The call of nature
Definition 1 of 1
Pro Tip 1/3Polite Euphemism
Used to avoid direct mention of toilets or bodily functions, seen as more courteous. 
During the meeting, he excused himself to answer the call of nature.
Pro Tip 2/3Humorous Tone
Can have a light-hearted undertone when said jokingly among friends or peers. 
At the campfire, Joe chuckled about needing to respond to the call of nature.
Pro Tip 3/3Avoiding Awkwardness
Reduces embarrassment in situations where saying 'toilet' might be awkward or too direct. 
In the crowded elevator, she murmured about the call of nature.