Body Fluids L03

Question 1

how is transcellular fluid different from interstitial fluid?

Answer 1

it’s separated by a membrane e.g. CSF, peritoneal fluid

molecules must pass through this epithelial barrier to enter/leave compartment

Question 2

what is the total body fluid (tbf) for 70kg person?

Answer 2

42L

Question 3

what percentage of tbf is in intracellular compartments?

Answer 3

35% (28L) *63% extracellular and 2% transcellular

Question 4

what % of tbf is in interstitial compartment?

Answer 4

16% (10L) *more than circulating volume

Question 5

what % of tbf is in plasma?

Answer 5

5% (3L)

Question 6

what % of tbf is in transcellular compartment?

Answer 6

2%

Question 7

what is ascites?

Answer 7

accumulation of fluid in the peritoneal cavity, causing abdominal swelling. causes include infection, heart failure, portal hypertension, cirrhosis, and various cancers.

Question 8

what are the effects of an increase of fluid in the cranium?

Answer 8

-increase in intracranial pressure
> shifting of brain in cavity
> damage to brain tissue
> restriction of blood flow due to closed/damaged cerebral vessels

Question 9

define the volume of distribution (Vd).

Answer 9

the volume of fluid required to contain the total amount of a substance/drug in the body at the same concentration as present in the plasma

or

Vd = Q/Cp *essentially volume = mass/conc

where Q = amount of drug, Cp = plasma concentration of drug (use plasma to measure conc as easiest compartment to access)

Question 10

Why can heparin and insulin be used to measure plasma volume?

Answer 10

heparin - large molecule, confined to plasma as cannot pass through endothelium

insulin - strongly bound to plasma proteins, again does not exit circulating plasma

therefore Vd of these drugs = plasma volume

Question 11

what compartment is Vd of Gentamicin used to measure and why?

Answer 11

extracellular space

very polar molecule so cannot cross membranes into intracellular space (unless there is a protein channel)

Question 12

Ethanol can be used as a measure of total body volume. It is not an accurate measure, why?

Answer 12

ethanol is both water and fat soluble so some ethanol moves into fat - not accurate measurement of fluid volume in body

Question 13

How can we measure total body water?

Answer 13

by using water that is ‘marked’ to distinguish it from unmarked water i.e. use radioactive deuterium (H with a neutrons) or tritium (H with 2 neutrons) instead of normal isotope of H in water. Then measure Vd in normal way.

Question 14

How can we measure plasma volume?

Answer 14

• Vd with substances such as heparin or insulin

- label plasma proteins with dye such as Evan’s Blue (binds to protein) and measure Vd

Question 15

How do we traditionally measure extracellular fluid?

Answer 15

• 36Cl- (some passes intracellularly)
• inulin (polysaccharide)
• thiosulfate/thiocyanate (most accurate)

*these will NOT measure transcellular volumes

Question 16

What percentage of blood volume is plasma?

Answer 16

55%

Question 17

Extracellular concentration of Na+?

Answer 17

140mM

Question 18

intracellular concentration of Na+?

Answer 18

15mM

Question 19

extracellular concentration of K+?

Answer 19

4mM

Question 20

intracellular concentration of K+?

Answer 20

140mM

Question 21

extracellular concentration of Ca2+?

Answer 21

2.4mM *only half of this is free Ca2+ ions

Question 22

intracellular concentration of Ca2+?

Answer 22

0.1microM

Question 23

extracellular concentration of Cl-?

Answer 23

110mM

Question 24

intracellular concentration of Cl-?

Answer 24

around 4mM

Question 25

extracellular concentration of HCO3-?

Answer 25

25mM

Question 26

intracellular concentration of HCO3-?

Answer 26

12mM

Question 27

extracellular concentration of glucose (fasting conc)?

Answer 27

4mM

Question 28

Which protein is Ca2+ bound to in plasma?

