Renal- body fluid compartments

Question 1

Describe the water distribution in different body fluid compartments

Answer 1

60% of body weight will be water (42 L of water).
o 40% of body weight will be water in the intracellular fluid (28 L of
water).

o 20% of body weight will be water in the extracellular fluid (14 L).

 80% present in interstitial fluid (11 L).
 20% present in plasma (3 L).

 Tiny bit present in transcellular fluid (e.g. aqueous humour, vitreous humour, synovial fluid).

Question 2

Give 3 markers for measuring body fluid volumes

Answer 2

1.1. Tritiated water
 Penetrates all body water fluid compartments and the total body water can then be measured.
 Takes 2 hours for an even distribution.
 Need to take into account the excreted Tritiated water in urine and sweat (i.e. the amount given – the amount lost).
 Toxic at extremely high doses (unlikely in the clinical setting because it is very expensive).

1.2. Inulin
 Plant polysaccharide which is a marker of extracellular fluid because it has small penetration into cells.
 Kidneys filtrate it so it is lost in the urine.
 Used for measuring GFR.

1.3. Evan’s blue, 125I albumin
 Binds to albumin so can only stay in the plasma. Does not cross the capillary wall.
 Does not enter erythrocytes.
 Has a short half-life (10-30 minutes) so measurements need to be taken quickly

Question 3

What are the equations for calculating dilution principles in vivo

Answer 3

 Beaker: Concentration =
Amount/
Volume

 In vivo: Volume =
Amount given−Amount lost
Concentration after equilibrium

Question 4

Calculate total body water from this example:
60 kg female
 995 μCi (μCurie (measurement of radioactivity)) of H2 3O administered IV.
 2 hours allowed for equilibrium, during which 5 μCi were lost through urine.
 Plasma sample then taken, the activity was 30 μCi/L.

Answer 4

Calculation: Volume =
Amount given−Amount lost
Concentration after equilibrium
=
995 μCi−5 μCi
30 μCi/L
= 33 L

Question 5

In this example work out the plasma volume:
70 kg male
 10 ml of a 1% solution (1 g/100 ml) of Evans blue administered IV.
 After 5 min a sample of blood was taken and the plasma was found to have 0.037 mg of dye per mL of blood.

Answer 5

Calculation: Volume =
Amount/
Concentration
=
100 mg
0.037 mg/ml
= 2702 ml

Question 6

What is the formula for converting plasma volume to blood volume

Answer 6

Blood volume =Plasma volume/ 1−Haematocrit %

Question 7

Describe the composition of different body fluids

Answer 7

 Plasma and
interstitial fluid is
separated by a
semipermeable
capillary membrane.

 Intracellular and
extracellular fluid is
divided by an impermeable cell membrane and requires specific
transporters to transport ions.

 The ion composition of the compartments allows for the movement
of water through these compartments via osmosis.
 High Na
+ in extracellular fluid and high K
+ in intracellular fluid. This
concentration gradient is maintained by Na
+/K
+ ATPase.

 Lots or organic phosphate and proteins produced in the cell (intracellular), and since they are impermeable, there is a high
concentration within the cell.
 To maintain the negative charge within the cell, Cl
− and HCO3
− are maintained in the plasma. These are low intracellularly.

 Low [Ca
2+] in extracellular fluid.
 High [Mg
2+] in intracellular fluid.

Question 8

what is osmosis and how do you measure osmotic pressure

Answer 8

Diffusion of water down its concentration gradient, across a semi-permeable
membrane.
 [Image right] No further osmosis occurs when the hydrostatic pressure, h, is
equivalent to the osmotic pressure of the solution.

The osmotic pressure is the pressure that is just
sufficient to prevent the movement of water into a
solution across a semi-permeable membrane.

Question 9

Define osmolarity, osmolality and osmoles

Answer 9

Rather than measure osmotic pressure directly, it is
more convenient to state the:

o Osmolarity: moles of solute particles per litre of water.
o Osmolality: moles of solute particles per kg of water.
 Unit: osmole. Defined as one gram molecular weight (1 mole) of any non-dissociable substance (such as
glucose).

 The osmolality of the solution is the number of osmoles of solute per kg of solvent. However, NaCl
dissociates in solution. Therefore 1 mole NaCl in 1 kg water has an osmolality of 2 Osmol/kg.

o In medicine, osmolality is preferred because it refers to the weight of the water.

Question 10

Describe the osmolality of the blood

Answer 10

The total osmolality of a solution is the sum of the osmolality due to each of the constituents of the solution.
 Plasma: 295 mOsmol/kg.
o Na
+, Cl
− and HCO3
− contribute to most of this.
o Glucose and other small molecules contribute < 10 mOsmol/kg.
o Plasma proteins contribute only about 1 mOsmol/kg (< 0.5% of total plasma osmolality).

