A man,instead of needing,as he does,only about 5 quarts of blood ,would need ,if he had no red blood cells,some 75 gallons of circulating fluid to carry oxygen to his tissues.

一个人在正常情况下只需要五夸特的血液来携带氧气到他的各部分组织。但是，若没有血红细胞，他将需要大约75夸特的循环液体来承担输送氧气的任务。as he does为插入语，强调事实上确实是如此。instead of needing only about 5 quarts of blood 为主句的状语。主句为A man would need some 75 gallons of circulating fluid to carry oxygen to his tissues，为虚拟语气。条件状语为if he had no red blood cells。像这样附加句子成分较多的句子，直接翻译不如拆开两句更能表达清楚。cypress的回答中有理解上的错误。