The document discusses advanced clinical biochemistry with a focus on renal function testing, highlighting the kidneys' role in synthesizing hormones, regulating blood composition, and filtering waste. It emphasizes the importance of multiple tests to accurately assess kidney function as no single test can capture all aspects. Key topics include glomerular filtration rate (GFR), tubular function and reabsorption processes, renal thresholds, and the mechanisms governing the reabsorption of various solutes.

1. ADVANCED CLINICAL BIOCHEMISTRY
RENAL FUNCTION TEST
Done by
P.S. YOGITHA
223056014
II M.Sc., Biochemistry
Dept. of Biochemistry & Biosciences
SRC, SASTRA
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Endocrine Function:
1.The kidney synthesizes hormones and also is the target organ for
other site formed hormones.
2.The kidney is the site of degradation of hormone-like insulin and
aldosterone.
3.The kidney produces hormones like :
*Erythropoietin, it stimulates RBCs production.
*Renin.
*Prostaglandins.
*Thromboxane.
*Vit. D. It converts 25-hydroxyvitamin D3 to form the biologically
active vitamin D.

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2,00,000 to as high as 2.7 million

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8.  The body has a considerable factor of safety in renal as well as hepatic tissues.
 One healthy normal kidney can do the work of two, and
 if all other organs are functioning properly, less than a whole kidney can be
enough.
 There are certain extra renal factors which can interfere with kidney function,
specially circulatory disturbances.
 Hence, methods that appraise the functional capacity of the kidneys are very
important.
 Such tests have been devised and are available,
 but it is stressed that no single test can measure all the kidney functions.
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9.  Consequently, more than one test is indicated to assess the kidney
function.
Main functions of the kidney are:
 To get rid of the body waste products of metabolism,
 To get rid of foreign and non-endogenous substances,
 To maintain salt and water balance, and
 To maintain acid-base balance of the body.
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 Glomeruli are the tiny network of blood
vessels that are the “cleaning units” of your
kidney.
 They filter waste and remove extra fluids
from your blood.
 When glomeruli are damaged and can't
function as they should, it's called glomerular
disease.
 Many diseases and conditions can damage
the glomeruli.
Glomerular Function:
e.g., Nephrotic syndrome

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The glomeruli act as filters
the fluid which passes from the blood in the glomerular capillaries into
Bowman’s capsule is of the same composition of protein-free plasma.
The effective filtration pressure which forces fluid through the filters is the
result of
(a) the blood pressure in the glomerular capillaries and
(b) the opposing osmotic pressure of
plasma proteins,
renal interstitial pressure and
intratubular pressure.

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Thus,
• Capillary pressure = 75 mmHg
• Osmotic pressure of plasma proteins = 30 mmHg
• Renal interstitial pressure = 10 mmHg
• Renal intratubular pressure = 10 mmHg
Hence,
net effective filtration pressure:
= 75–(30 + 10 + 10)
= 25 mmHg

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Rate of filtration is influenced by:
• Variations in BP in glomerular capillary
• Concentration of plasma proteins
• Factors altering intratubular pressure, viz.
– Rise with ureteral obstruction
– During osmotic diuresis
• State of blood vessels.
If the efferent glomerular arteriole is constricted,
the pressure in the glomerulus rises and the effective filtration pressure is
increased.
if the afferent glomerular arteriole is constricted, the filtration pressure is
reduced.

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The volume of glomerular filtrate formed depends on:
• The number of glomeruli functioning at a time,
• The volume of blood passing through the glomeruli per minute and
• The effective glomerular filtration pressure.
Under normal circumstances,
about 700 ml of plasma (contained in 1300 ml of blood or approximately 25 per
cent of entire cardiac output at rest) flow through the kidneys per minute and
120 ml of fluid are filtered into Bowman’s capsule.

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The volume of the filtrate is reduced in extrarenal conditions, such as
dehydration,
oligaemic shock and
cardiac failure
which diminish the volume of blood passing through the glomeruli, or lower the
glomerular filtration pressure, and
when there is constriction of
**the afferent glomerular arterioles or,
**changes in the glomeruli such as occurring in glomerulonephritis.
If the volume of glomerular filtrate is lowered below a certain point, the
kidneys are unable to eliminate waste products which accumulate in blood.

