S1: Body Fluids

Question 1

What percentage of our body weight is water?

Answer 1

60%

Question 2

What are the 2 reasons water is important?

Answer 2

Acts as a medium for nutrients and waste products

For diffusion of nutrients and water products

Question 3

Stable volume and composition of the body fluids is essential for metabolism (homeostasis)

How does water content vary?

Answer 3

Decreases with elderly

Increases with increased body fat

Question 4

State some ways how we gain and lose water

Answer 4

• we intake water through ingested fluids
• we intake water through metabolism
• we loose water primarily by urine (main regulator of fluid loss)
• water loss from skin/lungs
• loss of water through faeces and sweat

Intake of fluids should match output

Question 5

What is the 60 40 20 rule?

Answer 5

60% of body is water

40% is in the intracellular fluid
20% is in the extracellular fluid

Question 6

What is the barrier between blood vessel and interstitium?

Answer 6

Capillary wall

Question 7

What is the barrier between intracellular fluid and interstitium?

Answer 7

Plasma membrane

Question 8

Explain the different in permeability of the cell membrane and capillary wall?

Answer 8

CELL MEMBRANE
semi-permeable
-allows water only
It is impermeable to most ions and proteins but has pumps to AT molecules. Water can move freely usually through channels.
💧Osmotic pressure drives this movement of water

CAPILLARY WALL
More permeable
- allows water, electrolytes but not proteins
It is made of endothelial cells, only large molecules (e.g. proteins) can exert an osmotic pressure across it.
💧Hydrostatic pressure drives movement of water.

Question 9

What two factors affect the movement of between compartments?

Answer 9

Pressure

• ionic pressure
• osmotic pressure

The nature of the barrier

• cell membrane
• capillary wall

Question 10

Does the ECF or ICF have more protein?

Answer 10

ICF: Rich in protein
ECF: Poor in protein

Question 11

Compare the ionic compositions of ICF and ECF

Answer 11

ECF——
High in sodium (150mM)
High in chloride (120mM)
Some bicarbonate (26mM)

ICF——
Low in sodium (15mM)
Minute chloride (<1mM)
High in K+ (140mM)
High in phosphate

Question 12

Define diffusion

Answer 12

Movement of molecules down a concentration gradient due to random motion

Question 13

Define osmosis

Answer 13

Net diffusion of water across a selectively permeable membrane, from region of high water concentration to a region of lower concentration

Question 14

What is an osmole?

Answer 14

This is a unit of measurement to describe the concentration of solute particles

1 osmole= 1 mole= 6.02x10^23 osmotically active particles

Examples:
1 mol of glucose= 1 osmole (doesn’t dissociate in solution)
1 mol of NaCl= 2 osmole (Na+ and Cl-)
1 mol of sodium sulphate (2Na+ and SO4 2-)

Question 15

What’s the difference between osmolality and osmolarity?

Answer 15

Osmolality: osmole/kg of water

Osmolarity: osmole/L of solution

Question 16

What is tonicity?

Answer 16

Tonicity describes the effect of a solutions concentration on osmotic movement of water across a membrane (while osmolality is independent of any membrane).
Tonicity is influenced only by solutes that cannot cross the membrane, as only this can exert an effective osmotic pressure.

Question 17

What is osmolality?

Answer 17

It is a count of the number of particles present in a solution and is independent of the size or weight of the particles

Question 18

What is tonicity?

Answer 18

It is the effective osmolality and is equal to the sun is the concentrations of solutes which have the capacity to exert an osmotic force across the membrane

Question 19

What is isosmotic?

Answer 19

It refers to solutions that have the same number of dissolved particles/kg water

E.g.
300mmol glucose=300 mol urea= 150mmol NaCl
All have the same osmolarity

Question 20

What is isotonic?

Answer 20

It refers to solutions that do not cause changes to cell volume

Question 21

Explain the relationship of osmolality and tonicity

Answer 21

Osmolality is a property of a particular solution and is independent of any membrane

Tonicity is a property of solution in reference to a particular membrane

Question 22

What is the dilution method equation?

Answer 22

Concentration = mass/volume

Question 23

List some directly measured volumes

Answer 23

Total body water :radioactive water distributed in all compartments

Extracellular volume: radioactive inulin, excluded from cells

Plasma volume: radioactive albumin stays in plasma

Question 24

List 2 indirectly measures volumes

Answer 24

Intracellular volume: TBW-extracellular volume

Interstitial volume: extracellular volume - plasma volume