Electrolyte homeostasis

Question 1

What is the main extracellular cation?

Answer 1

Sodium

Question 2

How much sodium is consumed daily through a typical diet?

Answer 2

100-300 mmol

Question 3

How is sodium handles by the GI tract?

Answer 3

Almost all is absorbed - 5-10 mmil lost daily in faeces

Question 4

How is the sodium level within the body regulated?

Answer 4

Renal mechanisms and extra-renal mechanisms

Renal mechanisms:
-GFR
-RAS via angiotensin 2
-several prostaglandins

Extra renal mechanisms:
-RAS via aldosterone
-Atrial natriuretic peptide

Question 5

How is the sodium balance in the kidney determined?

Answer 5

■ glomerular filtration rate (GFR)
■ renin–angiotensin mechanism
■ several prostaglandins

Question 6

How does the RAS affect sodium balance within the kidney? What is the mechanism?

Answer 6

RAS intrarenal affect via angiotensin 2:
- stimulates Na reabsorption is most nephron segments
- constricts glomerulus arterioles

This favours Na +retention and restoration of ECF volume.

Question 7

What is the extra renal mechanism by which RAS influences sodium levels

Answer 7

Circulating angiotensin 2 stimulates aldosterone release from zone golmerulosa of adrenal glands

Aldosterone promotes sodium reabsorption in:
- DCT and collecting ducts
- colonic epithelium
- ducts of salivary and sweat glands

Question 8

How is sodium handled in nephrons?

Answer 8

99% of the filtered sodium is reabsorbed: 65% in the proximal tubule, 25% in the loop of Henle, and approximately 10% in the distal tubules and collecting ducts.

Question 9

Where is ANP released from? In response to what?

Answer 9

Released from cardiac atria in response to stretch

Question 10

How does ANP impact sodium levels?

Answer 10

ANP increases the excretion of Na+:

■ by increasing GFR
■ inhibiting Na + reabsorption in the collecting ducts
■ reducing the secretion of renin and aldosterone.

Question 11

What is the most common cause of high sodium?

Answer 11

Water depletion

Question 12

Categorise the causes of high sodium

Answer 12

Increased sodium or reduced water.

Causes of increased sodium:
- excessive IV sodium therapy
- Cushings
- conns
- steroids
- ccf
- cirrhosis

Causes of water depletion:
Reduced intake - coma, confusion
Increased loss - osmotic diuresis, diuretic phase of ARF, post obstructive diuresis, DI, fevers, burns, diarrhoea, fistulae

Question 13

What is the physiological mechanism for high sodium correction?

Answer 13

• The osmolality of ECF increases.
• This results in the release of ADH and retention of water in distal tubule.
• This increases the volume of ECF and restores osmolality to normal.

Question 14

How does high sodium present?

Answer 14

Sodium excess presents with dependent oedema, increase in body weight and eventually pulmonary oedema.

Question 15

How is high sodium treated?

Answer 15

Reduce intake
Treat the cause eg spironolactone in Conn’s
If reflection of water depletion, hydration with 5% dextrose sufficient

Question 16

What are the causes of sodium deficiency?

Answer 16

Low intake:
■ saline-free intravenous solutions
■ reduced oral intake, e.g. coma, dysphagia

• Excessive loss
- via gastrointestinal tract
■ diarrhoea
■ intestinal obstruction
■ fistulae
■ paralytic ileus

• Excessive sweating, e.g. fever
• Burns
• Drainage of ascites
• Addison’s disease
• Diuretics

Inappropriate water retention:
-SIADH (HI/bronchogenic carcinoma)

Question 17

What is physiological mechanism for correction of low sodium levels?

Answer 17

• decrease in osmolality of ECF

• as long as osmoregulation continues, loss of Na+ leads to loss of water at a rate of 1 L per 150 mmol of Na+

• water loss is shared between plasma and extravascular ECF

Question 18

What is the chief manifestation of body Na depletion ?

Answer 18

Peripheral circulatory failure

Question 19

How should hypo Na due to ECF depletion be treated?

Answer 19

usually treated with isotonic saline, as sodium loss is invariably accompanied by water loss. Infusion of normal saline requires close monitoring with checking of serum electrolytes and measurement of urinary sodium, which increases with adequate sodium repletion

Question 20

How should hypo na with apparently normal or high ECF be treated?

Answer 20

an apparently normal or high ECF volume should be treated by water restriction. Avoidance of a diuretic is advisable since this may remove nearly as much sodium as water

Question 21

How is severe hypo na defined? What can it cause and how should it be treated?

Answer 21

( < 119 mEq/L) with clinical symptoms such as fits, confusion or coma, should be treated with hypertonic saline.

Question 22

Name physiological factors which affect plasma potassium levels

Answer 22

• aldosterone: increases renal excretion by effects on distal tubule
• insulin: promotes entry of K + into cells
• acid–base balance: acidosis results in increased plasma K + due to reduced entry into cells and reduced urinary excretion. Alkalosis causes the opposite effect
• hydration: K + is lost from cells in dehydration and returns when the patient is rehydrated
• catabolic states, e.g. trauma, major surgery, infection: K + is lost from the cells.

Question 23

What are pathological causes of low potassium?

Answer 23

Inadequate intake
• Potassium-free intravenous fluids
• Reduced oral intake:
■ coma
■ dysphagia

Excessive loss
• Renal:
■ diuretics
■ renal tubular disorders

• Gastrointestinal:
■ diarrhoea
■ vomiting
■ fistulae
■ laxatives
■ villous adenoma

• Endocrine:
■ Cushing’s syndrome
■ steroid therapy
■ hyperaldosteronism (primary and secondary)

Question 24

What are pathological causes of high potassium?

Answer 24

• Excess administration of potassium, especially rapidly
• Renal failure
• Haemolysis
• Crush injuries
• Tissue necrosis, e.g. burns, ischaemia
• Metabolic acidosis
• Adrenal insufficiency (Addison’s disease)

Question 25

What ECG changes are seen in high potassium?

Answer 25

• peaked T-waves
• loss of P-waves
• widening of the QRS complex.

Question 26

How is high potassium treated?

Answer 26

• infusion of calcium gluconate
• infusion of glucose and insulin
• ion-exchange resins, e.g. resonium
• haemodialysis.

Question 27

What is the presentation of low potassium?

Answer 27

• clinical fatigue and lethargy with eventual muscle weakness
• low serum K + together with alkalosis
• ECG changes include low broad T-waves in the presence of U-waves

Question 28

How is low potassium treated?

Answer 28

• treatment is by correction with oral supplements, or in severe cases slow intravenous replacement with careful monitoring.