Why veterinary nurses should get excited to look at blood smears (and how to do it)
A couple of weeks ago, I asked you who looked at the blood smears in your practice. Here’s what you said:
74% of you said the vets do them all, and only 26% of you looked at them yourselves.
I wanted to know why so many practices weren’t using their nurses to do this. There’s no reason why we can’t, and it’s another new skill for us to develop! So I dug deeper - and you told me that it was all about not feeling comfortable with what you’re looking at on the slide.
So we’re starting a new series over here on the blog, and on instagram and facebook - all about haematology nursing!
We’re going back to the start to begin with - so that you can understand the different cells we see, how they work, and what they mean for our patients.
Our blood is made up of several major components. These include:
Red blood cells, which transport oxygen
White blood cells, which fight infection, maintain immunity, and are involved in allergic responses
Platelets, which stem haemorrhage
Clotting factors, which stem haemorrhage
Plasma, which contains a number of solutes dissolved in fluid, and is the fluid in which our blood cells are suspended
Red Blood Cells
Red blood cells or erythrocytes transport oxygen through the bloodstream. These cells travel from the lungs to cells and tissues. Oxygen is required for the production of cellular energy (the conversion of glucose into ATP). Their role is vitally important in maintaining normal function.
Erythrocytes have a specific shape and structure. This allows them to travel through blood vessels at high speeds and through tiny capillaries.
They have a classic bi-concave disc shape, and are enclosed within a lipid and carbohydrate-containing cell membrane. The lipids in this membrane are responsible for maintaining the shape and surface area of the cell. The types of carbohydrates present determine the patient’s blood group.
The presence of haemoglobin allows erythrocytes not only to transport oxygen around the bloodstream. It also maintains the structure and shape of the cell, by changing the viscosity of the cytoplasm within it.
Each red blood cell contains four iron-containing haem units, and four globin units. Suspending the haem units preventing them from coming into direct contact with each other, which reduces their function.
Oxygen reversibly binds to the haem units within the red blood cell, after gaseous exchange occurs in the capillaries around the alveoli in the lungs.
The oxygen-rich red blood cells (now full of oxyhaemoglobin) travel to their destination via the bloodstream. Here, they release the bound oxygen into the cells.
Carbon dioxide is a waste product of cellular respiration. It is deposited into the bloodstream at this point for transport to the lungs and removal from the body. Carbon dioxide is transported in three ways:
20-30% is reversibly bound in haemoglobin
7% is dissolved within the blood
70% is converted to carbonic acid and transported inside red blood cells.
Red cells last for approximately 2-5 months in the circulation in normal circumstances, after which they mature. Dead and dying red blood cells are removed from the body via the spleen.
New red cells are produced in the bone marrow (more on this later!). They evolve into a reticulocyte or immature erythrocyte, which are then released into the bloodstream.
White Blood Cells
White blood cells or leukocytes are differentiated based on the presence or absence of granules with their cytoplasm:
Granulocytes have granular cytoplasm. These are also known as polymorphonuclear leukocytes since their nuclei are lobulated.
Agranulocytes have no (or very small numbers of) granules within their cytoplasm. They are mononuclear (have only one nucleus).
There are five different white blood cells:
Neutrophils
Eosinophils
Basophils
Lymphocytes
Monocytes
Neutrophils
Neutrophils are the most common leukocyte in the bloodstream. Their primary function is to ingest and kill bacteria and fungi within the body.
They are granular (although we often do not see these granules when we examine a blood smear). They have a polymorphic nucleus which becomes more lobulated as the cell ages.
Within the blood vessels neutrophils exist in two ‘pools’.
The circulating pool, where neutrophils are circulating in the bloodstream and,
The marginal pool, where neutrophils have adhered to vessel walls.
Neutrophils last for 4-6 hours (dogs) or 10-12 hours (cats) within the circulating pool. All neutrophils within the blood vessels are replaced every 2 days.
‘Band’ or immature neutrophils are slightly larger than mature neutrophils. They do not have a lobulated/segmented nucleus. They are more ‘U’ shaped. They can be differentiated from a monocyte as these cells are larger, and have a thicker nucleus taking up a larger portion of the cell.
Eosinophils
Eosinophils are leukocytes with lobulated nuclei and characteristic pink granules within their cytoplasm.
Their functions include phagocytosis and destruction of bacteria, yeasts, mycoplasmas and other pathogens.
They are also involved in type 1 hypersensitivity reactions (allergic reactions), as well as responding to parasite burdens within the body.
