Blood is sometimes

BLOOD

Blood is sometimes considered to be a fluid connective tissue because of the mesenchymal origin of its cells and a low ratio of cells to liquid intercellular substance, the blood plasma. In human adults about 5 liter of blood contribute 7-8 % to the body weight of the individual. The contribution of red blood cells (erythrocytes) to the total volume of the blood (haematocrit) is about 43%.

Erythrocytes are the dominant (99%) but not the only type of cells in the blood. We also find leukocytesand, in addition, blood platelets. Erythrocytes, leukocytes and blood platelets are also being referred to as the formed elements of the blood. Erythrocytes and blood platelets perform their functions exclusively in the blood stream. In contrast, leukocytes reside only temporarily in the blood. Leukocytes can leave the blood stream through the walls of capillaries and venules and enter either connective or lymphoid tissues.

Erythrocytes

Erythrocytes do not contain a nucleus. They do contain haemoglobin, which fills almost the entire cytoplasm. Erythrocytes are unable to move actively, but they are remarkably elastic and can withstand deformation. They are typically biconcave disks although their shape is influenced by osmotic forces. The average diameter of the disk is ~7 µm. Since erythrocytes can be found in the vast majority of histological sections - in small numbers even in perfused tissues - they will often allow us to estimate the size of other structures or cells. Mature erythrocytes do not contain organelles, and their cytoplasm looks fairly homogenous - even in the EM.

At high magnification some granularity may be visible in EM images. The granular appearance is caused by haemoglobin molecules. Foetal erythrocytes (up to the 4th month of gestation) are larger than "adult" erythrocytes, and they are nucleated. The later feature they share with erythrocytes of other animal classes (e.g. amphibia and birds).

Functions

Erythrocytes function in the transport of oxygen. Haemoglobin, the oxygen binding protein in erythrocytes, contributes about 30% of the weight of an erythrocyte. The lifespan of an erythrocyte in the bloodstream is 100-120 days.

About 5×10¹¹erythrocytes are formed/destroyed each day.

Suitable Slides

blood smear - Leishman, Wright's, Giemsa or May-Grünwald-Giemsa stains

Where to look for cells in a blood smear The density of cells varies across the smear. Cells will be "heaped and piled" close to the point were the drop of blood was placed on the slide. White blood cells appear shrunken, and some types are difficult to distinguish from each other. There are fewer cells close to the tip of the smear. In this region, white blood cells are sometimes damaged and erythrocytes may be deformed. The best area to look at is between these two regions. Where it is located exactly and how wide it is will depend on the smear, but the middle of the smear is a good starting point.
Blood Smear, human - Leishman stain In a suitable region of the blood smear, the erythrocytes are rarely forming clumps or rows. Instead, they are more or less evenly spaced and occasionally forming goups of maybe 2 or 3 cells. Due to the biconcave shape of the erythrocytes, their center will look lighter than their periphery. Depending on the way in which the blood smear was prepared, blood platelets may be found singly, in small groups or large clumps. How does the shape of the erythrocyte facilitate its function? How would you expect an erythrocyte to look like if it is in an extracellular fluid of very low or very high osmotic pressure? Identify and draw a few erythrocytes. It is a good idea to do one large composite drawing for all types of blood cells.   Leukocytes Leukocytes can be further subdivided into granular leukocytes, i.e. neutrophils, basophils and eosiniphils, and non-granular leukocytes, i.e. monocytes and lymphocytes. In healthy individuals the relative numbers of circulating leukocyte types are quite stable. A differential leukocyte countwould typically produce the following cell frequencies (numbers in parentheses are the range of normal frequencies reported in different texts): ~ 60% neutrophils (50% - 70%) ~ 3% eosinophils (>0% - 5%) ~ 0.5% basophils (>0% - 2%) ~ 5% monocytes (1% - 9%) ~ 30% lymphocytes (20% - 40%) Changes in their relative numbers indicate that something abnormal is happening in the organism. A larger than usual number of neutrophils (neutrophilia) would indicate e.g. an acute or chronic infection. The number of basophils and eosinophils may increase (eosinophilia or basophilia) as a consequence of e.g. allergic disorders.