Clotting — answer key

1. CQ1 · Practice

Model answer. A clot seals a cut. It prevents blood loss, and it closes the opening in the skin, so it prevents pathogens from entering the body.

A full answer includes

• I chose: it prevents loss of blood at the cut.
• I chose: it prevents pathogens entering the body.
• I did not choose digesting bacteria: that is what phagocytes do.
• I did not choose stopping bacteria multiplying in the blood: a clot acts only at the cut.

Also accept

• prevents blood loss; prevents entry of pathogens

Watch for

• “It digests bacteria that enter the cut.” — That is what phagocytes do. A clot stops pathogens getting in; it does not digest them.
• “It prevents bacteria multiplying in the blood.” — A clot forms only at the cut and seals it. Bacteria that are already in the blood are destroyed by white blood cells, not by a clot.

2. CQ2 · Practice

Model answer. Platelets, fragments of cells, gather at a cut and help the blood to clot.

A full answer includes

• I chose platelets.
• I did not choose red blood cells: they are trapped in a clot, but platelets help it to form.

Also accept

• platelets

Watch for

• “Red blood cells.” — Red cells are trapped in a clot and give it its colour, but platelets are what help start it.
• “Lymphocytes.” — Lymphocytes produce antibodies. They do not make the blood clot.
• “Haemoglobin.” — Haemoglobin is the protein in red cells that carries oxygen. It plays no part in clotting.

3. CQ3 · Practice

Model answer. Before the cut, his plasma carried fibrinogen, a soluble protein, so it stayed dissolved and flowed with the blood. At the nick it was converted into fibrin, which is insoluble. The long fibrin threads formed a mesh that trapped blood cells: a clot that sealed the nick.

A full answer includes

• (1) I wrote fibrinogen.
• (2) I wrote soluble.
• (3) I wrote fibrin.
• (4) I wrote mesh (or: net).
• I did not swap fibrinogen and fibrin.

Also accept

• (1) fibrinogen; (2) soluble; (3) fibrin; (4) mesh

Watch for

• “Fibrin is converted into fibrinogen.” — The soluble protein in the plasma, fibrinogen, becomes the insoluble threads, fibrin: fibrinogen to fibrin.

4. CQ4 · Practice

Options as numbered on the pupil sheet: 1 — fibrinogen; 2 — a platelet; 3 — a white blood cell; 4 — a red blood cell; 5 — a fibrin thread.

Model answer. T is one of the long threads of fibrin that form the mesh across the cut. U is a red blood cell trapped in the mesh. V is a platelet, a fragment of a cell, stuck to the cut edge of the wall. Fibrinogen is not lettered: it is soluble, dissolved in the plasma, so it cannot be seen; at a cut it is converted into the insoluble fibrin threads. The white blood cell caught in the mesh has no letter.

A full answer includes

• I matched T to a fibrin thread.
• I matched U to a red blood cell.
• I matched V to a platelet.
• I did not match T to fibrinogen: fibrinogen is dissolved in the plasma, so it cannot be seen.
• I did not match V to a red blood cell: V is a small fragment stuck to the wall.

Also accept

• T fibrin; U red blood cell; V platelet

Watch for

• “T is fibrinogen.” — Fibrinogen is soluble: it is dissolved in the plasma and cannot be seen. The threads you can see are fibrin, made from it at the cut.
• “V is a red blood cell.” — V is a small irregular purple fragment stuck to the wall: a platelet. The red discs with paler centres are red blood cells.
• “U is a white blood cell.” — U is a red disc with a paler centre and no nucleus: a red blood cell. The cell with a purple nucleus in lobes has no letter.

5. CQ5 · Practice

Model answer. Cut B. A clot seals a cut, so it prevents pathogens entering the body. Cut A is sealed after about 4 minutes, but cut B keeps bleeding for about 20 minutes, and while it bleeds it stays open. So there is about five times as long for pathogens to enter through cut B, such as bacteria from the skin or from things the hand touches. Cut B also loses more blood.

A full answer includes

• I chose cut B.
• I wrote that cut B stays open for longer before a clot seals it.
• I wrote that pathogens, such as bacteria, can enter through an open cut.
• I wrote that cut B gives pathogens more time to enter.
• I did not write that cut B is safer because the blood washes the bacteria out.

Also accept

• cut B: it stays open for longer, so pathogens have more time to enter

Watch for

• “Cut B is safer, because the blood flowing out washes the bacteria away.” — Some dirt may be washed out, but a cut that is bleeding is still open. Until a clot seals it, pathogens can enter, and cut B stays open far longer.
• “Both are equally likely to be infected, because the cuts are the same size.” — The size is the same, but cut B stays open about five times as long. The longer a cut is open, the more time pathogens have to enter.

6. CQ6 · Practice

Options as numbered on the pupil sheet: 1 — not a role of clotting; 2 — prevents blood loss; 3 — prevents pathogens entering.

Model answer. The nosebleed stops because a clot seals the cut vessel: it prevents blood loss. Once a graze is sealed by a clot, soil bacteria cannot get in through it: it prevents pathogens entering. Phagocytes digesting bacteria and lymphocytes making antibodies are the work of white blood cells against pathogens that are already inside, not roles of clotting.

A full answer includes

• I matched the nosebleed stopping to preventing blood loss.
• I matched the sealed graze keeping out soil bacteria to preventing pathogens entering.
• I matched the phagocytes digesting bacteria to "not a role of clotting".
• I matched the lymphocytes making antibodies to "not a role of clotting".
• I did not match the phagocytes to preventing pathogens entering: those bacteria had already entered.

