Final answer:
Increased demand for RBCs can lead to the release of reticulocytes into the bloodstream, which mature into erythrocytes. These mature erythrocytes circulate for an average of 120 days. The production of RBCs is part of the continuous process of hematopoiesis, stimulated by erythropoietin.
Step-by-step explanation:
Increased demand for red blood cells (RBCs) can cause the release of immature cells known as reticulocytes into the bloodstream. Reticulocytes are immature red blood cells that have been released from the bone marrow into the blood. They mature into erythrocytes, which are fully developed red blood cells, within a day or two. These erythrocytes, which are the major formed element of the blood, circulate for an average of 120 days.
The process of blood cell production, called hematopoiesis, must occur continuously due to the short lifespan of the formed blood elements. Erythropoietin (EPO) is released by the kidneys to stimulate this process, especially when there is a decrease in blood flow or oxygen levels. The most numerous cells found in blood are erythrocytes, which are specialized for the transport of respiratory gases thanks to their high hemoglobin content.