Biochemistry

Question 1

Describe the RAAS system

Answer 1

• Juxtaglomerular cells sense low renal blood flow
• Kidneys produce renin
• Renin converts angiotensin to angiotensin I
• ACE converts angiotensin I to angiotensin II
• Angiotensin II vasocostricts and also stimulates the release of aldosterone from the adrenal cortex
• Aldosterone causes the renal tubules to increase the reabsorption of sodium and water and the excretion of potassium

Question 2

If you lose sodium from the body, what happens to the water?

Answer 2

You lose the water because it follows sodium

Question 3

What is syndrome of inappropriate ADH (SIADH)?

Answer 3

Excessive unsuppressible release of antidiuretic hormone (ADH) either from the posterior pituitary gland, or an abnormal non-pituitary source

Question 4

How do the kidneys regulate blood volume?

Answer 4

Renal excretion of sodium

Question 5

What is nephrogenic diabetes insipidus

Answer 5

Renal resistance to ADH

Question 6

What is central diabetes insipidus

Answer 6

ADH deficiency originating in the posterior pituitary

Question 7

What is Addison’s disease?

Answer 7

Primary adrenal insufficiency (usually autoimmune destruction) where the adrenal glands do not produce enough steroid hormones

Question 8

What happens when you give a patient with renal insufficiency saline?

Answer 8

Patients with adrenal insufficiency are less able to retain infused saline (sodium) than normal subjects

Question 9

What blood results might make you suspect Addison’s?

Answer 9

A hypotensive patient with low sodium and high potassium

Question 10

What causes secondary adrenal insufficiency?

Answer 10

Adrenal hypofunction due to a lack of adrenocorticotropic hormone (ACTH)

Question 11

What part of the kidney does autoimmune Addison’s attack?

Answer 11

Cortex

Question 12

How do you diagnose Addison’s?

Answer 12

Synacthen test (ACTH stimulating test)

Question 13

How do you diagnose Addison’s?

Answer 13

Synacthen test (ACTH stimulating test) - A basal cortisol of less than 100 is suspicious

Question 14

What can distinguish between primary and secondary insufficiency?

Answer 14

Measurement of ACTH

Question 15

Patients with malignancy-associated hypercalcaemia have high circulating concentrations of which hormone? (Hint - It’s similar to PTH)

Answer 15

PTHrP

Question 16

What is primary hyperparathyroidism?

Answer 16

Overactivity of parathyroid gland (eg in parathyroid adenoma)

Question 17

What is secondary hyperparathyroidism?

Answer 17

Physiological response to low calcium

Question 18

What does PTH do?

Answer 18

Secreted from parathyroid glands in response to low blood serum calcium. It indirectly stimulates osteoclast activity

Question 19

Why is primary hyperparathyroidism diagnosed earlier than it used to be?

Answer 19

Primary hyperparathyroidism is diagnosed much earlier than in the past because of hypercalcaemia; as a result, radiological changes like osteitis fibrosa cystica are rarely seen any more.

Question 20

Why is rehydration always instituted early in the management of severe hypercalcaemia

Answer 20

Hypercalcaemia interferes with proximal

tubular reabsorption of sodium and so causes loss of sodium and water – patients are usually dehydrated

Question 21

What is pseudohyperkalaemia?

Answer 21

Measured potassium levels is falsely elevated, due to mechanical trauma during blood drawing (potassium leakage out of the red blood cells due to haemolysis) etc

Question 22

Once haemolysis and renal failure have been excluded, what is the most likely cause of hyperkalaemia?

Answer 22

Antihypertensive drugs e.g. spironolactone

Question 23

Once haemolysis and renal failure have been excluded, what is the most likely cause of hyperkalaemia?

Answer 23

Antihypertensive drugs e.g. spironolactone, ACEI etc

Question 24

What does the finding of gross hyperkalaemia and hypocalcaemia suggest?

Answer 24

The finding of gross hyperkalaemia and hypocalcaemia suggests contamination with potassium EDTA, the anticoagulant used in the FBC (‘purple-top’) bottle

Question 25

What should happen to fluid intake in SIADH?

Answer 25

Fluid intake should be restricted

Question 26

What happens to the urine osmolarity in SIADH?

Answer 26

High urine osmolarity (small amounts of very concentrated urine)

Question 27

Why do you do a synacthen test if you suspect iatrogenic Cushing’s due to steroids?

Answer 27

It will show you that the kidneys are fine because they respond to the ACTH so it’s the pituitary axis that’s the problem