Small lymphocyte

Neutrophil

Erythrocyte

Eosinophil

Young (band)neutrophil

Monocyte

Neutrophil

Basophil

Largelymphocyte

Neutrophil

Smalllymphocyte

Platelets

Monocyte

674 part FOur Regulation and Maintenance

Components and General properties of BloodAll of the foregoing functions depend, of course, on thecharacteristics of the blood. Adults generally have about4 to6litersofblood.Itisaliquidconnectivetissuecomposed,likeotherconnectivetissues,ofcellsandanextracellularma-trix.Thematrixisthebloodplasma, a clear, light yellow fluid constitutingalittleoverhalfofthebloodvolume.Suspendedinthe plasma are the formed elements—cells and cell fragments including the red blood cells, white blood cells, and platelets (fig. 18.1). The term formed element alludes to the fact that these are membrane-enclosed bodies with a definite struc-turevisiblewiththemicroscope.Strictlyspeaking,theycannotallbecalledcellsbecausetheplatelets,asexplainedlater,aremerely fragments torn from certain bone marrow cells.

The formed elements are classified as follows:

Erythrocytes3 (red blood cells, RBCs)

Platelets

Leukocytes4(whitebloodcells,WBCs)

Granulocytes

Neutrophils

Eosinophils

Basophils

Agranulocytes

Lymphocytes

Monocytes

Thus, there are seven kinds of formed elements: the erythro-cytes, platelets, and five kinds of leukocytes. The five leuko-cyte types are divided into two categories, the granulocytes and agranulocytes,ongroundsexplainedlater.

The ratio of formed elements to plasma can be seen by taking a sample of blood in a tube and spinning it for a few minutes in a centrifuge (fig. 18.2). Erythrocytes, the densest elements, settle to the bottom of the tube and typically con-stitute about 37% to 52% of the total volume—a value called the hematocrit. WBCs and platelets settle into a narrowcream-orbuff-coloredzonecalledthebuffy coat just above theRBCs; theytotal1%or lessof thebloodvolume.At thetop of the tube is the plasma, which accounts for about 47% to 63% of the volume.

Table18.1listsseveralpropertiesoftheblood.Someofthe terms in that table are defined later in the chapter.

▶▶▶ a p p ly W h at yO u K n O WBased on your body weight, estimate the volume (in liters) and weight (in kilograms) of your own blood, using the data in table 18.1.

Figure 18.1 the Formed elements of Blood.

? What do erythrocytes and platelets lack that the other formed elements have?

3 erythro = red; cyte = cell4 leuko = white; cyte = cell

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