Bodily fluid compartments and challenges to homeostasis Flashcards
What is osmolarity?
Concentration of osmotically active particles in a solution
What are the units of osmolarity?
(M)osmol/l
Which two pieces of information are needed to calculate osmolarity?
Concentration of a solution
Number of osmotically active particles in a solution
How is osmolarity calculated?
Multiplying to concentration of a solution by it’s number of osmotically active particles
What is the difference between osmolality and osmolarity?
Units osmolality = osmol/kg and osmolarity = osmol/l (no difference when talking about weak salts)
What is the rough osmolarity of most body fluids?
300 mosmol/l
What is tonicity?
The effect a solution has on cell volume
What are the three types of tonicity and how do they affect cell volume?
Hypotonic - increase cell volume
Isotonic - no effect
Hypertonic - decrease cell volume
Is total body water high on average in men or woman? Why?
Men because they have less fat
What two compartments can total body water be broken down into?
Intracellular volume
Extracellular volume
What categories can extracellular fluid volume be broken down into?
Plasma
Interstitial (biggest proportion)
Transcellular
Lymph
What separates the intracellular and extracellular fluid compartments?
Cell membrane
How can the volume of body fluid compartments be measured?
Distribution volume of tracers
Name three useful tracers and which compartment they measure
Tritiated water - total body water
Inulin - extracellular fluid volume
Labelled albumin - plasma
How can intracellular fluid volume be measured?
If the volumes of the total body water and extracellular fluid are known intracellular fluid volume can be measured using the equation:
TBW = ECF + ICF
How can the volume of water in a container be measured using the dilution principle?
Add a dose of tracer to the fluid volume in question (the body) and allow equilibrium to be reached. Sample the fluid and measure the tracer concentration (equilibrium volume). Use this equation:
Volume = dose of tracer (mol) / sample concentration (mol/l)
In simple terms, what is homeostasis?
The maintenance of a variable within an acceptable range
What are the inputs and outputs involved in water balance?
Input - fluids, food, metabolism
Output - faeces, sweat, skin, lungs, urine
What is an insensible loss (in terms of water balance)? Give two examples
A loss of water which cannot be regulated by physiological means
Skin & lung water loss
Water imbalance manifests as changes in fluid tonicity. T/F
False - changes in fluid osmolarity
Can decreased excretion from the kidneys (i.e decreased urine output) maintain water balance alone?
No - physiological dysuria does not exist
Describe the isotonic composition of the ICF and ECF repectively
ICF - high potassium, high magnesium and high negatively charged proteins
ECF - high sodium, high chlorine, high bicarbonate
What allows the difference in environment between the ICF and ECF?
Proteins in the plasma membrane
How do the osmotic concentrations differ between the ICF and ECF?
They don’t - both 300 mosmol/l
Why is regulation of fluid balance and electrolyte concentration related?
Changes in solute concentrations cause changes in water distribution (osmotic gradient)
Do kidneys regulate composition of ICF or ECF?
ECF
What is fluid shift?
Movement of water between the ICF and ECF due to changes in the osmotic gradient
What happens when the osmotic concentration of the ECF increases?
Concentration increase > hypertonicity > fluid loss from the ICF
What happens when the osmotic concentration of the ECF decreases?
Concentration decreases > hypotonicity > fluid movement into the ICF
List the three possible changes to fluid homeostasis
Gain/loss of water
Gain/loss of NaCl (salt)
Gain/loss of isotonic fluid
What happens when there is a gain/loss of water?
Change in fluid osmolarity causes parallel changes in ICF and ECF volumes (both increase or decrease)
What happens when there is a gain/loss of NaCl?
Change in fluid osmolarity causes opposite changes in fluid volumes in response to active transport of sodium into the ECF and osmotic gradient changes
What happens when there is a gain of salt in the ECF?
ECF volume increases
ICF volume decreases
What happens when there is a loss of salt in the ECF?
ECF volume decreases
ICF volume increases
What happens when there is a gain/loss of isotonic fluid?
No change in fluid osmolarity but change in ECF volume
Give an example of an isotonic fluid
0.9% saline
Regulation of ICF volume is vital for long term blood pressure control. T/F
False - ECF volume
Give two reasons why electrolyte balance is important?
Electrolyte concentrations affect water balance
Concentrations of individual electrolytes affect cell functioning
What are the two most important electrolyte concentrations to maintain?
Sodium and potassium
What is an electrolyte?
A substance which dissociates into free ions when put in solution
What is sodium a major determinant of in terms of fluid balance?
ECF volume (water follows salt)
Sodium is an accurate measure of the balance between input and output of water. T/F
True
Minor fluctuations in plasma potassium can have serious affects. T/F
True
Why must potassium concentration be so tightly regulated?
It is important for the membrane potential (K plasma)
What are the two effects of deranged plasma potassium concentration?
Muscle weakness –> paralysis
Cardiac irregularities –> cardiac arrest
What are the inputs and outputs of salt balance?
Input - diet
Output - sweat, faeces & urine
How does salt imbalance manifest?
Changes in extracellular volume
