red blood cells are made in the bone marrow in adults.
where are red blood cells made in adults?
in the bone marrow
the cancellous part of bones contain bone marrow. bone marrow produces blood cells in a process called hematopoiesis. blood cells that are created in bone marrow include red blood cells, platelets and white blood cells. progenitor cells such as the hematopoietic stem cell divide in a process called mitosis to produce precursor cells. these include precursors which eventually give rise to white blood cells, and erythroblasts which give rise to red blood cells. unlike red and white blood cells, created by mitosis, platelets are shed from very large cells called megakaryocytes. this process of progressive differentiation occurs within the bone marrow. after the cells are matured, they enter the circulation. every day, over 2.5 billion red blood cells and platelets, and 50–100 billion granulocytes are produced in this way.
bone
the mechanism by which topical progesterone in cream and water-based gel produces very high salivary and capillary blood levels in spite of low circulating levels is not well-understood. however, at least two hypotheses have been proposed. steroid hormones including progesterone have been found to be transported by red blood cells in addition to serum carrier proteins like albumin, sex hormone-binding globulin, and corticosteroid-binding globulin, and as much as 15 to 35% of total steroid hormone content in whole blood may be confined to red blood cells. according to the hypothesis, very high local concentrations of progesterone occur in skin capillaries after topical application and are taken up by red blood cells. the transit time of red blood cells from capillaries and the release of steroid hormones from red blood cells are both very rapid, so it is suggested that progesterone is delivered through circulation to tissues via red blood cells without having time to equilibrate with systemic blood. this could potentially explain the low levels of progesterone in venous blood in spite of very high levels in capillary blood and saliva. however, one study assessed progesterone levels in red blood cells with topical progesterone and found that they were significantly increased but still very low. nonetheless, according to other authors, "[a]lthough the investigators of that study concluded that the progesterone levels in red blood cells were too low to be important in the delivery of progesterone to target tissues, it should be realized that even small amounts of progesterone taken up by red blood cells might be important because the transit time of red blood cells from capillaries is very rapid. [...] however, the role of red blood cells in steroid hormone transport has not been studied thoroughly, and such studies are warranted."
pharmacokinetics of progesterone