Four - Fluids and Electrolytes

Main Fluid Compartments

1. Intracellular fluid (ICF) - 60% - major cation K+

2. Extracellular fluid (ECF) - 40% - major cation Na+

Three Locations:

1. ICF

2. ECF - Interstitial fluid (IF)

3. ECF - Plasma

Movement of Fluid Between Compartments

• Continuous mixing

• Freely permeable membrane

• Moves across capillary wall

• Regulated by osmotic and hydrostatic pressure

• Fluids pass plasma/IF to reach intracellular compartment

Fluid and Electrolyte Balance

Fluid balance in ECF maintained by feedback mechanisms.

Changes detected aim to change water intake, electrolyte reabsorption

Maintained by kidneys, thirst/appetite adjusted

Antidiuretic Hormone (ADH)

ADH made by hypothalamus and released by the posterior pituitary.

Inhibits diuresis (urine production) and works to maintain electrolyte balance of ECF.

Aldosterone

Secreted from adrenal cortex.

Integral part of RAAS.

Higher Na+ reabsorbed in presence of aldosterone

Maintains homeostatic levels of electrolyes in plasma.

Hypotonic Hydration

Fluid imbalance - excess water in ECF = low Na+

Water influx into cells via osmosis, causing cellular swelling and lysis.

Oedema

Abnormal accumulation of fluid in interstitial space (fluid distribution problem).

Caused by; increase in capillary hydrostatic pressure, permeability, decrease osmotic pressure

Results in; swollen tissue, impaired functioning, compromised cardiovascular functioning, decreased circulating blood volume.

pH Regulation

• Chemical buffer systems - most rapid, temporarily tie up H+ ions

  • Three: Carbonic acid-bicarbonate ion buffer system, Phosphate buffer system, Protein buffer system

• Respiratory regulations - acts within minutes

• Renal mechanisms - slowest (hours), most potent mechanism

Normal pH Range

• Arterial blood pH 7.4

• Venous blood and IF pH 7.35

• ICF pH 7.0

Acidosis - below 7.35

Alkalosis - above 7.45

Respiratory Acidosis

Acid-base disturbance (pH <7.35)

From hyperventilation causing retention of CO2,

Producing excess carbonic acid from impaired pulmonary gas exchange or central nervous system depression (acute cause).

Respiratory Alkalosis

Acid-base disorder (pH>7.45)

Triggered by hyperventilation (from panic, stress, or pain)

Causing excessive CO2 exhalation = rise in pH.

Metabolic Acidosis

Acid-base disorder,

Excess fixed acids decrease pH,

Caused by renal disease, ingestion of acids (alcohol), excessive loss of bicarbonate ions (persistent diarrhoea).

Metabolic Alkalosis

Acid-base disturbance,

Excess of bases or acid deficit increasing pH

(caused by vomiting, constipation, excessive urine production,)