Systems Pathology: Disordered Fluid And Electrolyte Balance

Question 0

Water regulation

Answer 0

Water intake->Thirst centre in hypothalamus-> sensitive to osmolality and blood volume changes
Water loss-> ADH-> high osmolality and low blood volume-> increases permeability of distal tubule-> increased water reabsorption, vasoconstriction-> increased blood pressure
-> RAA sensitive to change in renal blood flow-> aldosterone-> increase Na reabsorption in exchange for H and K
Net renal loss-> 1.5l water and 100mmol Na

Question 1

Fluid compartments

Answer 1

Total body water 45l osmolality 285-295mmol/kg
Extra cellular 15l (1/3)-> sodium 135-145 mmol/l, potassium 3.5-5.3
-> intravascular 5l
-> intersitial 10l
Intracellular 30l (2/3)-> Na 9 K 150
1100l of fluid transferred daily between plasma and interstitial fluid
ECF fluid largely determined by total body sodium
Distribution determined by hydrostatic pressure, colloid osmotic pressure, capillary

Question 2

Dehydration causes

Answer 2

Reduced fluid intake-> unavailability, impaired thirst mechanism, Dysphagia
Increased loss-> sweating, burns, hyperventilation, diuretics, diabetes mellitus and insipidus, adrenal failure, renal failure

Question 3

Dehydration clinical features

Answer 3

Tachycardia
Hypotension
Thirst
Rapid weight loss
Loss of skin elasticity
Oliguria
-> severe-> confusion, coma, renal failure

Question 4

Dehydration managment

Answer 4

Fluid +- electrolyte replacement

Treat cause

Question 5

Dehydration case study

Answer 5

High plasma sodium-> water loss exceeds electrolyte loss
High urine osmolality-> hypernatremia triggers ADH relase-> increased water reabsorption
Low urine Na-> dehydration causes decrease ECF volume-> decreased renal blood flow-> RAA-> aldosterone-> sodium reabsorption
Loosing Na via vomiting-> don’t be mislead by high plasma Na-> only because of water loss

Question 6

Dilutional Hyponatraemia

Answer 6

Post op IV dextrose-> water moves out of cells-> dilution
Fluid overload
Hyperglycaemia-> high blood glucose-> increased osmolality-> water out of cells
Hypovolomic-> low blood pressure-> water moves out of cells
Diuretics, antidepressants, anticonvulsants, ACEis

Question 7

Fluid overload causes

Answer 7

Excessive fluid intake
Impaired excretion
-> renal impairment
-> failure of regulatory mechanisms

Question 8

SIADH

Answer 8

Some cancers secrete ectopic hormones
Eg small cell carcinoma of lung-> ADH, ACTH
Unregulated ADH release-> water retention-> hypervolaemia-> Dilutional Hyponatraemia
All body fluid compartments effected
Hypervolaemia inhibits RAA-> high urine Na >20mmol/l
-> high urine osmolality >200 (inability to produce dilute urine)
Response to water restriction
Other causes-> cerebral trauma or infection, pulmonary embolus, drugs

Question 9

Conns syndrome

Answer 9

Aldosterone secreting tumour or adrenal Hypertrophy
Increased aldosterone-> sodium and therefore water retention-> K loss
Hypernatremia and hypokalaemia
Increased ECF-> hypertension

Question 10

Pseudo Hyponatraemia

Answer 10

Excessive proteins or lipids causes dilution
Total body water and sodium unchanged
Normal plasma osmolality

Question 11

Clinical features of fluid over load

Answer 11

Dependent oedema
Raised central venous pressure 
Pulmonary oedema 
Rapid weight gain 
Abdo cramps
Weakness
Impaired conciousness 
Hypertension

Question 12

Treatment of fluid overload

Answer 12

Fluid restriction
Diuretics
Treat cause