Ch. 26 - Fluids

Body Fluid Compartments

• Intracellular Fluid Compartment (ICF)

• Extracellular Fluid Compartment (ECF)

Extracellular Fluid Compartment

• One-third of water outside of cells

• Body’s “internal environment”

• External environment of cells

• Divided into:

  • Plasma – fluid portion of blood

  • Interstitial Fluid (IF) – fluid in spaces between cells

Intracellular Fluid Compartment

• Two-thirds by volume in cells

• 25 L (of 40 L body water) in average male

Electrolytes

• Chemical compounds that dissociate into ions in water

• Can conduct electrical current

• Includes inorganic salts, inorganic & organic acids/bases, some proteins

• Greater osmotic power than nonelectrolytes

• Water moves according to osmotic gradients

• Water moves from lesser osmolality to greater osmolality (concentration)

• Electrolytes have greatest ability to cause fluid shifts

Nonelectrolytes

• Chemical compounds with covalent bonds that prevent them from dissociating in solution

• Not electrically charged

• Most are organic molecules (glucose, lipids, creatinine, urea)

Fluid Movement

sources

of body water:

• Typically 2500 ml per day

• Ingested liquids and foods

• Metabolic water (water of oxidation)- body water produced by cellular metabolism

Metabolic water

body water produced by cellular metabolism

routes

of water loss:

• Insensible Water Loss

  • Vapor in expired air from lungs

  • Diffusion through skin

• Sensible Water Loss (measurable)

  • Urine (60%)

  • Obvious sweat

  • Feces

Insensible

• Vapor in expired air from lungs

• Diffusion through skin

Sensible

• Urine (60%)

• Obvious sweat

• Feces

regulation

of water intake/output:

• Healthy people – osmolality of body fluids = 280-300 mOsm

• Rise in plasma osmolality triggers thirst and release of ADH

• Decline in plasma osmolality inhibits thirst and ADH release

• Water and Na⁺ are closely tied together

• Na⁺ acts as “water magnet”

• Thirst and ADH mechanisms act independently of Na⁺

Thirst Mechanism

• Driving force for water intake

• Governed by hypothalamic thirst center

  • Osmoreceptors – detect ECF osmolality through changes in plasma membrane stretch

  • Dry Mouth – increase in blood osmotic pressures causes decrease in saliva production

  • Decrease in blood volume (or pressure) – signaled by baroreceptors

• Collective cause of thirst sensation

Antidiuretic Hormone

• Low ADH levels = most water not reabsorbed in kidneys = dilute urine = reduced volume of body fluids

• High ADH levels = nearly all water is reabsorbed in kidneys = concentrated urine

• Osmoreceptors of hypothalamus trigger or inhibit ADH release from posterior pituitary gland

Low ADH

most water not reabsorbed in kidneys = dilute urine = reduced volume of body fluids

High ADH

nearly all water is reabsorbed in kidneys = concentrated urine

Dehydration

fluid loss

Edema

accumulation of fluid in interstitial space leading to swelling