Week 6: Fluids & Electrolytes Flashcards
Describe the distribution of body fluids, including aging considerations.
Body fluids are divided into fluid compartments, these compartments include the intracellular compartment (ICF: 2/3 of total body fluid inside the cells) and the extracellular compartment (ECF: 1/3 of total body fluid outside of the cell). The ECF is composed of plasma (intravascular) and interstitial fluid (between the cells - ‘bathes them’). Fluid moves continuously between the ICF and ECF compartments through semi-permeable cell membranes but move in ‘net’ amounts to maintain balance and homeostasis. Distribution of fluids is different across the life spans - females, for example have more fat cells and therefore hold less water than men. Older adults and newborns have more extreme changes in fluid because they have a greater internal volume. Older adults lose muscle mass and have more fat proportionally (therefore less water). They are also at greater risk of hydration due to decreased thirst response, reduced kidney function, etc. Newborns are also at increased risk due to frequent intake and output and an under-developed thirst response.
Describe factors that control water movement between the intracellular and extracellular compartments.
- Diffusion: particles move from high to low solute concentrate. Simple and facilitated diffusion are both passive and do not require ATP, but diffusion using a facilitated process requires a carrier/transport cell membrane molecule.
- Active Transport: movement of particles across a cell membrane against a concentration gradient - from lower to higher particle concentration. As such, will require ATP energy (i.e., the sodium-potassium ATPase pump)
- Osmosis: water moves from low to high solute concentration. Osmolality is typically at equilibrium, however, when ECF osmolality changes, water will move from one compartment to another until osmotic equilibrium is re-established.
- Filtration: refers to fluid movement out of the capillary and into the interstitial space. As plasma flows from the arterial to the venous end of the capillary, four forces, sometimes called Starling forces, determine whether the net effect is filtration or reabsorption.
Compare and contrast the causes, diagnostics, manifestations, and treatment of alterations in water movement.
Accumulation of fluid in the interstitial spaces will result in edema and is typically caused by increased capillary hydrostatic pressure (venous obstruction), decreased plasma oncotic pressure (losses of diminished production of albumin), increased capillary permeability (inflammation and immune response) and/or lymphatic obstruction (lymphedema). Treatment includes elevating oedematous limbs, using compression stockings or devices, avoiding prolonged standing, restricting salt intake and diuretics.
Discuss the importance of sodium
Sodium is the main ECF electrolyte and can combine with chloride or bicarbonate to provide electroneutrality. Its roles include neuromuscular excitability, acid-base balance, cellular reactions, and transport of substances. The hormonal regulation of sodium is controlled by aldosterone and natriuretic peptides. It regulates osmotic forces and has a narrow range of 135-145 mmol/L.
Hypernatremia: sodium serum level of >145 mmol/L associated with a hypertonic alteration where water moves from the ICF to the ECF and cells shrink
Hyponatremia: sodium serum level of <135 mmol/L associated with a hypotonic alteration where ECF osmotic pressure decreases as water moves into the cells and they swell
Discuss the importance of potassium
Potassium is the main ICF electrolyte. It is kept within a very narrow range of 3.5-5 mmol/L and is related to acid-base balance.
Hyperkalemia: serum level of >5 mmol/L caused by excessive consumption or renal pathology.
Hypokalemia: serum level of <3.5 mmol/L often caused by use of non-potassium sparing diuretics.
Differentiate between isotonic, hypotonic, and hypertonic solutions in relation to fluid and electrolyte balance
Hypotonic solution: low solute concentration, less water. ECF <09.% salt solution due to water gain or solute loss; lower concentration of solutes than ICF; cells swell.
Hypertonic solution: high solute concentration, less water. ECF >0.9% salt solution; water loss or solute gain; higher concentration of solutes than ICF; cells shrink.
Isotonic: ECF equivalent to 0.9% salt solution; not net water movement (i.e., no cell shrinking or swelling)
How much of total body weight is fluid? How is that fluid distributed?
60% of total body weight is fluid, 40% of that is in the cells and 20% is located in the ECF.
Discuss the Starling Forces
1.Capillary hydrostatic pressure (blood pressure) facilitates the outward movement of water from the capillary to the interstitial space.
2.Capillary (plasma) oncotic pressure osmotically attracts water from the interstitial space back into the capillary.
3.Interstitial hydrostatic pressure facilitates the inward movement of water from the interstitial space into the capillary.
4.Interstitial oncotic pressure osmotically attracts water from the capillary into the interstitial space.
Filtration is favoured by capillary hydrostatic pressure (pushing water out) and interstitial oncotic pressure (pulling water out)
Where does filtration and reabsorption take place in a capillary?
The pressure at the arterial end of the capillary is different than the pressure at the venous end of the capillary, there is more pressure at the arteriole end and therefore filtration occurs (pushing water out). The venous end experiences less pressure and therefore reabsorption (movement of particles back into the vessel) occurs.
What is dependent edema?
Dependent on gravity, typically lower extremity swelling due to long hours standing, being on an airplane, etc.
Define “Third Spacing”
Occurs when too much fluid moves from intravascular space into interstitial or “third”, non-functioning spaces where fluid is not easily exchanged with the rest of the ECF. Increase in this fluid occurs at the expense of fluid in other compartments as the fluid is now trapped and non available for functional use (e.g., edema, pleural effusion, ascites)
Differentiate between osmolality and tonicity
Osmolality is the number of dissolved particles in 1 kg of water (N = Sodium = 280-294 mmol/kg (mOsm).
Tonicity is relative concentrations of two fluids separated by a membrane. The difference in concentrations creates an osmotic gradient promoting fluid movement (e.g., isotonic, hypertonic, hypotonic)
Osmotic forces are controlled by sodium for the ECF and potassium for the ICF (bags of potassium in a big salty sea)
Differentiate between hypovolemia and hypervolemia
Hypovolemia is a concentration of particles too great or water content too little and is a value greater than 294 mmol/kg it is referred to as a “water deficit”. May be caused by water deprivation or water loss through diarrhea or vomiting, diabetes (osmotic effects of glucose), excessive diuresis or diaphoresis.
Hypervolemia is a concentration of too little solute for amount of water or too much water for amount of solute and is a value less than 280 mmol/kg it is referred to as “water excess”
How is fluid balance maintained?
- intake and output regulated by hormones
- antidiuretic hormone
- aldosterone
- natriuretic peptides
Describe how the ADH system maintains fluid balance
Anti-diuretic hormone is released when there is an increase in plasma osmolality and decrease in circulating blood volume (a hypotonic alteration - where the water moves into the cells). When this occurs the cells of the hypothalamus shrink, triggering the release of ADH which tells the kidneys to retain water through reabsorption.
