The pipes could leak. This is the equivalent of bleeding in the human circulatory system. The pipes could get blocked. One or more valves in the pumps could become faulty. Again this is found in the human sytem.
Heart valves can and do develop faults.
One or both pumps could stop working. This is the same as a faulty heart in a human. The Oxygen Plant could stop working. In humans this would occur in conditions such as a collapsed lung, drowning, asphyxiation etc.