Blood bone marrow in a process called hematopoiesis, which includes erythropoiesis, the production of red blood cells; and myelopoiesis, the production of white blood cells and platelets. During childhood, almost every human bone produces red blood cells; as adults, red blood cell production is limited to the larger bones: the bodies of the vertebrae, the breastbone (sternum), the ribcage, the pelvic bones, and the bones of the upper arms and legs. In addition, during childhood, the thymus gland, found in the mediastinum, is an important source of T lymphocytes. The proteinaceous component of blood (including clotting proteins) is produced predominantly by
Tissue remodeling In the proliferation phase, inflammatory cells die by apoptosis, being replaced by myofibroblasts which produce large amounts of collagen. In the maturation phase, myofibroblast numbers are reduced by apoptosis, allowing for infiltration by endothelial cells (for blood vessels) and cardiomyocytes (heart tissue cells). Usually, however, much of the tissue remodeling is pathological, resulting in a large amount of fibrous tissue. By contrast, aerobic exercise can produce beneficial cardiac tissue remodeling in those suffering from left ventricular hypertrophy. Vascular remodeling refers to a compensatory change in blood vessel walls due to plaque growth. Vascular expansion is called positive remodeling, whereas vascular