Water Transport

Question 1

Explain the principles and concepts of water transport (osmosis).

Answer 1

Osmosis = water moving down its concentration gradient. Aquaporins are membrane proteins that are water channels & work via facilitated diffusion (passive transport). Simple diffusion of water without membrane protein interactions may also occur (although this is much slower than facilitated diffusion). Osmosis is driven by particle number, not particle size.

Question 2

Explain the principles and concepts of cell volume regulation.

Answer 2

Isotonicity = isoosmolarity, hypertonicity = hyperosmolarity, hypotonicity = hypoosmolarity (RBC’s crenate in this solution).

Question 3

Where solute concentration is high & osmolarity is high, water concentration is?

Answer 3

LOW. Water concentration is inversely proportional to solute concentration & osmolarity.

Question 4

Osmolarity definition & normal value

Answer 4

Osmolarity is the collective sum of all of the solute concentrations in the solution (both intra & extracellular). Osmolarity is equal inside & outside the cell at an osmolarity of 300 mili osmoles (normal).

Osmosis is the movement of water across a semipermeable or water permeable membrane from a less concentrated solution (lower osmolarity) to a more concentrated solution (higher osmolarity).

Question 5

What is the distribution of total body water, intracellular fluid, & extracellular fluid.

Answer 5

60% body weight, 40% body weight-intracellular & 20% body weight-extracellular

ICF is 2/3 of TBW & the ECF is 1/3 TBW

Question 6

Osmotic pressure

Answer 6

The hydrostatic pressure develops in a compartment of higher osmolarity where water has rushed into. The pressure is sufficient to stop the net flow of water into the right compartment = osmotic pressure difference of the solutions in the two compartments. Osmotic pressure can also be viewed as the amount of pressure that would need to be applied to the vessel to force water back into the original vessel. Concentrated solutions have higher osmotic pressure.

Question 7

Electrogenic

Answer 7

A transporter that leads to the net translocation of charge across the membrane is said to be electrogenic. For example, with a coupling stoichiometry of 2 Na+ ions to 1 glucose/galactose molecule, Na+/glucose cotransport by SGLT1 represents an electrogenic process.

Question 8

Electroneutral

Answer 8

When no net charge is transported across the membrane per transport cycle, the process is said to be electroneutral.

Question 9

ADH

Answer 9

Stimulates thirst & triggers water retention by the kidneys.

Question 10

Which of the following statements is true for an uncharged solute (S) that enters a cell by facilitated diffusion concerning extracellular & intracellular [S] @ equilibrium?

Answer 10

At equilibrium, the extracellular [S] will be equal to the intracellular [S]. @ equilibrium, there is no net flux of S.

Question 11

The most rapid form of transport over a distance greater than 1 cm is:

Answer 11

bulk flow (flow due to pressure differences)

Simple diffusion and membrane transport are rapid over short distances. When distances reach 1 cm or greater a significant movement can occur only with pressure differences, which is also referred to as bulk flow.

Question 12

Under steady-state conditions the intracellular osmolarity and extracellular osmolarity are _____.

Answer 12

Under steady-state conditions the intracellular osmolarity and extracellular osmolarity are equal.

If there was a difference, water would diffuse until they were equal.

Question 13

The intracellular compartment does have a _____ concentration of organic anions, mainly proteins, than the extracellular fluid.

Answer 13

The intracellular compartment does have a higher concentration of organic anions, mainly proteins, than the extracellular fluid.

Question 14

Does diffusion require a semipermeable membrane?

Answer 14

No, just a gradient.

Question 15

Which set of the following best illustrates the consequences of the infusion of plasma:

Intracellular volume

Extracellular volume

Extracellular osmolarity

Extracellular sodium conc.

Hematocrit

Answer 15

Intracellular volume- no change

Extracellular volume- increase

Extracellular osmolarity- no change

Extracellular sodium conc.- no change

Hematocrit- decrease

Question 16

An excessive intake of tap water over a short period of time would be expected to produce which of the following:

Extracellular volume

Intracellular volume

Extracellular sodium conc.

Intracellular osmolarity

Answer 16

Extracellular volume- increase

Intracellular volume- increase

Extracellular sodium conc.- decrease

Intracellular osmolarity- decrease

Ingesting excessive amounts of tap water will increase the volume of the extracellular fluid. Since tap water contains few if any electrolytes, it will dilute the extracellular sodium and reduce extracellular osmolarity. The decreased extracellular osmolarity will cause a diffusion of water into cells, increasing intracellular volume but reducing intracellular osmolarity.

Question 17

A young man was found after wandering the Arizona desert for several hours under a hot sun with no source of drinking water. Which of the following solutions given intravenously would best return the individual’s intracellular volume toward normal?

Answer 17

Isotonic saline

The loss of hypotonic fluid due to sweating would produce a hyperosmotic osmolarity and an extracellular and intracellular contraction, given that isotonic saline would lower the individual’s osmolarity and expand the intracellular space. Whole blood (choice C), and plasma (choice D), contain a solute (albumin) that does not penetrate a capillary membrane; thus, less fluid would enter the interstitial space. This is not the case with isotonic saline.

Question 18

A solution is prepared by adding 10g of NaCl (formula weight=58.5) to 1 L of distilled water. An isotonic solution is 300 mOsm. The prepared solution is ____mOsm.

Answer 18

342 mOsmolar

Question 19

Drinking an isotonic saline solution will _____ the ECF volume and _____ the ECF or ICF osmolarity or ____ ICF volume.

Answer 19

Drinking an isotonic saline solution will expand the ECF volume and neither increase nor decrease the ECF or ICF osmolarity or neither increase nor decrease ICF volume.