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/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.

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.

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.

A rather lame

illustrationYou 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