Electrolyte homeostasis Flashcards
What is the main extracellular cation?
Sodium
How much sodium is consumed daily through a typical diet?
100-300 mmol
How is sodium handles by the GI tract?
Almost all is absorbed - 5-10 mmil lost daily in faeces
How is the sodium level within the body regulated?
Renal mechanisms and extra-renal mechanisms
Renal mechanisms:
-GFR
-RAS via angiotensin 2
-several prostaglandins
Extra renal mechanisms:
-RAS via aldosterone
-Atrial natriuretic peptide
How is the sodium balance in the kidney determined?
■ glomerular filtration rate (GFR)
■ renin–angiotensin mechanism
■ several prostaglandins
How does the RAS affect sodium balance within the kidney? What is the mechanism?
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.
What is the extra renal mechanism by which RAS influences sodium levels
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
How is sodium handled in nephrons?
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.
Where is ANP released from? In response to what?
Released from cardiac atria in response to stretch
How does ANP impact sodium levels?
ANP increases the excretion of Na+:
■ by increasing GFR
■ inhibiting Na + reabsorption in the collecting ducts
■ reducing the secretion of renin and aldosterone.
What is the most common cause of high sodium?
Water depletion
Categorise the causes of high sodium
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
What is the physiological mechanism for high sodium correction?
- 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.
How does high sodium present?
Sodium excess presents with dependent oedema, increase in body weight and eventually pulmonary oedema.
How is high sodium treated?
Reduce intake
Treat the cause eg spironolactone in Conn’s
If reflection of water depletion, hydration with 5% dextrose sufficient
What are the causes of sodium deficiency?
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)
What is physiological mechanism for correction of low sodium levels?
• 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
What is the chief manifestation of body Na depletion ?
Peripheral circulatory failure
How should hypo Na due to ECF depletion be treated?
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
How should hypo na with apparently normal or high ECF be treated?
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
How is severe hypo na defined? What can it cause and how should it be treated?
( < 119 mEq/L) with clinical symptoms such as fits, confusion or coma, should be treated with hypertonic saline.
Name physiological factors which affect plasma potassium levels
• 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.
What are pathological causes of low potassium?
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)
What are pathological causes of high potassium?
• Excess administration of potassium, especially rapidly
• Renal failure
• Haemolysis
• Crush injuries
• Tissue necrosis, e.g. burns, ischaemia
• Metabolic acidosis
• Adrenal insufficiency (Addison’s disease)
