Water Transport and Cell Volume

Question 1

How do the driving forces for water movement differ across plasma membrane vs capillary endothelium?

Answer 1

Plasma membranes do not have the mechanical strength to withstand physical differences in pressure, thus, we only consider osmotic pressure due to difference in concentration of water.

We also factor in hydrostatic pressure due to the heart pumping when talking about capillary endothelium.

Question 2

What are two ways water moves across the membrane, and when is the faster one preferred?

Answer 2

1. Diffusion between spaces in phospholipids

2. Diffusion via pore (aquaporin) - bulk flow is preferred when there is a concentration gradient for water

Question 3

What is the osmotic concentration? How does this vary for osmolarity vs osmolality?

Answer 3

Osmoles solute / volume of solution

Osmolarity - moles solute / liter solution

Osmolality - moles solute / kg water

Osmolality is preferred because proteins take up space in plasma and in cells

Question 4

How is osmotic concentration related to water concentration?

Answer 4

As osmotic concentration goes up, water concentration goes down.

Question 5

How is osmotic concentration calculated?

Answer 5

Osmotic coefficient (nonideal) * i * C

i = number of moles per mole dissolved
C = molal concentration (moles / kg water)

Question 6

What is the threshold for hyper osmolal vs iso-osmolal vs hypo-osmolal?

Answer 6

290 mOsm. How the solution’s osmotic concentration compares to this

Question 7

How is osmotic pressure defined?

Answer 7

The amount of force a piston must apply to a chamber of a given osmolality in order to make the net flow of water across a semi-permeable membrane equal to zero, when the solution is separated from pure water

Question 8

How do you calculate osmotic pressure?

Answer 8

Pi = Osmotic concentration * R * T

Pi = osmotic pressure
R = Gas constant 0.082 Latm/molK
T = absolute temp in Kelvin

Question 9

What is oncotic colloid pressure?

Answer 9

Hydrostatic pressure due to presence of protein

Question 10

What happens when a solute is added that can cross the membrane vs cannot?

Answer 10

Cannot: If added outside, cell will shrink to reach equilibrium

Can: Cell will initially shrink, but once the solute diffuses across, cell will increase in size again

Question 11

What is tonicity? What are the three tonicity states of extracellular solutions?

Answer 11

Effective osmolality, it takes into consideration whether a solute is permeable to the barrier or not.

Urea would be counted in total osmolality, but not effective osmolality

Extracellular states:
Hypertonic: Equilibrium cell volume will decrease
Hypotonic: Equilibrium cell volume will increase
Isotonic: Equilibrium cell volume stays the same

Question 12

What are major clinical causes of changes in tonicity of ECF?

Answer 12

hyponatremia or hypernatremia (changes in plasma Na+)
hyperglycemia (plasma glucose)

Urea can be increased in kidney failure, causing death

Question 13

What is the function of the Na,K-ATPase relative to cell volume maintenance? What happens when it is impaired?

Answer 13

Keeps the membrane effectively impermeable to sodium, with sodium staying outside of the cells.

Na+ is the only solute with high enough concentration to balance effect of impermeable anions, so sodium will rise if it is impaired, and the cell will swell.

Question 14

How does Na,K-ATPase vary?

Answer 14

The higher the intracellular sodium levels, the more it acts

Question 15

What cells exhibit regulatory volume decrease or increase?

Answer 15

Intestinal enterocytes, for example. Both of these are acute responses (seconds to minutes) to changes in plasma osmolality.

Question 16

What is the long-term response to hyperosmolality?

Answer 16

Over hours to days, cells accumulate organic solutes (osmolytes) including alcohol derivatives of sugar, and amines. These are synthesized or transported into the cell to prevent cell shrinkage, and is especially important in brain cells.

Question 17

What happens to the brains of the elderly during dehydration?

Answer 17

There is an increase in plasma osmolality, causing brain cells to shrink, tearing blood vessels and causing hemorrhage. Brain cells will respond by accumulating organic osmolytes to restore brain volume over a few days

Question 18

Why should a patient not be rehydrated too quickly?

Answer 18

Accumulated organic osmolytes in brain cells will lead to cell swelling if the patient is hydrated too quickly, potentially leading to brain swelling (cerebral edema) and herniation of the brain stem.