Bladder

From The Sims Wiki, a database for The Sims series that anyone can edit

The Bladder meter increases naturally throughout the day, and can only be decreased by going to the toilet. Upgrading to a better toilet can boost Comfort whilst the Sim is relieving him or herself. If a Sim’s bladder starts to get full, they become extremely cranky; when it gets to the red, they make a wild dash for the toilet with urgency, and if they can’t get there in time, the poor Sim causes an embarrassing accident on the floor, which will severely effect the Sim’s Hygiene bar. Drinking a lot of coffee can make the Bladder meter decrease more rapidly. If a Sim does not flush the toilet, it can become clogged and will result in a sloppy mess that someone will have to clean up.

Even in places such as Downtown and Studio Town, there are public toilets where Sims can go to relieve themselves should they need to.

Outgoing and messy Sims are also able to "use" shrubs and the AquaGreen Hydroponic Garden to relieve their bladder needs.

[edit] Pets

While a cat will instinctively use the litter box when it needs to go, dogs aren't as smart. Until they are properly trained and house broken, dogs are happy to go wherever they are, usually creating a mess in the house in the process.