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Tubular function: reabsorbtion
Whereas, the glomerular cells act only as a passive semipermeable membrane,
the tubular epithelial cells are a highly specialised tissue able to reabsorb
selectively some substances and secrete others.
About 170 litres of water are filtered through the glomeruli in 24 hours, and
only 1.5 litre is excreted in the urine.
Thus nearly 99 per cent of the glomerular filtrate is reabsorbed in the
tubules.

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Glucose is present in the glomerular filtrate in the same concentration as in the
blood
but practically none is excreted normally in health in detectable amount in urine
and
the tubules reabsorb about 170 GM/day.
At an arterial plasma level of 100 mg/ 100 ml and a GFR of 120 ml/mt,
approximately 120 mg of glucose are delivered in the glomerular filtrate in each
minute.
Maximum rate at which glucose can be reabsorbed is about 350 mg/mt (Tm G)
maximum glucose tubular transport, which is an ‘active’ process.

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About 50 gm of urea are filtered through the glomeruli in 24 hours,
but only 30 gram are excreted in the urine,
this is a passive diffusion.
Certain substances foreign to the body,
e.g. diodrast, para-amino hippuric acid (PAH) and phenol red are:
(1) filtered through the glomeruli and in addition are
(2) secreted by the tubules.
Thus the amount of these substances excreted per minute in the urine is
greater than that filtered through the glomeruli per minute.
Substance excreted/min in urine > filter through glomeruli/min

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At low blood levels, the tubular capacity for excreting these compounds is so great
that
the plasma passing through the kidneys is almost completely cleared of them.
Another group of substances, e.g. inulin, thiosulphate, and mannitol are eliminated
exclusively by the glomeruli and
are neither reabsorbed nor secreted by the tubules.
Hence, amount of these substances excreted per minute in the urine is the same
as the amount filtered through the glomeruli per minute, thus they give the
glomerular filtration rate (GFR).
So the amount in glomeruli/min = excreted/min

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Glomerular filtration rate
(GFR):
1.This is the quantity of blood cleared of substances like creatinine and urea per
unit time.
2.It is the rate in milliliter (mL) per minute that substances like creatinine and
urea filtered through the kidney glomeruli.
3. GFR depends upon:
1. Plasma concentration of the substance.
2. The excretion rate of the kidney, this will reflect:
**Renal plasma flow.
**Glomerular filtration rate.
3. This substance used should not be reabsorbed, secreted, synthesized, or
degraded in the nephron.

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4.GFR is the most reliable measure of kidney function.
**GFR may be measured by giving exogenous substances like Inulin clearance.
**GFR may be measured by endogenous substances like creatinine and urea
nitrogen.
90 to 120 mL/min

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 Kidney function tests measure how efficiently your kidneys are working.
 Most of these tests check how well your kidneys clear waste from your system.
 A kidney test may involve a blood test, 24-hour urine sample or both.

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Psp test

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What is Meant by Clearance Test?
As a means of expressing quantitatively the rate of excretion of a given substance
by the kidney, its “clearance” is frequently measured.
This is defined as a volume of blood or plasma which contains the amount of
the substance which is excreted in the urine in one minute.
It is also defined as that volume of blood or plasma cleared of the amount of the
substance found in one minute excretion of urine.

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Functions of the Tubules
When the glomerular filtrate is formed, it contains almost all the crystalloids
of plasma.
In the proximal convoluted tubules, about 70% water, Na+ and Cl– as well as
100% glucose, amino acids and K+ are reabsorbed.
Urea, phosphate and calcium are partially absorbed.
The major processes occurring in renal tubules are the reabsorption or
secretion of solutes and reabsorption of water.

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Functions of different parts of the renal
tubules

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Renal Threshold and Tubular Maximum
Compounds whose excretion in urine is dependent on blood level are known as
threshold substances.
At normal or low plasma levels, they are completely reabsorbed and are not
excreted in urine.
But when the blood level is elevated, the tubular reabsorptive capacity is
saturated,
so that the excess will be excreted in urine.
The renal threshold of a substance is the plasma level above which the
compound is excreted in urine.

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The maximum reabsorptive capacity of the substances is known as the
tubular maximum or Tm.

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For glucose, the renal threshold is 180 mg/dl and Tm is 375 mg/min.
In other words, glucose starts to appear in urine when blood level is more than
180 mg/dl, and
all the glucose molecules above 375 mg are excreted in the urine.
In abnormal conditions, the renal threshold may be lowered
so that even at lower blood levels, compounds are excreted in urine,
e.g.
renal glycosuria (glucose); and
renal tubular acidosis (bicarbonate)

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Reabsorption of Solutes in
Tubules
1. Sodium
In the proximal convoluted tubules, the reabsorption of sodium is by co-
transport mechanism, accompanied by glucose, amino acids.
These mechanisms are coupled with the activity of sodium-potassium-
ATPase.
There is passive transport of equivalent amounts of chloride to maintain the
electrical neutrality.
The net effect is the reabsorption of sodium chloride along with glucose,
amino acids, etc.

