Intravascular volume consists of plasma and cellular elements.
- Intravascular volume consists of glycocalyx volume, plasma volume, and red cell volume. This is important because there is an erythrocyte exclusion zone preventing direct contact with endothelial cell membranes. The exclusion zone includes a surface layer we loosely call the glycocalyx volume.
Capillaries separate plasma with high protein concentration from ISF with low protein concentration.
- Sinusoidal tissues (marrow, spleen, and liver) have discontinuous capillaries and their ISF is essentially part of the plasma volume.
- Open fenestrated capillaries in the kidney produce the renal glomerular ﬁltrate.
- Diaphragm fenestrated capillaries in specialized tissues can absorb ISF to plasma.
- Continuous capillaries have a glycocalyx layer that is semi-permeable to anionic proteins. Protein concentration in intercellular clefts below the glycocalyx is regulated by filtration of protein-poor aqueous solvent into the subglycocalyx spaces.
The important Starling forces are the transendothelial pressure difference and the plasma–interstitial COP difference.
- The important Starling forces are the transendothelial pressure difference and the plasma–subglycocalyx space colloid osmotic pressure difference. General interstitial fluid colloid osmotic pressure difference is not a direct determinant of Jv.
Fluid is ﬁltered from the arterial end of capillaries and absorbed from the venous end. Small proportion returns to the circulation as lymph.
- Jv is much less than predicted by Starling’s traditional principle, and lymph is the major route for return of extracellular fluid from interstitium to the circulation.
Raising plasma COP enhances absorption and shifts ﬂuid from ISF to plasma.
- Raising plasma COP reduces Jv but cannot cause steady state absorption of fluid to the tissues; that is to say, negative Jv occurs only transiently in continuous capillaries.
At subnormal capillary pressure, net absorption increases plasma volume.
- At subnormal capillary pressure, Jv approaches zero. (See the flat section of the J curve.)
At supranormal capillary pressure, net ﬁltration increases ISF volume.
- At supranormal capillary pressure, when the COP difference is maximal, Jv is proportional to transendothelial pressure difference. (See the sloping section of the J curve.)
- An abrupt reduction of the transendothelial pressure difference, or an abrupt increase of the plasma–subglycocalyx space COP difference, causes a transient and limited reversal of Jv until the subglycocalyx space COP approaches plasma COP. This ‘auto transfusion’ lasts no more than 30 minutes and accounts for no more than 500ml aqueous solvent.
- Plasma volume recovery from hypovolaemia includes both aqueous solvent and proteins, and is predominantly achieved by accelerated efferent lymph flow entering the great veins.
Infused colloid solution is distributed through the plasma volume.
- Infused colloid solution is distributed through the plasma volume (single compartment kinetic model).
Infused isotonic salt solution is distributed through the extracellular volume.
- Infused isotonic salt solution is initially distributed through the plasma volume and glycocalyx volume and into the ISF of sinusoidal tissues (the central volume of distribution). The second volume of distribution (tissue volume of distribution) is to the expansile volume of tissues with continuous capillaries. The tissue volume of distribution for an isotonic salt solution is typically 2x the central volume of distribution, and much smaller than the total extracellular fluid volume.