A SCOTTISH appeal for blood donors will be launched in a fortnight's time - all because of a football match on June 10. Blood transfusion authorities are not suggesting that the Scotland versus Brazil World Cup match is going to lead to bloodshed and medical emergencies, but they do anticipate a drop in donations on the day of the match.

Before then, the Scottish Blood Transfusion Service will be looking for extra supplies to cover next weekend's May Bank Holiday. Its next shortage - although crises can hit at any time - is likely to be over the summer holidays when its regular pool of donors, made up of only 5% of the Scottish population, takes its annual break.

It is a service that tends to lurch from emergency to emergency, although Gordon Redpath, recruitment manager, acknowledged yesterday that the situation had never become so critical that operations had had to be cancelled. Frequently, however, supplies do drop below the three-day minimum level - that is when the 5% pool of donors that keeps the NHS in Scotland going is called upon to give emergency donations.

Yesterday, because national supplies in England and Wales had sunk below the minimum level, the Government announced it was expanding the age range of donors from 18-to-65 to 17-to-70. It said it hoped this move would bring forward at least 40,000 new donors, but warned that unless the number of donations increased, hospitals might have to postpone operations within weeks.

Such an option is not open, however, to the Scottish service. It first dropped the age from 18 to 17 (with parental permission) in 1989, in 1992 withdrawing the need for a 17-year-old donor to have a parent's permission. In April 1991, it raised the upper age limit to 70.

The thinking behind lowering the age of blood donation to 17 was not simply to increase the number of donors, but to give the service the opportunity of going into secondary schools and to put across its message to school-leavers.

Although some young people reach physical maturity at the age of 15 and others at 19, it was also felt that 17 was a reasonable threshold.

At the upper age limit, new donors are not recruited over 60, and, indeed, many long-standing donors are indignant when they have to stop donating blood at the age of 70. Again, that age is chosen somewhat arbitrarily - the quality of blood given by a young person is not necessarily any better than a

70-year-old or even 95-year-old.

However, the Scottish Blood Transfusion Service argues that, as people grow older, they become more vulnerable to illness and can therefore ill-afford to lose some of their own blood supply, even on a temporary basis.

Because of repeated blood donation appeals, the service inevitably suffers from a ''crying wolf'' syndrome. But, as medical technology improves, demand increases. On top of that, hospitals under pressure to reduce their waiting lists to meet Government targets are putting the service under pressure.

Medical trends are also raising demand for red blood cells - 3% up last year - and there are increasing calls for blood platelets, which can only be stored for up to five days.

''It is just medical science going forward. We are finding new ways of utilising blood,'' says Mr Redpath.

As if growing demand was not enough of a problem for the service, earlier this week the Government banned the use of blood products made from British plasma because of a ''theoretical risk'' they are contaminated with new variant CJD, the human form of mad cow disease. It means that the products used to treat haemophiliacs will now be imported from abroad, but the move still has the effect of reducing the pool of donors, albeit only slightly, as some regular donors' blood can be used only as a plasma source, for instance if they have visited countries with a high incidence of blood-related diseases such as HIV or malaria.

Other barriers to giving blood can be the use of aspirin or flu infection, but the trend among young girls to have their bodies pierced prevents them from giving blood for a year after the procedure because of the risk of infection. The same regulations apply to tattooing.

''It is all a matter of trying to make blood as safe as possible - it is a belts-and-braces scenario,'' says Mr Redpath.

Meanwhile, the service is gearing up for yet another public appeal for new blood donors - long term if possible, but in the short term to cover the expected shortages likely to occur during the football frenzy of the World Cup.

''Our big problem is that because the existing 5% of donors do tend to respond to emergency appeals, the vast majority of the general public does not think there is a great problem, and unfortunately that 5% is keeping the NHS going,'' said Mr Redpath.