your body’s pretty good at fixing itself

June 6, 2009

Macrophage taking down some bacteria (the small, cylindrical shapes) (http://www.dimethaid.de/wundheilung/informationen_en.html)

Macrophage taking down some bacteria (the small, cylindrical shapes) (http://www.dimethaid.de/wundheilung/informationen_en.html)

I did most of my undergraduate research in the area of wound healing, so I have some experience with the process. But I recently sustained some scrapes and pulled tendons on my hand while white water rafting, and my observations of the healing process renewed my interested in the macroscopic process. The places where skin used to be scabbed over and turned red around the edges, and my thumb/hand swelled up a good deal. Watching our flesh repair itself is something we all do every now and again, but seldom do we actually think about what’s going on in there.

I did some research and found a good review paper on the topic of soft tissue wound repair. What follows is a general overview of how your body responds to a soft-tissue (skin, muscle, tendon, etc) wound. Those of your who like more details should check out the review article.

One of the most fascinating things about the response to tissue damage is that it’s pretty much a one-size-fits-all policy. Whether you get a cut, or a bruise, or a twisted ankle, your body does basically the same steps each time, just for different amounts of time.

The four steps to wound repair are basically 1) bleeding, 2) inflammation, 3) proliferation, and 4) remodelling.

Bleeding obviously occurs in almost every wound, since pretty much any wound damages vascularized tissue (most tissue needs blood supply to function, but some like muscle needs more than other like tendon). Upon injury, platelets in your blood stream begin to stick together to staunch both the flow of blood out and the flow of foreign microorganisms and other debris in. Later, other proteins are laid down to reinforce the clot. Your body knows it’s been injured when either macrophages (defense cells) come in contact with foreign cells it doesn’t recognize and/or proteins and other stuff inside your cells leak into the blood stream, where they shouldn’t be if the cell is healthy. For both cases, the inflammation response is the same.

In inflammation, both vascular and cellular processes occur. Your blood vessels dilate. Previously unused capillaries are opened to allow more blood to enter the wound region, bringing in new supplies and cells used in infection defense and tissue repair. Blood vessels become more leaky, which allows fluids and other things to flow into the surrounding tissue, which causes most of the swelling that occurs in any wound inflammation. The increased fluid can dilute any harmful substances at the wound site and prevent them from spreading to the rest of the body.

One the cellular front, macrophages and other types of defense cells gobble up any dead, dying, or foreign cells. In doing so, they release lactic acid, which stimulates the next phase of the process, repair. The lymphatic system slowly removes the detritus over a few days.

In tissue repair, fibroblasts (basically, the general contractors of the body) fill in new connective tissue and promote new growth from surrounding, healthy tissue. Fibroblasts lay down new collagen (a protein that basically acts like a scaffold for other tissues), which is contracted over time to make the area of the wound smaller. Interestingly, in the final remodelling phase, these collagen fibers are reorganized in the direction of the tensile stress on the tissue. As time progresses, the wound area becomes even smaller, as the collagen continues to contract, which ensures that the weaker area of the wound site is minimized to avoid future damage.

So there you have it. Our bodies are remarkably good at what they do. So next time you get a cut on your hand or leg, think about all the amazing stuff your body’s doing to repair itself.

Now, if only we could make our machines do all that.

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