Fluid, Electrolyte and Acid-Base Balance
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Transcript of Fluid, Electrolyte and Acid-Base Balance
Water content varies with age & tissue type
1. Infants – 73%
2. Adult male – 60%
3. Adult female – 50%
4. 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/LNa+ 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.
DehydrationChronic 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.