  The architecture of a large interstellar spaceship, which is capable of
serving as a living environment for a population over many generations, is
mainly dictated by the needs of said population in terms of food, water and
breathable gases. These resources cannot be stored for the entire duration of a
journey that goes on for several centuries, so they must be produced in situ.
In order to determine the quantities necessary for the survival of the
population, it is imperative to precisely estimate their needs. In this
article, we focus on accurate simulations of the water and air (oxygen)
requirements for any type of population in order to be able to provide precise
constraints on the overall architecture of an interstellar ark (the
requirements in terms of food having already been studied in a previous
publication). We upgrade our agent-based, numerical, Monte Carlo code HERITAGE
to include human physiological needs. Our simulations show that, for a crew of
about 1100 crew members (each characterized with individual anthropometric and
biological data), 1.8 $\times$ 10$^8$ litres of oxygen are annually required,
together with 1.1 $\times$ 10$^6$ litres of water. Those results do not account
for oxygen and water used in growing plants, but they give us an insight of how
much resources are needed in the spaceship. We also review the best methods for
generating water from waste gases (namely carbon dioxide and dihydrogen) and
how such system could complement the oxygen-supplying biospheres inside
multi-generational spaceship to form a closed and controlled environment.
