1. Water and electrolytes are located in 2 major compartments – the ICF (2/3 of the body’s water) and the ECF (subcompartments of interstitium, plasma, and transcellular "third" space).

  3. A variety of transport mechanisms – diffusion, osmosis, active transport and others – maintain the life-supporting balances inside (ICF) and outside cells (ECF).
  4. Examples:

    Osmosis of water – no increase in cell size in an isotonic environment but increase in cell size if ECF is hypotonic (osmolality less than 275 mOsm/kg) and decrease in cell size if ECF is hypertonic (osmolality greater than 295 mOsm/kg). Common osmotic agents include sodium, chloride, glucose, urea, plasma proteins – particularly, albumin.


    What happens to cells if serum glucose triples its normal value (in diabetes mellitus which is out of control)?


    In a critical situation in which hemorrhaging occurred, the client was given 5 L of sodium-free IV fluids in 2 hours. What happened to the plasma and the cells?

    Na+-K+ pump maintains increased concentration of K+ inside of cells and increased Na+ outside in ECF. This "pump" requires energy.

    A reduction in oxygen delivery to the cells – tissue hypoxia – causes cells to swell. Why?


    Electrical neutrality is maintained by equating + and – charged particles between ICF and ECF. Like charges repel and opposite charges attract. In acidosis of the ECF, an increase in H+ charges outside the cells leads to the excess H+ inside the cells which pushes another cation out of the cell.


    Which cation normally higher in the ICF reaches higher than normal levels in the ECF during acidosis? Hint: Because of the role of this cation in repolarization of the heart muscle, its’ accumulation in the serum is reflected in a "tenting" of the T wave.


  5. Movement of fluid within ECF
  6. 2 major capillary factors and 2 major tissue factors – "Starling factors" determine the flow of fluid between interstitium and plasma compartments.

    These factors include capillary and tissue colloidal osmotic pressure determined by the level of proteins – i.e. plasma proteins – which pull fluid in their direction. Capillary and tissue hydrostatic pressure oppose each other so that the pressure in the capillaries (highest on arteriolar end) pushes fluid into the tissues and the pressure of fluids in the interstitium (tissue gel) resists the flow of fluid out of the capillaries.

    Tissue factors are influenced by disorders that increase capillary permeability – enabling fluids and proteins to leave capillaries and by conditions that impede lymphatic circulation (the only means of placing proteins and some fluids back into circulation).



    Describe edema – what is it, where is it, what changes in fluid movement are associated it and conditions associated with it. What types of edema are life-threatening?


    Third space shift (abnormal accumulation of fluid in the transcellular space) removes fluid from active movement between extracellular and intracellular compartments. Fluid accumulation between the pleura, in the pericardial sac or within the peritoneal cavity not only removes it from circulation but creates situations in which accumulated fluid compresses vital organs. Fluid may accumulate increasing synovial fluid, cerebrospinal fluid, amniotic fluid, etc.


  7. The homeostatic balances in fluids volume and pressure, serum electrolyte concentrations, and osmolality are achieved by the following mechanisms –
Thirst – osmoreceptors and baroreceptors sense osmolality, blood pressure/volume. Thirst occurs in response to increased osmolality and decreased blood pressure and/or volume.

ADH – secreted from posterior pituitary in response to the same factors causing thirst. The hormone increases the reabsorption of water in the collecting tubules of the nephrons, increasing blood volume and decreasing plasma osmolality.


What causes SIADH and diabetes insipidus? Contrast their clinical manifestations. What other factors influence the secretion of ADH?


Factors increasing the reabsorption of sodium influence the reabsorption of water and ultimately, the blood volume, blood pressure, osmolality and electrolyte balance. The factors include the secretion of atrial natriuretic hormone when the atria of the heart are overstretched by an increased end-diastolic volume as in congestive heart failure. ANH secretion increases the excretion of sodium in the urine. The other homeostatic mechanism activated when the arterial circulation to the kidneys is reduced by a fall in blood volume, pressure, and/or perfusion is the renin-angiotensin-aldosterone mechanism.


Describe the steps of this mechanism. Name 3 major actions of angiotensin II. What effect does aldosterone secretion have on the serum levels of sodium and potassium?