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Ass. Lec.
Wafaa H. M. Alhashimy
Dentistry College
Second Stage
Blood is a unique from of connective tissue that consists of three major cell types :
erythrocytes (red blood cells) , leukocytes (white blood cells) and platelets
(thrompocytes) . these cells which anew also called the formed elements of blood are
suspended in a liquid medium called plasma .
Blood cells transport gases , nutrients , waste products , hormones , antibodies ,
various chemicals , ions and other substances in the plasma both to and from different
cells in the body .
Major blood cell types
Erythrocytes or red blood cells : are the most numerous blood cells , about 4-6
millions/mm3. erythrocytes are devoid of a nucleus and have the shape of a biconcave
lens . The red cells are rich in hemoglobin, a protein able to bind in a faint manner to
oxygen. Hence, these cells are responsible for providing oxygen to tissues and partly
for recovering carbon dioxide produced as waste. However, most CO 2 is carried by
plasma, in the form of soluble carbonates.
Platelets are cytoplasmic remnants of larger bone marrow cells called megakaryocytes
. erythrocytes and platelets both perform their major functions within the blood vessels
. The main function of platelets, or thrombocytes, is to stop the loss of blood from
wounds (hematostasis). Their density in the blood is 200000-300000 /mm3.
Leukocytes or white blood cells , perform their major functions outside the blood
vessels . ,leukocytes migrate out of the blood vessels through capillary walls and enter
connect live tissue , lymphatic tissue and bone marrow . leulocytes are nucleated cells
and they are subdivided into granulocytes and agranulocytes , depending on the
presence or absence respectively of the granules in their cytoplasm .
The primary function of leukocytes is to defend the body against bacterial invasion
or the presence of foreign material . consequently , most leukocytes are concentrated
in the connective tissue .
Neutrophils are very active in phagocyting bacteria and are present in large amount in
the pus of wounds.
Eosinophils attack parasites and phagocyte antigen-antibody complexes.
Basophil secrete anti-coagulant and vasodilatory substances as histamines and
Lymphocytes are cells being present in the blood, populate the lymphoid tissues and
organs too, as well as the lymph circulating in the lymphatic vessel. The lymphocytes
are the main constituents of the immune system which is a defense against the attack
of pathogenic micro-organisms such as viruses, bacteria, fungi and protista.
Lymphocytes yield antibodies
Monocytes are the precursors of macrophages. They are larger blood cells, which
after attaining maturity in the bone marrow, enter the blood circulation where they
stay for 24-36 hours. Then they migrate into the connective tissue, where they become
macrophages and move within the tissues.
Platelets or thrombocytes , are the smallest formed elements in the blood . they are
present in the blood of all mammals and are the nonnucleated , cytoplasmic fragments
of megakaryocytes which are the largest cells in the bone marrow . platelets are
produced when small uneven portions of the cytoplasm separate or fragment from the
peripheries of the megakaryocytes .
The main function of platelets is to continually monitor the vascular system and
detect any damage to the endothelial lining of the vessels . if a break is detected in the
endothelial lining of a blood vessels , the platelets adhere to the damage site and
initiate a complex process that results in formation of ablood clot .