Page 126 - 84 human physiolofy part-1
P. 126

Chapter 7

                 WBCs are classified by phenotype which can be identified by looking at the WBCs under a
            microscope. The Granular phenotype are able to stain blue. The Agranular phenotype are able to stain
            red. Neutrophils make up 50-70% of Granular cells Eosinophils make up 2-4%, and Basophils 0-1%.
            Monocytes make up 2-8% of Agranular cells. B and T Lymphocytes make up 20-30%. As you can see,
            there is a great deal of differentiation between WBCs. These special cells help our bodies defend
            themselves against pathogens. Not only do they help our immune system but they remove toxins,
            wastes, and abnormal  or   damaged  cells.  Thus,   we  can  say   that  WBCs'  main   function is  being
            Phagocytic which means to engulf or swallow cells.



            Platelets


                 Platelets, also  called  thrombocytes,  are membrane-bound  cell
            fragments.   Platelets   have   no   nucleus,   are   between   one   to   two
            micrometers in diameter, and are about 1/10th to 1/20th as abundant
            as   white   blood   cells.   Less   than   1%   of   whole   blood   consists   of
            platelets.   They   result   from   fragmentation   of   large   cells   called
            Megakaryocytes - which are cells derived from stem cells in the bone
            marrow. Platelets are produced at a rate of 200 billion per day. Their
            production is regulated by the hormone called Thrombopoietin. The
            circulating life of a platelet is 8–10 days. The sticky surface of the
            platelets allow them to accumulate at the site of broken blood vessels
            to   form   a   clot.   This   aids   in   the   process   of   hemostasis   ("blood
            stopping").   Platelets   secrete   factors   that   increase   local   platelet
            aggregation (e.g., Thromboxane A), enhance vasoconstriction (e.g.,
            Serotonin), and promote blood coagulation (e.g., Thromboplastin).
                                                                                 A 250 ml bag of newly collected
                                                                                 platelets.
            Hemostasis (Coagulation or Clotting)


                 Hemostasis is the natural process of stopping blood flow or loss of blood following an injury.
            (hemo = blood; stasis = standing). It has three stages: (1) vascular spasm, vasoconstriction, or intense
            contraction of blood vessels, (2) formation of a platelet plug and (3) blood clotting or coagulation.
            Once the flow of blood has been stopped, tissue repair can begin.


                 Vascular spasm or Vasoconsriction: In a normal individual, immediately after a blood vessel has
            been cut and endothelial cells are damaged, vasoconstriction occurs, thus slowing blood flow to the
            area. Smooth muscle in the vessel wall goes through spasms or intense contractions that constrict the
            vessel. If the vessels are small, spasms compress the inner walls together and may be able to stop the
            bleeding completely. If the vessels are medium to large-sized, the spasms slow down immediate
            outflow of blood, lessening the damage but still preparing the vessel for the later steps of hemostasis.
            These vascular spasms usually last for about 30 minutes, long enough for the next two stages of
            hemostasis to take place.

                 Formation of a Platelet Plug: Within 20 seconds of an injury, coagulation is initiated. Contrary to
            popular belief, clotting of a cut on the skin is not initiated by air or drying out, but by platelets adhering
            to and activated by collagen in the blood vessels endothelium. The activated platelets then release the
            contents   of   their   granules,   which   contain   a   variety   of   substances   that   stimulate   further   platelet
            activation and enhance the hemostatic process.


            126 | Human Physiology
   121   122   123   124   125   126   127   128   129   130   131