Fluid, Electrolyte, and Acid-Base Balance

Body Water Content and Distribution

Water is the most abundant component of the human body, playing a critical role in physiological processes. The percentage of body water varies with age, sex, and body composition.

• At birth: Babies have about 75% of their body weight as water.

• Adult men: 55-60% water.

• Adult women: Slightly less water due to higher adipose tissue content, which contains little water.

Body water is distributed between intracellular and extracellular compartments:

• Intracellular fluid (ICF): 40% of body weight, contained within cells.

• Extracellular fluid (ECF): 20% of body weight, includes interstitial fluid and plasma.

• Interstitial fluid (IF): 80% of ECF.

• Plasma: 20% of ECF.

Water Movement in Fluid Compartments

Water moves between compartments primarily due to the presence of electrolytes, which influence osmotic gradients and water distribution.

• Electrolytes are ions such as Na+, K+, and Cl- that regulate water movement and content.

• Water moves from areas of low solute concentration to high solute concentration to maintain balance.

Fluid Balance and Water Loss

Fluid balance is maintained by matching water intake with water output. Water loss varies with environmental conditions and physical activity.

• Perspiration increases with hot air or heavy work.

• Respiratory loss increases with cold, dry air or heavy work.

• Vasoconstriction conserves heat, increases blood pressure, decreases ADH, and increases urine output.

Dehydration, Thirst, and Rehydration

Dehydration occurs when water loss exceeds intake, leading to increased blood osmolarity and decreased blood volume and pressure. The body responds by activating thirst mechanisms.

• Thirst center in the hypothalamus is stimulated.

• Salivation is inhibited via sympathetic signals.

• Rehydration involves cooling and moistening the mouth, distension of the stomach and intestine, and restoration of blood osmolarity.

Regulation of Water Output

Water output is regulated by sodium (Na+) levels and the action of antidiuretic hormone (ADH).

• Na+ reabsorption or excretion changes urine volume; water follows sodium.

• ADH changes urine concentration and is most influential in output regulation.

• ADH secretion is stimulated by dehydration.

Disorders of Water Balance

Imbalances in water can lead to volume depletion (hypovolemia), dehydration, or fluid excess.

• Volume depletion (hypovolemia): Total body water decreases, osmolarity remains normal. Causes include hemorrhage, burns, vomiting, or diarrhea.

• Dehydration: Total body water decreases, osmolarity rises. Causes include lack of drinking water, diabetes, sweating, diuretics. Infants are more vulnerable.

• Serious effects: Circulatory shock, neurological dysfunction, infant mortality.

Fluid Excess

Fluid excess is less common due to effective kidney regulation. It can occur as volume excess or hypotonic hydration.

• Volume excess: Both Na+ and water are retained, often due to aldosterone hypersecretion or renal failure.

• Hypotonic hydration (water intoxication): More water than sodium is retained or ingested, leading to hypotonic blood and cellular swelling.

• Serious effects: Pulmonary and cerebral edema.

Electrolytes

Electrolytes are ions that are chemically reactive in metabolism, determine cell membrane potentials, affect osmolarity, and influence water distribution.

• Major cations: Na+, K+, Ca2+, H+

• Major anions: Cl-, HCO3-, PO43-

Acid-Base Balance

Acid-base balance is essential for homeostasis. Acids lower pH, bases raise pH, and metabolism depends on pH-sensitive enzymes. The normal pH range of extracellular fluid (ECF) is 7.35 to 7.45.

• Metabolism continually lowers pH through production of lactic, phosphoric, fatty, ketone, and carbonic acids.

• Key equation:

Disorders of Acid-Base Balance

Acid-base disorders are classified as respiratory or metabolic, depending on their origin.

• Respiratory acidosis: Excess CO2 (hypercapnia).

• Respiratory alkalosis: Low CO2 (hypocapnia).

• Metabolic acidosis: Increased production of organic acids (lactic acid, ketones), ingestion of acidic drugs, or loss of base.

• Metabolic alkalosis: Overuse of bicarbonates or loss of acid (vomiting).

Respiratory Control of pH

The respiratory system adjusts ventilation to regulate blood pH.

• Hypercapnia: Increased ventilation expels CO2 and raises pH.

• Hypocapnia: Reduced ventilation allows CO2 to accumulate and lowers pH.

Renal Control of pH

The kidneys provide a slow but powerful buffer system for acid-base balance.

• Acidosis: Increased H+ secretion.

• Alkalosis: Increased bicarbonate secretion.

• Renal mechanisms are effective for imbalances lasting days or longer.