DSA - K, Ca, P, Mg Flashcards
Normal Plasma K+ concentration
3.5-5.0 mEq/L
What is the distribution of K+ throughout the body?
98% intracellular
(80% muscle cells; 20% other cells)
2% extracellular
What channels regulate K+ concentrations?
Na-K-atpase
NKCC2
K+ Channels
How is Hypokalemia defined and what is the most common cause?
Plasma [K+] <3.7 mEq/L
Caused by vomiting/diarrhea, insulin excess, dietary deficiency, alkalosis
How is hyperkalemia defined and what is the most common cause?
Plasma [K+] > 5.2 mEq/L
> 10 mEq/L is lethal
Excessive intake (dietary), tissue release (rhabdimyolosis, burns, hemolysis), shifts from ICF to ECF (acidosis, insulin deficiency, tissue damage, hyperglycemia)
How is pseudohyperkalemia defined and what is the most common cause?
Artificially high plasma [K+] due to lysis of RBCs while blood is drawn
How much K+ should we intake per day and where does it go once ingested?
70 mmol/day
Muscle has largest amount of K+, followed by bone, then liver and RBCs
Of the 70 mmol/day, 10 mmol/day is excreted in feces while 60 mmol are absorbed through the gut into the ECF
60 mmol of K+ is excreted in urine per day
How does extracellular K+ levels affect normal nerve and muscle function?
HYPOkalemia causes resting membrane potential to hyperpolarize (become more negative) in neuromuscular cells
HYPOkalemia causes muscle weakness, muscle paralysis, intestinal distention, respiratory failure
HYPERkalemia causes resting membrane potential to hypopolarize (become more positive) in neuromuscular cells
HYPERkalemia causes an initial increase in muscle excitability, but later causes muscle weakness/paralysis
How does K+ levels affect cell volume?
Net loss of K+ causes cell shrinking
Net gain of K+ causes cell swelling
How does K+ levels affect intracellular pH?
Low plasma [K+] causes cellular acidosis
High plasma [K+] causes cellular alkalosis
How does K+ levels affect vascular resistance?
Low Plasma [K+] causes vasoconstriction
High Plasma [K+] causes vasodilation
How does extracellular K+ levels affect normal cardiac function?
Cardiac Cells respond in the opposite direction to neuromuscular cells
HYPOkalemia causes hypopolarization (more positive) of cardiac cell resting membrane potential
HYPOkalemia causes prolonged repolarization, slowed conduction, abnormal pacemaker activity, leads to tachyarrhythmias
HYPERkalemia causes hyperpolarization (more negative) of cardiac cell resting membrane potential
HYPERkalemia causes enhanced repolarization, slowed conduction, leading to bradyarrhytmias and cardiac arrest
What is the effect of insulin on K+?
Enhances cell uptake (reduces ECF K+ levels)
What is the effect of Beta-Catecholamines on K+?
Enhanced cell uptake (reduces ECF K+ levels)
What is the effect of Alpha-Catecholamines on K+?
Impaired cell uptake (increases ECF K+ levels)
What is the effect of Acidosis on K+?
Impaired cell uptake (increases ECF K+ levels)
What is the effect of Alkalosis on K+?
Enhanced cell uptake (reduces ECF K+ levels)
What is the effect of Cell damage on K+?
Impaired cell uptake (increases ECF K+ levels)
What is the effect of Hyperosmolality on K+?
Enhanced cell efflux (increases ECF K+ levels)
What is the normal approximate value of daily Ca2+ intake?
~1000 mg/day (adults)
Absorption is best at doses <500 mg, thus splitting up Ca2+ intake is preferable
What are the major storage pools for Ca2+ in the body and what are the major routes of loss from the body?
The general body and Bones pool Ca2+
Majority of Ca2+ is lost through Stool (~800 mg) and the rest is lost in urine (~200 mg)
PTH and calcitriol inhibit urinary excretion of Ca2+ by promoting Ca2+ renal tubular resorption. Additionally they both promote calcium resorption from bone.
Calcitriol promotes absorption of Ca2+ from gut
Calcitonin promotes bone formation and also inhibits Ca2+ excretion by promoting renal tubule resorption of Ca2+
What are the effects of Hypocalcemia on neuromuscular excitability?
Hypocalcemia hyperpolarizes the membrane threshold potential, causing a reduction in the potential difference between resting membrane potential and threshold potential, thus INCREASING neuromuscular excitability (hypercalcemic tetany/spasticity)
What are the effects of Hypercalcemia on neuromusclar excitability?
Hypercalcemia hypopolarizes the membrane threshold potential, causing the potential difference between resting membrane potential and threshold potential to increase, thus DEPRESSING neuromuscular excitability
What differential diagnoses could cause an elevated serum calcium?
Primary hyperparathyroidism
Malignancy
What differential diagnoses could cause a reduced serum calcium?
Hypoparathyroidism
Renal disease
Vitamin D deficiency
What is the normal daily dietary intake of Phosphate?
1500 mg
What are the major storage pools for Phosphate?
Bone (majority)
Cells
Serum
What are the major routed of phosphate loss from the body?
Stool
Urine
Bone formation
What are the 4 main regulators of phosphate metabolism?
Dietary
Calcitriol - increases phosphorus resorption from bone and absorption form the gut
PTH - increases phosphorus resorption from bone and indirectly promotes absorption from intestines by activating calcitriol
Renal Tubular Resorption - Stimulated by tubular filtered load of phosphorus; inhibited by PTH
What are the major storage pools for Mg2+?
50% in bone
49% in ICF (especially muscle)
1% in ECF
What are the major routes of Mg2+ absorption and excretion from the body?
Most ingested Mg2+ is excreted in stools; the remaining Mg2+ is absorbed from the gut into the blood
Mg2+ leaves blood to enter cells where it is stored and used
Remainder in blood gets excreted through urine
What is the state of serum Mg2+ and how much is there?
Total serum Mg2+ = 1.8 mEq/L (1.8-2.2 mg/dL)
Free serum Mg2+ = 0.8-1.0mEq/L (not bound to protein)
Non-diffusable (protein bound Mg2+) makes up almost 1/3 of serym Mg2+ while DIffusible and Ionized Mg2+ make up a little over 2/3.
