Lecture 3: Body fluid compartments

Question 1

Describe roughly the composition of people with water:

Answer 1

Babies = 70%
Men = 60%
Women = 50%
Elderly = 50%
Fat people = less

Question 2

Describe the water compartments:

Answer 2

ICF (2/3)

ECF (1/3) = Interstitial fluid (80%) and plasma (20%)

Question 3

All membranes of the body are permeable to water except:

Answer 3

Kidneys, ureters and bladder

Question 4

What drives water movement?

Answer 4

Water moves from low to high concentration of osmotically active molecules

Question 5

What is osmolality?

Answer 5

Osmolality is used clinically and is the number of osmotically active particles per a unit weight of solvent…. osmoles per kg of water

Osmolarity is number of osmotically active particles per litre of total solution (Osmol/L)

Molal and molar are very similar between the two i.e theyre much of the same.

Question 6

What is tonicity?

Answer 6

NOT OSMOLALITY

Describes the osmotic pressure a solute exerts across a cell membrane (thereby causing movement of water)

Question 7

What does tonicity account for?.

Answer 7

Accounts ONLY for osmotically active IMPERMEABLE solutes i.e proteins rather than all osmotically active solutes.

• In reference to a particular membrane (osmolality is indp. of a membrane)
• Not measurable

Question 8

Describe the permeability of the plasma membrane and how this relates to tonicity:

Answer 8

Plasma membrane of cells is semi permeable and thus is permeable to water but NOT permeable to charged molecules

i.e cells are full of proteins which are osmotically active but impermeable to the membrane

Question 9

Define hypotonic, isotonic, hypertonic

Answer 9

Hypotonic: Makes cells swell (water moves into the cells)
Isotonic: Water equilibrium, cell remains same
Hypertonic: Makes cells shrink (Water moves out of the cell)

Question 10

What is the gibbs-donnan equilibrium:

Answer 10

In the presence of a non-diffusible ion i.e protein, charged particles can fail to distribute evenly across a semi-permeable membrane

Question 11

What is the gibbs-donnan equilibrium responsible for?

Answer 11

Competing electrical and concentration gradients mean that at equilibrium the side with the proteins is more negatively charged = voltage gradient

More osmotically active molecules are on the protein side (greater osmolality) therefore water flows into the protein side (oncotic pressure)

Question 12

How do cells deal with the osmotic pressures across their membrane?

Answer 12

Na/K ATPase

Intersitial and ICF are isotonic because of this

Question 13

How does ECF and ICF osmolality compare?

Answer 13

Different compositions but osmolality identical

ICF: Lots of negatively charged protein and K+
ECF: Lots of Na

Question 14

What does hypotonic ECF cause?

Answer 14

Cells to swell via osmosis

Question 15

What is ECF osmolality dominated by?

Answer 15

Na

Question 16

What does ECF hypertonicity cause? and what is the implication of this

Answer 16

Cells to shrink via osmosis

ECF osmolality control is critical. ECF osmolality is largely regulated by altering water levels

Question 17

What is the variation of osmolality of the ECF?

Answer 17

1-2%, tightly regulated via water

Question 18

Is ECF volume tightly controlled?

Answer 18

Less tightly controlled, 15% varitiation….

i.e gain Na = gain water and vice versa

Question 19

What is the major regulator of salt and water? and what is an additional regulator..

Answer 19

The kidney i.e ECF osmolality and volume

Additional: Starling forces = movement b/w vasculature and ECF, oncotic and osmotic pressures..

Question 20

What is an example of abnormal interstitial fluid compartment expansion?

Answer 20

Oedema, localised or general - results from a change in starlings forces

Question 21

Describe the balance of water for intake and output; and the role of urine in this:

Answer 21

Input is completely balanced by output

Urine output and osmolality varies to balance water and salt levels

Question 22

What happens following a five day high salt diet?

Answer 22

Salt excretion is delayed compared to intake therefore water is retained to maintain ECF osmolality

Question 23

Summise the impacts of a five day high salt diet:

Answer 23

• Transient increase in plasma osmolality
• Increased renal salt excretion (but few days lag)
• Increased thirst
• Plasma osmolality returns to normal but at expense of larger ECF volume
• Larger ECF continues while high salt diet continues
• ECF volume returns to normal if less Na is ingested or renal Na excretion increases i.e diuretic

Question 24

Describe how increased ECF volume contributes to Na regulation

Answer 24

• As ECF volume increases, BP increases and renal Na increases (Pressure natriuresis) (helps resotre ECF volume back to baseline)

Question 25

Describe the rate of clearance of free water vs saline

Answer 25

Free water is cleared very rapidly,

saline takes a long time to be cleared.

Question 26

What is the value of giving 5% dextrose?

Answer 26

Glucose is metabolised or becomes bound to glycogen. So infusing 1L of 5% dextrose will ultimately dilute all compartments (as it causes water to be shifted into cells)

Question 27

Whats the value of isotonic saline?

Answer 27

Temporarily expands ECF (doesnt go into ICF because isotonic) (given to expend circulating volumes and replace actual fluids losses)

Question 28

What is the value of hypotonic saline fluids such as 0.45fi NaCl?

Answer 28

Hypotonic saline fluids such as 0.45% NaCl dilute the ECF first then expand the intracellular compartment causing cells to swell.

Question 29

In summary what regulates osmolality?

Answer 29

• Regulated by renal water handling
• Tightly regulated
• Controlled by ADH

Question 30

In summary what regulates ECF volume?

Answer 30

• Regulated by renal Na handling (b/c osmolality is roughly constant, thus Na (in) = Na (out) to maintain volume
• Varies continuously (15%)
• Controlled principally by the renin-angiotensin system and symp. nervous system