Living on the Seafloor. (Design Issues on Underwater Habitats. 3rd part). The Basics

Knowledge of ocean conditions is important to development of underwater habitats to confront the forces of the environment. Basic needs for the human being to live on extreme and limited conditions have to be fulfilled.
Depth is the main driving mechanism that will affect the basic design and behavior of the human body for living on those conditions.

Pressure and Depth
External pressure product of the weight of the water column will increase with depth. Once the operational depth of the habitat has been established, the pressure at that depth will determine the design and characteristics of the systems for the habitat.
Access to the seafloor by divers and the breathing gases will also depend on depth as the physiology of the human body will change under pressure
The ocean life will also vary with depth and the ecosystems that lives at the coastal shelf will be completely different from the ones that live in the deepest areas of the oceans. As a consequence, the research purpose should be also considered for the design and the operational depth.

The human body under pressure [7]
As the human body is 95% composed of water, including bones, muscles, solid organs, nerves, etc., the effect of the increase on pressure is transmitted and undiminished in all directions making the pressure tolerated at any depth. The body also contains air filled compartment with flexible walls, like the lungs that will compress until certain limits. The air filled compartment with rigid walls, like sinuous and inner ear have to be equalized to avoid damage.

The human body needs air to breath at atmospheric pressure, the increase in depth causes changes to the breathing gas and these changes will have a direct effect on the tissues of the body. Because of these changes, different proportions of gases, like Helium, have to be added to the breathing mix to support the human body at greater depth. The saturation of the tissues with gases at depth is used to prolong the stay of people underwater and long decompression procedures are needed to take the aquanauts back to surface.
Even with saturation diving using mix gases, there is still a limit of depth that the human body can stand and exploration of the ocean at more that 500 meters will required closed habitats. The closed habitats will maintain the internal pressure lower that outside to support appropriated conditions for the human body and the structure of the habitat will have to resist that difference. [10]

Environmental Constraints [8]
Major environmental factors of the ocean will change with pressure and life on the ocean had adapted to those changes. Light, temperature, currents, salinity, density, bottom conditions are major environmental constraints that have to be recognized and will influence visibility, living conditions, communications and safety among others.