Body Fluids II

Question 1

What percentage of body mass consists of water for the standard reference individual?

Answer 1

60% of body mass

Question 2

State the major compartments and subcompartments that make up body water.

Answer 2

2 major compartments: intracellular fluid (ICF), extracellular fluid (ECF)
ECF is subdivided into 2 major subcompartments and 2 minor subcompartments:
- Plasma (major)
- Interstitial fluid (ISF) (major)
- Lymph (minor)
- Transcellular fluid (minor)

Question 3

What is the difference between intracellular fluid and extracellular fluid?

Answer 3

Intracellular fluid is all the fluid inside the cells of the body and extracellular fluid is the fluid that is moving around between cells of the body.

Question 4

What proportion of total body water and total body mass do intracellular fluid and extracellular fluid make up (for the standard reference individual)?

Answer 4

ICF occupies 2/3 of total body water. Since total body water makes up 60% of body mass, ICF is 40% of body mass. ECF occupies 1/3 of total body water and therefore 20% of total body mass.

Question 5

In an individual with 42 L of body water total, how many litres of intracellular fluid and extracellular fluid are in their body?

Answer 5

ICF = 2/3 of total body water, so 2/3 of 42 = ~28 L
ECF = 1/3 of total body water, so 1/3 of 42 = ~14 L

Question 6

What is plasma?

Answer 6

Plasma is the fluid medium in which blood cells are suspended.

Question 7

What are the components of blood? How can we isolate them?

Answer 7

The three components of blood are red blood cells (erythrocytes), the buffy layer (consisting of white blood cells and platelets), and plasma. To isolate these things, we have to centrifuge blood.

Question 8

What is hematocrit (Ht)?

Answer 8

It is the percentage of blood volume that is occupied by red blood cells, also known as the packed cell volume. It can be thought of as the height of the erythrocyte column in centrifuged blood.

Question 9

What is the formula to calculate hematocrit?

Answer 9

Height of erythrocyte column/height of whole blood column.

Question 10

How can the hematocrit value be used?

Answer 10

It can be used to calculate how much of the blood is composed of plasma. Plasma (% of blood) = 100% - hematocrit (%)

Question 11

What is the normal hematocrit value?

Answer 11

Around 45%

Question 12

What is the percent breakdown (% body mass) of the different compartments of extracellular fluid?

Answer 12

Of the 20% of body mass occupied by extracellular fluid, 15% is interstitial fluid and 5% is plasma.

Question 13

What is interstitial fluid?

Answer 13

It is the fluid that moves between cells in the body, making it the true “milieu interieur”.

Question 14

In a 70 kg male with 60% body water, how much interstitial fluid and plasma does he have?

Answer 14

0.6 x 70 kg = 42 L of body water total
ISF: 0.15 x 42 = 10.5 :
Plasma; 0.05 x 42 = 3.5 L

Question 15

Explain the interaction between the fluid in the capillaries, lymphatic system, and interstitial space.

Answer 15

The blood (plasma) flows through the capillaries and exits into the interstitial space, where it becomes interstitial fluid. Some interstitial fluid reenters the capillaries. Excess fluid gets picked up by the lymphatic system, at which point it becomes lymph (lymph does not exit at this point again - drains into the chest later).

Question 16

Explain what the lymphatic system is (briefly).

Answer 16

It is a network of terminal tubules that form larger lymphatic vessels, which themselves form lymphatic ducts. These ducts drain into the large veins in the chest.

Question 17

What proportion of the ECF does lymph volume make up?

Answer 17

About 1-2% of the ECF volume.

Question 18

What is trancellular fluid?

Answer 18

It is an aggregate of fluid found in specific areas of the body, secreted by specific epithelial cells and with specialized functions depending on the body part.

Question 19

Give 3 examples of trancellular fluid.

Answer 19

1. Intraocular fluid (in the eye)
2. Cochlear fluid (in the ear)
3. Synovial fluid (in joints)

Question 20

What proportion of the ECF does transcellular fluid make up? How significant is transcellular fluid in the body as a whole?

Answer 20

It makes up less than 1-2% of the ECF volume. While it does not contribute significantly to overlal water, however, it is very important in local function.

Question 21

How does the total body fluid volume and the relative distribution of compartments vary within an individual (not over a long period of time)?

Answer 21

They both remain constant to remain in good health. That being said, there is always water movement between the compartments.

