3 - Renal Physio: Body Fluid Compartments

Question 1

What percentage of humans is water?

Answer 1

60-70%
Depends on sex, age and body fat 
Woman are 50% water
Men are 60% water
Elderly are 50% water 
Babies are 70% water

Question 2

What are the 2 water compartments?

Answer 2

Intracellular fluid and extracellular fluid (interstitial fluid and plasma)

Question 3

How is water distributed in a 70kg man?

Answer 3

• 60% water
• 2/3 ICF
• 1/3 ECF
  > 80% ISF
  > 20% plasma

Question 4

What is the most osmotically active particle

Answer 4

Na+

Question 5

Difference between osmolality and osmolarity

Answer 5

Osmolarity: Per L of solution
Osmolality: Per kg of solvent

Question 6

What is tonicity?

Answer 6

Tonicity describes the osmotic pressure a solute exerts across a cell membrane and therefore causes the movement of water. It only accounts for the osmotically active particles in a solution that are impermeable to a membrane. It is NOT readily measureable and is a property of the solution WITH reference to a membrane

Question 7

Describe the cell membranes permeability

Answer 7

Semi - permeable to water but not charged molecules

Question 8

Hypotonic, isotonic, hypertonic

Answer 8

Hypo: The solution has less osmotically active particles than the cell so water moves into the cell and causes it to swell
Iso: Cells stay the same size (water moves in and out at the same rate)
Hyper: Makes cells shrink

Question 9

What is the Gibbs Donnan Equilibrium

Answer 9

Describes how charged particles separated by a semi-permeable membrane can fail to distribute evenly across the membrane in the presence of a non-diffusible ion

Question 10

Primary way cells balance the osmotic pressures across their membrane so they don’t burst?

Answer 10

Na/K ATPase (pump out osmotically active particles). Net result is that there is K and proteins inside the cell that balances the Na outside of the cell. Results in the interstitial fluid and the ICF being isotonic (the opposite concentration gradients of Na+ and k+ balance the osmotic pressures

Question 11

Hypotonic ECF

Answer 11

Drinking lots of water. Causes cells to swell

Question 12

Hypertonic ECF

Answer 12

Causes cells to shrink

Question 13

How much does osmolarity vary

Answer 13

1-2% (essentially Na conc)

Question 14

ECF volume?

Answer 14

Much less tightly regulated. 15% range, Depends mostly on amount of Na

Question 15

What regulates maintenance of compartment sizes?

Answer 15

The kidney is the major regulator of both water and salt homeostasis i.e. both ECF osmolarity and volume. Starling forces are also important for governing the movement of fluid across compartments.

Question 16

Oedema?

Answer 16

Is abnormal expansion of the interstitial fluid compartment (ECF). It results from changes in the starlings forces in the plasma leading to movement of fluid into the interstital space. Can be caused by inflammation. lymphatic or venous obstruction, sodium retention, low serum albumin (fluid is moving form the plasma to the interstitial space)

Question 17

What is our daily intake and output of fluids?

Answer 17

INTAKE
- drink 1.2L
- food 1L
- Metabolism 0.3L (2.5L)
OUTPUT
- insensible (breathe) 700mL
- sweat 100mL
- faeces 200mL
- urine 1.5L

Urine output and osmolarity (CONC) varies in order to balance salt and water levels and maintain homeostasis and regulate blood volume, pressure etc.

Question 18

What happens when you consume a high salt diet for 5 days?

Answer 18

• ECF osmolarity increases and there is weight gain due to water retention in the ECF in order to maintain constant ECF osmolarity
• salt excretion increases but lags behind the increase in salt intake but eventually a new balance/equilibrium is reached where intake = excretion
• will feel thirsty

Question 19

Give a summary of what happens when we consume a high sodium diet

Answer 19

• there is a transient increase in plasma osmolarity
• consequently there is increased renal salt excretion (lags few days)
• increased thirst; water retained to maintain ECF osmolality proportional to the extra salt load
• plasma osmolality returns to normal but at the expense of a LARGER ECF VOLUME
• the larger ECF volume continues while the high na diet continues
• ECF volume will return to normal if less na is ingested or renal na excretion increases i.e. by a diuretic
• as ECF volume increases, bp increases and renal sodium loss increases (‘pressure natriuresis’ i.e. increased bp increases renal bf and so na excretion) and this helps to restore ECF volume back to baseline

Question 20

Give 1L water IV fluids?

Answer 20

• dilute ECF and reduce osmolality
• kidney responds by excreting some of that water to maintain osmolality within 1-2% (responds quickly)
• urine production/output increases within minutes

Question 21

Give 1L 0.9% NaCl

Answer 21

• isotonic
• urine output doesn’t change much
• haven’t really altered osmolality, only volume which can change 10-15%
• get fluid retention and increase in BV, isn’t immediately excreted

Question 22

5% dextrose?

Answer 22

• 280 mOsm glucose = isosmotic
• glucose is metabolised to water or becomes bound to glycogen and so infusing 1L of 5% dextrose will ultimately dilute all body compartments and is essentially adding water to the system

Question 23

IV fluids summary?

Answer 23

• isotonic saline fluids temporarily expand the EC compartments
• hypotonic solutions like 0.45% NaCl expand the IC compartment and so cells swell and you need to be careful with this

Question 24

Osmolality vs ECF volume summary

Answer 24

Osmol
- regulated by renal WATER handling
- tightly regulated within 1-2% largely by ADH
Vol
- regulated by renal SODIUM handling
- varies continuously (10-15%) in order to keep osmol constant controlled largrely by RAAS system and the sympathetic nervous system