Blood

Platelets are small cell fragments involved in hemostasis. Thrombopoiesis takes place primarily in the bone marrow through a series of cell differentiation and is influenced by several cytokines. Platelets are formed after fragmentation of the megakaryocyte cytoplasm. As a result, platelets have a diameter of 2–3 μm. Nuclei are not present; however, a variety of organelles are present and aid in different platelet functions.

Bone marrow, the primary site of hematopoiesis, is found in the cavities of cancellous bones and the medullary canals of long bones. There are 2 types: red marrow (hematopoietic with abundant blood cells) and yellow marrow (predominantly filled with adipocytes). Human marrow composition changes with age. In the young, the entire bone marrow is red, as there is increased blood cell production. As age increases, there is gradual change to the yellow marrow variety. The yellow marrow can revert to red marrow when hematopoiesis is needed (e.g., anemia). The red marrow, by hematopoiesis, produces about 6 billion cells per kilogram per day. The process relies on hematopoietic cells (stem cells and progenitors) producing mature effector cells (lymphocytes, platelets, granulocytes, erythrocytes) with the aid of nonhematopoietic elements. Production is regulated by cytokines released in the bone marrow environment and feedback from target tissues. The marrow structure allows hematopoiesis to take place in the extravascular area, and after staged differentiation, blood cells are released into circulation.

The myeloid lineage produces a diverse group of blood cells (leukocytes) involved in the defense of the body against foreign substances or infectious agents. Monocytes, neutrophils, eosinophils, basophils, and mast cells all arise from the common myeloid progenitor (CMP), which develops from bone marrow stem cells. However, leukocytes have different functions. Neutrophils kill invading bacteria and fungi, eosinophils increase in parasitic infections, basophils and mast cells are key to allergic response as sources of histamine, and monocytes are released from bone marrow to become macrophages in the tissues. The development in the bone marrow is regulated by different cytokines.

Trace elements are minerals required in small amounts (1–100 mg/day in adults) to carry out biologic functions. These elements act as cofactors for essential enzymes as well as being components of hormones and antioxidant molecules. Trace elements have essential roles in oxygen transport, energy metabolism, and immune responses. Iron, chromium, copper, and iodine are among these elements. Overt deficiencies in resource-rich countries are rare because of varied food supply, food fortification, and supplement use. However, deficiencies can develop in certain medical conditions. Toxicity can also occur when there is exposure to concentrations larger than those physiologically required.