If pregnancy does not occur, the corpus luteum degrades and so progesterone levels fall. Without progesterone, the _____ (which supply blood to the endometrium) spasm and the _____ dies. Ultimately, these spasms cause the dying tissue to detach.

3000+ review questions on anatomy and physiology. 

Anatomy refers to the internal and external structures of the body and their physical relationships, whereas physiology refers to the study of the functions of those structures.