AS Human Bio Unit 1 OCR

Taking a blood sample

- Tie tourniquet around elbow > vein stands out
- Clean area around vein > using alcohol-based solution
- Push needle into vein
- Needle is attached to syringe > pull back (blood is sucked up) and remove needle when you have the vol. of blood you want
- Hold cotton over wound > remove once it's stopped bleeding > apply dressing

Making & Staining a blood flim

- Place drop of blood on microscope slide
- Use another slide (the 'slider') and push it along slide to spread blood
- Allow to dry > so cells stick to slide
- Add alcohol > preserves the cells
- Add differential stain > Some parts appear darker and stand out

Leishman's and Wright's stains are examples of a differential stain.

Check that you know this without using notes (tick): O

Haemocytometer
A special counting chamber used to count erythrocytes and leucocytes.
Volume: 0.1 x 0.1 x 0.2 = 0.004mm³

Counting erythrocytes

- Draw pipette with blood up to 5 mark
- Draw Dacie's Fluid up to 101 mark > Bead will make blood & Dacie's mix=1 in 200 dil.
- Let first few drops run out & place blood in haemoc. and count red blood cells
- Use North-West Rule if necessary
- Only count cells in small 5 triple-lined squares and use following formula:

1/0.02 x no. of cells x 200
(or no. of cells x 10 000 = the same)

Counting leucocytes

-Draw blood up to 5 mark and diluting fluid up to 11 mark {mix} = 1 in 20 dilution
- Diluting fluid causes erythrocytes to burst, but leucocytes remain intact
- Use large squares which measure 1 x 1 mm. Each square has vol: 0.1x1=0.1mm³
1/0.4 x no. of cells x 20 OR 50 x no. of cells