EVERY day, hospitals in the North-East and North Yorkshire need at least 400 units of blood.

From hip replacements to emergency surgery, every major operation carried out requires plentiful supplies of blood.

Apart from surgery, blood is also needed to treat patients with anaemia, those with serious burns and women who have given birth.

Blood products are also essential in the treatment of bone marrow failure, leukaemia, after organ transplants and for patients undergoing chemotherapy.

Nationally, there are 1.7m donors, including approximately 180,000 in the North-East and Yorkshire.

Every day, the NHS requires donations from 9,000 of those registered donors, each patient giving a single unit (just under a pint) of blood at designated centres.

While red blood cells will keep in a chiller cabinet for up to 35 days, blood products, such as platelets, which are vital to treat conditions where the blood will not clot properly, only last five days.

Daily collections are made from public buildings across the region.

Mobile blood collection units can set up and take blood at various venues within a few hours.

Elsewhere in the region, a Bloodmobile, a fully-equipped mobile blood collection unit, is a familiar site in supermarket car parks.

Once collected, blood is transferred to one of the two regional blood processing centres.

Blood collected in the North-East is taken to the Barrack Road centre, in Newcastle, while donations from North Yorkshire volunteers are taken to a similar centre in Leeds.

At the centres, the blood is divided into red blood cells and platelets, and carefully screened for potentially harmful organisms, such as the Aids virus (HIV), hepatitis B and C, and syphilis. Each unit of blood is barcoded to ensure there is no chance of different donations being mixed up.

It is particularly important to ensure that different blood groups are kept separate.

Platelets extracted from donated blood have to be kept constantly moving to avoid premature clotting.

Every day, the region's hospitals collect a pre-booked allocation of blood from their nearest processing centre.

In an emergency, such as the aftermath of a major road accident, extra supplies of blood products can be rushed to hospitals by ambulance.

Laura Summers, the National Blood Service's communications officer for Yorkshire and the North-East, said that donors need to be in good health, aged between 17 and 60 and weigh more than 7lb 12oz.

"You should be fit and well on the day. We ask you not go give blood if you have a cold or anything that is potentially infectious," she said.

"If you have access to the Internet, you can find out about your nearest collection points by going to our website - www.blood.co.uk - and tapping in your postcode. It is as easy as that," said Miss Summers.

Donors are asked to fill in a routine questionnaire designed to stop someone who may have an infectious condition, such as hepatitis, from giving blood.

Nurses also carry out a quick finger-prick test to check whether potential donors are anaemic.

"Women sometimes have lower levels of haemoglobin in their blood because of their period," said Miss Summers.

Women who are pregnant should not give blood for at least 12 months after the birth of their baby.

Once donors pass through this screening process, they are asked by a nurse whether they wish to have a local anaesthetic injected into their arm before the blood is taken.

"Some people prefer this, others are quite happy to go ahead with the donation without an anaesthetic," Miss Summers added.

Giving blood usually takes between ten to 15 minutes, and donors are given a drink and snack before being allowed home or back to work.