nules. Histologically they consist of three layers; an inner lining of flattened en -
dothelial cells lying on a basement membrane surrounded by an adventitial layer
of cells. The individual endothelial cells are separated by intercellular regions consisting of a 'ground substance' to which some of the special properties of capillaries
have been attributed.
Fluids mainly move across capillaries by hydrodynamic or 'bulk' flow that is
superimposed on diffusional movements of water and solutes. Water is transferred
1000 times more rapidly than across the red cell wall . Modern concepts about the
process of fluid transfer across capillary walls were initiated by STARLING and have
been called the STARLING-filtration-absorption principle (LANDIS and PAPPENHEIMER, 1963). Our current knowledge about such processes is substantially due to
LANDIS and PAPPENHEIMER and the review cited should be consulted for a detailed
account of the subject. The transfer of fluid between the plasma and the interstitial
solutions is the result of the balances of hydrostatic and colloid osmotic pressures
on either side of the capillary wall. Filtration is assisted by the hydrostatic pressure
of the plasma and the osmotic pressure of the interstitial fluid, and is opposed by
the corresponding pressures in the interstitial and plasma fluids respectively. The
situation can be summarized as follows:
Fluid movement = filtration coefficient (Pc - n; pi - Pif + n; if)
P = hydrostatic pressure n; = osmotic pressure; c = capillary
if = interstitial fluid
Average corresponding pressures mm Hg in man; 32 - 25 - (1 to 9) + (0.1
to 5)
In the proximal (arteriolar) end of the capillaries the forces result in filtration.
Water and small solutes pass out of the plasma while the larger protein molecules
are retained, a process of ultrafiltration. After passage to the distal end of the capillary, the hydrostatic pressure drops sufficiently to permit a reabsorption of fluid
to take place from the interstitial solution. The hydrostatic and osmotic pressures
cited vary considerably in the different regions of the body. In reptiles, Amphibia
and fishes the osmotic pressure of the plasma proteins is usually equivalent to only
about 10 mm Hg, but in such species the blood pressure (and presumably capillary
pressure) is also lower, so that a similar balance is maintained.
Water and solute molecules up to the size of inulin (M.W. 5500, molecular
radius, 12 to 15 A) readily cross the capillary wall but larger molecules such as serum albumin (M. W. 67000, molecular radius 36 A) do so only with great difficulty.
This explains why fluid movement is influenced by the colloid osmotic pressure
of the surrounding fluids, as small molecules with their relatively unrestricted
movements do not produce an osmotic pressure in such a system. From physicochemical data based on differences of the movements of fluids and solutes by diffusion and hydrodynamic flow, PAPPENHEIMER (1953) calculated that if (as appears
likely) fluid movements take place through water-filled 'pores', then their radius
would be about 30 A. This figure corresponds rather well with the limitations of
solute movement which are dictated by their molecular size . Such 'pores' have not
been seen under the electron microscope. It is considered, that if they exist, they
most likely are in the intercellular regions between the endothelial cells. Lipid-so18
dothelial cells lying on a basement membrane surrounded by an adventitial layer
of cells. The individual endothelial cells are separated by intercellular regions consisting of a 'ground substance' to which some of the special properties of capillaries
have been attributed.
Fluids mainly move across capillaries by hydrodynamic or 'bulk' flow that is
superimposed on diffusional movements of water and solutes. Water is transferred
1000 times more rapidly than across the red cell wall . Modern concepts about the
process of fluid transfer across capillary walls were initiated by STARLING and have
been called the STARLING-filtration-absorption principle (LANDIS and PAPPENHEIMER, 1963). Our current knowledge about such processes is substantially due to
LANDIS and PAPPENHEIMER and the review cited should be consulted for a detailed
account of the subject. The transfer of fluid between the plasma and the interstitial
solutions is the result of the balances of hydrostatic and colloid osmotic pressures
on either side of the capillary wall. Filtration is assisted by the hydrostatic pressure
of the plasma and the osmotic pressure of the interstitial fluid, and is opposed by
the corresponding pressures in the interstitial and plasma fluids respectively. The
situation can be summarized as follows:
Fluid movement = filtration coefficient (Pc - n; pi - Pif + n; if)
P = hydrostatic pressure n; = osmotic pressure; c = capillary
if = interstitial fluid
Average corresponding pressures mm Hg in man; 32 - 25 - (1 to 9) + (0.1
to 5)
In the proximal (arteriolar) end of the capillaries the forces result in filtration.
Water and small solutes pass out of the plasma while the larger protein molecules
are retained, a process of ultrafiltration. After passage to the distal end of the capillary, the hydrostatic pressure drops sufficiently to permit a reabsorption of fluid
to take place from the interstitial solution. The hydrostatic and osmotic pressures
cited vary considerably in the different regions of the body. In reptiles, Amphibia
and fishes the osmotic pressure of the plasma proteins is usually equivalent to only
about 10 mm Hg, but in such species the blood pressure (and presumably capillary
pressure) is also lower, so that a similar balance is maintained.
Water and solute molecules up to the size of inulin (M.W. 5500, molecular
radius, 12 to 15 A) readily cross the capillary wall but larger molecules such as serum albumin (M. W. 67000, molecular radius 36 A) do so only with great difficulty.
This explains why fluid movement is influenced by the colloid osmotic pressure
of the surrounding fluids, as small molecules with their relatively unrestricted
movements do not produce an osmotic pressure in such a system. From physicochemical data based on differences of the movements of fluids and solutes by diffusion and hydrodynamic flow, PAPPENHEIMER (1953) calculated that if (as appears
likely) fluid movements take place through water-filled 'pores', then their radius
would be about 30 A. This figure corresponds rather well with the limitations of
solute movement which are dictated by their molecular size . Such 'pores' have not
been seen under the electron microscope. It is considered, that if they exist, they
most likely are in the intercellular regions between the endothelial cells. Lipid-so18