2. Hemopoiesis  Production of most formed elements found in the blood  First occurs in the yolk sac of an embryo and later in the liver, spleen, thymus, and lymph nodes of a fetus.  Last three months before birth, red bone marrow becomes primary site of hemopoiesis and continues as the source of blood cells after birth and throughout life.

3. Erythrocytes  A.k.a. red blood cells (RBCs)  Amount in blood: 4-6 million  Life Span: 120 Days  Biconcave disc without nucleus  Function: transport oxygen in hemoglobin

4. Leukocytes  A.k.a. white blood cells (WBCs)  Amount in blood: 4,000 to 10,000  Life Span: 13-20 Days  Nucleated? Yes  Function: fight viruses and bacteria-immunity- move out of capillaries to site of infection  Classified as granular or agranular

5. Granulated - Neutrophil  Respond 1 st to bacterial invasion  Function: Phagocytosis (engulf bacteria) and releasing enzymes (i.e. lysozyme)  Multi-lobed nucleus  54 - 62% of white cells present

6. Granulated - Eosinophil  orange histamine granules – released during allergic reactions  Effective against certain parasitic worms  Helps to detoxify foreign substances  Secretes enzymes that break down clots  1 - 3% of white cells present

7. Granulated - Basophil  Blue granules  Releases anticoagulant heparin  < 1% white cells

8. Agranulated cells - lymphocytes  Large nucleus  Makes antibodies  Fights viruses  B cells, T cells, and natural killer (NK) cells  20 - 40% of white cells

9. Agranulated cells - monocytes  Pac Man Cells  Phagocytic cells  Garbage man cell of the blood  3 - 9%

10. Platelets - thrombocytes  Amount in blood: 150 to 400,000  Life Span: 5-9 Days  Nucleated? In bone marrow – not peripheral blood  Function: enable clotting

11. Blood Clotting  A.k.a. coagulation  Clotting factors: Ca 2+, enzymes made by liver cells, and various molecules associated with platelets or released by damaged tissues  Three stages: 1. Prothrombinase is formed. 2. Prothrombinase converts prothrombin into the enzyme thrombin. 3. Thrombin converts soluble fibrinogen into insoluble fibrin. Fibrin forms the threads of the clot.

12. Blood Vessel Repair  Once a clot is formed, it plugs the ruptured area of the blood vessel and thus stops blood loss.  Clot retraction- consolidation or tightening of the fibrin clot.  Fibrin threats attached to the damaged surfaces of the blood vessel gradually contract as platelets pull on them.  As the clot retracts, it pulls the edges of the damaged vessel closer together.  Fibroblasts form connective tissue.  Endothelial cells repair vessel lining.