Body Fluids

Question 1

what is the purpose of body fluid?

Answer 1

acts as an exchange medium for the transport of nutrients and waste

Question 2

how is body fluid organised?

Answer 2

it  is separated by membranes  into compartments that have differing electrolyte compositions and volumes due to their different functions in the body

Question 3

what are the main body fluid compartments?

Answer 3

The main 2 compartments of body fluid are the  

Intracellular fluid – fluid contained within the cell membranes, also known as the cytoplasm 

Extracellular fluid – fluid outside of the cell membranes 

Question 4

what are the sub compartments of extracellular fluid?

Answer 4

Interstitial fluid – found in connective tissue and surrounds most cells 

Plasma  component of blood – the liquid part of blood, in the capillaries 

Transcellular fluid – fluid contained within epithelial lined space, e.g. cerebrospinal fluid 

Question 5

give the relative volumes of each compartment

Answer 5

total 42L (70kg man where 60% of mass is body fluid)

intracellular = 28L

interstitial = 10L

plasma = 3L

transcellular = 1L

Question 6

how are the concentrations of the interstitial and intracellular fluid compartments maintained?

Answer 6

 The plasma  membrane is a phospholipid bilayer that is hydrophobic and separated the intracellular fluid to the interstitial.

As such,  hydrophilic  ions and  molecules are transported via channels

channels also act as concentration regulators in the way they operate. 

aquaporins (intrinsic protein channels specifically for water) allow water through the membrane

Question 7

why is it important to maintain the concentrations of the interstitial and intracellular fluids?

Answer 7

1. maintain the membrane potential
2. Transport  
   Processes that occur within the cell require nutrients which must be transported into the cell from outside. Similarly, these processes produce waste products that need to be transported out of the cell. Body fluid  is the  medium that  facilitates  this transport (diffusion, osmosis, active transport etc.) 
    
3. The optimum pH 
   Enzymes  involved  in cell processes have a pH in which they work at the optimum, this pH must be maintained within the cell

Question 8

what are the solute concentrations In the ICF (mM)?

Answer 8

sodium = 5-15
potassium = 140
chlorine = 5
calcium = 0.1 micromolar
bicarbonate = 10-20
hydrogen = 50-100 nanomolar

Question 9

what are the solute concentrations n the ECF?

Answer 9

sodium = 140
potassium = 4
chlorine = 100
calcium = 2.4
bicarbonate = 25
hydrogen = 40 nanomolar

Question 10

what maintains these sodium and potassium concentrations?

Answer 10

Na+-K+-ATPase pumps in the membrane that actively transport  3Na+ ions out of the cell and  2K+ ions  in  simultaneously, against the concentration gradient. 

Na+ cannot passively diffuse back through the membrane  because it is impermeable to sodium ions,  however the membrane is slightly leaky to K+ ions. This means, although the negative proteins and macromolecules inside the cell exert a force to keep the positive K+ in,  some K+ ions  diffuse into the extracellular fluid, down the concentration gradient maintained by the pumps.

This makes  the  intracellular fluid slightly more negative, creating the membrane potential. 

Question 11

what maintains these chlorine concentrations?

Answer 11

Cl- ions are actively brought into the intracellular fluid by Na+-Cl- pumps but then  passively  diffuse down the concentration gradient through channels in the membrane. 

Question 12

what is donan equilibrium?

Answer 12

Taking a closer look at Cl- and K+, both are are ions that can move across the cell membrane.
They contribute to the electrical potential as well as having their own concentration gradients.

Donnan  equilibrium  sets out that the product of the concentration of these permanent ions in the interstitial fluid must be equal to that in the intracellular fluid. This is so that the electrical potential across the cell balances with the concentration gradients of these ions.  

Question 13

what maintains the calcium concentrations?

Answer 13

The extracellular volume of calcium is much greater than the intracellular 

The cell membrane is permeable to Ca2+ ions and thus calcium ions diffuse into the cell. 

Some of these calcium ions are taken into intracellular stores but mostly  ATP energy is used to actively pump  the ions  against the concentration gradient.

This is done via Ca2+-ATPases and Na+/Ca2+ exchangers  that transport ions  from the intracellular fluid to the extracellular. 

The low Ca2+ levels are used by the cell for  signaling.  

Question 14

what maintains the concentrations of hydrogen carbonate?

Answer 14

The concentration of HCO3- is greater in the interstitial fluid than the intracellular 
This is because both fluids are maintained at a pH around neutral (7).
Primary H-ATPase  removes most of the excess H+ ions from the intracellular fluid, where they are neutralised by the higher concentrations of HCO3- (buffer) in the interstitial fluid. 
As such, the pH of the interstitial  fluid  is around 7.4 and the intracellular is 7. 

Question 15

compare the compositions of the interstitial and plasma fluids

Answer 15

similar composition to the interstitial fluid. 
This is because interstitial fluid is formed from the water pushed out of the capillaries due to the hydrostatic pressure  generated by the heart. 
Both these extracellular  fluid sub-compartments are only separated by the  capillaries  that hold the plasma, which are freely permeable to many small ions.
Despite their similarities, the plasma contains more proteins with a negative charge. These proteins cannot move across the walls of capillaries and  attract cations, increasing their concentration in the plasma. 

Question 16

what are staring forces?

Answer 16

the net movement  changes on either end of a capillary due to a change in starling forces. 

At the arterial end of the capillaries, the hydrostatic pressure is greater than the  osmotic  pressure, as such the net movement of fluid and solutes is out of the capillary. The heart  generates the hydrostatic  pressure; thus, it is greater at the arterial end which is closer to the heart.

In comparison, at  the  venous end the osmotic pressure is the greater force, so 
fluid and solutes move back into the plasma. 

The hydrostatic pressure  pushes water out of the fenestrations in the capillaries. This reduces the water potential in the capillaries and thus the osmotic pressure increases  down the vessel  as water moves from  a  high water  potential  (outside the capillary)  to an area of low  water potential (in the capillary). This constant flow of blood in the capillaries means that this concentration gradient is maintained, and  equilibrium  is not reached. 

Question 17

how do we experimentally prove the volumes of compartments?

Answer 17

Compartment volumes are measured by determining the volume of distribution of a tracer substance.
A known amount of a tracer is added to a compartment. The tracer concentration in that compartment is measured after allowing sufficient time for uniform distribution throughout the compartment.

The compartment volume is calculated as:
Volume = Amount of tracer / Concentration of tracer

Question 18

what is the clinical significance of starling forces?

Answer 18

Hydrostatic edema refers to accumulation of excess interstitial fluid which results from elevated capillary hydrostatic pressure

Excessive accumulation of interstitial fluid is generally viewed as detrimental to tissue function because edema formation increases the diffusion distance for oxygen and other nutrients, which may compromise cellular metabolism in the swollen tissue. For the same reason, edema formation also limits the diffusional removal of potentially toxic byproducts of cellular metabolism.