Question 22

What method is used to determine body fluid compartment volumes in an individual?

Answer 22

The indicator dilution method.

Question 23

Explain how the indicator dilution method works, including the formula.

Answer 23

You add a known quantity of some agent, allow time for it to equilibrate in the body, and then calculate its concentration. The formula for the volume of the body compartment (ml) is quantity of indicator (g)/concentration of indicator after equilibration (g/ml)

Question 24

Describe the 5 steps of the indicator dilution procedure

Answer 24

1. Introduce a known quantity (Q) of indicator into the vein
2. Allow time to equilibrate
3. Remove known volume of blood, and centrifuge to obtain plasma
4. Measure concentration (c) in unit volume of plasma
5. Calculation V = Q/c

Question 25

Name 4 considerations when choosing an indicator for the indicator dilution process.

Answer 25

1. Must be non-toxic
2. Must be able to diffuse and distribute itself evenly throughout the compartments being measured
3. It must induce no changes in the distribution of water between compartments (i.e. reduce permeability somewhere)
4. Must be easy to measure

Question 26

If you want to measure the volume of total body water using the indicator dilution method, describe the key property of the indicator that must be chosen. Give two examples of possible indicators.

Answer 26

The indicator must be able to cross the capillary wall (dividing the plasma and the rest of the ISF) and the cell membrane to be able to get into both the whole ECF and the ICF.
Possible indicators: Antipyrine, H2O (heavy water), T2O

Question 27

Given that you inject 50 ml of D2O into a man and find that, after some time, its concentration is 0.001 ml/ml plasma.
a) What body fluid compartments are we calculating for?
b) How much of that fluid is present in the man?

Answer 27

a) Total body water, because D2O can cross both the plasma membrane and the cell membrane.
b) 50/0.001 = 50,000 ml = 50 L

Question 28

If we want to measure only extracellular fluid volume using the indicator dilution method, what key property must the indicator have? Give 2 examples of possible indicators.

Answer 28

We need an indicator that can cross the plasma membrane into the interstitial fluid, but not cross the cell membrane into the intracellular fluid.
Possible indicators: radioactively labeled sugars like inulin, sucrose, mannitol

Question 29

If we wanted to measure the plasma volume using the indicator dilution method, what is the key property that the indicator would need? Give an example of a possible indicator.

Answer 29

The indicator would have to be unable to pass through the capillary wall or the cell membrane.
Possible indicator: Evans’ blue (dye)

Question 30

What factors are not taken into consideration by the indicator dilution method calculation that would have to be taken into account in a clinical setting?

Answer 30

Fluid leaving the body by metabolism or excretion.

Question 31

How do you determine the volume of the intracellular fluid using the indicator dilution method? What key properties must the indicator have? Give two examples of possible indicators.

Answer 31

You can’t measure intracellular fluid directly. You have to first measure extracellular fluid, which requires an indicator that can pass through the capillary wall but not the cell membrane, like sucrose or inulin. Then, you would measure total body water using an indicator that can pass through both barriers, like D2O or antipyrine. You would then find the difference between these two results to get the ICF.

Question 32

How do you measure the interstitial fluid volume using the indicator dilution method? What key properties must the indicator have? Give examples of possible indicators.

Answer 32

It cannot be directly measured. First, you would need to measure the total extracellular fluid volume using an indicator that can pass through the capillary wall but not the cell membrane using an indicator like sucrose or inulin. You would then have to calculate the volume of plasma using an indicator that can pass through neither barrier, like Evans’ blue. You would then take the difference to find the interstitial fluid volume.

Question 33

If someone’s total plasma volume = 3L and their Hematocrit (Ht) = 40%, what is their total blood volume?

Answer 33

Plasma occupies 100-40 = 60% of the blood. 3 L = 60% of the blood, so (100x3)/60 = 5 L total.
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Question 34

Explain the main ways in which the ICF and ECF vary in ionic composition.

Answer 34

ICF is high in potassium and magnesium and low in sodium and chloride.ECF is high in sodium and chloride and low in potassium.

Question 35

What is the difference in composition between interstitial fluid and plasma?

Answer 35

There are more or less the same except that plasma contains more proteins.

Question 36

How can extracellular fluid by artificially simulated if an individual needs their fluids replaced? Why does this work?

Answer 36

It can be simulated with a 0.9% NaCl solution, called physiological saline. This works because extracellular fluid is naturally high in Na and Cl.