discusses the effects of space travel on the human body. The whole passage can be divided into two sections. The first section is composed of three paragraphs: Paragraph 1 to Paragraph 3, which is a general introduction to the passage. The second section can be subdivided into 4 parts. Paragraph 4 is the first part, referring to weightlessness as a dominant factor which leads to various related responses in human bodies. The second part consists of 4 paragraphs, from Paragraph 5 to Paragraph 8, which is about how space travelers feel in their spacecraft and why this is so. The third part is from Paragraph 9 to Paragraph 12, which deals with the actual consequences weightlessness brings about for human bodies and offers some explanations. Paragraph 13 is the last part, which points out the practical use of this research. 
