Just as in his previous visit last December, Ned Kelly captured the attention of the audience from the start. This time his topic was the clotting of blood: how, why, when and where it occurs.

The first stage of blood clotting is when a cut on the skin or an internal injury creates a small tear in a blood vessel wall causing blood to flow. In response to the injury, platelets in the blood are activated and migrate to the site where they stick to the vessel wall. The activated platelets develop a spiky surface enabling them to stick together to form a plug. They also release molecules to turn on clotting factors, the most important of which is fibrin, a long, thin and sticky protein. This forms a mesh to hold the platelet plug in place. The mesh attracts red blood cells to form a blood clot and the body’s immune system kicks in to repair the damage, the clot disappearing when the repair is complete.

This mechanism was strikingly demonstrated with visual aids, to the amusement of the audience. A toy hedgehog was produced to represent the spiky platelets and a string of cooked spaghetti representing the fibrin. These will have a new significance for the audience in future! Slides were shown tracing the multiple chemical stages and different blood clotting factors, including calcium ions, involved in the production of blood clots.

Blood clots can differ in the number of platelets and fibrin they contain. Arterial clots block arteries which can cause a heart attack or stroke. They have more platelets than fibrin and require anti-platelet drugs, such as aspirin, for dispersal. Venous clots, which block veins can cause pulmonary embolisms and strokes. They have more fibrin than platelets. An anti-coagulant such as warfarin is widely used to treat these, though more effective (and expensive) drugs are available. A thrombosis is simply a clot of either type in the wrong place at the wrong time.

Haemophilia is a hereditary condition affecting males where the ability to clot blood is severely reduced. Females are carriers of the gene which causes this condition (notably Queen Victoria), but they are not affected by the disorder themselves.

Animals can be a source of pro and anti-coagulants. Leeches, for example, have hirudin, an anti-coagulant in their saliva which keeps the host’s blood flowing after the skin has been punctured; another one, draculin, is found in the saliva of vampire bats. An example of a naturally occurring pro-coagulant is found in Russell’s Viper which has a clotting agent in its venom. This venom is used in coagulation blood tests.

A lively question and answer session followed the interesting and informative presentation.