If a Kohen donates a kidney, is he considered a Bal Moom for purposes of doing an Avodah in the future Bais hamikdash? On the one hand, the missing organ is not visible. On the other hand, a cataract is also not visible and I think it would make a Kohen a baal moom.
The Rambam (Bias Mikdash 6:7) states clearly that a Kohain whos Kidney was removed is NOT considered a Baal Mum.
אֵין פּוֹסֵל בָּאָדָם אֶלָּא מוּמִין שֶׁבְּגָלוּי. אֲבָל מוּמִין שֶׁבַּחֲלַל הַגּוּף כְּגוֹן שֶׁנִּטַּל כֻּלְיָתוֹ שֶׁל אָדָם אוֹ טְחוֹל שֶׁלּוֹ אוֹ שֶׁנִּקְּבוּ מֵעָיו אַף עַל פִּי שֶׁנַּעֲשָׂה טְרֵפָה עֲבוֹדָתוֹ כְּשֵׁרָה שֶׁנֶּאֱמַר (ויקרא כא יט) "שֶׁבֶר רָגֶל אוֹ שֶׁבֶר יָד" מָה אֵלּוּ בְּגָלוּי אַף כּל בְּגָלוּי:
Only blemishes that are apparent disqualify a person. Blemishes that are within the cavity of the body, e.g., a person's kidney or his spleen was removed or his intestines were perforated, even though he becomes a treifah,18 his service is acceptable. [This is derived from the mention, Leviticus 21:19, of] "a broken leg or a broken arm." Just as these are apparent, all [disqualifying blemishes] must be apparent.
The deciding factor is NOT whether it is visable or not, but rather if it is considered בַּחֲלַל הַגּוּף - within the cavity of the body.