6. Fluid + Electrolyte K+ Flashcards
Describe K+ distribution in the body
K+ = the major intracellular cation (98% intracell)
- ICF K+ conc ~20x greater than ECF
- only 2% total K+ present in plasma
Many cell functions require maintenance of low ECF K+ conc
How is K+ regulated?
K+ not as closely regulated as Na+
Kidneys have important role in K+ regulation;
- prox tubules reabsorb nearly all K+
Aldosterone from RAAS or directly from adrenal cortex stimulates additional K+ secretion into the urine in exchange for Na+
K+ uptake from ECF to ICF important - acts to normalise acute rise in K+ due to increased intake
In healthy pop these mechanisms robust + cope with wide variations in K+ intake
What are the K+ reference ranges? And what do they indicate about total body K+?
Serum/plasma ref range = 3.5-5.0mmol/L
Intracellular concentration = ~130mmol/L
Serum level = late indicator of total body depletion/excess
What are the functions of K+?
Integral component of electrochemical gradient across cell membranes;
- created by Na+/K+ ATPase pump
Responsible for correct function of all human cells;
- maintains electrical neutrality + osmolality of cells
- regulates neuromuscular excitability
- contraction or skeletal, cardiac + smooth muscles
- maintenance of ICF volume
- BP control
- acid-base balance (through K+, H+ exchange)
List the sources of K+ intake
Obtained from diet: beans, dark green leafy veg, potatoes, bananas, melon, fish, yoghurt, avocados, mushrooms
What influences the distribution of K+ between ICF + ECF?
- K+ loss due to inhibition of Na+/K+-ATPase pump (hypoxia)
- Insulin promotes acute entry of K+ into skeletal muscle + liver;
- increases ATPase pump activity - Catecholamines, e.g. adrenaline, promote cellular entry of K+/ propanolol impairs entry of K+ into cells
Others;
Exercise: K+ released during, changes reversed after several mins rest
Hyperosmolality: causes H20 to diffuse into cells carrying K+ with it
Cellular breakdown: release K+ into ICF, e.g. trauma, tumour lysis syndrome, blood transfusion
How is K+ measured?
Serum, plasma + urine used for analysis
ISE for K+ = valinomycin membrane;
- selectively binds K+ = impedence that correlates with K+ conc
- KCl used as inner electrode ref solution
Ref ranges;
Serum: 3.5-5.1mmol/L
Plasma: (male) 3.5-4.5/ (female) 3.4-4.4mmol/L
Urine 24H: 25-125mmol/day
What are the common problems with K+ analysis?
Due to low conc dilutional errors (like with Na+) do not tend to arise even with indirect ISEs
Leakage from cells;
- hemolysis: lysis of RBCs + release of contents into surrounding fluid, e.g. blood plasma w/ delayed processing
- Abn cellularity
K+ EDTA contamination from vacutainer tube
When should problems with K+ measurements be suspected?
Sample visibly hemolysed Sample dated previous day Grossly abn result Urea/creatinine normal Doctor reports no accompanying ECG changes
Describe the relationship between K+ and acid-base balance
Acid-base disturbances cause K+ to shift in and out of cells
Hyperkalemia linked to acidosis;
- acidosis causes K+: ICF > ECF (plasma) in exchange for H+ ions
- H+ ions moved into cells to decrease pH
Hypokalemia linked to alkalosis;
- reverse movement
- cells release H+ into blood to increase acidity
What is hyperkalemia?
Increased plasma K+ >4.5mmol/L (male)/ >4.4mmol/L (female)
Pts usually have underlying disorders;
- renal insufficiency, diabetes mellitus, metabolic acidosis
- most common in hosp pts = therapeutic K+ admin
Describe the steps of K+ metabolism
Primary events;
- K+ intake = transient increase in plasma K+
- alpha + beta pancreas cells sense increased plasma K+
Initial response;
- pancreas secretes;
- insulin = increased K+ storage in muscle
- glucagon = increased K+ secretion at renal tubules = increased urinary K+ excretion
- overall decrease in plasma K+
Associated responses;
- decrease in plasma K+ induces;
- K+ release from muscle
- muscle + plasma K+ values return to normal
List the broad causes of hyperkalemia
- Decreased renal excretion
- Cellular shift
- Increased intake
- Artefactual (pseudo)
What are the causes of decreased renal K+ excretion?
Acute/chronic renal failure (GFR<20ml/min) Hypoaldosteronism Addison's Diuretics Adrenal failure Drugs (some anti-hypertensives)
What are the causes of cellular shift leading to hyperkalemia?
Acidosis, e.g. lactic acidosis Muscle/cell injury Chemo Leukemia Hemolysis
