Fluid, Electrolyte and Acid-Base Balance Chapter 27 (26)
Water content varies with age & tissue type • Infants – 73% • Adult male – 60% • Adult female – 50% • Elderly – 45% Fat has the lowest water content (~20%). Bone is close behind (~22 – 25%). Skeletal muscle is highest at ~65%.
Electrolyte concentrations are calculated in milliequivalents mEq/L = ion concentration (mg/L) x number of charges on one ion atomic weight Na+ concentration in the body is 3300 mg/L Na+ carries a single positive charge. Its atomic weight is approximately 23. Therefore, in a human the normal value for Na+ is: 3300 mg/L= 143 mEq/L 23 Note: One mEq of a univalent is equal to one mOsm whereas one mEq of a bivalent ion is equal to ½ mOsm. However, the reactivity of 1 mEq is equal to 1 mEq.
Regulation of water balance • It is not so much water that is regulated, but solutes. • osmolality is maintained at between 285 – 300 mOsm. • An increase above 300 mOsm triggers: • Thirst • Antidiuretic Hormone release
The Thirst Mechanism An increase of 2 – 3% in plasma osmolality triggers the thirst center of the hypothalamus. Secondarily, a 10 – 15% drop in blood volume also triggers thirst. This is a significantly weaker stimulus.
Dehydration • Chronic dehydration leads to oliguria. • Severe dehydration can result in hypovolemic shock. • Causes include: • Hemorrhage • Burns • Vomiting • Diarrhea • Sweating • Diuresis, which can be caused by diabetes insipidus, diabetes mellitus and hypertension (pressure diuresis).
Hypotonic hydration • A severe drop in osmolality • Caused by: • Excessive water intake • Renal dysfunction • Major consequence is hyponatremia. • Hyponatremia results in: • Cerebral edema (brain swelling) • Sluggish neural activity • Convulsions, muscle spasms, deranged behavior. • Treated with I.V. hypertonic mannitol or something similar.