Testosterone (increased # of rbcs) stimulates the production of erythropoietin from the kidneys which increases erythropoiesis which is the production of rbcs. Also, in hens, the liver is usually yellow due to yolk proteins/precursors which are transported via the blood which will take up a lot of blood components. Serum: remain fluid when blood clots & the solids are removed. Identical to plasma except for the absence of fibrinogen. Difference between avian & human erythrocytes: nucleated, larger than human rbcs, shorter life span, shorter life span because: birds have a very high metabolic rate which requires high caloric intake, oxygen intake, metabolic rate, high body temperature, etc. Human erythrocytes: form & function: arterioles capillary (gas exchange) venule, human rbcs are too large to fit through capillaries, so they must squeeze through, flattening them out. Quantities of erythrocytes & hemoglobin: males have higher hematocrit than females due to size (testosterone), rbc #, etc.