AMcG wrote:
Hi Malden
Seems like you have stirred up a bit of a hornet’s nest.
My point 4. Venous blood can easily open a capillary is based on this statement from Krogh’s Nobel Lecture:
“very low venous pressure is sufficient to fill capillaries whose walls are flaccid, while the high arterial pressure cannot force entry into a contracted capillary.”
Alan
Seems like you extract one sentence from context. Its part of experiment on frog tongue. Totaly different context than you implied.
Whole paragraph is:
Quote:
First, I have attempted to solve the question as to whether capillary changes in the tongue of the frog are independent of the arteries. When the tongue of a frog is spread across a glass slide, it is a particularly suitable specimen for investigation. It is translucent and, on one side, a smooth mucous membrane is found, with a widely distributed capillary network, in which the individual capillaries are readily responsive to excitation, and where both the small arteries and veins can be observed directly. So long as the tongue is not excited, the great majority of these capillaries are closed, and the tongue is extremely pale and bloodless. It is well known that mechanical irritation of human skin by, for example, a scratch from a needle, produces a red streak, because the vessels fill with blood. A corresponding reaction can be produced very easily on the tongue of the frog, and under the microscope one can show that this depends, in the first instance, on capillary dilation, and one can bring about dilation of a single capillary, or part of a capillary. Fig. 8 shows such a capillary, part in uninfluenced condition, part after it has been lightly scratched in the middle. Fig.9 shows how it is further possible to find a completely closed capillary by careful scratching along a tiny venous branch (v). Blood passes out from the vein, and fills a part of the capillary, but does not flow through. By further scratching, one can obtain the capillary filled bit by bit, until it makes connection with an open capillary or artery, when flow suddenly commences. This experiment is important because it shows that the very low venous pressure is sufficient to fill capillaries whose walls are flaccid, while the high arterial pressure cannot force entry into a contracted capillary.
Fig. 9. Effect of repeated mechanical irritation on the tongue of the frog (v = tiny venous branch).
...conclusion:
Quote:
This behaviour is of great theoretical and practical importance. One can conclude from this and from many other analogous facts that, when appreciable capillary dilation occurs, it cannot be because of a simple rise in arterial blood pressure but must depend on change in condition of the capillary walls - a relaxation of their contractile elements. Widening of the arteries, on its own, only leads to a higher pressure in the capillaries, and a more rapid flow of blood through them. The greater or lesser red colouration of an organ depends, in the first instance, on the blood content of the capillaries, and one is therefore justified in concluding that, in all such cases, where there is marked redness, we are dealing with widened capillaries. However, only closer examination can give information as to whether, at the same time, the arteries are widened, and capillary pressure is also high. The capillaries in human skin become, for example, widened with strong heat and by excitation with strong light, and with different emotions, while cooling to a certain level and psychological depression results in contracted capillaries. The detailed mechanism of these reactions has not yet been studied. By various chemical agents, the capillaries can be made to dilate, and urethane, for example, in fairly strong solution, causes very great dilation of the capillaries of the frog's tongue, while it has no effect on the arteries. With urethane, one can produce the situation depicted in Fig. 10, where blood is pumped out from a very narrow artery into a capillary, which widens more and more, and retains all the blood corpuscles it receives. The capillary walls widen so forcefully that they become permeable to blood plasma, so that the capillary is at last filled by a solid mass of corpuscles. This observation is not irrelevant to the problem of oedema, and is at the moment being more closely investigated by my colleague, Dr. Harrop, along the lines of a determination of the size of openings which appear in the walls of the capillaries. Suitable substances are introduced into the blood through a vein. A group of capillaries in the tongue of the frog can be caused to dilate themselves, and observations made on the substance passing out through the capillary walls. If the colloidal dye "vital red" was administered, followed by application of urethane, a fine red stripe appeared alongside the widened capillary. If Indian ink, whose particles are on the borderline of the microscopically visible, was administered, the widened capillaries showed themselves closed towards that substance. We have similarly ascertained that molecules of starch can pass through.
....So, your point:
My point 4. Venous blood can easily open a capillary
can be:
My point 4. Venous blood can easily open a capillary after it has been lightly scratched (with a needle).
But we don't stratch our brain with a needle, aren't we?
M.
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