Blood
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 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 B12
and 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 erythrocytes.
- 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
- Agranulocytes include monocytes and lymphocytes
- 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