Counting red blood cells calculation from practical 7

A lot of you seemed to struggle with the Red Blood Cell calculations in practical 7...let me try to clarify things a bit for you.

First of all, you had to calculate the volume of one large square on the haemocytometer:

So, when we say one large square, we mean the square I have highlighted with green.  You know from the diagram in your notes that the sides of the square are 1 mm long, and if you read the information about the haemocytometer, you will know that the depth of this square is 0.1 mm.

So, to calculate the volume of this square you have to do:

1 mm x 1 mm x 0.1 mm = 0.1 mm³ 

Questions 2a, b, c and d want you to covert 0.1 mm³ to other units.  The one I want to focus on is expressing 0.1 mm³ in μl.  This is actually very simple, you just need to know that:

                                                             1 mm³ = 1 μl

Which means that

0.1 mm³ = 0.1 μl

This is important, because Question 2e wants to you say how many red blood cells you had in a specific volume.  Well, lets imagine you counted, on average, 50 red blood cells in one large square, this simply could be written as 50 cells per 0.1μl, seeing as you know that the volume of the large square is 0.1μl. So that would be your answer to question 2e.

Great, so what about Question 2f.  This question asks you to determine the number of red cells per ml of your original sample. So, there are two things going on here:

 First, you have to remember that you diluted your sample before you counted any cells.  Remember, you were told to do a 1 in 100 dilution of your red blood cells, before you did any counting.  This means, to find out how many red blood cells were in your original undiluted sample, you have to undo this dilution.....seeing as you diluted 100x to get your diluted sample, this means your diluted sample will have 100x fewer cells in it than your original sample - so you have to multiply your number of cells by 100:

50 cells per 0.1 μl of diluted cells x 100 = 5000 cells per 0.1 μl of undiluted cells.

Fantastic, now you know that you had 5000 cells per 0.1 μl of undiluted cells, but remember, the question is asking for how many cells were in 1 ml.  For this you have to know how much bigger 1 ml is than 0.1 μl.  The way I would figure this out is:

I would convert 0.1 μl to ml

0.1 μl / 1000 = 0.0001 ml

(remember, 1 μl is 1000 times smaller than a ml, 

so to convert from μl to ml you divide by 1000)

Then I would see how many times 0.0001 ml fits into 1 ml, as this will tell me how much bigger that 1 ml is than 0.0001ml:

1ml / 0.0001ml = 10,000

Ok, I now know that the volume I have counted cells in (0.1 μl) is 10,000 times smaller than 1 ml, so to find out how many cells are in the bigger volume of 1 ml, I have to multiply my cell count by 10,000:

5000 cells per 0.1μl  x 10,000 = 50,000,000 cells per 1 ml

which could be written as

5 x 10⁷ cells ml^-1

Moving on....

Question 2g: Estimate the number of red cells present per ml of whole blood 

To answer this question you need to know two things: the red blood cell sample you were given was a 2 % sample, and whole blood contains 45 % red blood cells

So, from the previous question you know that there are 50,000,000 cells per 1ml original sample which was actually a 2 % sample.

So, to find out how many red blood cells there are in whole blood, which is 45 % red blood cells you can:

do 50,000,000/2 = 25000000 cells in 1 ml of a 1 % sample

and then  25000000 x 45 = 1125000000 cells in 1 ml of a 45 % sample

And there you have it!