There is always the consideration (albeit unlikely) that one of them may suffer a heart attack or a stroke during the transmission. If that happens, there is nothing that anybody can do about it. Presumably the producers have a contingency plan in case it ever does.
A call of nature, however is something that is within their control. As somebody previously mentioned, they are professionals. They will have ensured that they are physically comfortable before the transmission begins and they will drink little or nothing while on air. If someone has an upset stomach, there are medications to calm it down for the duration.