One of the main functions of the circulatory system is to pump blood throughout the body so that oxygen and nutrients can reach the tissues. The heart works as the pump, and is it actually considered a “double pump”, since the right side distributes blood towards the lungs for oxygenation, while the left side forces blood through the aorta so that oxygenated blood can circulate to the tissues. Blood vessels within the circulatory system include an arterial system and a venous system. Arteries carry blood away from the heart and towards tissues, while the venous system carries deoxygenated blood back to the heart.
Enteroendocrine cells have traditionally been recognized by their affinity for certain metal stains, hence the older terms chromaffin cells (having an affinity for chromium) and argentaffin cells or argyrophil cells (having an affinity for silver). With sufficient resolution, these cells can sometimes be recognized in routine light microscopic preparations by their relatively pale cytoplasm with a broad base and a basal concentration of secretory vesicles (in contrast to the apical concentration of secretory vesicles for exocrine serous cells or mucous cells ). Immunocytochemical methods are preferred for demonstrating and properly identifying the various types of enteroendocrine cells.
The mechanisms of mineralization are not fully understood. Fluorescent, low-molecular weight compounds such as tetracycline or calcein bind strongly to bone mineral, when administered for short periods. They then accumulate in narrow bands in the new bone.  These bands run across the contiguous group of bone-forming osteoblasts. They occur at a narrow (sub- micrometer ) mineralization front. Most bone surfaces express no new bone formation, no tetracycline uptake and no mineral formation. This strongly suggests that facilitated or active transport , coordinated across the bone-forming group, is involved in bone formation, and that only cell-mediated mineral formation occurs. That is, dietary calcium does not create mineral by mass action.