Also accept

• nosebleed: blood loss; sealed graze: pathogens; phagocytes and antibodies: not clotting

Watch for

• “Phagocytes digesting bacteria in the cut shows clotting preventing pathogens.” — Those bacteria have already got in. Clotting keeps pathogens out by sealing the cut; destroying them inside is the work of white blood cells.
• “A nosebleed stopping shows clotting keeping pathogens out.” — The nosebleed stopping is about the blood: the clot prevents blood loss.

7. CQ7 · Practice

Options as numbered on the pupil sheet: 1 — Fibrin threads form a mesh that traps blood cells.; 2 — Fibrinogen is converted into fibrin.; 3 — The top of the clot dries into a scab.; 4 — Platelets gather at the cut.; 5 — Blood escapes from the cut vessel..

Model answer. A cut lets blood escape. Platelets gather at the cut. Soluble fibrinogen is converted into insoluble fibrin, whose threads form a mesh that traps blood cells: the clot. Its top dries into a scab.

A full answer includes

• I put "blood escapes from the cut vessel" first.
• I put "platelets gather at the cut" second.
• I put the conversion of fibrinogen into fibrin before the mesh traps blood cells.
• I put the scab last.
• I did not put the scab before the fibrin mesh.

Also accept

• escape; platelets; fibrinogen to fibrin; mesh traps cells; scab

Watch for

• “The scab forms first, then the fibrin.” — The scab is the dried top of the clot, so the fibrin mesh must form first.

8. CQ8 · Practice

Model answer. Because fibrinogen is soluble, it flows along with the plasma and forms no threads while blood is in the vessels. If insoluble fibrin formed there, its mesh would make clots that could block vessels and stop blood reaching organs. Converted only at a cut, the mesh forms where it is needed, to seal the cut.

A full answer includes

• I wrote that soluble fibrinogen flows along with the blood.
• I wrote that fibrin forming inside the vessels would make clots that could block them.
• I wrote that the mesh is needed only at a cut, to seal it.
• I did not write that fibrinogen is soluble so that it can carry oxygen.

Also accept

• soluble so it flows and does not block; fibrin in vessels would block them; mesh needed only at the cut

Watch for

• “Fibrinogen is soluble so that it can carry oxygen.” — Oxygen is carried by haemoglobin. Fibrinogen's job is to become fibrin at a cut; being soluble keeps it from forming clots everywhere else.

9. CT1 · Apply

Model answer. (a) Preventing blood loss: the clot seals the torn vessels, so less blood leaks out of them into the tissue, and the bruise stays smaller. (b) The other role, preventing pathogens entering the body, is not needed: the skin is not broken, so there is no opening through which pathogens could get in.

A full answer includes

• (a) I suggested preventing blood loss.
• (a) I explained that the clot seals the torn vessels.
• (a) I explained that less blood then leaks out of them into the tissue.
• (b) I wrote that the other role is preventing pathogens entering the body.
• (b) I explained that the skin is not broken.
• (b) I explained that there is then no opening for pathogens to get in through.
• I did not write that the clot stops the bruise becoming infected.

Also accept

• (a) preventing blood loss: the clot seals the torn vessels; (b) preventing pathogens entering is not needed: the skin is not broken

Watch for

• “The clot stops the bruise becoming infected.” — Pathogens get in through an opening in the skin. Here the skin is not broken, so there is no opening for a clot to close.
• “No role matters, because no blood leaves the body.” — Blood is still lost from the vessels into the tissue. Sealing the torn vessels stops more of it leaking out.

10. CT2 · Apply

Model answer. In tube B nothing stopped clotting. The soluble fibrinogen in the plasma was converted into insoluble fibrin, whose threads formed a mesh through the blood. The mesh trapped the red blood cells, so the blood set into a red jelly: a clot. In tube A the chemical prevented this.

A full answer includes

• I wrote that soluble fibrinogen in the plasma was converted into insoluble fibrin.
• I wrote that the fibrin threads formed a mesh.
• I wrote that the mesh trapped the red cells.
• I wrote that this set the blood into a jelly: a clot.
• I did not write that the red cells alone stuck together to make the jelly.

Also accept

• fibrinogen to fibrin; mesh traps red cells; jelly is a clot

Watch for

• “The red cells stuck together and formed the jelly.” — The red cells give the colour, but the fibrin mesh holds them in place: that is what makes the jelly.

11. CT3 · Apply

Model answer. At a cut, a clot seals an opening. Inside a coronary artery there is no opening, so the clot only blocks the artery. Blood cannot flow past it, so the heart muscle beyond it gets no oxygen or glucose. That muscle cannot respire, so it stops working and may die: this is a heart attack.

A full answer includes

• I wrote that the clot blocks the artery.
• I wrote that blood cannot reach the heart muscle beyond the clot.
• I wrote that this muscle gets no oxygen.
• I wrote that without oxygen the muscle cells cannot respire.
• I wrote that the muscle may stop working or die (a heart attack).
• I did not write that the clot helps by stopping blood loss: there is no cut to seal.

Also accept

• blocks the artery; no blood, so no oxygen, to the heart muscle; the muscle cannot respire and may die

Watch for

• “A clot is always useful, because it stops blood loss.” — Inside an artery there is no opening to seal. The clot only stops blood flowing to the muscle beyond it.