Topic 2: Body Fluids [6 & 7]

Question 1

What is the result of:
a) Hypotonic solution
b) Isotonic solution
c) Hypertonic solution

Answer 1

a) Water enters cells, cell swells and bursts
b) Relative equilibrium
c) Water leaves cellscausing shrivelling

Question 2

In a male, 60% of the body is made up of fluids (55% in female). This fluid can be divided up into 2 groups. What are they and what percentage of the body fluid do they take up?

Answer 2

Intracellular fluid (2/3): the fluid within cells

Extracellular fluid (ECF) (1/3):
- Interstitial fluid (80% of ECF)
- Plasma (20% of ECF)

Cell membranes separate ICF and ECF
Capillary walls separate interstitial fluid and plasma

Question 3

What are the volumes of fluid in an average man (70kg):
a) Total body
b) ICF
c) ECF
d) Interstitial Fluid
e) Plasma
f) Transcellular fluid

Answer 3

a) 42L
b) 28L
c) 14L
d) 10.5L
e) 3L
f) 0.5L

Question 4

What are some examples of transcellular fluid

Answer 4

• cerebrospinal fluid
• intraocular fluid
• gastrointestinal fluid
• bile
• sweat

Question 5

What is an osmole and what is osmotic pressure

Answer 5

1 Osmole = amount of solute that exerts an osmotic pressure of 1 atm when placed in 22.4L of solution at 0 degrees C. They are the measurement for the number of particles of a solute that contribute to osmotic pressure.

Osmotic pressure is the pressure that is required to prevent the flow of water across a semipermeable membrane due to differences in solute concentration on either side of the membrane

Question 6

What is the difference between osmolarity and osmolality?

Answer 6

Osmolarity
The the measure of the number of osmoles of solute per litre of solution (mmol/L).

Osmolality
the measure of the number of osmoles of solute per kilogram of solvent, taking into account the mass of the solvent.

Question 7

Osmolality is crucial to the regulation of body fluid.
What happens to the a) osmolality and b) total body water (TBW) that precedes thirst and water retention?

Answer 7

a) Increase in osmolality
b) Decrease in TBW

These things lead to ADH release from hypothalamic osmoreceptors causing thirst and water retention

Question 8

What is oncotic pressure (plasma colloid osmotic pressure)?

Answer 8

Oncotic pressure is the osmotic pressure inside the plasma of blood vessels. It is caused by proteins (most commonly albumin) that cannot permeate the membrane that binds it

In plasma this is 25 mm Hg

Question 9

Define electroneutrality

Answer 9

In a close compartment, the number of positive charges = the number of negative charges

Question 10

What is the Gibbs-Donnan equilibrium equation and what does it mean

Answer 10

Cation A x Anion A = Cation B x Anion B

Basically means that charged particles are influenced by the presence of impermeable charged particles such as proteins that create electrostatic potential across the semipermeable membrane

Question 11

Van Hoff’s equation is:

Osmotic Pressure = RT(φ iC)

What do the symbols mean

Answer 11

R = Gas constant
φ = Osmotic coefficient (close to 1)
i = Number of particles formed when substance dissolves
C = Concentration (mol/l)

Question 12

Bit of an abstract question but,, when looking at a volume-osmolarity graph, what 3 things should you examine?

Answer 12

• ECF volume
• ECF osmolarity
• ICF volume and osmolarity

Any changes to ECF osmolarity will cause subsequent changes in volume distribution between the ICF compartment and the ECF compartment. Eventually they will equilibrate after change

Question 13

A patient arrives at the ER on a hot, humid July day complaining of a headache. An ER resident places him on IV Fluids, assuming dehydration (rapid infusion of Normal Saline-286 mOsm/L NaCl). It is later found that the patient was not dehydrated, but was suffering from a migraine headache.

What would the Volume-Osmolarity diagram most likely look like?

Answer 13

ECF volume: Increase
ECF osmolarity: No change
ICF volume: No change
ICF osmolarity: No change

Question 14

A man arrives at the ER after collapsing at the top of a mountain. It is a hot July day, and his friend indicates that they were hiking and they have had very little to drink during the past couple of days.

What would the Volume-Osmolarity diagram most likely look like?

Answer 14

** NEEDS CHECKING - Answer in Qs section **

ECF volume: Decrease
ECF osmolarity: Increase
ICF volume: Decrease
ICF osmolarity: Increase

Question 15

How does water travel across a membrane (hint = 2 proteins)

Answer 15

Some moves through the membrane lipid but most moves through selective water channels:
- Aquaporin I (AQPI), found in most cells
- Aquaporin 2 (AQP2), found exclusively in the kidney collecting duct and is sensitive to antidiuretic hormone

Question 16

Cell volume will remain constant if net water movement is prevented as there is no osmotic pressure across the membrane. In addition, ECF osmolality is kept remarkably constant by regulation of thirst mechanisms + kidneys.

How can cell volume be regulated?

Answer 16

By altering the cells solute load. Metabolism drives solute movement in and out of the cell as well as breaking down polymers in the cytoplasm

Question 17

What is the role of the sodium pump and solute transport in cell volume regulation?

Answer 17

• The Na-pump, thru active transport, prevents the composition of the cytoplasm coming into equilibrium with the extracellular medium.
• It generates ion gradients that can be used to regulate cell volume very rapidly (sodium and potassium ion gradients)
• Solute uptake driven by the Na gradient causes swelling, whereas solute loss driven by the K gradient causes shrinkage
• Accumulation and release of solutes such as taurine, betaine are also used to stabilise cell volume

Question 18

Describe the composition of Plasma, Interstitial Fluid and Intracellular Fluid (cell) in terms of Na+ ions and K+ ions

Answer 18

Na+ is high and about even in plasma and Interstitial fluid (~145) but low in ICF (10).
K+ is low in plasma and interstitial fluid (~4) but hagh in ICF (~160)

Question 19

What are Starling Forces?

Answer 19

Hydrostatic and osmotic pressures

Question 20

What is Starling’s hypothesis?

Answer 20

• The full effect of colloid osmotic gradient is only experienced if the barrier is totally impermeable to proteins, which it is not.
• This reduces the effects of colloid osmotic pressure by a factor σ (reflection coefficient: 1 when membrane impermeable to protein, 0 when totally permeable)

Therefore:
Jv = k{(Pcap – Pi) – σ(Πcap – Πi)}

• If Jv is positive, fluid leaves the capillary (filtration)
• If Jv is negative, fluid enters the capillary (reabsorbtion)

Question 21

What is the basic organisation and functions of the Lymphatic system?

Answer 21

Basic Organisation:
- Lymph fluid in lymph vessels
- Structures: organs with lymphatic tissue, red bone marrow, liver and spleen

Functions:
- Return interstitial fluid and proteins to the blood
- Transport dietary fats to adipose tissue
- Protect against cancer & infection
- Non-specific resistance + specific immunity to disease

Question 22

We want to measure body fluid compartment volumes in human subjects using the indicator dilution technique. Which of the following statements about the technique are correct?

a.
We can use a single indicator that enters cells to measure the size of the intracellular fluid (ICF) compartment

b.
We can use a single indicator, 35S-thiosulphate, to measure extracellular fluid (ECF) volume

c.
We cannot measure the volume of the interstitial fluid (ISF) using the indicator dilution technique

d.
Following the injection of indicator into a vein we can take a blood sample about 30 min later to measure its concentration and calculate its volume of distribution

Answer 22

b