Physiology: Body Fluid Compartments

Question 1

What is osmolarity and what are its units for body fluids?

Answer 1

The concentration of osmotically active particles in a solution

Units: osmol/l, or mosmol/l for body fluids as they are weak salt solutions

Question 2

How is osmolarity calculated?

Answer 2

Molar concentration of the solution
x
Number of osmotically active particles present

Question 3

What is the osmolarity of 100mM MgCl2?

Answer 3

Molar conc. x no. of osmotically active particles
= 100 x 3 (Mg + Cl + Cl)
= 300 mosmol/l

Question 4

What is tonicity?

Answer 4

The effect that a solution has on cell volume

Question 5

Describe how cell volume is affected when submerged in a...
- Isotonic
- Hypotonic
- Hypertonic 
... solution

Answer 5

• Isotonic: no overall net change
• Hypotonic: solution contains more water than the cell, water enters the cell by osmosis, cell volume increases, may lead to cell lysis
• Hypertonic: solution contains less water than the cell, water leaves the cell by osmosis, cell volume decreases, the cell shrinks

Question 6

What 2 factors does tonicity depend upon?

Answer 6

• Osmolarity of the solution

- Permeability of the cell membrane to the solute

Question 7

Total body water (TBW) makes up what proportion of total body weight in males vs females?
Why is there a difference?

Answer 7

Males: ~60%
Females: ~50%

This difference is due to the higher fat distribution in females - fat contains little water volume

Question 8

What are the 2 major compartments that make up total body water?

Answer 8

Intracellular fluid (ICF) - ~2/3rd of TBW
Extracellular fluid (ECF) - ~1/3rd of TBW

Question 9

What is the estimated...
- Total body water volume
- ICF volume
- ECF volume
... for a 70kg male?

Answer 9

TBW: 42L (60% of weight)
ICF: 28L (2/3rds of TBW)
ECF: 14L (1/3rd of TBW)

Question 10

What is extracellular fluid composed of?

Answer 10

Interstitial fluid (~80%)
Plasma (~20%)
Lymph and transcellular fluid (negligible)

Question 11

How can the body fluid compartments be measured?

Answer 11

By obtaining the volume of distribution of a tracer

Question 12

Name a suitable tracer for each of the following body fluid compartments:
- Total body water
- ECF
- Plasma
...?

Answer 12

• Total body water: 3H2O (titriated water - a radioactive form of water)
• ECF: Inulin (an exogenous plant protein)
• Plasma: Labelled albumin (a plasma protein)

Question 13

How can ICF be calculated?

Answer 13

By using tracers to measure TBW and ECF then TBW = ECF - ICF

Question 14

What is the equation used to work out the distribution volume of a tracer?

Answer 14

Distribution volume = known dose of tracer added (mol or mg) / concentration of the tracer in a sample (mol/litre)

Question 15

Whole-body fluid homeostasis relies on a balance between input and output of water and salts from the body. What is the main way that water balance is maintained?

Answer 15

By increasing and decreasing water ingestion

Increased and decreased excretion of water by the kidneys alone is insufficient to maintain water balance

Question 16

How does water imbalance in the body affect the body fluid compartments?

Answer 16

Water imbalance changes body fluid osmolarity

This results in fluid shift between the ECF and ICF compartments by osmosis

Question 17

What are the main ions of the…
- ICF
- ECF (interstitial fluid + plasma)
…?

Answer 17

• ICF: K+

- ECF (interstitial fluid + plasma): Na+, Cl- and HCO3-

Question 18

What are the osmotic concentrations of the ECF and ICF?

Answer 18

Both are identical (usually ~300 mosmol/L)

Question 19

What is fluid shift?

Answer 19

Movement of water between the ICF and ECF in response to an osmotic gradient

Question 20

Describe fluid shift in response to increased osmolarity of the ECF e.g., during dehydration

Answer 20

• ECF becomes hypertonic
• Water from the ICF leaves the cells to restore the fluid and electrolyte balance between the ICF and ECF
• Osmotic concentrations are identical again

Question 21

Describe fluid shift in response to decreased osmolarity of the ECF e.g., during excess H2O intake

Answer 21

• ECF becomes hypotonic
• Water is taken into the ICF of the cells to restore the fluid and electrolyte balance between the ICF and ECF
• Osmotic concentrations are identical again

Question 22

List 3 challenges to fluid homeostasis

Answer 22

1. Gain or loss of water
2. Gain or loss of NaCl
3. Gain or loss of isotonic fluid

Question 23

How does gain or loss of water disturb fluid homeostasis?

Answer 23

It changes fluid osmolarity so causes fluid shift, this results in a similar change in ECF and ICF (e.g., both increase or both decrease)

Question 24

How does gain or loss of NaCl disturb fluid homeostasis?

Answer 24

It changes fluid osmolarity as…

• Na+ is excluded from the ICF (as cells are relatively impermeable to Na+)
• Water follows Na+ by osmosis

This results in opposite changes in ECF and ICF e.g., NaCl gain causes increased ECF volume and decreased ICF volume and vice versa

Question 25

How does gain or loss of isotonic fluid disturb fluid homeostasis?

Answer 25

There is no change in fluid osmolarity but ECF volume increases
e.g., 0.9% NaCl IV infusion

Question 26

The kidneys alter the composition and volume of the ECF. What is regulation of ECF volume essential for?

Answer 26

Long-term regulation of blood pressure

Question 27

Why is ECF volume important for regulating BP?

Answer 27

Because plasma is a component of the ECF so increased ECF increases plasma volume and decreased ECF decreases plasma volume

Question 28

Why is electrolyte balance so important (2)?

Answer 28

• Electrolyte concentrations can affect water balance due to changes in osmolarity
• Concentrations of individual electrolytes can affect cell function

Question 29

What are the 2 most important electrolytes?

Answer 29

Na+ (>90% for ECF)

K+ (>95% for ICF)

Question 30

What is the relationship between total body sodium and volume of ECF?

Answer 30

Na+ is the major determinant of ECF volume

ECF volume is determined by the balance of sodium intake (mainly dietary) and sodium output in the urine, as water follows Na+

Question 31

Why is potassium balance important for cells?

Answer 31

K+ plays a key role for establishing membrane potential

Question 32

Why must plasma K+ be closely regulated?

Answer 32

Even small changes in membrane potential in cardiac and muscle cells can have severe consequences e.g., paralysis, cardiac arrest