One of the basic objectives of Project Mercury was to evaluate man’s responses to the space-flight environment.  The stresses of this environment which would elicit physiological responses included, according to Dr. Berry,¹⁰ the wearing of the full-pressure suit although not pressurized in flight, confinement and restraint in the Mercury spacecraft with the legs at 90° elevated position, the 100-percent oxygen atmosphere at 5 psi pressure, the changing rather to cabin pressure through powered flight and reentry, variation in cabin and suit temperature, the acceleration forces of launch and reentry, varying periods of weightless flight, vibration, dehydration, the performance required by the flight plan, the need for sleep and for alertness, changes in illumination inside the spacecraft, and diminished food intake.

Data showed that the peak physiological responses were closely related to critical inflight events.  The six astronauts who flew a mission showed themselves capable of normal physiological function and performance during the accelerations of launch and reentry; they tolerated the vibration of launch and reentry well; mine there was no evidence of motion sickness.  The heat loads imposed however, caused discomfort upon occasion but. did not become a limiting factor in the missions.
 
Since the Mercury missions were planned for altitudes that would not involve contact with the Van Allen radiation belt, radiation was not considered to be a problem until the manmade radiation belt was noted prior to the MA-8 mission.  At that time, personal dosimeters were added within the astronaut's suit and inside the spacecraft.  The MA-8 and MA-9 flights revealed that the astronauts received no greater radiation dose than would have been received on earth, and even less than that received during a chest X-ray.

Weightlessness caused no problems, according to the astronauts.  They were able to conduct complex visual-motor coordination tasks proficiently in the weightless state.  No evidence of body system dysfunction was discovered during the flights.  Urination occurred normally in time and amount, and there was no evidence of difficulty in intestinal absorption in the weightless state.

Signs of orthostatic hypotension were noted after the last two missions; they persisted for between 7 and 19 hours after landing.

10.  Ibid., p. 206.