Preventing blood loss

When a blood vessel is damaged

• Bleeding occurs• A cascade of reactions occur in

the blood• A blood clot forms to stop

continued blood loss

Excessive bleeding – what do you do?

Seek medical help – but while you’re waiting.....

• If possible put on some disposable gloves• Reassure the person – get them to sit / lie down• Look carefully at the wound – it might be necessary

to cut the clotting away to see it clearly, e.g. make sure there is no glass there.

• If there is nothing in the wound, place a large pad of clean cloth onto the wound and press it down firmly using your hand.

• Use a bandage to hold the pad in place

If there is an object stuck in the wound..

• Don’t remove it!• Make a pad in the shape of a ring and place it on

the wound so that it surrounds the object• Use a bandage to apply pressure on the ring

around the sides of the wound. The pressure should push the edges of the wound together.

• If the wound is in an arm or leg, raise it higher.• If the blood soaks through the first pad, don’t

remove it but put another on top.

How blood clots

The actual process..• When tissues are damaged they are exposed to

the air. Collagen fibres (in the connective tissue) are exposed and platelets stick to them.

• The platelets release a substance that makes them sticky, the platelets clump together to form a plug – this forms an initial barrier. Calcium is needed for this process.

• Leucocytes collect at the site and the exposed tissues just below the endothelium release an enzyme called thromboplastin.

• Platelets also break down and release thromboplastin.• Thromboplastin catalyses the conversion of an inactive

plasma protein, prothtrombin into thrombin. This reaction also requires calcium ions.

• Thrombin is an active enzyme. It hydrolyses a large soluble plasma protein called fibrinogen into smaller units.

• The smaller units join together (polymerise) to form long, insoluble fibres of fibrin (a protein)

• This process also requires calcium ions.• The fibrin fibres pile up and form a mesh over the wound.

Blood cells become trapped in the mesh and form a blood clot. The clot dries to form a scab, this prevents further blood loss. It also stops pathogens getting into the wound.

Thrombin• Thrombin is an enzyme• Enzymes are proteins

that catalyse a specific reaction in the body.

• It does this by reducing the activation energy needed for the reaction to occur.

Enzyme facts• They are globular proteins with a highly specific

tertiary structure. • Enzymes have an active site. This is exactly the

right shape for one specific substrate to fit in. It works like a key fits into a lock.

• An enzyme-substrate complex is formed. Due to the very close fit the enzyme exerts forces on the substrate and the activation energy required is lowered.

• After the reaction has taken place, the enzyme is unchanged and can be used over and over again.

What affects enzyme activity?

• Enzyme concentration: low concentration = low rate of reaction. For the rate of reaction to remain high there also needs to be a constantly high concentration of substrate molecules.

Fixed concentration of substrate

If substrate concentration is also increased