Erythrokinetics1% of erythrocytes, aka, red blood cells, are removed and replaced every day. The production of new red blood cells, called erythropoiesis, occurs in the red marrow of spongy bone. Regulated by hypoxia, sensed by kidney.On average, erythrocytes travel in the circulatory system for approximately 120 days; recall that they are responsible for oxygen and carbon dioxide transport. Once they are no longer viable, splenic macrophages remove RBCs via phagocytosis. Regulation of ErythropoiesisRegulation:Tissue oxygen level, aka, hypoxia, regulates erythropoiesis via negative feedback. When the kidney senses hypoxia, hypoxia-inducible factor 1 (HIF-1) activates the gene to increase erythropoietin (EPO) synthesis (EPO is also synthesized by other organs, including the liver, but in quantities insufficient to maintain homeostatic erythropoiesis). In the red marrow of spongy bone, erythropoietin stimulates erythrocyte formation.Consequently, oxygen delivery to the tissues increases to homeostatic levels (aka, normoxia), which, in turn, downregulates erythropoietin synthesis. EPO production is also initiated by testosterone and thyroid hormones.Key characteristics of healthy mature red blood cellsBiconcave center is referred to as the "central pallor," this is region is relatively thin, and, therefore, appears lighter in color. Cell membrane is deformable, which allows passage through small spaces, and is permeable to gases and ions. The cytoplasm of the erythrocyte contains abundant hemoglobin, which binds iron and maintains fluid osmolarity. Healthy mature erythrocytes lack nuclei, mitochondria, and endoplasmic reticulum.Cytoskeleton proteins interact with the cell membrane to allow deformation.Cytoplasmic enzymes maintain the cell membrane and keep iron in its ferrous form. Degradation of ErythrocytesBecause erythrocytes are lacking in protein-synthesizing organelles, they cannot replace these enzymes as they are used; hence, the limited supply of cytoplasmic e...