Respiratory
The respiratory of the hippo is very similar to that of all other mammals. They exchange carbon dioxide with oxygen through their lungs which are vascularized, supplied richly with blood vessels. The breathe through their mouth and nostrils (nares), but close them when they submerge themselves underwater. They can hold their breathe a remarkable amount of time, but no exact time has been recorded as no one has ever tried to drown one. Reports range from 10 to 30 minutes.
Circulatory
The hippopotamus has a closed circulatory system. It's heart has four chambers (atria and ventricles), just like a human. Blood is pumped through the veins and arteries to supply the hippo with oxygen and nutrients stored in the blood, and then is returned back to the heart to be oxygenated again.
Digestive
The digestive system of the hippopotamus is very similar to other hooved grazers except in the fact that it isn't as effective when coming to utilization of it's food. They have long lips (70 cm!) that clip grass and other vegetation off and grind it into a pulp that can be swallowed. Since they do not regurgitate this pulp their system is ineffective compared to other grazers. The food then travels down the esophogus and is digested in the stomach. After digestion, the waste is excreted.
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