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I fell over on my bicycle trying to avoid running over a rattlesnake, and ended up badly skinning my knee. It immediately began to bleed, but soon clotted.

It has been 2 days now and it has formed a thin scab and is healing well as far as I can tell. I was chilly from the air conditioning, and I noticed the wounded knee was much warmer than the healthy knee. It was very warm, while the other was rather cold to the touch.

I read here about skin healing, and thought, maybe it is because the blood rushed there to clot the wound. But, the wound is already clotted and sealed and the worst is over. So my question is:

  • Why is my wound and the area around it so much warmer days after the injury?
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  • $\begingroup$ This is personal medical question territory. Your knee is not yet fulled healed so it is reasonable to expect the ongoing healing process would produce heat. Heat can also be a sign of infection. $\endgroup$
    – Amory
    Commented Sep 25, 2013 at 4:37
  • $\begingroup$ The wound is warm because of increased blood flow to the area. This may be part of the natural inflammatory process, or you may have infection. Wait and see what happens. $\endgroup$
    – Kenshin
    Commented Sep 25, 2013 at 5:02
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    $\begingroup$ @Amory: Cryptic didn't ask what to do about it or if it is unhealthy. I think it is perfectly fine to ask about the biological reason for this, even if a personal example is used. $\endgroup$
    – skymningen
    Commented Sep 25, 2013 at 6:15
  • $\begingroup$ @Amory: I'm not asking for medical advice. I want to know why wounds are warmer than normal, please don't misread the question. $\endgroup$
    – kittycat
    Commented Sep 25, 2013 at 6:22

3 Answers 3

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This is a normal part of the inflammatory process. Inflammation is your body's localised defensive response to tissue injury of any kind, and it is characterised by four cardinal signs- redness, swelling, heat and pain (severe inflammation involves a fifth, loss of function).

Your body is performing three main processes here:

  1. Mobilisation of the body's defences.

    Involves release of vasoactive chemicals, which cause the blood vessels to dilate and increase blood flow to the area. This causes the pain and heat, The blood vessels also produce cell-adhesion molecules which make immune cells (leukocytes) stick to the walls of the your blood vessels.

  2. Containment and destruction of pathogens.

    The immune system then goes to work clearing up any bacteria which have got into the wound. The leukocytes release chemicals called cytokines to attract more immune cells, to contain and destroy bacteria.

  3. Tissue cleanup and repair.

    Dead cells, cell fluid and other debris (pus) is then absorbed by the lymphatic system. Platelets and endothelial cells then secrete growth factors which stimulate cells called fibroblasts to multiply and produce collagen. The heat also increases metabolic rate and increases the rate of tissue repair.

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  • $\begingroup$ Do you think you could provide times to your answer as well (e.g. after 3 hours this happens). I'm curious as to the timeline of normal inflammation compared to the questioner's timeline of heat after 2 days. $\endgroup$
    – Kenshin
    Commented Sep 26, 2013 at 0:25
  • $\begingroup$ there is increased blood flow during healing as well. also there is some secretion (perhaps histamine) which makes wounds itchy.. the reason for which i dont know myself.. and some involuntary rubbing can again trigger blood flow.. $\endgroup$
    – WYSIWYG
    Commented Sep 27, 2013 at 17:02
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The increase in temperature is due to dilatation of the capillaries in an area of inflammation, here brought about by trauma and is one of the five cardinal signs of inflammation'. This is a natural process which helps to bring in more blood supply for repair and removal of damaged tissue components.

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Skin wound healing is a dynamic and regulated process of cellular and molecular mechanisms and starts directly after skin injury.

  1. The first stage of physiological or acute wound healing is about the hemostasis and the formation of a provisional wound matrix. It occurs immediately after injury. When skin injury breaks up the epidermal layer, blood and lymphatic vessels are traumatized, flushing the wound to remove microorganisms and antigens. The different clotting cascades are then initiated by clotting factors from the injured skin (extrinsic system), and thrombocytes get activated for aggregation by exposed collagen (intrinsic system). At the same time the injured vessels follow a 5- to 10-min vasoconstriction, triggered by the platelets, to reduce blood loss and fill the tissue gap with a blood clot comprised of cytokines and growth factors. This blood clot contains fibrin molecules, fibronectin, vitronectin and thrombospondins, forming the provisional matrix as a scaffold structure for the migration of leukocytes, keratinocytes, fibroblasts and endothelial cells and as a reservoir of growth factors.
  2. The vasoconstriction is then followed by vasodilation in which thrombocytes invade the provisional wound matrix as discussed in this article. In addition, platelets influence the infiltration of leukocytes by the release of chemotactic factors. Both platelets and leukocytes release cytokines and growth factors to activate the inflammatory process which results in local warmth (hyperemia) and edema (swelling) of the wound.
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