Blood Cells

Connective tissue, composed of ~45% red blood cells and ~55% plasma. It is thicker than water. When separated, the liquid plasma rises to the top and is separated from the red blood cells by the buffy layer

Red Blood Cells : a biconcave disk filled with hemoglobin. When mature, it has no nucleus
Production : hematopoiesis occurs exclusively in the bone marrow (in the fetus, it occurs in the yolk sac, liver and spleen). A red blood cell lives an average of 120 days and the total RBC count is relatively stable. The older a blood cell gets, the more fragile it becomes, and when it is damaged, it is phagocytized by macrophages.

Erythropoietin is a renal hormone that regulates red blood cell production by a negative feedback mechanism
Vitamin B₁₂and folic acid also also important in blood cell production, as they are needed for DNA synthesis
Iron is needed for hemoglobin production
All blood cells start from hematopoietic stem cells called hemocytoblasts. They become proerythroblasts if they are destined for red blood cell production.

White Blood Cells : or leukocytes use the circulatory system for transportation. They can be generally divided into granulocytes, which have granular cytoplasm, and agranulocytes , which don’t.

Granulocytes include neutrophils, eosinophils and basophils. Typical granulocytes are larger than erythrocyte

Neutrophils have a lobed nucleus and are active phagocytes
Eosinophils are bilobed, and make up 1-3% of all leukocytes. They function in allergic reactions.
Basophils are very rare (<1% of leukocytes) and their granules include a clotting substance called heparin

Monocytes are 2-3 times larger than red blood cells. They are highly phagocytic.
Lymphocytes are similar in size to red blood cells and some of them form antibodies.

All white blood cells function in immunity and are considered part of the lymphatic system
Production : WBCs also arise from hemocytoblasts, but those precursors change into myeloblasts (which form granulocytes), monoblasts (which form monocytes) or lymphoblasts (which form lymphocytes).

Platelets : or thrombocytes are clotting factors in the bloodstream. They are not actually cells, but are pieces of cells that are filled with clotting chemicals, including serotonin, calcium ions, a variety of enzymes, ADP, and PDGF (platelet derived growth factor).

Production : platelets also come from hemocytoblasts, which change into megakaryoblasts as they commit to platelet production
Platelet production is mediated by the hormone thrombopoietin