Principal cation in _____-cellular fluid
- total body K
- ICF
- skeleton
- ECF
intra
3800 mmol
3200 mmol
300 mmol
80 mmol
Potassium Functions
- Maintains cell integrity, fluid & electrolyte balance and pH
- Key role (with sodium) in resting membrane potentials and action potentials
- nerve impulse transmission and muscle contraction
- So influences contractility of smooth, skeletal & cardiac muscle & nerve tissue
excitability - Also influences electrolyte & pH balance
- Maintaining consistent extracellular fluid (ECF) K [ ] in the blood is CRITICAL
- Small increases in ECF [ ] depolarizes membranes
- Small decrease in ECF [ ] increases the ratio, hyperpolarizes membranes &
raises threshold for excitation - Therefore aberrations can paralyse muscle & stop the heart
Absorption & Transport
- Absorption throughout the small intestine and to a lesser extent the
large intestine - > 85% is absorbed
- K+/H+ ATPase pump mediated absorption
- Absorption managed hormonally to a large extent by the action of
insulin, which promotes active potassium uptake by hepatic and muscle
cells. - Diffuses across basolateral membrane into circulation via a K+ channel
Potassium balance
- Controlled by uptake into cells (promoted by
insulin) and excretion by kidneys - Kidney excretion increased by higher
extracellular potassium concentration,
aldosterone - Controlled by the same hormones as Na but in
the opposite direction - Balance is compromised by kidney diseases-
potassium content of diets important - K intake DECREASES
Calcium excretion
Sources of potassium
- Found in all living cells (plant and animals)
- Rich sources include avocados, bananas,
mango, leafy green vegetables, honeydew
melon. Other good sources include milk
yoghurt (see p. 466 textbook for details) - Can be added to processed foods as salt
substitute (potassium chloride) - Fresh foods higher than processed foods
Potassium and Blood Pressure
- Potassium intakes may affect blood pressure
- Particularly in those who are salt sensitive
- K causes the kidneys to excrete excess Na from
the body - High potassium intakes in conjunction with low sodium
intakes may ↓ blood pressure
Potassium deficiency
- Potassium depletion- rarely caused by inadequate intakes
alone, - Most often results from situations causing profound loss of
fluids and electrolytes, such as excessive vomiting or diarrhoea - Also caused by magnesium deficiency (promotes potassium
excretion), use of diuretic drugs (blood pressure treatment) or
glucocorticoids (used to suppress inflammation), nephropathies - “Refeeding syndrome”: when malnourished persons
(starvation, anorexia etc.) are provided with nutritional
treatment insufficient in potassium
Hypokalemia
Usually due to high GI tract loss –
severe vomiting and diarrhoea.
Can also be part of refeeding
syndrome
* Fatigue, malaise, muscular
weakness, nervous irritability,
disorientation, cardiac arrythmia
* Maybe rectified by food
consumption, potassium infusion
Hyperkalemia
- Rare in general population-
often occurs in conjunction with
renal damage/disease - May occur in shock (cells
release K, decrease blood flow
to kidneys), crush injuries (cells
release K and haemoglobin-
block glomerular filtration),
anuria and Addison’s disease - Dangerous- severe cardiac
arrhythmia and possibly death
Assessment of Potassium Status
Biochemistry
- Plasma K [ ]
* Usual K 3.5 to 5.0 mmol/L
* Influenced by multiple factors besides intake
- Urinary K excretion: 24-hr urine collection
* Amount excreted in 24 hr urine reflects intake
* Substantial intra individual variation, so multiple samples
needed to accurately estimate intake
- Clinical symptoms
