Fluid, Electrolyte and Acid- Base Balance Chapter 27 (26)

Where’s the water?

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/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.

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 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.

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