Fluid Balance and Electrolyte Regulation in the Body

Body Fluid Percentage

Human body fluid ranges from 45% to 75%.

Adipose Tissue Water Content

Adipose tissue contains about 20% water.

Skeletal Muscle Water Content

Skeletal muscle contains about 75% water.

Intracellular Fluid (ICF)

Fluid within cells, two-thirds of total body fluid.

Extracellular Fluid (ECF)

Fluid outside cells, includes interstitial fluid and plasma.

Interstitial Fluid (IF)

Fluid surrounding cells, two-thirds of extracellular fluid.

Blood Plasma

Fluid within blood vessels, more permeable than ICF.

Cerebrospinal Fluid

Fluid surrounding the brain and spinal cord.

Synovial Joint Fluid

Fluid in joints, lubricates and nourishes cartilage.

Osmolarity

Concentration of solutes in body fluids.

Fluid Intake

Addition of water to the body, 2500 mL/day.

Fluid Output

Loss of water from the body, 2500 mL/day.

Sensible Water Loss

Measurable loss through urine and feces.

Insensible Water Loss

Not measurable, includes expired air and sweat.

Obligatory Water Loss

Water loss that always occurs, essential for waste.

Facultative Water Loss

Controlled loss regulated by hydration status.

Fluid Imbalance

Occurs when fluid output does not equal intake.

Volume Depletion

Loss of body fluid leading to decreased volume.

Volume Excess

Excess body fluid leading to increased volume.

Dehydration

Deficiency of body fluid, affects osmolarity.

Hypotonic Hydration

Excess fluid dilutes body fluids, alters osmolarity.

Fluid Sequestration

Abnormal accumulation of fluid in body compartments.

Capillary Permeability

Allows selective passage of substances between fluids.

Isotonic Fluid Loss

Loss of fluid without changing osmolarity.

Examples of Volume Depletion

Includes hemorrhage, burns, vomiting, diarrhea.

Fluid Intake Normal

Decreased kidney fluid loss causes volume excess.

Osmolarity Changes

Alterations in solute concentration in fluids.

Causes of Dehydration

Includes sweating, diabetes, alcohol intake, low water.

Hypertonic Plasma

Increased solute concentration in blood plasma.

Water Shift

Movement of water from cells to plasma.

Water Intoxication

Excessive water intake leading to hypotonic plasma.

ADH Hypersecretion

Excess antidiuretic hormone causing water retention.

Cerebral Edema

Swelling of brain cells due to fluid imbalance.

Na+ Loss During Sweating

Sodium loss occurs alongside water loss in sweat.

Edema

Puffiness with fluid accumulation in interstitial.

Ascites

Accumulation of fluid within peritoneal cavity.

Pericardial effusion

Accumulation of fluid in pericardial cavity.

Pleural effusion

Accumulation of fluid in pleural cavity.

Fluid intake is less than output

Decreases blood pressure and blood volume; increases blood osmolarity if more water is lost than solutes.

Fluid intake is greater than output

Increases blood pressure and blood volume; decreases blood osmolarity if water gain exceeds solute gain.

Stimuli that activate the thirst center

Decreased blood volume and blood pressure, increased blood osmolarity, decreased salivary secretions.

Renin

Released from kidney, results in production of angiotensin II.

Angiotensin II

Stimulates thirst center.

Stimuli that inhibit the thirst center

Fluid intake is greater than fluid output, decreased blood osmolarity, increased salivary secretions, distension of stomach.

Regulating fluid output

Regulated through kidneys by controlling urine output.

Hormones involved in regulating urine output

Angiotensin II, antidiuretic hormone (ADH), aldosterone decrease urine output; Atrial natriuretic peptide (ANP) increases urine output.

Nonelectrolytes

Molecules that do not dissociate in solution.

Electrolytes

Dissociate in solution to form cations and anions; ability to conduct electrical current when dissolved.

Concentration of electrolytes

Given as milliequivalents/L (mEq/L).

Sodium ion (Na+)

99% in ECF and 1% in ICF; principle cation in ECF; exerts greatest osmotic pressure.

Sodium balance

Normal concentration 135 to 145 mEq/L; daily requirement about 2 g/day; intake may vary from 0.5 g to 20 g.

Sodium's role in blood plasma osmolarity

Most important electrolyte in determining blood plasma osmolarity and regulating fluid balance.

Hypertonic

Condition if Na+ concentration increased due to increased Na+ or decreased water content.

ECF hypotonic

Condition if Na+ concentration decreased due to decreased Na+ or increased water content.