Salt and Water Disorders

Question 1

What causes the hypothalamus to cause “thirst”?

Answer 1

Increase in plasma osmolality

Decrease in body water

Question 2

How does the posterior and anterior pituitary differ in their management of hormones?

Answer 2

Anterior pituitary MAKES the hormones: 
TSH
GH
ACTH
FSH
LH
PRL
Endorphins

Posterior pituitary STORES hormones made in the hypothalamus:
Oxytocin
ADH

Question 3

Aside from causing us to feel ‘thirst’, what else does the hypothalamus do to increase blood volume?

Answer 3

It produces ADH which is transported to the posterior pituitary and released

Question 4

How does ADH work?

Answer 4

Opens aquaporin 3/4 on basement membrane causing AQPN2 to open on apical membrane and reabsorb water from the collecting duct

Question 5

What are the disorders of ADH secretion called?

Answer 5

Diabetes Insipidous

Syndrome of Inappropriate ADH Secretion

Question 6

What is the pathophysiology of Diabetes Insipidus?

Answer 6

Cranial Diabetes Insipidous is when the pituitary gland releases too little vasopressin (ADH)

Nephrogenic Diabetes Insipidous is when the kidneys are insensitive to the vasopressin.

Both mean the aquaporin channels in the collecting duct don’t open and so large amounts of dilute urine are produced. This also leads to hypernatraemia.

Question 7

Which test diagnoses diabetes insipidus?

Answer 7

Water restriction test

In DI it leads to hypernatraemia and dehydration

Question 8

What is SIADH?

Answer 8

Syndrome of inappropriate ADH secretion

Question 9

What is the pathophysiology of SIADH?

Answer 9

There are many stimuli that can cause over secretion of ADH. Lots of ADH causes high reabsorption of water from collecting ducts, which can lead to high urine sodium content and low serum sodium content (hyponatraemia)

Question 10

What are the symptoms/signs of Diabetes Insipidous?

Answer 10

Polyuria
Polydipsia

Hypernatraemia
Dehydration

Question 11

What test diagnoses SIADH?

Answer 11

Comparing urine osmolality to serum osmolality

Find that urine sodium osmolality is high whilst serum sodium osmolality is low

Question 12

What are the causes of SIADH?

Answer 12

Lung disease - cancer, pneumonia

Brain Lesions - tumour, SAH, trauma, stroke, meningitis

Hypothyroidism

Drugs - carbamazipine, SSRIs, morphine, amitiptyline

Misc

Question 13

What are the symptoms of hyponatraemia?

Answer 13

Headache
Nausea
Dizziness

–> COMA

Question 14

What are the signs of hyponatraemia?

Answer 14

This depends on the hydration status of the patient.  
If hypovolaemic:
Prolonged CRT
Tachycardia
Postural hypotension
Confusion
Dry mucous membranes
Reduced skin turgor

If hypervolaemic:
Tachycardia
Raised JVP
Pulm Oe or effusions
Ascites
Periph Oe

Question 15

How do you decide what is causing the hyponatraemia?

Answer 15

First you decide whether or not they are hyper or hypo volaemic.

Hypovolaemia:

If urinary Na>20mmol/L then the cause is RENAL NA LOSS:
Addison's 
Renal failure
Diuretic xs
Osmolar diuresis

If urinary NA<20mmol/L then loss is elsewhere:
D/V
Burns
Fistulae
SBO
Trauma
CF
Heat exposure

Hypervolaemia:

If oedematous:
Nephrotic syndrome
Cardiac failure
Cirrhosis
Renal failure

If not oedematous - is urine osmolality <500mmol/kg?

No:
Water overload
Sev

Question 16

What causes hypernatraemia?

Answer 16

Usually only seen in dehydration with little access to water.

Uncommon but seen in the elderly and in medically induced coma patients on ITU, also in extreme weather conditions

Question 17

Which receptors affect ADH secretion?

Answer 17

1. Special osmolarity receptors in the hypothalamus monitor the plasma concentration and stimulate ADH as and when it is neded
2. Stretch receptors in the atria inhibit ADH. The receptors are stimulated when there is a large volume of blood returning from veins and so the response is to stop ADH and allow water excretion
3. Carotid and Aorta stretch receptors that are stimulated when blood pressure falls, stimulate ADH to maintain a higher blood pressure.

Question 18

Why is the osmolarity of plasma so tightly regulated?

Answer 18

Because if there are extreme variances, the cells can shrink or swell, causing damage and destruction and disrupting the normal cell function

Question 19

Which is the major solute of the extracellular fluids?

Answer 19

Sodium

Question 20

Balance of which substances regulates osmolarity?

Answer 20

Sodium and H2O

Question 21

Which hormones affecting the kidney are responsible for regulating osmolarity?

Answer 21

Aldosterone and ADH

Question 22

Why does dehydration affect your osmolarity, but blood loss does not?

Answer 22

In dehydration, you have lost more H2O proportional to normal serum than you have Na, so the osmolarity increases. If you just lose blood you have lost both H2O and Na in the same proportion to blood volume.

Question 23

What effect does ADH have on osmolarity?

Answer 23

Decreases osmolarity

Question 24

What effect does aldosterone have on osmolarity?

Answer 24

Increases osmolarity

Question 25

What is the net effect of aldosterone secretion inhibition/ADH secretion stimulation on urine?

Answer 25

Decreases the volume of urine but increases the concentration of it

Question 26

Which receptors affect aldosterone secretion?

Answer 26

Adrenal receptors can sense the osmolarity Kidney senses low blood pressure --> juxtaglomerular cells produce reni--> more AT2 --> adrenal cortex produces aldosterone

Question 27

When the body is trying to conserve volume, which hormones does it release to the kidneys?

Answer 27

Both ADH and aldosterone because it needs to absorb fluid at the same osmolarity as the bodily fluid so needs both Na and H2O

Question 28

What are the symptoms of SIADH?

Answer 28

Mild - nausea, vomiting, headache, anorexia, lethargy Moderate - muscle cramps, weakness, confusion, ataxia Severe - drowsiness, seizures, coma

Question 29

What are the signs of SIADH?

Answer 29

``` Decreased level of conciousness Cognitive impairment Focal/generalised seizures Brain stem herniation in severe Hypervolaemia ```

Question 30

What features must be present to diagnose SIADH?

Answer 30

``` Hyponatraemia Low plasma osmolality Elevated urine osmolality Urine >40mmol/L Na Euvolaemia Normal thyroid and adrenal function ```

Question 31

What are the causes of diabetic insipidous?

Answer 31

Cranial - damage to hypothalamus or pituitary via infection, operation, tumour or trauma, sometimes no cause Nephrogenic - medication (lithium), inherited, kidney damage (trauma)