Question 11

Describe tonicity (isotonic, hypotonic and hypertonic) solutions

Answer 11

Tonicity of a solution refers to the influence of its osmolality on the
volume of cells.
 Isotonic solution: concentration of 295 mOsmol/L in solution, therefore
no net movement of water. Erythrocyte maintains normal shape.
 Hypotonic solution: concentration < 295 mOsmol/L in solution, therefore
osmotic pressure is generated, driving water into the erythrocyte.
Erythrocyte increases in size and potentially haemolysis.
 Hypertonic solution: concentration > 295 mOsmol/L in solution,
therefore, water is drawn out of the erythrocyte. Erythrocyte shrivels (crenation).

Question 12

Explain the difference between iso-osmotic vs isotonic

Answer 12

Two solutions with the same osmolality are said to be iso-osmotic.
 Since cell volumes normally remains stable, it suggests that intracellularly fluid must have the same osmolality as the
extracellular fluid. The two fluids are isotonic with each other.
 Fluids which are isotonic are also iso-osmotic.
 However, not all iso-osmotic solutions are isotonic with cells.

Question 13

What is the difference between effective and ineffective osmoles

Answer 13

Effective osmole: a non-permeating solute (e.g. sucrose, NaCl) that cannot cross the cell membrane and is said to be an effective osmole at 295 mOsmol/L.
 Ineffective osmole: a permeating solute (e.g. urea, ethanol) that can cross the cell membrane and is said to be an ineffective osmole at 295 mOsmol/L. The cell will eventually swell and burst.

Question 14

Describe the distribution of different fluids:
1L water
1L of 0.9% saline solution
1L of 5% albumin

Answer 14

1. 1 L of water
    Total body water increases by 1 L:
   o 2/3 into the intracellular fluid: 670ml
   o 1/3 into the extracellular fluid: 330 ml
    80% interstitial fluid: 264 ml
    20% plasma: 66 ml
2. 1 L of 0.9% saline solution
    Same as having a 154 mmol/L NaCl solution. Dissociates and generates a solution containing 300 mOsmol/L solution.
    Na
   + is impermeable to the cell membrane, so it is retained in the ECF, and acts as an effective Osmol. 1 L of fluid is retained
   in the ECF:
   o 80% interstitial fluid: 800 ml
   o 20% plasma: 200 ml
3. 1 L of 5% albumin
    Albumin is too large to cross the capillary membrane. It acts as an effective Osmol, retaining the 1 L of fluid in the plasma
   compartments.
    This is effective in bleeding, to maintain an effective blood pressure.
    1 L of fluid is retained in the plasma: 1 L.

Question 15

What are the different ways fluid can move between the blood and interstitial fluid

Answer 15

1. Mean blood pressure (mmHg)
    Blood pressure is high in the arteries to protect other vessels from
   the pressure generated by the heart.
    There is a rapid decrease in pressure in the arterioles.
    Pressure of 10-40 mmHg in the capillaries drives filtration.
2. Starling forces
   The difference between the hydrostatic and oncotic pressures within the
   capillaries determines whether there is filtration or reabsorption.
    Hydrostatic pressure within the capillaries is generated by the blood
   pressure.
    Hydrostatic pressure within the interstitium.
    Oncotic pressure within the capillary opposes the oncotic pressure in the
   interstitium.
    At any point on the capillary:
   Net filtration pressure = ∆Hydrostatic pressure (H) −
   ∆Oncotic pressure (π) = (Pc − Pi) −(πc − πi)

Question 16

What happens to body fluid movement in a normal situation, haemorrhage, congestive heart failure and liver failure

Answer 16

2.1. Normal situation
 Along the capillary:
o Decrease in hydrostatic pressure.
o Slight increase in oncotic pressure as plasma proteins
become more concentrated.
o Net filtration: hydrostatic pressure > oncotic pressure.
o Net reabsorptionb: hydrostatic pressure < oncotic pressure.

2.2. Haemorrhage
 The hydrostatic pressure within the capillary decreases (decreased
blood pressure).
 The oncotic pressure more quickly opposes the hydrostatic pressure.
This favours more fluid moving into the capillary, increasing blood
volume, at the expense of the interstitial volume.
 As oncotic pressure > hydrostatic pressure for a greater distance of
the capillary, there is net reabsorption of fluid into the capillary.

2.3. Congestive heart failure
 The hydrostatic pressure within the capillary increases (reduced
venous drainage).
 Hydrostatic pressure increases, and opposes the oncotic pressure for
a longer period of time.
 As hydrostatic pressure > oncotic pressure for a greater distance of
the capillary, there is net filtration into the tissue, causing oedema.

2.4. Liver failure
 The oncotic pressure within the capillary decreases (reduced plasma
protein concentration due to the liver not producing protein).
 The hydrostatic pressure opposes the oncotic pressure for a longer
period of time.
 As hydrostatic pressure > oncotic pressure for a greater distance of
the capillary, there is net filtration into the tissue, causing oedema.

Question 17

How does this fluid movement return back to normal?

Answer 17

Normal filtration (20 L/day) at the arteriole end of a capillary exceeds
reabsorption (18 L/day) at the venule end. The difference (2-4 L/day)
is returned to the circulation via the lymphatic system, preventing
accumulation in the interstitium,
which would cause oedema.
 Blockage of lymphatics leads to
oedema, leading to elephantiasis
of the legs and/or scrotum.