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The Kidney FunctionBy
M. Rasjad Indra
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Main function
Excretion of metabolic waste products &foreign chemicals
Regulation of: water & electrolyte balances.
body fluid osmolarity & electrolyteconcentration.
acid-base balance.
arterial pressure.
Secretion, metabolism, and excretion ofhormones
Gluconeogenesis
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Renal Blod Flow
1200 ml/minute or 20-25% of cardiac out put
Both kidney weigh: 300 gr or 0.5% b.w. Blood flow per grams of kidney tissue: 4 ml /
minute => 1200 ml / 300 gr, why?
Blood flow is highest in the renal cortex, why? RBF & GFR change relatively little if arterial blood
pressure between 80 - 180 mmHg, why?
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The nephron ~ Functional Unit
Each kidney contains about 1 million nephrons
The kidney cannot regenerate new nephrons.
After age 49 the number usually decrease 10 %
every 10 years.
Regional differences in nephron structure:
Cortical nephrons: they have short loops.
Juxtamedullary nephrons: they have long loops. Urine formation results from: Glumerular filtration,
tubular reabsorption, and tubular secretion.
Urinary excretion rate = Filtration rate- Reabsorption rate + Secretion rate
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Ke Counter Current
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Two capillary beds: The glomerular & Peritubularcapillaries
Are arranged in series
Separated by the efferent arterioles
Regulate the hydrostatic pressures in both sets of capillaries.
Hydrostatic pressure: The glomerular (high ~ 60 mmHg) => for filtration.
The peritubular (low ~ 13 mm Hg) => for reabsorption.
By adjusting the resistances of afferent and efferent arterioles
The kidneys regulate the hydrostatic pressure of the glomerular &
peritubular capilaries.
Changing the rate of filtration and / or tubular reabsorption.
Response to body homeostatic demands.
KEMBALI
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Urine formation startwith the filtrationof plasma inthe glomeruli:
GFR determined by: The balance of hydrostatic & colloid osmotic forces across the
glomerular membrane
The glomerular filtration coefficient (Kf)
Net Filtr.Pressure= PG- PB- G+ B.
GFR= Kfx Net Filtration Pressure
Glomerular filtration is rather non selective: Proteinare mostly retained in the plasma
Low-molecular weightsubstance are freely filtered(excepts that are bound to the plasma protein).
Negative charged large molecules are filtered less easilythan positively charged molecules of equal molecules size
Glomerular Filtration
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Macula densa
Juxtaglomerular cells
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Reabsorption Secretion Tubular reabsorption includes Passive & Active
mechanism.1.Across the tubular epithelial cells into interstitiel
2.Through the peritubular capillary membrane back intothe blood
Active transport (against electrochemical gradient& requires energy.1.Primary active transport
Expl: Sodium transport in luminal membrane prox. Tub.
2.Secondary active reabsorption Expl.: Glucose & amino acid reabs.
Secondary active Secretion: Expl: Hydrogen ion: Counter-transportwith sodium
reabsorption in luminal membrane
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Glucose:Allof the filtered are activelyreabsorbed and sodium dependent.
Urea & Chloride arepassively reabsorb.
Active absorb. of Na+--> the driving forcefor tubular reabsorb. of water, glucose,amino acids, chloride and phosphate.
Some organic compoundsare secreted
from the blood into the tubular urine.
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Transport Maximum
Transport maximum for substances that are activelyreabsorbed:
Glucose 320 mg/min.
Phosphate 0.10 mM/min.
Sulfate 0.06 mM/min.
Amino acid 1.5 mM/min. Uric acid 15 mg/min.
Lactate 75 mg/min
Plasma protein 30 mg/min
Transport maximum for substances that are actively
secreted:
Creatinin 16 ng/min
Para-aminohipuric acid 80 ng/min
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Constituent Filtered Reabsorbed Excreted
Water 167.5 liters 166 liters 1.5 liters
Sodium 24,000 mmoles 23,900 mmoles 100 mmoles
Potasium 720 mmoles 630 mmoles 90 mmoles
Chloride 19,500 mmoles 19,400 mmoles 100 mmoles
Bicarbonate 4,500 mmoles 4,498 mmoles 2 mmolesPhosphate 6 g 5 g 1 g
Glucose 150 g 150 g 0 g
Urea 50 g 25 g 25 g
Uric acid 8 g 7.2 g 0.8 g
Creatinine** 1.5 g 0 g 1.8 g
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Reabsorption of glucose
Glucose is cotransport with sodiumacross the luminal cell membrane(uphill)
the energy from:
the sodium gradient, how?
the electrical gradient
Glucose leave the cell membrane to
peritubular capillary blood byfacilitated difussion
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Glucose Threshold
The ability to reabsorb is limited
At normal plasma glucose levels(65-90 mg/dl) => completelyreabsorb.
At 180-200 mg/dl => glucose firstappear in the urine (threshold).
Tubular transport maximum (Tm)for glucose: the maximal rate ofglucose reabsorption.
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Sodium (Na+):
Most filteredsodium is reabsorbed. The proximal tubules: 70%.
The loop of Henle: 20%
The distal tub. and collecting duct: 9%
The quantity of Na+excreted =>importantrole in body sodium balance.
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CountercurrentMechanism
Loop of Henle (countercurrent multipliers)&Vasa recta (countercurrent ex-changers)
Loops of Henle: establish an osmotic gradientin the medulla.
The descending limb: water permeable
The ascending limb:
Active sodium transport
Low water permeability The vasa recta: remove water from the
medulla.
Ke Slide 8
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The collecting ducts:
Final regul. of Na
+
excretion.AldosteroneandADH:increase Na+and
water reabs. by the collecting duct.
Potasium (K+):
Filtered, reabsorband secreted
The cortical collecting tubules:important site of K+ secretion.
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Ke Slide 4
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Clearance(CX)= UXx V (ml plasma/ minute)
PX
The Inulin clearance (CIN
)= GFR .....Why ?
Endogenous Creatinine Clearance also = GFR ..... Why ?
Clearance Ratio = Cx
CInulin
PAH clearance (CPAH)=Effective Renal Plasma Flow (ERPF) Renal Plasma Flow (RPF) = CPAH EPAH =PPAH-VPAH(Extraction Ratio)
EPAH PPAH
Renal Blood Flow (RBF) = RPF
1-Hematocrit Excretion Rate = Uxx V
Reabsorption Rate = Filtered Load Excretion Rate
= (GFR x Px) (Uxx V)
Secretion Rate = Excretion Rate Filtered Load
The Clearance Concept (CX) to Quantify Kidney Function
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Inulin:
Not be reabsorbed or secreted by the kidney
Not be metabolized, synthesized, or stored
Pass through the glomerular filtration
barrier unhindered
Nontoxic
Be able to measure in plasma and urine
Clearance Inulin ~ GFR
PAH is avidly secreted by tubules that it is almost completely
cleared from all of the plasma in one passage of blood through
the kidneys
Clearamce PAH ~ ERPF
uteplasmaml
plasmamlxmg
uteurineXmlurinemlxmgCx min/)(
)(/)(
min/)()(/)(
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Urea Recirculation in RenalTubules
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The two ureters are muscular tubesthat carry the urine from the
kidneys to the bladder. The urinary blader functions as a
reservoir for urine and is
periodically emptied (micturition).
MICTURITION
A complex act involving autonomicand somatic nerves, spinal reflexes,and higher brain centers.
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Ke slide 4
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