5. PU/PD and concentration disorders

Question 1

what is the normal urine production in dogs and cats

Answer 1

26-44 ml/kg/day

Question 2

what are maintenance fluids in dogs and cats

Answer 2

2ml/kg/hr

Question 3

what is polyuria

Answer 3

excreting an increased volume of urine

Question 4

what is pollakiuria

Answer 4

passing urine more frequently

Question 5

normal water intake is affected by what factors

Answer 5

• ambient temp
• respiratory evaporative loss
• exercise level
• water content of food
• faecal water content
• age
• physiological state (pregnancy/lactation)

Question 6

what are the most common causes of PUPD in dogs and cats

Answer 6

• renal insufficiency
• chronic renal disease
• diabetes mellitus
• hyperadrenocorticism
• hyperthyroidism

Question 7

the production of concentrated urine is dependent on which three mechanisms

Answer 7

• the ability to make and secrete ADH
• functional kidneys that are able to respond to ADH
• the presence of an osmotic gradient in the renal medulla

Question 8

list causes of primary polyuria

Answer 8

• central diabetes insipidus = rare, partial or complete deficiency of ADH due to issue with pituitary
• primary nephrogenic diabetes insipidus = very rare congenital disorder = kidneys cant respond to ADH due to aquaporin channel insertion defect
• secondary nephrogenic diabetes insipidus = acquired inability to respond to ADH, due to many conditions which interfere with the normal interaction of ADH and tubular receptors affecting tubular function or decreased hypertonicity of the medullary interstitium resultin in loss/reduction of concentration gradient
• in both cases of nephrogenic DI, ADH levels will be high but no response to synthetic vasopressin or water deprivation

Question 9

list causes for renal insufficiency

Answer 9

• osmotic diuresis in remnant nephrons (water followig solutes that are not resorbed)
• disruption of medulalry hypertonicity
• decreased functional nephrons
• decreased number of functional nephrons in CKD must excrete the normal daily solute load leading to solute diuresis with impaired urinary concentrating ability

Question 10

how does hyperadrenocoricism caus PUPD

Answer 10

• defective ADH release
• impaired tubule response to ADH action

Question 11

how does hypoadrenocorticism cause PUPD

Answer 11

• loss of the osmotic gradient in renal medulla due to chronic sodium wasting
• loss of renal medullary hypertonicity leading to inadequate urine concentration
• also loss of aldosterone

Question 12

how does diabetes mellitus cause PUPD

Answer 12

• glucose normally freely filtered by the glomerulus so large amounts enter the ultrafiltrate for active resorption (none or very little should be present in urine)
• in DM blood glucose is high and therefore glucose levels in ultrafiltrate will be high
• active transport of glucose in the tubule reaches maximum (TMAX) and is overloaded
• excess glucose remains in the filtrate and is excreted in the urine
• PUPD in DM is via osmotic diuriesis = high osmolality filtrate (lots of glucose in the filtrate) prevents water moving out of the tubule via osmosis. ADH is present and may be working fine but the large concentration gradient for water to move down is significantly lessened

Question 13

how does hyperthyroidism caus PUPD

Answer 13

• primary polyuria may occur as a result of thyrotoxicosis = increasing cardiac output and GFR as well as increased renal medullary blood flow wich has the potential to decrease renal medullary hypertonicity and urine concentrating ability
• some cats with hyperthyroidism may also have primary polydipsia secondary to the effect of high thyroid hormone concentrations on the thirst centre

Question 14

how does liver disease cause PUPD

Answer 14

• due to loss of medullary hypertonicity
• urea is made in the liver
• in chronic/severe liver disease decreased production of urea leads to decreased renal medullary hypertonicity
• increased levels of corticosteroids (mimics HAC) that inhibit the release of ADH thus causing a degree od central diabetes insipidus

Question 15

what is psychogenic polydipsia

Answer 15

• psychiatric disturbance leading to huge drive to drink
• result is hypo-osmolality when water intake exceeds the ability of kidneys to remove it
• very large amounts of very dilute urine are produced
• ADH not the problem - controlled water deprivation is the only definitive test (to see if the kidneys can concentrate urine when animal is given less water)