By the way, the reason fish gills are bright red is that, like lungs, many capillaries are gathered there.
Literal
By-the-way, fish [genitive-の] gills [subject-が] bright-red [literary-copula-である] [reason-のは], lungs [same-as-と同じように] capillaries [subject-が] many gathered-[because-からである].
An explanatory science sentence using the cleft construction ~のは~からである ('the reason X is because Y'). 真っ赤 uses the intensifier prefix 真っ (まっ) — 'bright red / vivid red.' えら (gills) is usually written in hiragana. 毛細血管 is a four-kanji compound: 毛 (hair) + 細 (thin) + 血 (blood) + 管 (tube) = 'capillary.' ちなみに ('by the way / incidentally') signals supplementary information. The sentence ends with からである, the literary copula form of から, typical of textbook explanations.