal failure.

Appropriate tests to distinguish between these two are

therefore:

PRE-RENAL INTRA-RENAL

U Na conc * < 20 > 40 mmol/l

Urine/plasma osmo ratio > 1.4 < 1.1

Urine/plasma urea ratio > 14 < 10

The urinary Na+ is low in pre-renal failure because the low blood volume causes a

marked stimulation of aldosterone-mediated Na+ uptake. In Intra-renal failure the

damaged tubules can't respond fully.


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MANAGEMENT OF ACUTE RENAL FAILURE

1. Post-renal - relieve the obstruction, but then

watch out for subsequent polyuria .

2. Pre-renal - Restore blood volume, then blood

pressure and GFR will return to normal levels. 3. Acute tubular necrosis -

Water : if blood volume is low, replace with care,

since fluid overload can lead to

cardiac failure, maintain balance with 500 ml/day for

insensible loss, plus previous day's volume of urine

output, monitor by weighing patient daily


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Na+ : if oliguricrestrict, in diuretic phase - may need to

administer Na+ ++.

K+ :if oliguric - restrict - may even have to dialyse, in diuretic

phase - administer if hypokalaemic.

H+ :high anion gap metabolic acidosis, may need bicarbonate

to neutralise a severe acidosis

Dialysis is indicated if

blood urea is greater than 50 mmol/l and rising

bicarbonate is less than 10 mmol/l

K+ is greater than 7.0 mmol/l (or ECG changes)


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CHRONIC RENAL FAILURE (CRF)

It is a progressive loss in the number of functioning

nephrons.

Causes: The most common are glomerulonephritis,

diabetes mellitus, and hypertension

The key characteristic of well developed CRF is polyur ia -

the opposite to the oligur ia or anur ia of ARF.

Features : are those of decreasing glomerular function


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Increase in urea ( azotaemia)

Increase in creatinine and progressive decrease in Cr

clearance

Increase in urate, phosphate, sulphate, etc.

Features of decreasing tubular function (Developed later):

- Polyuria with fixed output

- Loss of concentrating and diluting abilities

- Metabolic acidosis with increased anion gap

- Sodium instability - overload or deficiency can easily occur


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MANAGEMENT OF CRF

1. Water intake is controlled by thirst since output is

fixed.

2. Careful control of Na+, K+ and protein intake.

3. Treatment of anaemia with erythropoietin.

4. Oral bicarbonate if acidosis is severe.

5. In end-stage CRF : dialysis - haemodialysis or

peritoneal dialysis , renal transplantation


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RENAL ACIDOSIS

2 components to H+ excretion by the kidney:

(i) Reabsorption of filtered bicarbonate in the proximal

tubule

(ii) H+ secretion in the distal tubule.

2 types of acidosis:

1. URAEMIC ACIDOSIS.

Seen in acute or chronic renal failure.

Decreased H+ excretion due to both glomerular and tubularfailure.

Increased anion gap due to retention of phosphate, sulphate

and other anions.


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2- RENAL TUBULAR ACIDOSIS (RTA).

A group of disorders characterized by tubular dysfunction,

with normal or perhaps slightly decreased glomerular function.

Normal anion gap (i.e. hyperchloraemic)

Metabolic acidosis, in the presence of a normal or near-normal

plasma creatinine.

The urine pH is often inappropriately high in the face of the

systemic acidosis (but NOT always)

4 types


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Type 1 (distal) RTA:

Due to inability of distal nephron to excrete H+.

The urine pH is inappropriately high (pH > 5.5), but does not

contain significant bicarbonate.

Associated with hypokalemia, nephrocalcinosis and rickets.

There are genetic and acquired forms (e.g. heavy metal

toxicity).

Type 2 (proximal) RTA: Due to defective proximal bicarbonate reabsorption.

The renal threshold for bicarbonate is decreased (normal

value 24 mmol/l).


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Plasma bicarbonate level exceeds the (lowered) renal threshold

(e.g. 16 mmol/l)

Urine pH is inappropriately high (called renal bicarbonate

wasting).

Associated with hypokalemia due to the increased delivery of

Na+ to the distal tubule.

Proximal RTA may be isolated or may be associated with

other proximal tubular defects: glycosuria, phosphaturia and

amino aciduria - the Fanconi syndrome. Can be genetic (e.g.with cystinosis) or acquired (e.g. toxins).


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(Type 3 RTA is a mixed form of types 1 and 2 - not

recognised nowadays as a specific entity.)

Type 4 RTA :

The acidosis due to mineralocorticoid deficiency or torenal resistance to mineralocorticoid action is

Associated with hyperkalemia.

Mineralocorticoid resistance may be a specificgenetic entity (pseudohypoaldosteronism) or may

result from generalized tubular damage.


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A 6-month old male infant was investigated for poor feeding

and failure to gain weight. There was occasional regurgitation

of food but no diarrhoea. There were no problems during

labour and the infant appeared normal after birth. The

following biochemical results were obtained:

Serum: Na+ 134 mmol/l, K+3.0mmol/l, Cl- 115mmol/l,

HCO3- 12mmol/l, creatinine 75 mol/l, pH 7.2, pCO2 3.8 kPa,

Bicarbonate 11 mmol/l, URINE pH 6.7

Discuss the biochemical findings in this case, and suggest a

diagnosis. Would any further investigations be useful? Discuss

treatment.


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30 year old male. 48 h after motor accident. Multiple injuries.

Serum: urea 20.5 mmol/l, creat 450 mol/l, Na+ 140 mmol/l,

K+ 6.0 mmol/l HCO3 15 mmol/l Urine:urea 74 mmol/l (N

250-600 mmoles/day), creat 1500 mol/l (N 7-17

mmoles/day), Na+ 90 mmol/l, Cl- 100 mmol/l, urine osmol350 mosm/l, urine output 400 ml/24 h

i. Comment on the urea and creatinine output.

ii. What diagnosis is suggested, and by what specific tests ?

iii. Why is the plasma potassium elevated?


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iv. What is the danger associated with hyperkalemia?

v. What can be done to treat the hyperkalemia acutely?

vi. Comment on this patient's fluid requirements. Would an

infusion of several litres of intravenous fluid be beneficial?

renal chemistry 2

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