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Type: Multiple-Choice
Category: Nature and Science (Stories)
Level: Grade 8
Author: szeiger
Last Modified: 5 months ago

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Nature and Science (Stories) Question

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Have you ever had a cut or scrape? While you watched the wound bleed, did you wonder where the blood was coming from or how the blood would eventually stop coming out of the cut so that you didn’t bleed to death?

The blood came out of a little tube in your body. Just like water pipes carry water to your kitchen and bathroom, little pipes called blood vessels carry blood around the body.

The two main kinds of blood vessels are arteries and veins. The difference between the two is that, in the big round trip that blood makes around the body before returning to the heart, arteries carry blood away from the heart and veins carry blood back to the heart. There is also another kind of blood vessel, capillaries.

Capillaries are smaller than arteries and veins. They allow things to be exchanged between blood and the other things in the body so that both the blood and other body tissues have what they need.

A cut or scrape is a wound that slices into one of these kinds of blood vessels. A tear in a blood vessel is bad, because blood, which is necessary for your body to work correctly, can leak out. The body is programmed to know that and to try to fix the damage through a process called hemostasis.

The first step in hemostasis is called the vasoconstriction stage. In the vasoconstriction stage, muscles around the wound make the hole in the body smaller by constricting it like a boa constrictor. By making the hole smaller, the muscles make the situation a lot better for the body and ensure it has less of a chance of losing a lot of blood.

After the first step, the vasoconstriction stage, it is time for the second step, the platelet plug stage. This step deals with platelets, or little circles in the blood. Because a cut in the skin exposes them to the collagen in an outer layer in their blood vessel, platelets get sticky. They stick to each other and the blood vessel, and make a roadblock for blood. The roadblock keeps blood from getting close enough to the hole in the blood vessel to leak out of the body.

The third stage of hemostasis, which is called the coagulation stage, is the most complicated. It has three steps of its own. The first step is the combination of two chains of chemical reactions. Chemical reactions are ways that materials in nature interact with each other. When they come into contact with each other, they trade ions. Materials with too many ions try to give them away and materials with too few ions try to steal ions from other materials. When several materials called chemicals participate in multiple chemical reactions together, the combination of those reactions is called a chain of reactions.

As mentioned before, there are two chains of reactions in the first step of the third stage of hemostasis. One of the chains, which is called the extrinsic pathway, is between chemicals released by the area injured by the cut and chemicals in the blood. The other chain of reactions, called the intrinsic pathway, is between collagen, the stuff that also causes blood platelets to stick to each other, and other chemicals in the blood. Both of the chains of chemical reactions have many chemicals trading ions with each other to make new chemicals. Those chemicals, which react with each other, are called coagulation factors.

In the second step of the coagulation stage, the reaction makes thrombin. Thrombin makes the blood clot stronger. In step three, thrombin activates a protein called fibrinogen. Fibrinogen is usually in the blood, but it doesn’t do its job in the coagulation stage unless thrombin wakes it up by trading ions with it. That exchange of ions turns fibrinogen into fibrin threads, which are like tiny ropes. All of the ropes are woven together to make a net which catches blood and doesn’t let it go near where it can leave the body through the wound.

After turning fibrinogen into a net of fibrin threads, thrombin still isn’t done with its blood-clotting duties. Until the end of the blood clot-making, thrombin participates in chemical reactions that make the body keep working on the blood clot. When the body doesn’t need to keep clotting the blood around the wound because it has healed enough that blood isn’t in danger of leaving the body, the body makes anticoagulants, which make the process of blood-clotting stop.

The body has successfully kept its blood inside and worked to fix a wound in itself.

Good job, body.

Shannon, Marilyn M., & Yunis, Rachael L. (2013) Exploring Creation With Advanced Biology: The Human Body, 2nd Edition. Anderson, Indiana: Apologia Educational Ministries, Inc.

This piece was written by Katharine Voightschild.

Grade 8 Nature and Science (Stories)

What are capillaries?
  1. heart tubes
  2. skin covers
  3. main arteries
  4. blood vessels
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