Movement of Ions and Body Fluid Dynamics

Flashcards covering ion movement, osmosis, osmolarity, tonicity, fluid shift disturbances, Gibbs-Donnan effect, and membrane potentials based on Mrs Sinkala's lecture notes.

What is the definition of osmosis?

Osmosis is the diffusion of solvent molecules into a region in which there is a higher concentration of a solute to which the membrane is impermeable.

How is one osmol ($1\,Osm$) calculated?

It equals the gram-molecular weight of a substance divided by the number of freely moving particles that each molecule liberates in solution.

What is the key difference between osmolarity and osmolality?

Osmolarity is the number of osmoles per liter of solution (e.g., plasma), whereas osmolality is the number of osmoles per kilogram of solvent.

Why is osmolarity used instead of osmolality when assessing osmotic pressure in body fluids?

Because measuring kilograms in water is difficult, and the difference between osmolality and osmolarity in body fluids is negligible.

What is the average freezing point of normal human plasma and its corresponding plasma osmolarity?

The freezing point averages $-0.54\,^\circ\text{C}$, which corresponds to a plasma osmolarity concentration of $290\,mosm/L$.

Why does an initially isotonic $5\%$ glucose solution have the net effect of infusing a hypotonic solution?

Because the glucose is metabolized after intravenous infusion, leaving behind water without the solute.

How many milliosmoles ($mOsmol$) are represented by $1\,mmol$ of sodium citrate ($Na_3C_6H_5O_7$)?

$1\,mmol$ of sodium citrate represents $4\,mOsmol$ ($3\,Na^+ + C_6H_5O_7$) of total particles.

What is the actual measured osmolarity of a $0.9\%$ $NaCl$ injection, and why is it not $300\,mOsmol/L$?

The actual measured osmolarity is about $286\,mOsmol/L$ because interionic bonding forces cause the dissociation factor $n$ to be slightly less than $2$.

Which plasma components contribute all but about $20\,mOsm$ of the $290\,mOsm/L$ in each liter of normal plasma?

$Na^+$ and its accompanying anions, principally $Cl^-$ and $HCO_3^-$.

Why do plasma proteins contribute less than $2\,mOsm/L$ to total plasma osmolarity despite their high concentration in grams per liter?

Because of their very high molecular weights.

How is concentration expressed in milliequivalents per liter ($mEq/L$) for divalent ions?

$\text{mEq/L} = 2 \times \text{mmol/L}$.

What accounts for the small ionic composition differences between plasma and interstitial fluid?

The Gibbs-Donnan effect of plasma proteins, which causes interstitial fluid to have a slightly higher concentration of small anions (such as $Cl^-$) and a slightly lower concentration of small cations (such as $Na^+$) than plasma.

What defines the Gibbs-Donnan effect?

The unequal distribution of permeable charged ions on either side of a semipermeable membrane occurring in the presence of impermeable charged ions.

What would happen to cell volume and resting membrane potential if a Donnan equilibrium were to become fully established?

The cell would swell due to the osmotic influx of water, and the resting membrane potential would be only about $-20\,mV$.

What active mechanism maintains the resting membrane potential in living cells to prevent a full Donnan equilibrium?

Constant $Na^+/K^+$ ATPase activity, along with selective membrane permeabilities for potassium and sodium.

Which equation is usually a better choice than the Nernst equation for explaining the electrochemical behavior of biological membranes?

The Goldman-Hodgkin-Katz equation.

What are the equilibrium potentials for $Na^+$, $K^+$, and $Cl^-$ based on standard internal and external concentrations given in the lecture?

The equilibrium potential for $Na^+$ is $+60\,mV$, for $K^+$ is $-90\,mV$, and for $Cl^-$ is $-70\,mV$.

Which clinical conditions cause hyperosmotic volume contraction?

Sweating, fever, and diabetes insipidus.

What are the changes in ECF volume, ICF volume, and plasma protein concentration during diarrhea (isosmotic volume contraction)?

ECF volume decreases, ICF volume shows no change, and plasma protein concentration increases.