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How do capillaries constrict?

  1. Apr 20, 2010 #1
    Hello everyone,

    Capillaries are just a single layer of endothelial cells with no muscle tissue so how do these constrict? Thanks :smile:
     
  2. jcsd
  3. Apr 21, 2010 #2
    hey sameeralord, i'll do my best.

    what you are asking about has to do with vasculogenesis and angiogenesis. there is an important difference between the two processes.

    vasculogenesis refers to the formation of blood vessels when there are no pre-existing vessels present. at first this was thought to only occur during development, when endothelial cell precursors (angioblasts) migrate and differentiate in response to local cues (growth factors or ECM) to form new blood vessels. later on it was discovered that it can also occur in adult organisms. circulating endothelial progenitor cells (derivatives of stem cells) were identified and it was shown that they were able to contribute to neo-vascularization occurring in tumor growth and also the neo-vascularization in response to trauma after suffering cardiac ischemia.

    angiogenesis refers to the formation of new blood vessels from pre-existing blood vessels. a monolayer of endothelial cells will begin sprouting to form capillaries. endothelial cells proliferate and differentiate resulting in the formation of a tube structure (this process is mediated by VEGF, FGF, and TGFbeta). after this vascular branching and remodeling occurs. then pericytes are recruited and differentiate into smooth muscle cells, which will line the vessel in order to strengthen it so it is capable of holding flowing blood. what is important to remember is angiogenesis is induced by hypoxia, which results in new capillaries being formed.

    i'll also point out a third process which is comparable, yet different, known as areteriogenesis. this refers to the adaptation of an existing blood vessel to the flow of blood, often also associated with increase in smooth muscle. an experimental example of this process is when researchers removed a rabbit's jugular vein (low pressure) and put it in place of the carotid artery (high pressure). after 7 days the structure became more rigid, showing an increase in smooth muscle, cell proliferation, and apoptosis of the old cells, allowing them to be replaced by newer artery-like cells. what is important to remember is that arteriogenesis is induced by physical forces, especially sheer blood flow.
     
  4. Apr 21, 2010 #3
    WOOPS, just noticed you asked how they constrict, i read constrUct
     
  5. Apr 21, 2010 #4
    I do not believe the capillaries themselves constrict.
    The other arteries of the body go through a process known as "Vasoconstriction". Diverting or limiting blood from wherever the capillaries in question you are talking about are located.
     
  6. Apr 22, 2010 #5
    Thanks for the response :smile: However I'm still bit uncertain. For inflammation for an example they say capillaries dilate and become more permeable. Are you saying that when the artery attached to capillary constrict or dilate, the capillary follows in the same direction? Or when artery constrict or dilate they regulate how much blood flows to capillary?
     
  7. Apr 23, 2010 #6
    capillaries cannot constrict because they consist only of endothelial cells, and no muscle cells at all.
     
  8. Apr 24, 2010 #7
    It is correct that capillaries do not constrict because they are covered in a single endothealial layer, but further upstream pre-capillary arterioles contain a small myocyte layer and are the last vessel which have the ability to constrict before reaching the post-capillary venules. So blood control to the capillary bed is controlled via upstream constriction.
     
  9. Jun 17, 2011 #8
    Yes! Something I know about (my mother is a physiologist). Capillaries are a bit of an anomoly because they give the illusion of constriction, while they themselves are not the ones that create the movement. The information in this thread should be able to guide anybody who is interested from start to finish.
     
  10. Jul 16, 2011 #9
    That's my opinion as well.. capillaries, so far as we know, do not exhibit any from of constriction or dilatation. Its the precapillary sphincters containing smooth muscle in their walls that constrict and hence regulate the flow of fluid through them.
     
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