Clinical Pathology of the Urinary System Flashcards
RENAL FUNCTION
URINE PRODUCTION- to regulate body water, eliminate waste, maintain normal blood Na, Ca, K and P concentrations, and maintain pH.
ENDOCRINE FUNCTION- erythropoetin, vitamin D, renin.
URINE PRODUCTION
30% of the plasma that reaches the kidney gets filtered at the GLOMERULUS- forms ultrafiltrate.
- PCT- REABSORPTION of protein (nutrients)
- DESCENDING arm of Loop of Henle- WATER REABSORPTION.
- ASCENDING arm of Loop- ION REABSORPTION.
- DCT- HORMONE (aldosterone, PTH) MEDIATED reabsorption.
- Collecting duct- hormone mediated water reabsorption, acid base balancing, and collection of urine
PRODUCING CONCENTRATED URINE
Requires:
- Appropriate concentration gradient
- Antidiuretic hormone (ADH)
- Kidney must respond to signal (hormones etc)
ie. the kidney must be functioning in order for concentrated urine to be produced.
Excess water intake -> dilute urine production
Insufficient water intake -> concentrated urine production.
ASSESSMENT OF URINE PRODUCTION
Refractometer used to measure urine specific gravity (USG)- read right hand side!
USG is a ratio of solution’s (urine) weight:weight of an equal volume of water.
Refractometry is indirect measurement of USG
Change in light refraction is proportional to concentration of solute.
Artefacts- glucosuria (glucose in urine) and proteinuria can falsely elevate USG.
Some scales are temperature compensated, others must be adjusted depending on room temperatures.
HYPOSTHENURIA
Diluted urine.
Urine osmolality is LESS than isosthenuric values.
<1.007
ISOSTHENURIA
Urine osmolality is EQUAL TO plasma osmolality.
Non-functioning kidney.
1.007- 1.012
HYPERSTHENURIA
Concentrated urine.
Urine osmolality is high; the kidney is actively concentrating urine.
Rarely used term.
DIABETES INSIPIDUS
Lack of ADH OR kidney is not responding to normal ADH.
GLOMERULAR FILTRATION RATE
BEST PREDICTOR OF RENAL FUNCTION.
Rate at which fluid move through glomerular filtration barrier from plasma to glomerular filtrate.
Measured by determining the rate at which a substance that is not excreted or reabsorbed is cleared from plasma- expensive and difficult.
An estimate is used in practice.
UN and creatinine can be used in practice to estimate GFR.
CREATININE
Used to estimate GFR.
A byproduct of muscle metabolism.
Muscle mass and therefore production rate are relatively constant.
Freely filtered by glomerulus and not reabsorbed by renal tubules.
AN INCREASE IN CREATININE CONCENTRATION REQUIRES 75% LOSS IN FUNCTIONING NEPHRONS.
Specific, especially in ruminants and horses (where non-renal factors are not as significant)- creatinine is used for GFR instead of urea in ruminants, as urea is excreted in to the rumen so blood levels are relatively constant.
INCREASED CREATININE
Due to decreased urinary excretion eg. in dehydration, hypovolaemia, kidney dysfunction, obstruction of outflow.
High muscle mass- elevated creatine can be seen normally in certain breeds (greyhound)
DECREASED CREATININE
Rare, but can be seen in animals with low muscle mass/cachexia.
UREA (UN)
Used to estimate GFR.
Synthesised from protein breakdown in the liver.
Excreted by kidneys.
RUMINANTS- Urea is excreted in to the rumen to be used for amino acid synthesis by urease bacteria. Therefore, creatinine should be used to estimate GFR in ruminants.
INCREASED UREA
Due to decreased urinary excretion- dehydration, hypovolaemia, kidney dysfunction, outflow obstruction.
Increased protein catabolism or digestion will also increased urea- eg. GI haemorrhage, high protein diets, starvation.
DECREASED UREA
Liver insufficiency, low protein diets, urea cycle problems.
AZOTAEMIA
Increased non-protein nitrogenous compounds in the blood (creatinine and/or nitrogen)
Can be PRE-RENAL, RENAL or POST-RENAL. USG is required to differentiate between these types.
Types can occur alone or in combination.
URAEMIA
CLINICAL SIGNS reflecting renal failure- vomiting, diarrhoea, coma, ulcers, ammoniac breath.
PRE-RENAL AZOTAEMIA
- Reduced renal blood flow eg. dehydration, hypovolaemia.
- > decreased GFR
- > concentrated urine production- the kidney is actively concentrating to preserve water.
- > USG increased- >1.025 (cattle/horses), >1.030 (dogs) >1.035 cats -hypersthenuria.
-Increased urea production- high protein diet, GI haemorrhage etc. Creatinine is less affected.
RENAL AZOTAEMIA
Any renal disease that causes a major decrease in GFR.
Caused BY THE KIDNEYS.
75% non functioning nephrons are needed to see increased creatinine/UN and renal azotaemia.
USG is usually 1.007-1.012- ISOSTHENURIC
POST-RENAL AZOTAEMIA
Azotaemia caused after urine has been formed by the kindeys.
USG is variable.
Caused by urinary tract blockage or urine leakage.
FUNCTIONAL RENAL MASS
Two-thirds of functional renal mass can be lost before urine concentrating ability diminishes and isosthenuria is seen.
Three-quarters (75%) of functional renal mass must be lost before increased creatinine/UN- azotaemia- are seen.
OTHER LAB ABNORMALITIES IN AZOTAEMIC PATIENTS
-Total calcium- variable, usually low in dogs/cats/cattle with chronic renal disease (decreased Ca reabsorption due to decreased vitamin D production by kidneys).
HORSES excrete much Ca through the kidneys (and have a different active form of vitamin D), so calcium carbonate crystals in the urine and hypercalcaemia are normal.
-Phosphorous- Decreased renal clearance in dogs/cats leads to increased blood phosphorous (hyperphosphataemia). Cattle/horses show extrarenal excretion of phosphorous (GI tract), so may not show hyperphosphataemia.
-Potassium- Kidneys are main route of K excretion from the body, so anuric/oliguric (no/low urine) kidney disease will cause hyperkalaemia.
Cattle are more likely to have hypokalaemia.
SECONDARY RENAL HYPERPARATHYROIDISM
Hyperphosphataemia and hypocalcaemia in renal failure causes PTH stimulation.
- > INCREASED PHOSPHATURIA (phosphate in urine)
- > INCREASED Ca REABSORPTION by kidney, bone.
Parathyroid hyperplasia may develop.
Consequences- Tissue mineralisation (due to high phosphorous), fibrous osteodystrophy (‘rubber jaw’)
ACUTE RENAL FAILURE
- Reversible or irreversible- the magnitude of the azotaemia does not differentiate between these.
- Isosthenuria (urine is more concentrated if it was formed before the insult that lead to failure)
- Anuria- no urine, or oliguria- little urine. Leads to hyperkalaemia.
eg. Ethylene glycol poisoning- causes vomiting, ataxia, CNS depression. Can be detected early on with UV fluorescence (look for staining round mouth of animal).
Metabolism of ethylene glycol produces CALCIUM OXALATE MONOHYDRATE crystals 3 hours/6 hours post exposure in the cat/dog.
Causes titrational metabolic acidosis with increased anion gap, hypocalcaemia (Ca is precipitated in to crystals), hyperphosphataemia (some antifrezes contain phosphorous)
