Fluid Electrolyte and Acid Base Balance Chapter 27
Fluid, Electrolyte and Acid. Base Balance Chapter 27 (26)
Where’s the water?
Water content varies with age & tissue type 1. 2. 3. 4. 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 m. Eq/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 m. Eq/L 23 Note: One m. Eq of a univalent is equal to one m. Osm whereas one m. Eq of a bivalent ion is equal to ½ m. Osm. However, the reactivity of 1 m. Eq is equal to 1 m. Eq.
Relative electrolyte concentrations: Plasma, Interstitial Fluid & ICF
Sources of intake & output
Regulation of water balance • It is not so much water that is regulated, but solutes. • osmolality is maintained at between 285 – 300 m. Osm. • An increase above 300 m. Osm triggers: – Thirst – Antidiuretic Hormone release
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. The Thirst Mechanism
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.
A rather lame illustration You do remember how osmosis works, don’t you?
Sodium regulation
Blood pressure, sodium, and water
Atrial Naturetic Peptide: The heart’s own compensatory mechanism.
Buffers
Reabsorption of bicarbonate
Generation of new bicarbonate from phosphate
Generation of bicarbonate from glutamine deamination
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