CV Physiology | Tissue Edema and General Principles of Transcapillary Fluid Exchange
The relation between the capillary pressure, Pc, in a tube and. the tube diameter is. (7) . permeability and the. measured permeability using the formation factor correlation depicted Charity Quiz Night, was held on 28 November. Now in its . Tutorials/Quizzes · Glossary · Search · Author. Tissue Edema and General Principles of Transcapillary Fluid Exchange Increased capillary permeability caused by proinflammatory mediators (e.g., histamine, bradykinin) reduces venous and capillary pressures, thereby decreasing filtration and promoting reabsorption of. It is also possible to estimate capillary pressure by direct cannulation using glass micropipettes.  who used a video-photometric cross correlation technique which allowed continuous the transcapillary diffusion of tracer as a marker of capillary permeability to small solutes . Mar; 73(2); quiz 2.
Tissue Interstitial Pressure Pi This hydrostatic pressure is determined by the interstitial fluid volume and the compliance of the tissue interstitium, which is defined as the change in volume divided by the change in pressure.
The more fluid that filters into the interstitium, the greater the volume of the interstitial space Vi and the hydrostatic pressure within that space Pi.
In some organs, the interstitial compliance is low, which means that small increases in interstitial volume lead to large increases in pressure. Examples of this include the brain and kidney, which are encased by rigid bone brain or by a capsule kidney. In contrast, soft tissues such as skin, muscle and lung have a high compliance and therefore the interstitial space can undergo a large expansion with a relatively small increase in pressure.
As interstitial volume increases, interstitial pressure increases, which can limit the amount of filtration into the interstitium because this pressure opposes the capillary hydrostatic pressure. In other words, as the hydrostatic pressure gradient PC - Pi decreases owing to the rise in interstitial pressure, fluid filtration will be attenuated.
Capillary pressure -
However, large increases in tissue interstitial pressure can lead to tissue damage and cellular death. Normally, Pi is near zero. In some tissues it is slightly subatmospheric, whereas in others it is slightly positive. Therefore, instead of speaking of "osmotic" pressure, this pressure is referred to as the "oncotic" pressure or "colloid osmotic" pressure because it is generated by colloids.
This pressure is typically mmHg.
The oncotic pressure increases along the length of the capillary, particularly in capillaries having high net filtration e. Normally, when oncotic pressure is measured, it is measured across a semipermeable membrane that is permeable to fluid and electrolytes but not to large protein molecules. In most capillaries, however, the wall primarily endothelium does have a finite permeability to proteins. The actual permeability to protein depends upon the type of capillary as well as the nature of the protein size, shape, charge.
Because of this finite permeability, the actual oncotic pressure generated across the capillary membrane is less than that calculated from the protein concentration.
Tissue Edema and General Principles of Transcapillary Fluid Exchange
The anatomy of the cutaneous microvasculature The skin blood supply originates from perforating vessels rising from the underlying muscles and subcutaneous fat to form a plexus, the lower horizontal plexus, at the dermal—subcutaneous interface. From the lower plexus, paired arterioles and venules rise to form direct connections with a second plexus, the subpapillary plexus which is situated in the papillary dermis, and from this the capillary loops of the dermal papillae arise.
The majority of the microvasculature of the skin resides in the papillary dermis 1—2 mm below the surface of the skin. Changes in microvascular anatomy with site and age Microvascular anatomy, particularly the capillary loops, may vary according to the skin area examined and the age of the subject. Uniquely in the toe and finger nail fold, the terminal row of dermal capillary loops lie parallel to the surface of the skin. Moving proximally along the digit the orientation of capillaries changes to become perpendicular or oblique to the surface.
Both orientations of capillaries are also found at other skin sites although the relative numbers vary. The development of the capillary loop can also vary, for example in forearm skin where the dermal papillae are not well developed the arterioles connect to capillaries which course close to the dermal—epidermal interface before joining a post capillary venule of the subpapillary plexus.
Total capillary density varies according to the area of skin examined and differences over small areas such as the dorsum of the foot have been reported [ 10 ]. Ageing is accompanied by a loss in dermal volume, a reduction in capillary density [ 11 ], shortened capillary loops, and rarefaction of larger microvessels [ 12 ] Examination of human capillaries-a historical perspective Thirty years after William Harvey elegantly first described the blood circulation, the tiny vessels which link the arterial and venous tree were identified [ 13 ].
Visualization of blood flow in these vessels, which are similar in diameter to red blood cells, was first made in frog capillaries by Anthony van Leeuwenhoeck and in the first microscopic examinations of human skin capillaries were conducted [ 14 ].
The initial measurements of capillary blood velocity were made in by Basher [ 16 ]. Subsequent measurements were reported in by Zimmer and Demis who used a microscope-television system to study the flow dynamics in human skin capillaries [ 17 ]. A new television-microscopy system introduced in by Bollinger and colleagues [ 18 ] showed that it was possible, using a frame-to-frame analysis of the movement of plasma gaps along a capillary, to examine capillary dynamics in healthy controls and patients.
This was improved and simplified by Fagrell et al. Cannulation of human capillaries and direct measurement of capillary pressure were first performed by Carrier and Rehberg in [ 19 ]. Even before this time indirect methods to estimate capillary pressure were reported [ 20 ] although the values obtained do not agree with direct measurements.
In Landis [ 21 ] published his seminal paper on measurement of capillary and venous pressure and the effects of physiological and pharmacological interventions.
- Relative Permeability Curves
- Comparison Between Capillary Pressure and Relative Permeability
- Hydrostatic and Oncotic Pressures
His technique involved introducing a micropipette attached to a water manometer into a capillary and adjusting manometric pressure until blood did not enter the micropipette tip. Manometric pressure at this equilibrium point represented mean capillary pressure. Further early studies using the manometric system in patients with hypertension [ 22 ], heart failure [ 23 ], and glomerulonephritis [ 24 ] were published and then interest in capillary pressure dwindled until the late s.
The disadvantage of the manometric technique is that only averaged pressure can be determined.