What is blood?

It's red stuff that flows around the body. The end. /s

Understand the components of blood

Blood contains cells (e.g. red blood cells, white blood cells, and platelets) which are suspended in plasma. Plasma is mostly made up of water along with proteins and electrolytes. The percentage of blood that is made up of red blood cells (a.k.a. erythrocytes) is known as the haematocrit.

Know the cells that are present in normal blood: red blood cells, leucocytes and platelets

Red blood cells (erythrocytes)

Red blood cells, also known as erythrocytes, are the oxygen-carrying cells of the blood. This is because they contain haemoglobin, which has iron groups that can bind oxygen. The percentage of blood that is made up of erythrocytes is known as the haematocrit. Usually the haematocrit is somewhere around 45%.

White blood cells (leucocytes)

Leucocytes are important in defending us from infections. There are many different types of leucocytes, as described here.

Platelets

Platelets are important in forming blood clots. I have described some of the pathway here but as our understanding of coagulation has improved a bit (for instance, we now know that the extrinsic pathway is far more significant than the intrinsic pathway when it comes to clotting in vivo), I will be writing about it again in a later post.

Understand the blood count

The blood count is a common test used to investigate disease. It's basically what it says on the box: a count of all of the cells present in the blood. As well as counting all of the cells, some characteristics of cells are also considered, such as the size of red blood cells (a.k.a. Mean Corpuscular Volume, or MCV for short) and the amount of haemoglobin present. MCV is important as it can help to determine the cause of anaemia. For instance, microcytic (small) erythrocytes may be due to iron deficiency or a chronic disease, whereas macrocytic (large) erythrocytes may be caused by too much alcohol or liver disease.

If there are abnormalities on the blood count, a blood film can be examined. When looking at the blood film, it is important to look at the cells that are present and whether or not they look normal. Some blood cell disorders can be determined by looking at the cell morphology. Speaking of which, there are a few terms that might be helpful in describing cell morphology:

• Anisocytosis- Variation in erythrocyte size.
• Poikilocytosis- Variation in erythrocyte shape.
• Hypochromasia- Pale cell colour (hypo: few, chromasia: colour)
• Polychromasia- Many cell colours
• Schistocyte- Fragmented erythrocyte.
• Spherocyte- Spherical erythrocyte (rather than a flat disc). Erythrocytes should have a pale area in the middle where the nucleus has been removed. Spherocytes lack this pale area.
• Acanthocyte- Erythrocyte with a spiky shape.