Electrolytes

1) SODIUM serum: 135 - 145 mEq/L

· Principle cation in extracellular fluid [ECF] ‑>

i) ensures isotonicity of ECF

ii) provides correct electrochemical environment for muscle & nerve activity

iii) role in acid-base balance in kidney (exchanged for H⁺)

Sodium Balance:

1) Aldosterone:

· the most potent inhibitor of renal Na⁺ excretion

reduced [ECF] Na⁺ &/or depressed water

↓

kidneys: increased renin

↓

plasma protein ‑> angiotensin I

↓

lung: angiotensin I ‑> angiotensin II

↓

adrenals: release aldosterone

↓

kidney tubule: reabsorption of H₂O & Na⁺[in exchange for K⁺ &/or H⁺]

2) ADH

increased ECF Na⁺ [increased ECF tonicity]

↓

shrinkage of osmoreceptors in hypothalamus

↓

posterior pituitary increased release ADH

↓

DCT & collecting ducts: increased H₂O reabsorption

2) POTASSIUM serum: 3.5 - 4.5 mEq/L

· Principle cation in ECF ‑>

· provides correct electrochemical environment for muscle & nerve activity

Potassium Balance:

· kidney will conserve Na⁺ at the expense of K⁺

1) Aldosterone:

increased ECF K⁺

↓

aderenal cortex: increased aldosterone

↓

kidney tubules: increased K+ excretion

3) CALCIUM serum: 4.5 - 5.5 mEq/L

· Total body Ca^2+:99% in bone; 1% in ECF

· In ECF: half bound with plasma proteins (2.5 mEq/L), rest freely ionized (2.3 mEq/L)

· provides correct electrochemical environment for muscle & nerve activity

· important in bone metabolism & coagulation

Calcium Balance:

1) Parathormone

reduced ECF Ca²⁺

↓

parathyroid gland: increased PTH

↓

1) bone: increased osteoclastic (bone breakdown) activity

2) kidney: increased Ca²⁺ resorption

3) GIT: increased Ca²⁺ resorption

2) Calcitonin

increased ECF Ca²⁺

↓

Thyroid: increased Calcitonin

↓

Bone inhibits osteoclastic activity

3) Vitamin D

· normally ingested or synthesized in skin

· promotes PTH activity by facilitating:

1) bone: increased osteoclastic activity

2) GIT: increased Ca²⁺ resorption

· in absence of vitamin D, PTH cannot function