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The hematocrit, also known as packed cell volume (PCV) or erythrocyte volume fraction (EVF), is the volume percentage (%) of red blood cells in blood. It is normally 45% for men and 40% for women.

so,Why do we have hematocrit for men?

First,I thought it might be caused by different hormones but I couldn't find adequate evidence to prove this Hypothesis.Therefore I decided to share it.

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I found a nice article that has insights to this matter. As posted before me it is true that only adults, but not juvenile individuals show this difference. Direct and indirect effects of sex hormones on bone marrow erythropoiesis and renal erythropoietin production has been proven, but it works in both sexes the same way. Females are capable to elevate their haemoglobin / red blood cell levels to external stimuli, yet at same conditions (same altitude etc) the difference remains. Also:

Men, premenopausal women and postmenopausal women have similar plasma erythropoietin levels, indicating that women do not attempt to compensate for their lower haemoglobin levels by increasing erythropoietic drive. These observations show that the prevailing lower haemoglobin level in females cannot be ascribed to a lack of bone marrow or renal erythropoietic capability: they indicate that adult females maintain their venous haemoglobin levels at a lower level than adult males as a physiological steady state — they do not try under physiological conditions to maintain the same levels as adult males.

This could be achieved by modulation of the juxtaglomerular apparatus signal (JGA) of the kidney. The JGA has important role in the regulation of the haemoglobin level - it is the main input for erythropoietic red blood cell production by regulating renal filtration and thus regulating the amount of oxygen that gets to the EPO producing peritubular cells.

It is noteworthy to say that similar difference exist in mammals and even there are examples among birds and reptiles.

It is described in the linked paper that both men and women achieve the same or very similar microcirculatory haematocrit level inspite the different mean venous haemoglobin levels.

This implies the very interesting theory / reasoning in the article:

(...) that both sexes set their mean optimum level separately and to some degree independently.(...) the red cell mass is an enormous resource in animals — one third of the body's cells by number in adult humans, and demands enormous effort to establish and maintain.(...)This price must be paid for by increased fitness.(...)The excess red cells over the critical minimum that circulate in the large vessels (and slowly through in the spleen in some species) and that constitute half or more of the venous haemoglobin level probably provide a storage function for the reserve red cell mass for use when the need arises for increased work — fight, flight, food and reproduction. They may also act as reserves for heat exchange and iron storage. It is likely that the return in fitness from this reserve differs between the sexes, and therefore that the optimal size for maximal cost benefit also differs.

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  • $\begingroup$ This is a very interesting point. It shows that my answer was obviously too simple. In fact, reference ranges for erythropoietin depend on age but not sex. See emedicine.medscape.com/article/2087846-overview for details. Hemoglobin homeostasis in women seems to adapt to the repetitive loss of iron by lowering its set point. The benefit of this mechanism could be that it saves valuable resources that would only be lost by menstruation. Thanks again for your valuable answer. $\endgroup$ – jwdietrich Jun 6 '15 at 17:50
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Reference values for hematocrit are identical in male and female children and as well in elderly men and women. Differences in hematocrit between fertile women and men are caused by repetitive loss of blood - and therefore iron - in menstruating women.

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    $\begingroup$ Could you maybe add references to support your claim that it is caused by menstruation? $\endgroup$ – fileunderwater Jan 12 '15 at 12:53
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    $\begingroup$ There is an interesting study that investigated menstrual blood loss in filipino women (Cheong RL, Kuizon MD, Tajaon RT. Menstrual blood loss and iron nutrition in Filipino women. Southeast Asian J Trop Med Public Health. 1991 Dec;22(4):595-604. PMID 1820649). The authors found a significant correlation between menstrual iron loss and transferrin saturation index. Interesting information is also available from this site by American Regent, but it is not without competing interests. $\endgroup$ – jwdietrich Jan 12 '15 at 21:36
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    $\begingroup$ Nice. Add them to your answer, so that the statement is supported. $\endgroup$ – fileunderwater Jan 13 '15 at 10:35
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males have testosterone that increase thyroxin secretion.thyroxin is importnat for ATP production,then if there is more energy production more oxygen is needed that directly increase more epo production.

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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

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    $\begingroup$ Please could you add some references to your answer $\endgroup$ – rg255 Dec 31 '15 at 8:34
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Males have more muscle mass,so they require more blood.Also females undergo menstrution so they lose more red blood cells.

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We're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. Answers that don't include explanations may be removed.

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    $\begingroup$ Welcome to Biology.SE! I think this is a good start but we're looking for long answers that provide some explanation and context. Don't just give a one-line answer; explain why your answer is right, ideally with citations. $\endgroup$ – arboviral Jun 23 '16 at 11:25

protected by Chris Jun 23 '16 at 9:24

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