WGU D236: Pathophysiology Notes | Homeostasis and Cellular Response | 2026 Update

Homeostasis and Cellular Response


 Fluid and electrolyte balance
o Albumin is the major solute in the bloodstream.
o Major Solutes in body
 the major solutes are albumin, sodium (Na+), potassium (K+), phosphate
(PO4−), magnesium (Mg++), calcium (Ca++), bicarbonate (HCO3−), and
glucose.
 Magnesium plays an important role in enzymatic systems within the body
 Calcium plays an important role in neuromuscular irritability, blood
clotting, and bone structure.
 Bicarbonate is responsible for acid–base balance.
o Body fluid = solvent
 Main functions of electrolytes, solutes and protein is to carry waste and
deliver nutrients and electrolytes to cells
o Intracellular Fluid
 40% of total body weight
 K+ is the major ion
 Phosphate is an intracellular negative ion (anion)
o Extracellular Fluid
 20% of total body weight in an adult
 Most found in blood vessels
 Contains electrolytes, oxygen, glucose, and other nutrients to be
delivered to cells, as well as cellular waste products
 Na+ is major ion
o Interstitial fluid
 Filtrate of blood
 Located between cells and between cells and capillaries
 contains water and electrolytes, mainly sodium (Na+)
o Hydrostatic pressure (PUSH)
 the pushing force exerted by water in the bloodstream.
 heart’s pulsatile pumping action is the source of hydrostatic pressure
o Diffusion
 molecules passively spread from areas of high concentration to areas of
low concentration.

o Osmosis
 molecules of a solvent to pass through a semipermeable membrane from
a less concentrated solution into a more concentrated one
o Osmotic pressure (PULL)
 the pressure exerted by the solutes in solution
 exerted by electrolytes, mainly sodium ions and plasma proteins
 pulls water into the bloodstream from the ICF and ISF and opposes
hydrostatic pressure at all capillary membranes

, Homeostasis and Cellular Response


 determined by the number of particles or their concentration within the
solution.
 A solution with a greater number of particles has a higher osmotic
pressure.
 Decrease is osmotic pressure = fluid moving OUT of the blood vessels in
to the ISF
 High osmotic pressure = fluid movement from ICF and ISF INTO the blood
stream
 Oncotic Pressure aka colloidal osmotic pressure
 a type of osmotic pressure exerted specifically by albumin in the
bloodstream
 Albumin attracts water and helps keep it inside the blood vessel
o Albumin Range 3.1-4.3
 With reduced albumin, the oncotic pressure is low and the force
exerted by hydrostatic pressure overwhelms the oncotic
pressure edema, hypotension due to H2O not being in
circulation water is in ISF and ICF
o Osmolality
 a measurement of the concentration of solutes per kg of solvent
 Normal plasma osmolality is 282 to 295 milliosmoles
o Starlings Law of Capillary Forces
 osmotic pressure pulls water from the ICF into the ECF at every cell–
capillary interface.
 The osmotic pressure opposes the hydrostatic pressure; in healthy
conditions, each force balances out the other.
 when osmotic pressure is lower than hydrostatic pressure, osmotic
pressure is overwhelmed and hydrostatic pressure is an unopposed force,
causing water to flow from the ECF to the ICF
o Tonicity he concentration of solutes in solution compared with the
bloodstream, also used to describe the various intravenous (IV) solutions
 Isotonic 0.9% NaCl
 Hypotonic 0.45% NaCl (half normal saline)
 Hypertonic 3.0% NaCl, Mannitol
o Osmoreceptors, ADH, and thirst.
 Changes in plasma osmolarity are responsible for both the sensation of
thirst and the release of ADH
 High plasma osmolarity stimulates osmoreceptors in the hypothalamus
ADH released from Posterior Pituitary
 osmoreceptors respond to changes in both blood osmolarity and blood
fluid volume.
 When there is an increase in blood osmolarity, ICF shifts into ECF and the
cells shrink
 ADH stimulates water reabsorption from the nephron tubule fluid at the
collecting duct into the bloodstream.