Editor's note: Part two will appear on Wednesday, May 11.

Did you ever wonder why you don't "bleed to death" even with minor cuts? Why don't all breaks in the skin continue to bleed? How does blood know when it is OK to clot to prevent hemorrhage, but at other times lead to potential death when if a clot forms at the wrong time or in the wrong places? This article will be the first of two having to do with blood clotting.

In this first installment I'll be describing the actual process of clotting while in next week's installment I'll describe different situations that lead to clotting.

The blood that courses through our veins and arteries is composed of myriad entities. Besides red blood cells and white blood cells that have their own job descriptions, there are things called platelets. Their job is to do nothing unless alerted that blood vessel walls have been disrupted by a cut to the skin or deeper laceration.

These blood entities then go into action, which is to say they get "sticky" as they race to the bleeding site and gather their forces together to plug up the breach in the vascular system.

Another entity of blood that plays a role in clotting is called the thrombin system. Thrombins are several blood proteins that also get called into action when there is bleeding.

The whole process is called a cascade reaction because one protein activates the next, which activates the next one and so on until finally a substance called fibrin is produced.

You could think of fibrin as sticky strands. These strands also find their way to the breach in the blood vessel wall to adhere and clump up on it. The fibrin part of the clot binds up the platelet portion and voila! The bleeding has been staunched.

But now, back to my original question. If platelets and the thrombin system are so smart, why do clots sometimes form inappropriately when there is no breach of a blood vessel?

There are three general reasons that come into play and each to varying degrees:

1) Venous stasis. This term refers to a "slowing down" in blood flow. Most commonly you would identify this condition in situations that involve immobility, like sitting in an airplane for five or more hours, or being bedridden after major surgery or a with a severe illness. Platelets and thrombins get sort of restless with immobility and start looking for trouble.

2) Vessel wall injury might occur during certain surgeries or major trauma, especially those that involve any part of the body below the waist. Platelets and thrombins get activated appropriately, but the cascade is such that they don't know when to stop.

3) Other situations that inadvertently cause blood to get "sticky" are outside influences, most notably tobacco use and/or hormonal contraceptives or menopausal hormone replacement.

So am I recommending that you don't take that plane trip to Europe? Or that you don't use birth control pills if that is your choice of contraception? Or that you don't get that hip replacement you've been planning for? Of course not.

What I am recommending is that you and your health care provider assess your own personal risk for blood clots and do whatever you need to minimize those risks. If you smoke, quit. If you are sedentary most hours of the day, get up and exercise. If you need to take some medication to help prevent the tendency to clot, then do so.

• Agnes Oblas is an adult nurse practitioner with a private practice and residence in Ahwatukee Foothills. For questions, or if there is a topic you would like her to address, call her at (602) 405-6320 or email her at aoblas@newpathshealth.com. Her website is www.newpathshealth.com.

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