The Laws of Krias Shema part 3 vol. 3, no. 23
One should not interrupt his recitation of Shema.
If one did interrupt his recitation, by speaking or even by remaining silent, for the amount of time it would take to complete reciting the Shema, the halachic requirements will depend on the cause of the interruption. If the interruption was caused by something beyond his control he should return to the beginning of the Shema. If the interruption was caused by something within his control he does not have to recite Shema over.
For example, if one did not check if he had to relieve himself before starting to recite Shema, and while in the middle of his recitation felt the need to relieve himself, he must stop reciting Shema until he takes care of his needs. This is considered an interruption beyond his control and he would have to return to the beginning of the Shema if he interrupted for more than the time it would take to recite the entire Shema. ( However if he did check himself before davening and he did not need to relieve himself, but during Shema he needed to do so, he may complete the Shema and take care of his needs before reciting Shemone Esreh. However it is preferable to stop between the paragraphs of Shema and relieve himself.)
One should not speak to other people during his recitation of Shema. This includes speaking to his father, Rabbi or other prominent people. However, if his silence would result in physical or monetary damage he may speak to the person, except while he is reciting the first verse and baruch shem kavod.
Between the paragraphs of Shema one may answer to the following parts of the davening: bar’chu; amen yehei shmei raba; k’dusha; the amen after the blessings HaKel Hakodosh and Shomeiah T’fillah; and the words modim anachnu lach in the modim d’rabanan.
