Body Fluid Compartments

Question 1

What proportion of the human body is made up of water?

Answer 1

60-70%
Varies with age, sex, body fat
Higher in babies, lower in elderly, lower in females, lower in increased weight due to fat being hydrophobic.

Question 2

Where in the human body is water located, and what quantity is located at each of the areas?

Answer 2

60% water (42 litres)
2/3 intracellular fluids (28L) + 1/3 extracellular fluid (14L)

Intracellular fluids are within cells

Extracellular fluids are outside cells
4/5 80% interstitial fluid (11L) + 1/5 20% plasma (3L)

Question 3

What are the two ways that water moves between compartments?

Answer 3

1. All the membranes in the body are permeable to water (except kidneys, ureters, and bladder).
2. Water flows to higher concentrations of osmotically active molecules (osmotically active molecules attract H2O)

Question 4

Explain osmolality, in 3 points.

Answer 4

1. Osmolality is the number of osmotically active particles per unit weight of water.
2. The units are milli-osmoles/Kg of solvent (eg water) - 13 solute particles per 1kg water.
3. Osmolality determines the osmotic pressure exerted by a solution across a membrane.

Question 5

Explain tonicity, in 3 points.

Answer 5

1. Describes the osmotic pressure a solute exerts across a cell membrane (thereby causing movement of water).
2. Tonicity is the relative concentration of 2 environments separated by a semipermeable membrane, thereby determining the direction and extent of diffusion.
3. Tonicity is not readily measurable.

Question 6

What is the main difference between osmolality and tonicity?

Answer 6

Osmolality is a property of a particular solution, independent of a membrane.
Tonicity is a property of a solution in reference to a particular membrane.

Question 7

Describe the plasma membrane of cells.

Answer 7

Semi-permeable membrane which is permeable to water but not permeable to charged molecules.

Question 8

What is hypotonic?

Answer 8

Solution has fewer osmotically active particles than the cells - so water moves in to cells, making them swell.

Question 9

What is isotonic?

Answer 9

Solution has equal osmotically active particles to the cells - so cells stay the same size, water moves in at the same rate it moves out.

Question 10

What is hypertonic?

Answer 10

Solution has more osmotically active particles than the cells - so water moves out of the cells causing them to shrink.

Question 11

Discuss the relationship between cells and proteins?

Answer 11

Cells are full of proteins which are osmotically active (mostly negatively charged) but impermeable to the membrane.

Question 12

Explain the Gibbs-Donnan Equilibrium

Answer 12

Charged particles (ie. salts) separated by a semi-permeable membrane can fail to distribute evenly across the membrane in the presence of a non-diffusible ion (ie. proteins)

Question 13

How do ions flow?

Answer 13

1. Negative ions want to move down their concentration gradient.
2. Positive ions want to follow negative ions (to balance charge).

Question 14

Discuss the voltage gradient?

Answer 14

Competing electrical and concentration gradients mean that at equilibrium the side with the proteins is more negatively charged - so, because it’s more negative in cell than outside and so that generates the voltage gradient.

Question 15

Discuss oncotic pressure?

Answer 15

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

Question 16

Cells need to balance the osmotic pressures across their membrane or they burst - how is this achieved?

Answer 16

Na+/K+ ATPase transporter pumps out osmotically active ions (Na+).
The net result is K+ and proteins inside balance Na+ outside - so interstitial fluid and ICF are isotonic (fluid is balanced).
Osmolality is balanced despite different composition between the compartments - inside the cell has lots of negatively charged protein and K+, while outside the cell has lots of Na+.

Question 17

What is it when there are less osmotically active molecules outside the cell (ECF) than inside?

Answer 17

Hypotonic - so the cells will swell.

Question 18

What is when there are more osmotically active molecules outside the cell than inside?

Answer 18

Hypertonic - so the cells will shrink.

Question 19

Why is it critical to control ECF osmolality?

Answer 19

Cell survival - there can only be a 1-2% variation in osmolality.

Question 20

What is the primary determinant of ECF volume?

Answer 20

Na+ (the major osmotically active solute in the ECF).

Question 21

What are the three methods of maintenance compartment size?

Answer 21

1. Kidneys - major regulator of water and salt homeostasis.
2. Starling Forces.
3. Oedema - abnormal expansion of the interstitial fluid compartment.

Question 22

Identify the main components of water intake, and the volume for adults?

Answer 22

1. Drinking 1200ml
2. In food 1000ml
3. Metabolism 300ml
   TOTAL 2500ml

Question 23

Identify the main components of water output, and the volume outputs for adults?

Answer 23

1. Insensible (breathing, lungs) 700ml
2. Sweat 100ml
3. Faeces 200ml
4. Urine 1500ml
   TOTAL 2500ml

Question 24

Intake is completely balanced by output, how is this?

Answer 24

Urine output and urine osmolality varies to balance water and salt levels.

Question 25

What occurs in the body when on a high sodium diet for 5 days?

Answer 25

1. Increase in plasma osmolality.
2. Increased renal salt extraction (with a few days lag)
3. Increased thirst (water retained to maintain ECF osmolality proportional to extra salt loaded).
4. Plasma osmolality returns to normal, due to larger ECF volume.
5. ECF volume returns to normal if less Na+ ingested or renal Na+ excretion increases (eg. diuretic).
6. As ECV volume increases - BP increases, and renal Na+ loss increases (pressure natriuresis. Helps restore ECF volume back to baseline.

Question 26

Fee water (no salt) is cleared rapidly, saline is regulated much more slowly - discuss this in relation to osmolality, outflow, and ECF.

Answer 26

With water ECF osmolality is diluted, but outflow (urine) increases.
With saline ECF osmolality is unchanged, and outflow of urine is only marginally increased.

Question 27

Is glucose salt?

Answer 27

No, glucose is not salt.

It is metabolised to water or becomes bound to glycogen.

Question 28

What does isotonic saline fluid do?

Answer 28

Isotonic saline fluids temporarily expand the extracellular compartment.

Question 29

What does hypotonic saline fluid do?

Answer 29

Hypotonic saline fluids expand the intracellular compartment.

Question 30

Discuss the regulation of osmolality, 3 points?

Answer 30

1. Regulated by renal water handling
2. tightly regulated (1-2%)
3. Controlled by antidiuretic hormone

Question 31

Discuss the regulation of ECF volume, 3 points?

Answer 31

1. Regulated by renal Na+ handling
2. Varies continuously (15%)
3. Controlled principally by the renin-angiotensen system and sympathetic nervous system.

Question 32

How do you maintain a constant ECF, 2 points and 1 example?

Answer 32

1. The amount of Na+ excreted needs to match Na+ input by the diet.
2. The osmolality of the ECF is kept near constant, the amount of Na+ in the body determines the volume of the ECF.
3. “If you eat more salt you retain more water”