Fluid compartments Flashcards
What is a fluid?
A substance that deforms under a shear stress.
Important fluids are those in which water or lipids are the solvent.
What are key body fluid compartments?
Intracellular water
Interstitial water - inbetween cells
Fat
Blood plasma
Transcellular fluid - separated from extracellular by a membrane
What are examples of transcellular fluid?
Cerebral Spinal Fluid
Peritoneal fluid
Aqueous humour
Pleural cavity
Sinovial fluid
What is the body water content?
Total water is approx 42L for a 70kg person.
Proportionally greater in men than women.
Reduces with age.
Mostly intracellular
How does water enter and leave cells?
Facilitated diffusion - requiring aquaporins and a driving force - osmosis.
This is because phospholipid membranes are water impermeable.
How is cell volume regulated?
Expresses aquaporins - this affects the rate of change but not the equilibrium position.
Change the driving force - changes the concentration of solutes.
Why is water movement in and out of cells not due to hydrostatic pressure?
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.
What is hydrostatic pressure?
The force per unit area in a fluid.
It is generated mainly by the heart, but also by other places such as the gut.
How do cells change the concentrations of solutes and therefore water movement?
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.
How do cells regulate solute concentrations through breaking down?
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.
How are proteins broken down to regulate water movement?
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.
How can glycogen be broken down to regulate water movement?
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.
What does an increase in solute concentration do?
Increasing solute molecules in the cell increases the osmotic pressure inside the cell so drives water in.
What are volume regulated anion channels?
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.
What happens when Cl- leaves the cell?
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.
What are stretch activated cation channels?
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.
What is plasma?
This is the fluid component of the blood and is about 55% of blood.
What is haematocrit?
A measure of the proportion of the blood occupied by cells.
How do you measure total body water?
Add labelled water into the body and measure the factor by which it is diluted.
What is labelled water?
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.
How can you measure plasma volume?
Inject labelled substances which bind to plasma proteins.
Evan’s blue - the more diluted the larger the volume of plasma.
How do you measure the volume of extracellular fluid?
Add a substance that distributes in the volume you are interested in measuring.
e.g. Cl-36, thiosulfate, inulin
What are the constituents of body fluids?
Na+ has a high concentration extracellularly and low intracellularly.
K+ has a low concentration extracellularly and high intracellularly.
What is an osmole?
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.
