Do blood capillaries expand? If yes, through what means can it be done? And can warm water also be used to expand blood capillaries?

  • $\begingroup$ It's considered a requisite on biology.se that at least some reading/research is carried out and presented with your question. For example, have you read about vasodilation and capillaries? Otherwise it will be closed as homework. The site tour and the help center provide guidance on how to use this site. Please take a few minutes to read about the kind of questions which are on topic here $\endgroup$ Feb 2, 2015 at 16:15
  • $\begingroup$ @paul What do you mean by "expand"? Do you mean "increase the number of capillaries by angiogenesis" or "vasodilate"? Given the warm water example, I'm guessing it is the latter? $\endgroup$
    – Raoul
    Feb 4, 2015 at 8:36

2 Answers 2


Capillaries are the smallest blood vessels. They are unique in that:

  1. They don't have any muscular component like arterioles or venules do.
  2. They are lined by a single layer of endothelium (cells lining blood vessels are called endothelial cells) overlying the basement membrane.

Due to these reasons the capillaries are very permeable facilitating nutrient and gas exchange.

The capillaries don't expand as they do not have muscular or elastic components. However exposure to heat do increase the circulation. This is due to the following facts:

  1. Exposure to heat dilates the pre-capillary arterioles - this would inevitably cause improved capillary circulation in capillaries that are already open.
  2. Increase in pre-capillary flow opens up collapsed capillaries. In the human body roughly only one-fourth of the capillaries are open during rest1. As the pre-capillary arterioles dilate, the collapsed capillaries open up thus further improving circulation.

To improve capillary perfusion:

  • Temporary improvement can be achieved by activity (exercise), exposure to warmth, emotions (flushing is due to vasodilation), even exposure to cold causes vasodilation after the initial vasoconstriction
  • Permanent improvement of capillary circulation can be achieved by increasing capillary density. Capillary density refers to the number of capillaries present in 1 cm2 area. Capillary density can be increased by consistent anaerobic (isometric and to some extent isotonic) exercise.


1: Boron and Boupaep text book of medical physiology, 2nd Edition, p.467

For more:

  1. http://corporate.dukemedicine.org/news_and_publications/news_office/news/6597
  2. http://www.ncbi.nlm.nih.gov/pubmed/8852622

Yes, some blood vessels change their diameter, they dilate or constrict. This is termed 'vasodilation' and 'vasoconstriction'. Large and mid-sized vessels contain smooth muscle in their walls. The walls of arteries tend to be stiffer than those of veins, to withstand high pressure coming from the heart.

Cross-sectional cartoon of the walls of arteries and veins.

Smooth muscle, when stimulated by the nervous system, can sustain contractions for long periods of time, and often with fine control. When smooth muscle in the walls of your blood vessels relaxes, those vessels will dilate.

Small capillaries do not have smooth muscle in their walls, but the flow of blood through them can be controlled by the larger vessels that feed them. When arterioles constrict, the capillaries they feed can have reduced flow.

Cartoon of a capillary bed.

Our nervous system takes in a lot of signals relevant to controlling blood pressure and flow, and controls these smooth muscles. Flow can be increased or decreased to individual organs when needed, like the intestine (after a meal), the muscles (during exercise), the skin (for heat exchange).

The question about warm water is actually quite interesting. Placing your arm in warm water will cause vasodilation in your other arm (the one not in the water), in order to lose some heat to the cold air and maintain your core temperature. Normally, the blood vessels in our 'extremities' (our hands and feet) are kept quite constricted to reduce heat loss with the cold air. If we exercise or take a hot bath our core temperature will rise, some people will notice their skin gain a red hue after these activities, due to vasodilation.

  • 2
    $\begingroup$ Very little of this answer addresses the OP's question. Can you expand on what, exactly, increases capillary flow and how? $\endgroup$ Feb 2, 2015 at 16:26
  • $\begingroup$ OP asks "by what means [can capillary expansion] be done?". As C Rags says capillaries don't have muscular or elastic components to their walls, but I (and he) have answered that the flow through them is controlled by contraction and relaxation of the smooth muscle of the arterioles that feed capillary beds. Flow resistance is inversely proportional to the diameter to the power 4 - decreases of diameter increase resistance steeply. Are you asking what environmental stimuli & nervous input causes increased capillary flow? I can't answer that - there's many (google), and OP didn't ask that imo. $\endgroup$
    – Teige
    Feb 3, 2015 at 12:37
  • $\begingroup$ As I said (I did not miss the above), very little of your answer was a direct response to the OP's question. That remains a fact. $\endgroup$ Feb 3, 2015 at 18:29
  • $\begingroup$ You're right. Honestly, I don't see much point in improving it: OP ought to read a textbook, as you commented. I tried to give some background since it can't be assumed from the question. $\endgroup$
    – Teige
    Feb 3, 2015 at 23:02
  • $\begingroup$ On 2nd thought, I disagree with you. All paragraphs of the answer contribute, having assumed OP knows little: -Larger vessels have smooth muscle -This smooth muscle changes their diameter -Precapillary vessels control capillary flow -Circulation is influenced by many things -Warm water is one The issue is purely that the question lacks any background awareness of vasodilation. I think the answer is an appropriate one for a bad question, but is not a good read for yourself. :) $\endgroup$
    – Teige
    Feb 3, 2015 at 23:17

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