Friday, 17 February 2017

The consistency of blood

Arragh, boys, blood is thicker than water.
True, dat! Even in the metaphorical sense that family ties mean something. In my human physiology course, I preface our investigations of the circulatory system - blood-pressure, homeostatic control of - with an inventory of components. Mammalian blood - yours, mine, Fungie-the-dolphin's - is about 54% plasma; 45% red blood cells and 1% white-cells+ platelets = 100%. Other sources reconfigure these data as 99% of blood cells are RBCs or erythrocytes and 1% white cells. This 1% is trotted out so widely and with such certainty, that my skeptical hat starts whirling around on my head.

One of my other courses is 1st Year Cell Biology where I have a couple of lab sections most years. These classes meet, awkwardly, alternate weeks, so the course runs in two-week chunks with half the students off and half on at any one time. This is awkward because there are an odd number of weeks in the effective teaching term, so one lot of students gets ahead before Christmas. But that's okay because in the middle of term 2 is Rag Week - when Classes Continue As Usual [stated Institute policy] but nobody turns up. I carefully rejigged the order of practical classes so that Rag Week would complement the 'missing' week over Christmas and all the students would be back on a regular 2 weeks schedule. But then the Student's Union shifted Rag Week forward a week and my cunning plan for equity of class-time went down the toilet. Dang! those failures of communication.
My colleagues and I agreed that we could do something with prepared blood smears, which would build on the regular class on histology [microscopic structure of different tissues] which we've just finished. My soul rebelled against a strictly come microscopy plan of draw a T-cell, draw a neutrophil, compare and contrast an eosinophil with a basophil. Although the different sorts of white cell do have beautifully different structures, some of which can be cleverly related to function. I thought, contrariwise that we could all generate some data about the relative proportions of each type. Data is good, without data there is no science.  This is what we did:
Get a prepared blood-smear slide on your microscope; in focus; under a 40x objective [as see L]. Flip the field of view to some random place and count the white cells. The white cells are conveniently stained with blue nuclei and I've flagged them with red ticks so you know what I'm talking about. The red cells are the pale pink lads - pale pink because they have no nuclei; no nuclei so that can be packed full of oxygen carrying haemoglobin. There that's easy: there are four (4) white blood cells. But we want a % - to compare to Wikipedia's 1% figure - and that means counting the red blood cells. Whoa not so easy. I'll tell you that in the picture [L] there are 650 +/- 10; I know I counted them . . . by printing the picture out and putting a dot with a marker on each cell as I counted it.
Down the microscope that's not possible, so you have to make an informed estimate of the number of cells. This is what I did. I counted the cells across the diameter of the circular field of view. N=~50! That's probably correct-ish because the field of view at 40x is 0.45mm = 450μm. That says that my rbcs are each 9μm across - suspiciously close to the known diameter at 8μm.

The area of the whole field is πr2 25 * 25 * 3.14 = ~2000 cells but that's only true if the cells are cheek-by-jowl, wall-to-wall without spaces. I'd say that, in the picture, as on our prepared slides, about half the picture is cells and half spaces between cells, So the total cell count is about 1000, the class's indepnedent estimates of white cells in view were 1, 6, 4, 10, 8, 5, 4, 10, and 7 [average 6] so the answer is that 0.6% of blood cells are blue white. Which to the nearest whole % is 1% ! Well done us, we've verified something that everybody knew to be true. But . . .
It ain't what you don't know that gets you into trouble. 
It's what you know for sure that just ain't so.” Mark Twain
I've had occasion to cite that before.

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