What Are Blood Collection Tubes?

When you have to take a blood test, do you notice that the lab person collects your blood into a special tube marked ‘heparinised’? Let’s find out why a special vial is needed.

How blood clots
When you prick a finger, you notice blood comes out. If you leave it alone, the blood soon
dries into a thick, brownish ‘clot’. How does this happen?

If your blood didn’t clot, so much blood would come out of the prick that you would become very ill. But blood has its way of self protection. When there is an injury, the damaged tissue sends special chemical signals to the body. These signals are received by a kind of blood cells known as platelets. The platelets rush to the site of the prick.

Platelets are tiny storehouses of various chemicals, which do many things. Some of them signal to those cells which will start the healing process. But several of these chemicals (together called ‘clotting factors’) cause the activation of a molecule called thrombin. This is an enzyme present in blood that converts fibrinogen to fibrin. Fibrinogen is a protein that is dissolved in blood plasma. When thrombin acts on it, it becomes fibrin, which is insoluble. Fibrin molecules get deposited at the prick, and in a short time, completely seal off the wound. No more blood leaks now.

Many of the clotting factors need Vitamin K to work properly. Lack of vitamin K in the body causes unstoppable bleeding. Cabbages, grapes and green leafy vegetables are rich in vitamin K.

Heparin and other ‘anticoagulants’
When you give blood for a blood test, the blood will clot in the vial. This makes it
unsuitable for testing. Therefore, lab technicians use special vials that are coated with anticoagulants (Coagulation is the scientific name for clotting).

One such chemical is heparin (its CAS No. is 9005-49-6). This is a complex polysaccharide produced by white blood cells. When heparin is mixed with blood, it interferes with the action of thrombin. This prevents it from converting fibrinogen to fibrin.

Apart from use in blood tests, heparin is also used to treat people who have abnormal clotting within their blood vessels. It is used when open heart surgery is being done to keep the blood flowing, and also in other heart disease conditions.

Coagulation in the lab
Try this experiment in your biology lab. Take a glass slide, and put a drop of heparin* on
one side. Ask your teacher to prick your finger, and with a capillary, suck up some blood. Mix one drop with the drop of heparin, and on the other side of the slide, put a drop of plain blood. Leave the slide for a few minutes, and then watch under a microscope. What do you see?