Answer 28

albumin (half of Ca2+ bound to protein)

Question 29

What effect will changing albumin concentration (in plasma) have on Ca2+ concentration?

Answer 29

It will change the total Ca2+ plasma concentration without changing the free Ca2+ concentration.

Question 30

Does the body regulate the total Ca2+ concentration in plasma?

Answer 30

no. it regulates the FREE Ca2+ concentration as it is the free Ca2+ that is biologically active.

Question 31

How much albumin is normally present in the plasma?

Answer 31

40g/L

Question 32

Why do physicians use a 'corrected' total Ca2+ measurement rather than total Ca2+?

Answer 32

total Ca2+ values do not take into account the changes in free Ca2+ concentration that arise due to changes in albumin concentration (less albumin (pathologies?) = less Ca2+ binding sites = more free Ca2+). 'corrected' total Ca2+ does take into account these changes and helps clinicians to interpret results based on normal ranges of free total Ca2+ concentration - if albumin concentration decreases, corrected total Ca2+ concentration shown as increasing.

Question 33

What is an osmole?

Answer 33

the number of molecules that a compound dissociates into when dissolved in solution. e.g. 100mM NaCl --> 200mOsm

Question 34

what is osmolality?

Answer 34

number of osmoles per unit mass of solvent (Osm/kg)

Question 35

what is osmolarity?

Answer 35

number of osmoles per unit volume of solution (Osm/L)

Question 36

what is the relationship between osmotic pressure and water potential?

Answer 36

inversely proportional

Question 37

explain what osmotic pressure is

Answer 37

water moves by osmosis from areas of high water potential to areas of low water potential (across a semi-permeable membrane) The force (per unit area) required to oppose the movement of water by osmosis is osmotic pressure. 'the amount of pressure required to oppose osmosis'

Question 38

What two pressures must be equal to stop the movement of water?

Answer 38

hydrostatic pressure (water pressure) and osmotic pressure initially osmotic pressure is high (high water potential gradient so lot of force needed to oppose osmosis) and hydrostatic pressure is low (fluid is concentrated). as water moves, hydrostatic pressure builds up. when two pressures are equal --> point of equilibrium

Question 39

define isosmotic

Answer 39

if two solutions are isosmotic, they share the same osmolality

Question 40

define isotonic

Answer 40

if two solutions are isotonic, there is no net water movement across the membrane

Question 41

why is a plasma solution of urea isosmotic in relation to intracellular fluid but not isotonic?

Answer 41

urea can readily cross plasma membranes providing that the cell membrane expresses a urea transporter. Therefore the plasma solution and intracellular fluid may be isosmotic initially but not isotonic. This is because urea will move down its concentration gradient into cells, increasing intracellular osmotic pressure. As a result, water moves by osmosis into the cell (net movement of water - solutions are not isotonic). Once the urea concentration is equal on both sides of the membrane, the hydrostatic pressure balances and the solutions become isotonic/ in equilibrium. *movement of water can however cause cell swelling and rupture

Question 42

what is oncotic pressure?

Answer 42

the pressure exerted by plasma proteins, drawing water back into the capillary from tissue fluid.

Question 43

what are the causes of fall in plasma albumin?

Answer 43

- liver failure - decreased production | - renal failure - increased loss

Question 44

what are the effects of a fall in plasma albumin?

Answer 44

oedema, ascites

Question 45

in which compartment is oedema a critical problem?

Answer 45

brain

Question 46

How can oedema in the brain be treated?

Answer 46

Mannitol - injected intravascularly > increases osmolality of plasma and extracellular space > 'pulls' water from intracellular and transcellular spaces (e.g. CSF)

Question 47

What is mannitol used for?

Answer 47

- osmotic diuretic - intracranial haemorrhage - decrease intracranial pressure - inhaled for cystic fibrosis management

Question 48

What is the osmolarity of extracellular fluid?

Answer 48

around 285mOsm/kg