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The co-transport of glucose is inhibited by ouabain and phlorhizin.
The sodium-Pi co-transport system is inhibited by parathyroid hormone
and facilitated by calcitriol.
In addition, Na+ to H+ exchange system also exists in the PCT.
This is an antiport system, where sodium ions are reabsorbed, in exchange
for hydrogen ions.
This also achieves a net reabsorption of bicarbonate.
When hydrogen ions are to be conserved, sodium to potassium exchange
occurs.

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Most of the solute reabsorbed in
the proximal tubule is in the form
of sodium bicarbonate and sodium
chloride, and about 70% of the
sodium reabsorption occurs here.

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Detailed calcium reabsorption per segment.
A: proximal tubule. Calcium is mainly reabsorbed paracellularly, partially driven by the
activity of the sodium/proton exchanger 3 (NHE3 or SLC9A3), which allows transcellular sodium
entry at the apical brush border side of cells, while the Na-K-ATPase pumps sodium out of the
cell at the basolateral side.
B: thick ascending limb. Calcium is reabsorbed by specialized and controlled paracellular
pathways involving claudin 16, 19, and 14. The driving force is provided by sodium reabsorption
through the sodium/potassium/chloride cotransporter (NKCC2 or SLC12A1) and the basolateral
Na-K-ATPase. Rising interstitial calcium concentrations activate the basolateral calcium-sensing
receptor (CaSR), which reduces NKCC2 activity and directly modulates paracellular calcium
permeability.

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C: DCT-CNT. Calcium enters the cell at the apical side through TRPV5 channels,
binds intracellular calbindin D-28k, and exits the cell at the basolateral side by 2
pathways: the sodium/calcium exchanger (NCX1 or SLC8A1) and the calcium ATPase
PMCA4. Sodium enters the cell via NCC (DCT1) or epithelial sodium channel (ENaC;
DCT2 and CNT) and exits through the basolateral Na-K-ATPase.
D: collecting duct. No calcium reabsorption is taking place in this segment,
which totally depends on the calcium load delivered by the CNT. Apical CaSR-like
proteins sense urine calcium concentration. This leads to inhibition of water
reabsorption and stimulates urine acidification, decreasing the risk of stone formation.
AQP2, aquaporin-2.

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2. Calcium
90% of calcium is reabsorbed from the glomerular filtrate.
However, the regulation of calcium balance is achieved at the distal
convoluted tubules.
The major factors regulating calcium reabsorption are
parathyroid hormone and
vitamin D.
3. Uric acid
It is almost completely reabsorbed in the proximal convoluted tubules,
by both active and passive carrier mediated processes.

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The kidney is an
important regulator of
circulating uric acid levels, by
reabsorbing around 90% of
filtered urate, while being
responsible for 60–70% of
total body uric acid excretion.
Defective renal
handling of urate is a frequent
pathophysiologic factor
underpinning hyperuricemia and
gout.

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The drug, probenecid competes with the uric acid for reabsorption
(inhibits).
Since probenecid increases uric acid excretion, it is uricosuric.
There is also active secretion of uric acid into the tubules.
85% of the excreted uric acid is derived by tubular secretion.

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4. Urea
Urea is freely filtered by the glomerulus, but about 40% is reabsorbed
actively by the tubules.
Rate of reabsorption of urea varies inversely with tubular flow and
accounts for elevation of blood urea when renal function is low.
The concentration of urea in urine is about 70 times that of plasma.
Urea forms 80% of total urinary solutes.
Urine is roughly a 2% solution of urea.
reabsorption of nearly 40%

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5. Creatinine
Creatinine is neither reabsorbed nor secreted.
The urinary concentration is about 70% that of plasma.
6. Potassium
About 70% of potassium in the glomerular filtrate is reabsorbed by proximal
convoluted tubules.
Net secretion of K+ occurs at the distal tubules, in exchange for Na+
reabsorption, under the effect of aldosterone.

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However, when the H+ concentration is increased, H+ ions are exchanged for
sodium, instead of K+.
Urinary excretion of solutes under normal conditions.

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