Cell volume regulation and water

Question 1

Body composition of water

Answer 1

60% of body weight is water

40% of body weight is ICF

20% of body weight is ECF (75% interstitial and 25% plasma)

Question 2

Main solutes in ICF and ECF

Answer 2

ICF: K+, Mg2+, proteins, phosphates (ATP, ADP, AMP)

ECF: Na+, Cl-, HCO3-

Question 3

Why does body water as a percentage of body weight vary?

Answer 3

Because of variations in adipose tissue (due to sex or age)

Lean (fat-free) body weight is always 73% water

Males have higher percent of body weight as water

Question 4

Van’t Hoff Equation

Answer 4

pi = RT x osmolality gradient

RT = 19.3 mmGh/mOsm/kgH2O

Relationship between osmolality and osmotic pressure

Question 5

Difference between molarity and molality

Answer 5

Molarity: 1 mole glucose into flask then add water to get UP TO 1 L mark

Molality: 1 mole glucose into flask then add 1 L water

Question 6

What is an ineffective osmole?

Answer 6

Urea is an ineffective osmole because it does not affect the cell volume

Urea diffuses freely into and out of cell so can equilibrate to increase osmolality without changing volume

Question 7

Ideal osmometer

Answer 7

Cell changes volume when osmolality changes and maintains that new volume

Question 8

Non-ideal osmometer

Answer 8

Initial response to change in osmolality can be changed by regulatory volume decrease or regulatory volume decrease

In hypotonic solution: swell first, then dump out solute to shrink

In hypertonic solution: shrink first, then gain solute to swell again

Question 9

Mechanism for regulatory volume decrease (RVD)

Answer 9

Stretch sensitive Ca2+ channels cause increased Ca2+ –> Ca2+ is second messanger that activates K+ and Cl- channels –> KCl leaves the cell

Question 10

Mechanism for regulatory volume increase (RVI)

Answer 10

1) Short term: NaCl uptake
2) Long term: metabolic generation of AAs, sugal alcohols, trimethyl amines (all organic solutes)

Question 11

Why would it be bad for skeletal muscle to do RVD?

Answer 11

Skeletal muscle is a good osmometer

Skeletal muscle has 70% K+in the body, so dumping of K+ would be enough to raise extracellular [K+] enough to cause massive depolarization of the heart, arrhythmias

Question 12

Why is it good that the brain can do RVD?

Answer 12

Don’t want brain cells to swell because that can impair cerebral blood flow, or compress structures

Amount of K+ released in order to do RVD is insignificant to the body as a whole

Brain resists osmotic expansion by (1) squeezing interstitial fluid out of the brain and back into blood and CSF and (2) loss of K+ then loss of glutamine

Brain resists osmotic shrinkage by increasing K+ cell content and accumulating taurine, myoinositol (slow responses)