2.1 - fluid imbalances

What is TBW (Total Body Water)?

sum of all fluids within body compartments

TBW of infants?

70%

TBW of men and women?

Men: 60%

Women: 50%

TBW of older adults?

50%

Intracelluar Fluid

• fluid in cells

• 2/3 of TBW

Extracellular Fluid

• fluid outside of cells

Types of ECF

• Interstitial

• Intravascular

• Transcellular

Where is interstitial fluid found?

Between cells, but not blood vessels

Where is intra-vascular fluid found?

W/i blood vessels

Where is trans-cellular fluids found?

Contained in epithelial lined cavities of body

Why is TBW, ICF, ECF important?

• maintains fluid homeostasis

• balance depends on ECF levels of H₂O and Na

• adequate vascular volume → effective tissue perfusion → provide nutrients + remove waste

Abnormalities of BF

• Volume

  • excess

  • deficit

• Concentration of solutes

  • hypo-osmolar

  • hyperosmolar

• Specific Electrolyte Composition of Body Fluid

  • hyper

  • hypo

Mechanisms of Fluid Homeostasis

• Intake

• Absorption

• Distribution

• Excretion

What’s Intake & Absorption

• Entry of fluid into the body by any route

How is thirst triggered?

• Trigger - hypothalamus

  • Actions:

    • ↑ extracellular fluid osmolality via osmoreceptors

    • ↓ blood volume via baroreceptors, angiotensin II

• older adults thirst reduced → insufficient intake

What are other forms of intake and absorption?

• IV

• GI tubes and body cavity infusions

• SubQ or bone marrow infusion

• Rectal intake (enema)

Distribution

• Vascular ←→ Interstitial

• Interstitial ←→ Intracellular

What are the starling forces?

• Capillary Hydrostatic Pressure: Pushes fluid out of capillaries.

• Capillary Oncotic Pressure: Pulls fluid into capillaries. (by albumin proteins)

• Interstitial Hydrostatic Pressure: Pushes fluid into capillaries.

• Interstitial Oncotic Pressure: Pulls fluid out of capillaries.

What’s Hydrostatic Pressure?

“Leaf Blower” pressure

What’s Oncotic Pressure?

“Vacuum” Pressure

Tonicity

the concentration of solutes in a solution relative to the inside of a cell, affecting water movement via osmosis

Hypertonic

• Higher osmolality in interstitial fluid than inside the cell → water leaving cell → cell shrinks

Hypotonic

• Osmolality of interstitial fluid is less than in the cell → water enters the cell → cell swells

Isotonic

• No net water movement

• The cell remains the same size and shape

Excretion

• Sensible Fluid Loss

  • UT (0.5ish L/day)

    • based on adequate bp to perfuse kidneys + GFR (1ish mL/kg/hr)

  • Bowls

    • norm or increases bc diarrhea

• Insensible Fluid Loss

  • Lungs

  • Skin

    • perspiraiton

    • visible sweat

Hormonal Fluid Regulation

• primarily hormones controls amount excreted in urine

  • ADH

  • Aldosterone

  • Natriuretic Peptides: ANP and BNP

  • other minor hormones

Abnormal Loss

• Emesis

• Tubes in GI or other body cavities

• hemorrhage

• drainage from fistulas, wounds, or open skin

• paracentesis

ADH

• increases vasoconstriction and renal permeability/absorption

• hypothalamus → pituitary gland→ renal

• Factors:

  • ↑ osmolality of the extracellular fluid

  • ↓ circulating fluid volume

  • Pain, nausea

  • Physiologic/psychological stressors

• High ADH

  • small, concentrated urine

  • water reabsorption increase

• Low ADH

  • increases excretion

  • large, diluted

• dilutes body by retaining water

Aldostrone

• adrenal cortex in kidney

• renal tubes reabsorb Na and H₂O and excrete K

• decreased excretion

• Stimuli

  • Angiotensin II

    • from RAAS

    • bc decrease circ of BV, low blood flow to kidneys, low bp or low Na levels

  • increased concentration of plasma potassium

• expands ECF vol by sodium retention in kidneys

Natriuretic Peptides

• produced by myocardium

• opp of RAAS

• increase vasodilation and increase Na+H2O output → reduce BP

  • ANP: produced by atria

  • BNP: produced in ventricles

Albumin

• a plasma protein from liver

• maintains BV and maintain oncotic pressure

  • sustain bp and tissue perfusion

  • more albumin → more water pulled from interstitial to capillaries

• prevent fluid leakage/edema

Fluid Imbalances

• ECF volume (amount and vol)

  • deficit/excess saline or volume

• ECF concentration (sodium)

  • Hyponatremia/Hypernatremia

  • Water intoxication/Water deficit

  • Hypoosmolality/Hyperosmolality

Proportionate & Disproportionate Imbalances

Proportionate

• gains or losses of BOTH Na and H₂O

  • Isotonic fluid deficit/excess

Disproportionate

• gains or losses of Na OR H₂O

  • ↑ osmolality = Hypernatremia (too little Na or too much H₂O)

  • ↓ osmolality = Hyponatremia (too much Na or too little H₂O)

Assessing Hydration Status

Urine specific gravity

• Normal 1.003-1.030

Hematocrit

• Normal Male 40-50%;

• Normal Female 37-47%

Sodium

• 135-145 mEq/L

Serum Osmolality

• (275-295 mOsm/kg)

Syndrome of Inappropriate ADH (SIADH)

• Excess ECF vol and ADH

• Hyponatremia

• Causes

  • malignant tumors (lungs)

  • pulmonary TB

  • drug induced

Diabetes Insipidus (DI)

• ECF vol and ADH deficit

• Hypernatremia

• Causes

  • Idiopathic

  • surgical/non-surgical brain trauma

  • brain tumors

  • hypophysectomy