Final answer:
Blood banks usually don't pay for donations as blood donation is a voluntary act. The human body replaces plasma in 24 hours and blood cells in 4 to 6 weeks, which dictates the donation frequency. Hemopoiesis, which replenishes blood components, primarily occurs in the red bone marrow in adults.
Step-by-step explanation:
Typically, blood banks do not pay for donations of blood because it is encouraged as a voluntary service to help others in need. The replacement process for the blood components donated is known as hemopoiesis or hematopoiesis. After a person donates a unit of blood (approximately 475 mL, or about 1 pint), their body will replace the plasma within 24 hours, but replacing the blood cells takes about 4 to 6 weeks. This means that donors will need to wait for several weeks before they can donate blood again, to allow their body time to replenish their blood supply.
Sites of hemopoiesis, the production of new blood components, change throughout a person's life. Initially, during the fetal stage, this process occurs in multiple sites including the yolk sac and fetal liver. After birth most hemopoiesis occurs in the red marrow found within spongy bone tissue. In adults, this is mainly restricted to bones such as the cranial, pelvic bones, vertebrae, sternum and the proximal epiphyses of the femur and humerus. The age limit for blood donation typically ranges from 16 years and above, with no upper age limit as long as the donor is healthy. It's a common myth that there is an upper age limit or that those with high blood pressure cannot donate, but in reality these are not automatic disqualifiers. Blood banks have trained specialists who help ensure the safety and regulation of the blood donation process.