Only a small number of eosinophils are present within the circulation. Their survival time with the circulation is very short (approximately 30 minutes).
Basophils
Basophils are the most uncommon white blood cell in circulation. They have lobulated nuclei and blue granules within the cytoplasm and contain histamine.
Their function is similar to that of mast cells. They play an important role in allergic/hypersensitivity reactions in the body.
Basophils have a 4-6 hour average lifespan in the circulation, though some can live for as long as 2 weeks.
Lymphocytes
Lymphocytes are the second most common leukocyte in circulation. They play an important role in normal immune system function.
They are agranular and have a large, rounded nucleus which takes up the majority of the cell.
Only 10% of lymphocytes are present with the circulation. Others are present in lymphatic tissue in other locations such as
the lymph nodes,
spleen and,
gastrointestinal tract.
There are two types of lymphocyte in circulation – B and T lymphocytes.
B-lymphocytes play an important role in humoral immunity (the production of antibodies). T-lymphocytes are involved in cell-mediated immunity, meaning destruction of antigens that have been ‘tagged’ by antibodies. So essentially, the B-lymphocytes mark antigens for destruction by the T-lymphocytes.
Lymphocytes can live for months to years in the circulation. They can leave and re-enter the circulation via the lymphatic system.
Monocytes
Monocytes are large agranular leukocytes. They have a U or S-shaped nucleus, and vacuolar, purple cytoplasm.
They last in the circulation for up to 2-3 days and can move out of the circulation into peripheral tissues. Monocytes become macrophages when they enter the tissues. These phagocytose (‘eat’) pathogens, expired cells and cellular debris.
Platelets
Platelets or thrombocytes are cellular fragments and technically are not cells themselves. They play a key role in haemostasis (stemming haemorrhage).
Primary haemostasis is the formation of a platelet plug. This is the first step in haemostasis, prior to a blood clot forming. In order to perform this function, platelets must be present in sufficient numbers - but they must also adhere together properly.
Von Willebrand’s factor is the substance required to ensure platelets are able to function and adhere together correctly, to create the platelet plug.
Platelets are present in the circulation for 4-6 days on average. They are formed in the bone marrow from large platelet precursor cells called megakaryocytes. Fragments of these cells shear off, circulating as platelets.
Plasma
Plasma is the liquid component of blood. It is responsible for carrying and transporting blood cells, as well as other substances. Numerous biochemical substances are dissolved and transported within the plasma, such as:
Electrolytes
Hormones
Nitrogenous waste products
Glucose and many more.
Clotting factors are also present in plasma. They play a key role in secondary haemostasis. Which is where a fibrin-based blood clot forms over the plug our platelets have already made. This is to seal off areas of damaged blood vessels.
Blood Cell Formation
When we talk about the life and development of our blood cells. Regardless of the cell type, we’re interested in two main things - the blood, and the bone marrow.
New red blood cells, white blood cells and platelets are formed in the bone marrow. This is from one single cell called a haematopoietic stem cell.
This magical cell can then develop into either a lymphoid progenitor cell, or a common myeloid progenitor cell.
Lymphoid progenitor cells develop into new lymphocytes. Myeloid progenitor cells become either granulocyte progenitor cells, or megakaryocyte/erthyroid progenitor cells.
Granulocyte progenitor cells eventually become either neutrophils, basophils or eosinophils. While megakaryocyte/erythryoid progenitor cells become platelets or red blood cells, respectively.
So that’s a refresher of all of the cells we see on our blood smears, what they look like, what they do, and how they form!
Remember - you don’t need to be a vet to look at these samples and identify these cells. The more you look at them, the more you’ll identify what’s normal - making those abnormalities even easier to spot.
You don’t need to know the exact term for every morphological change (though if you want to, that’s great!) - you just need a good idea of what’s normal and what’s not - and you get that through practice.
So, what’s stopping you looking at these samples now? DM me on instagram and let me know - I’d love to chat with you about this more!
PS - I also have something coming up next month that will really help you with this, so keep your eyes peeled for that!
References
EClinPath. 2013. Haematology [Available from: http://eclinpath.com/hematology/].
Merill, L. 2012. Small Animal Internal Medicine for Veterinary Technicians and Nurses. 1st ed. Iowa: Wiley-Blackwell.
Sirois, M. 2020. Laboratory Procedures for Veterinary Technicians. 7th edition. Missouri: Elsevier.
