Body Water and Compartments

Question 1

How much total body water is there and what percentage of 70 kg male is this

Answer 1

60% of total body weight is water

= 42 L

Question 2

How is the body compartmentalised in terms of water distribution and what percentage of the bodies water lie in each of these compartments?

Answer 2

Intracellular fluid (ICF): 2/3’s total body water = 28 L

Extracellular Fluid (ECF): 1/3 total body water = 14 L

Separated by the cell membrane which is largely impermeable to ions but freely permeable to water

Question 3

What are the divisions of the ECF compartment? State the volume in each division

Answer 3

Fluid | volume (L)

Intravascular | 4

Interstitial | 9

Transcellular | 1

Bone and connective tissue | 1

Question 4

What is transcellular fluid and where is it found?

Answer 4

It is the portion of total body water contained between epithelial lined spaces
E.g.
CSF
Eyes - aqueous humour
Joints - synovial fluid
Bladder - urine
Gall bladder - bile

Question 5

Describe the contents of plasma and their proportions.

Answer 5

1. Water 90%
2. Proteins 7%
   - Albumin 60%
   - Globulins (alpha, beta, gamma) 35%
   - Fibrinogen 4%
3. Ions

Question 6

What is oncotic pressure

Answer 6

Colloid osmotic pressure (proteins) that contribute to the total osmotic pressure within a solution

Question 7

How does the ionic composition differ between plasma and interstitial fluid

Answer 7

It doesn’t. The only difference between these fluids is the protein content

Question 8

Define osmotic pressure

Answer 8

The hydrostatic pressure required to prevent movement of water across a semi-permeable membrane down its concentration gradient

Question 9

Define osmolality and osmolarity

Answer 9

Osmolality: number of osmoles per kg water
(osmoles = osmotically active particles)

Osmolarity: number of osmoles L of water
(this is dependent upon temperature so the term is not commonly used)

Question 10

What is the range of normal osmolality between plasma / interstitial fluid and intracellular fluid and what is the formula for plasma osmolality

Answer 10

As water can move freely between these compartments the osmolality in all 3 is usually the same: 280 - 300 mosmol/kg

Plasma osmolality = (2 x Na) + glucose + Urea

Question 11

Does the size of each compartment in the ECF depend on osmolality or number of solute particles

Answer 11

Number of solute particles - With more solute particles more water is required to maintain constant osmolality

More solute –> larger volume

Question 12

Draw a graph to demonstrate the net change in hydrostatic and oncotic pressure in arterioles –> capillaries –> venules

Answer 12

Y - axis is Pressure 15 - 25 - 30
X - axis: arterioles –> cappilaries –> venules

Straight line down from P of 35 in the arterioles (representing hydrostatic pressure) –> 25 (central ± mean in capillaries) then the straight line continues below mean to 15 in the venules (representing oncotic pressure)

Question 13

Describe the regulation of water intake

Answer 13

Sensor:
Osmoreceptors in the hypothalamus respond to changes in ECF osmolality and volume. Very small changes in ECF osmolality can be detected ( 1 - 2%)

Controller:
Hypothalamus (complex system of interconnecting pathways)

Effectors:

1. Thirst - conscious intake of more water
2. Pituitary ADH release - aquaporin insertion into the collecting ducts in the nephron leading to increased water reabsorption (prevents diuresis and reduces osmolality of plasma)

Question 14

What volume of a 1L Glucose 10% IV infusion remains in the:

1. Intravascular space
2. ECF
3. ICF

Answer 14

Draw flow diagram of divisions of TBW
TBW 42 L (100%) –> ICF 28 L (66%)
–> ECF 14 L (33%)

ECF 14 L (33%) –> Plasma 4L (9.5%)

                         - -> Interstitial 9L (21.5)
                         - -> Transcellular 1L (2%)
                         - -> Bone/Connective tissue 1L (2%)

As Glucose 10% is basically free water it will divide proportionally as per the percentages above

1. Intravascular space = plasma = 9.5% of 1000 mls = ± 100 mls
2. ECF = 33% of 1000mls = 330 mls
3. ICF = 66% of 100mls = 660mls

Question 15

Evaluate the final distribution of NaCl 0.9% after rapid infusion of 2L in the following compartments

1. ICF and ECF
2. Intravascular space

Answer 15

Sodium does not move into cells –> therefore none is distributed into the ICF

The 2000ml will be distributed throughout the ECF proportionally
–> Excluding transcellular and bone and connective tissue (takes long) then ECF volume is ±13L
Plasma makes up 4L and Interstitial 9L of this
Therefore about 30% will remain intravascular and 30 % of 2000ml = 600. The rest distributes into the interstitial compartment.