renal physiology and function ricki otten mt(ascp)sc [email protected]
TRANSCRIPT
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Objectives:
• Please review the objectives located on page 1 of the lecture handout
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Kidney Function• Form urine (excretory function)• Maintain acid-base balance• Regulates body water balance• Maintain electrolyte balance• Aids in maintaining BP• Excrete toxic (and) waste products• Reabsorb essential substances• Hormonal function:
– Target organ for aldosterone, ADH– Secretes renin (an enzyme)– Erythropoietin, vitamin D
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Kidney Function
• Depends upon proper– Renal blood flow (20-25% cardiac output)– Glomerular filtration– Tubular absorption– Tubular secretion
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Urinary System
Each kidney about the
size of your fist
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Inflammation/Infection
Pyelonephritis
Cystitis
Urethritis
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Nephron• Functional unit of kidney• ~1.5 million per kidney• Components
– Glomerulus– Tubules
(PCT, Loop of Henle, DCT)– Collecting ducts/tubules
• Functions to form urine– Filtration– Reabsorption– Secretion
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Glomerulus• ‘Capillary tuft’
• Bowman’s space
• Bowman’s Capsule
• Afferent arteriole
• Efferent arteriole
• Empties into PCT
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Glomerulus• Function: Filtration of blood
– Allows passage of water, electrolytes and low molecular weight substances (<70,000)
– Filtration based on solute’s size and charge
• Difference between glomerular filtrate and blood is absence of– Protein– Protein-bound substances: bilirubin, drugs– Cells (RBC, WBC)
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Glomerular Filtrate:
• Specific Gravity:
1.010
• Ultrafiltrate volume:
120 ml/min
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Glomerular Filtration
• Dependent upon– Basement membrane
cell structure
– Hydrostatic pressure
– Oncotic pressure
– RAA system
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Glomerular Filtration
• Dependent upon– Basement membrane
cell structure
– Hydrostatic pressure
– Oncotic pressure
– RAA system
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Renin-Angiotensin-Aldosterone
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Renal Tubules• PCT• Loop of Henle
– Descending limb– Ascending limb
• Not permeable to water
• DCT– ADH (controls water)– Aldosterone (controls Na+)
• Collecting ducts/tubules– ADH (controls water)
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Renal Tubules
• Functions– Reabsorption of essential substances– Secretion of waste, toxins, metabolites, drugs,
protein-bound substances, etc
• Final urine output: 1 ml/min
Range: 0.3 – 15 ml/min (dependent on hydration status)
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Tubular Transport Mechanisms
• Active transport: energy is required
• Passive transport: simple diffusion
• Renal Threshold– Glucose: 160-180 mg/dl (plasma)
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Normal Urine Composition
• Water: 94%
• Solutes: 6%– Urea– Sodium– Chloride
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Normal Urine Composition
• Water: 94%
• Solutes: 6%– Urea– Sodium– Chloride
Urea: end product of protein metabolism
Creatinine: end product of muscle metabolism
Uric acid: end product of purine metabolism
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Factors Influencing Concentration of Substances
• Dietary intake
• Physical activity
• Body metabolism
• Endocrine function (ADH, Aldosterone)
• Body position (orthostatic proteinuria)
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Urine Output• Average 24 hour volume:
– Normal: 1500 – 2000 ml/24 hour – ‘Extreme’: 600 – 2500 ml/24 hours
• Factors that influence urine output (volume)– Fluid intake– Non-renal loss (vomiting, diarrhea, sweating)– Secretion of ADH (too little, too much)– Body’s need to excrete solutes (glucose)
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Abnormal Urine Output
• Polyuria: >2500 ml/24 hr– Artificially induced by suppression of ADH
(diuretics, caffeine, alcohol)
– Diabetes mellitus: plasma glucose exceeds renal threshold
– Diabetes insipidus: • Neurogenic DI: lack of ADH• Nephrogenic DI: tubules unable to respond to ADH
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Abnormal Urine Output
• Oliguria (low)
• Anuria (absence)
• Nocturia
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Renal Function Tests
• Tubular Reabsorption
• Glomerular Filtration
• Renal Blood Flow
• Tubular Secretion
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Tubular Reabsorption Tests
• Can the kidneys concentrate urine
• “Concentration Tests”: control of fluid intake important in interpretation of test– Fluid deprivation– Free water clearance
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Tubular Reabsorption Tests
• Laboratory:– Specific gravity (number and density)
• Refractometer• Reagent strip
– Osmolality (number of particles in solution)• Better test• More accurate
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Glomerular Filtration Tests
• Assess filtering ability of glomerulus
• “Clearance tests”– Measures the RATE at which kidneys can
clear a ‘filterable’ substance from the blood
– This substance must not be reabsorbed or secreted by the tubules
– GFR = glomerular filtration rate
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Glomerular Filtration Tests
• Specimen requirements– Timed urine specimen: 24 hour collection– Plasma sample
• Clearance calculation: UV = ml/min
P
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Glomerular Filtration Tests
• Endogenous– Creatinine (most common)
• Exogenous– Inulin (considered the ‘standard’)
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Glomerular Filtration Tests
• Creatinine clearance test– Endogenous substance (found in the body)– Produced at a steady rate– Dependent on muscle mass (male, female,
child, adult)– Thus ‘normalized’ to body surface area using
a nomogram• Children• Obese
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Glomerular Filtration Tests
• Calculate creatinine clearance: UVP
Urine creatinine (U) 185 mg/dlPlasma creatinine (P) 0.9 mg/dlUrine volume (V) 1250 ml/24 hrHeight 6’0”Weight 260 lb
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Calculate Creatinine Clearance
CrCl = UV
P
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Calculate Creatinine Clearance
UV = 185 mg/dl x 1250 ml x 1 hour = 178.4 =
P 0.9 mg/dl 24 hours 60 min
CrCl = 178 ml/min
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Calculate Creatinine Clearance
UV = 185 mg/dl x 1250 ml x 1 hour = 178.4 =
P 0.9 mg/dl 24 hours 60 min
CrCl = 178 ml/min
Is this normal for a 20 year old male?
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Calculate Creatinine Clearance
UV = 185 mg/dl x 1250 ml x 1 hour = 178.4 =
P 0.9 mg/dl 24 hours 60 min
CrCl = 178 ml/min
Is this normal for a 20 year old male?
No, it is increased
Correct value for body surface area
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Corrected Creatinine Clearance
CrCl = 178 ml/min
Corrected CrCl =
CrCl x 1.73m2 = 178 ml/min x 1.73 BSA 2.39
= 128.8 = 129 ml/min
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Corrected Creatinine Clearance
CrCl = 178 ml/min
Corrected CrCl = 129 ml/min
Is this normal for a 20 year old male?
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Corrected Creatinine Clearance
CrCl = 178 ml/min
Corrected CrCl = 129 ml/min
Is this normal for a 20 year old male? Yes
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Renal Reserve
• Large margin of reserve in renal function
• Greater than 50% of kidney function must be lost before – Clinical symptoms apparent– Biochemical abnormalities evident
• Thus, not useful for screening for early renal disease
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• Renal Blood Flow
• Tubular Secretion No objectives
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