Blue whales have a complete digestion track so that means they have a mouth and anus. They are pretty picky I guess you could say in there diet as they tend to stick to the same thing, krill. Blue whale while they are carnivores are harmless to almost any thing larger then red crabs. Blue whales eat mainly krill, small crustaceans that while are found in all the oceans they are most numerous in artic seas. Its very rare for blue whales to eat anything but krill but have been known to eat other crustacean species like the red crab. Again it is very rare to see that in nature. Blue whales don't have teeth like many other creatures in the world. instead they use baleen to capture is prey. Baleen work like a filter. When a blue whale traps hundreds of krill in its mouth it, will push its tongue to the roof of its mouth, which forces mainly the water out through the baleen leaving mainly krill in the mouth of the whale which it then proceed to swallow. If your wondering what baleen is made of it's made of proteins that comprise our hair and nails. Baleen teeth can be found in all Mysticeti species. To consume the large amounts of krill the whale is required to eat on a daily basis it has specially modified throat pleats. These are large folds of skins in the throat that can expand into large pouches that can hold large volumes of water in the whales throat. These pouches hold the water taken in when the whale consumes krill. Once the mouth has closed it then forces the water out of these pouches and forces it through the baleen. A blue whales stomach is three chambered. It has the fore stomach, the main stomach, and the connecting channel. It also has a pyloric stomach which also has two chambers. The fore stomach just acts like a large organic storage container. It only holds the food consumed by the whale as it has no secretory glands. While it contains the food it will break down the food with muscular contractions and anaerobic bacteria in the walls of the stomach. in the main stomach the food is broken down further by pepsin and hydraulic acid glands. The pyloric stomach act in unison with the fore and main stomach as they have acid glands and other digestive solutions. These stomachs have a very heavy task that is not easy. The fore stomach can contain up to 1000 kg of food at a time. The hardest part of digesting the krill and other crustaceans are the hard exoskeletons that all arthropods have. Once the hard shells of the crustaceans are dissolved the digestive juices make quick work of the soft tissue that makes up the inside of the crustaceans. Once the crustaceans are broken down they travel through the connecting channel and into the small and large intestines. From here the nutriance is absorbed into the blood stream of the whales and all unusable matter is collected in the large intestines and expelled through the anus of the whale.