The amount of fluid in the intravascular space is only 3 liters, keep this in mind when giving patients fluids.

 

What Controls Intravascular Volume:

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Intravascular volume is controlled by Starling forces. Starling forces are the pressures that want to drive fluid into the intravascular space and the pressures that want to drive fluid out of the intravascular space.

 

Starling Forces:

1. Oncotic Pressure

2. Osmotic Pressure

3. Hydrostatic Pressure

 

Starling equation:

The Starling equation determines the rate of fluid either in or out of the intravascular space. This is done at the level of the capillaries. Jv is the rate at which it moves in volume per sec (m3/s). It is the difference between the hydrostatic forces minus the difference in oncotic forces.

 

Jv = LpS [(Pc - Pi] - σ(πc-πi)]

 

Jv = Rate of fluid entering or leaving the intravascular space

Lp = Hydraulic permeability coefficient

S = Surface area of membrane

Pc = Capillary hydrostatic pressure

Pi = Interstitial hydrostatic pressure (small)

πc = Capillary oncotic pressure

πi = Interstitial oncotic pressure (negligible)

σ = reflection coefficient for protein

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At equilibrium, the fluid is pushed out of the intravascular space at the capillary level because the Starling equation becomes positive, and then the fluid comes back in to the intravascular space at the venule level as the Starling equation becomes negative. 

 

By understanding this equation it is easy to see that in order to keep fluid in the intravascular space the capillary oncotic pressure must be increased or the capillary hydrostatic pressure decreased.

 

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Osmotic pressure:

Osmotic pressure occurs in the intravascular and interstitial compartments and opposes each other. The osmolarity in the intravascular space wants to pull fluid in, the same is true for the osmolarity in the interstitial space, but it wants to pull fluid into the interstitial space.

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The electrolytes completely dissociate into ions and are major contributors to the osmolarity of the interstitial and intravascular space. In fact, 95% of the osmolarity in the intravascular space is electrolytes. The reason that they do not contribute to the Starling equation is that electrolytes can cross the capillary membrane and, between diffusion and osmosis, the osmolarity equilibrates between the two spaces making the net movement due to electrolytes zero.

 

The van ‘t Hoff equation shows what determines the osmotic pressure

 

Osmotic pressure = π = iMRT

 

i = number of ions a solute will form when dissolved in water (van’t Hoff factor)

M = Molar concentration of the solution (mol/L)

R = Ideal gas constant (0.08206 L atm/mol K)

T = Temperature in Kelvin (K)

 

Again, since the electrolytes can diffuse across to the interstitial space, they normally do not participate much and the main determinate for keeping fluid in the intravascular space is oncotic pressure.

 

So why is osmotic pressure shown in figure 2?

 

The reason that osmotic pressure is represented in the forces for fluid movement is to represent opportunities for clinical intervention. This post is more about clinical opportunities for intervention more than most because physiologically electrolytes do not do much at baseline. But clinically, the osmolarity can be changed and used for resuscitation.

 

The logical next step in keeping volume intravascularly is giving hypertonic solutions. This is when a solution is given where the osmolarity is higher than the serum osmolarity. hypertonic solutions will change the equilibrium and, through osmosis, will drive fluid from the interstitial space to the intravascular space.

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The change in electrolyte concentration will also cause fluid movement out of the intracellular space as well.