Fluid compartments

Question 1

What is a fluid?

Answer 1

A substance that deforms under a shear stress.
Important fluids are those in which water or lipids are the solvent.

Question 2

What are key body fluid compartments?

Answer 2

Intracellular water
Interstitial water - inbetween cells
Fat
Blood plasma
Transcellular fluid - separated from extracellular by a membrane

Question 3

What are examples of transcellular fluid?

Answer 3

Cerebral Spinal Fluid
Peritoneal fluid
Aqueous humour
Pleural cavity
Sinovial fluid

Question 4

What is the body water content?

Answer 4

Total water is approx 42L for a 70kg person.
Proportionally greater in men than women.
Reduces with age.
Mostly intracellular

Question 5

How does water enter and leave cells?

Answer 5

Facilitated diffusion - requiring aquaporins and a driving force - osmosis.
This is because phospholipid membranes are water impermeable.

Question 6

How is cell volume regulated?

Answer 6

Expresses aquaporins - this affects the rate of change but not the equilibrium position.
Change the driving force - changes the concentration of solutes.

Question 7

Why is water movement in and out of cells not due to hydrostatic pressure?

Answer 7

Increasing hydrostatic pressure on the inside does not cause water to move in - it just increases the hydrostatic pressure on the inside of the cell.
This is because cells are compactable.

Question 8

What is hydrostatic pressure?

Answer 8

The force per unit area in a fluid.
It is generated mainly by the heart, but also by other places such as the gut.

Question 9

How do cells change the concentrations of solutes and therefore water movement?

Answer 9

Change concentration of small molecules through metabolic processes.
Change the influx of ions and small molecules.
But cells want to change the concentration of solutes without affecting the other functions of cells such as membrane potentials.

Question 10

How do cells regulate solute concentrations through breaking down?

Answer 10

Cells can break down large substances into smaller units - this changes the concentration per mol so there are more solutes.
This does not change the concentration per mass.

Question 11

How are proteins broken down to regulate water movement?

Answer 11

Proteins are quite large so can be broken down into their amino acids to make smaller molecules so there is a higher concentration of solute per moles.

Question 12

How can glycogen be broken down to regulate water movement?

Answer 12

Glycogen can be broken down into multiple glucose molecules, which can then be transformed into an unreactive substance - sorbitol.
Because glucose is easily moved across cells which influences water movement.

Question 13

What does an increase in solute concentration do?

Answer 13

Increasing solute molecules in the cell increases the osmotic pressure inside the cell so drives water in.

Question 14

What are volume regulated anion channels?

Answer 14

These channels are activated in the presence of cell stretch.
The channels sense increasing tension in the plasma membrane and open Chloride channels.
Cl is negative and the intracellular space is negative, so Cl leaves the cell.

Question 15

What happens when Cl- leaves the cell?

Answer 15

If the cell gets too big, Cl- channels open and Cl leaves.
This causes a loss of solute molecules from the cell, so there is a lower osmotic pressure and water leaves the cell and the cell shrinks.

Question 16

What are stretch activated cation channels?

Answer 16

The stretch channels let K+ out - which has a similar effect to Cl-, but let Na+ and Ca2+ in, which activates other signalling pathways.
Na+ causes depolarisation which activate other pathways which help regulate volume.

Question 17

What is plasma?

Answer 17

This is the fluid component of the blood and is about 55% of blood.

Question 18

What is haematocrit?

Answer 18

A measure of the proportion of the blood occupied by cells.

Question 19

How do you measure total body water?

Answer 19

Add labelled water into the body and measure the factor by which it is diluted.

Question 20

What is labelled water?

Answer 20

H2O usually has 0 neutrons in hydrogen.
Deuterium H2 has 1 neutron.
Tritium has 2 neutrons.
So if deuterated water is added into the body then the total body water can be measured.

Question 21

How can you measure plasma volume?

Answer 21

Inject labelled substances which bind to plasma proteins.
Evan’s blue - the more diluted the larger the volume of plasma.

Question 22

How do you measure the volume of extracellular fluid?

Answer 22

Add a substance that distributes in the volume you are interested in measuring.
e.g. Cl-36, thiosulfate, inulin

Question 23

What are the constituents of body fluids?

Answer 23

Na+ has a high concentration extracellularly and low intracellularly.
K+ has a low concentration extracellularly and high intracellularly.

Question 24

What is an osmole?

Answer 24

A measure of the number of moles that a compound dissociates into when dissolved in solution.
E.g. 2.4mmol of CaCl2 yields 7.2mOsm in solution.

Question 25

What is osmolality?

Answer 25

The number of osmoles per unit mass of solvent (Osm.Kg^-1)

Question 26

What is osmolarity?

Answer 26

The number of osmoles per unit volume of the solution (Osm.l^-1)

Question 27

What is the difference between osmolality and osmolarity?

Answer 27

In physiological ranges the difference is very small as the density of water is close to 1KgL^-1.
So the difference between the volume of the solvent and the volume of solution is very small.
But osmotic pressure is proportional to osmolality not osmolarity.

Question 28

What is osmotic pressure?

Answer 28

If the concentration of any species is different on either side of the semi-permeable membrane, there will be a net movement of molecules from one side of the membrane to the other.
The force per unit area required to oppose this movement is the osmotic pressure.

Question 29

What is the relationship between osmotic pressure and water potential?

Answer 29

Osmotic pressure is the inverse of water potential - if there is a high water potential there is a low osmotic pressure.

Question 30

How can osmotic pressure be measured?

Answer 30

The osmotic pressue is equal to the hydrostatic pressure generated.
P = ρgh
ρ = density of solution
g = acceleration due to gravity
h = height

Question 31

How can osmotic pressure be estimated?

Answer 31

Osmotic pressure = nCRT
nC = osmolality
R = ideal gas constant
T = temperature

Question 32

What does isomotic mean?

Answer 32

If two solutions are isomotic, they share the same osmolality.

Question 33

What does isotonic mean?

Answer 33

If a solution is isotonic, then applying the solution to cells will not cause net fluid movement.

Question 34

What is the relationship between isomotic and isotonic?

Answer 34

It is possible for a solution to be isomotic with respect to the intracellular fluid, but not isotonic.
e.g. urea

Question 35

What is movement of water across capillaries?

Answer 35

The net movement of water across the semipermeable membrane of capillaries will be a balance between hydrostatic force and osmotic pressure.
Across capillary membranes, the ions are in equilibrium, so the main osmotic forces are due to proteins - this force is sometimes called oncotic pressure.