Blood is a highly specialized circulating tissue consisting of several types of cells suspended in a fluid medium known as plasma. The cellular constituents are: red blood cells (erythrocytes), which carry respiratory gases and give it its red color because they contain hemoglobin (an iron-containing protein that binds oxygen in the lungs and transports it to tissues in the body), white blood cells (leukocytes), which fight disease, and platelets (thrombocytes), cell fragments which play an important part in the clotting of the blood.
Medical terms related to blood often begin with hemo- or hemato- (BE: haemo- and haemato-) from the Greek word "haima" for "blood." Anatomically, blood is considered a connective tissue from both its origin in the bones and its function.
Functions of the blood 
Blood performs two major functions:
* transport through the body of
o oxygen and carbon dioxide
o food molecules (glucose, lipids, amino acids)
o ions (e.g., Na+, Ca2+, HCO3-)
o wastes (e.g., urea)
o hormones
o heat
* defense of the body against infections and other foreign materials. All the WBCs participate in these defenses.
Blood Transfusions
In the United States, in 2001, some 15 million "units" (~475 ml) of blood were collected from blood donors.
* Some of these units ("whole blood") were transfused directly into patients (e.g., to replace blood lost by trauma or during surgery).
* Most were further fractionated into components, including:
o RBCs. When refrigerated these can be used for up to 42 days.
o platelets. These must be stored at room temperature and thus can be saved for only 5 days.
o plasma. This can be frozen and stored for up to a year.
Formation of Blood Cells
All the various types of blood cells
* are produced in the bone marrow (some 1011 of them each day in an adult human!).
* arise from a single type of cell called a hematopoietic stem cell — an "adult" multipotent stem cell.
These stem cells
* are very rare (only about one in 10,000 bone marrow cells);
* are attached (probably by adherens junctions) to osteoblasts lining the inner surface of bone cavities;
* express a cell-surface protein designated CD34;
* produce, by mitosis, two kinds of progeny:
o more stem cells (A mouse that has had all its blood stem cells killed by a lethal dose of radiation can be saved by the injection of a single living stem cell!).
o cells that begin to differentiate along the paths leading to the various kinds of blood cells.
Which path is taken is regulated by
* the need for more of that type of blood cell which is, in turn, controlled by appropriate cytokines and/or hormones.
Examples:
* Interleukin-7 (IL-7) is the major cytokine in stimulating bone marrow stem cells to start down the path leading to the various lymphocytes (mostly B cells and T cells).
* Erythropoietin (EPO), produced by the kidneys, enhances the production of red blood cells (RBCs).
* Thrombopoietin (TPO), assisted by Interleukin-11 (IL-11), stimulates the production of megakaryocytes. Their fragmentation produces platelets.
* Granulocyte-macrophage colony-stimulating factor (GM-CSF), as its name suggests, sends cells down the path leading to both those cell types. In due course, one path or the other is taken.
o Under the influence of granulocyte colony-stimulating factor (G-CSF), they differentiate into neutrophils.
o Further stimulated by interleukin-5 (IL-5) they develop into eosinophils.
o Interleukin-3 (IL-3) participates in the differentiation of most of the white blood cells but plays a particularly prominent role in the formation of basophils (responsible for some allergies).
o Stimulated by macrophage colony-stimulating factor (M-CSF) the granulocyte/macrophage progenitor cells differentiate into monocytes, macrophages, and dendritic cells (DCs).
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