ARTIPRESS
3
Of course variations of the capillary permeability caused by increase in inter-
capillary hydrostatic pressure or toxic-anoxic factors for example, mean that there is
an increase in that transfer, in a modification of the lymph flow or its composition.
For further clarification, we provide Rossing’s findings on the matter:
1)
the intravascular masses of albumin and immunoglobulin depend on the
amount of synthesis and the amount of fractionated catabolism;
2)
the ratios of intravascular masses to the total masses depend on the amount
of trans-capillary escape and on amount of extra-vascular return;
3)
the amount of trans-capillary escapes are inversely related to the molecular
weights of the proteins; the amount of trans-capillary escape increases with the
filtration pressure in the vessels, i.e. with the loss at micro-vessel level, as seen in
diabetes mellitus;
4)
the amount of extra-vascular return reflects the lymphatic protein transport
and is inverse to the extra-vascular transit time. It is of the same magnitude for
albumin and lg.G and perhaps a bit less for lg.M;
5)
the extra-vascular transit time includes a wide series of transit times: short
(liver, kidneys, lungs), long (skin, muscles, with maximum deposit of extra-vascular
proteins);
6)
in most cases of hypoproteinemia, the amount of intra-vascular/extra-
vascular plasmatic protein distribution changes in favour of the intravascular space;
7)
the extra-vascular pathological build-up of plasmatic protein occurs in a
few diseases, and when the trans-capillary escape increases without a corresponding
increase in the amount of lymphatic return. This can be seen in cirrhosis with
ascites, in untreated myxoedema and in some types of cancer, particularly those with
hepatopathy and ascites. If anything, the extra-vascular build-up of plasmatic protein
will occur in the tumoral and post-operative tissue in the wounds.
The main function of the lymphatic system is therefore to allow its penetration by
the previously mentioned molecules, prevent escape, and encourage progression.
By way of the capillary filtration, protein molecules and water are plentiful in blood
circulation, thus causing a build-up of liquids osmotically linked to the protein in the
interstitial tissue.
The liquid leads to an imbibition of the tissue, stretching the endothelial cells of the
initial lymphatics and further opening the lymphatic inter-endothelial junctions.
Other movements intervene in keeping the junctions “open” :
–
muscular movement;
–
rhythmic contractions of the arterial vessels;
–
the negative interthoracic pressure;
–
the cells and any other elementary corpuscles that are pushed through the
open junctions in the initial lymphatics
During their transit, these corpuscles act like dilators, keeping the passage free
through the junction of the initial capillary lumen .
In the most active areas of the body, the products of cellular metabolism increase
blood flow and the capillary permeability, so liquid contained in the interstitial
tissue increases further, and its pressure keeps the input routes accessible to the
lymphatic capillaries.
An increase in local tissue pressure follows this initial stage, caused by muscular
contraction that compresses the initial lymphatics, and therefore pushing the lymph
to close the intercellular junctions.
In this phase, a certain quantity of water spreads out from the lymphatics and the
lymph itself becomes more concentrated than the interstitial liquid.
The higher compression releases the system from fibrils attached to the lymphatic
endothelium.
The lymphatics that have been compressed like this will obviously be smaller with
lower diameters, being adhesive endothelial cells and therefore overlapping with the
junctions hermetically closed.
At this point, the third phase begins: a further compression on the initial lymphatics
pushes the lymph through the first valve, the sudden lowering of pressure means that
the lymphatics expand again, and the intercellular junctions open up again.
This mechanism is called the “CASLEY-SMITH lymphatic pushing pump”.
Intercellular lymphatic junctions have been defined as “aspiration valves” while the
first lymphatic valve is known as the “escape valve”.
The initial lymphatics comprise several aspiration and pushing pumps whose
function is not strictly mechanical but can be adapted to different needs.
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Mc Master measured the pressure values both of lymphatic capillaries, and of the
interstitial space, finding:
lymphatic capillary pressure = 0,7 ± 0,3 cm. H
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interstitial space pressure = 1,9 ± 0,5 cm. H
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