Four investigations fell in the discipline of regulatory physiology, studying the renal/endocrine and the hematological systems. The renal/endocrine system consists of the kidneys and endocrine glands that control hormones regulating blood volume, blood pressure and essential electrolytes (dissolved salt and minerals, such as sodium, potassium, calcium and phosphate) in the blood. The regulatory function of the renal/endocrine system exhibits both acute (hours to days) and adaptive (days to weeks) responses to microgravity in conjunction with the cardiovascular response to the headward fluid shift. Renal/Endocrine and hematology investigations studied two components of blood: plasma and red blood cells. By analyzing plasma, the fluid portion of the blood, investigators can identify the types of nutrients circulating throughout the body to determine if an astronaut is well-nourished and hydrated. Researchers also studied the noted decrease in red blood cell production that is regarded as a potential space flight problem that could acquire increased significance with inflight illness or injury, particularly on long-duration missions.
Scientists hypothesize that there are many ways weightlessness affects the cardiovascular and cardiopulmonary system. Results from SLS-1 showed an increase in the heart rate, heart size and cardiac output, presumably in response to the initial increase in central blood volume caused by the fluid shift. Three SLS-2 investigations assessed the function of this system by monitoring cardiac output (volume of blood pumped per minute), heart rate, arterial and venous blood pressure, blood volume and the amount and distribution of blood and gases in the lungs.
In microgravity, the musculoskeletal system is not used as intensively as it is on Earth. The absence of gravitational force results in changes to load-bearing tissues, causing a reduction of bone and muscle. Astronauts float instead of using their legs for locomotion. In addition, the metabolic state of the musculoskeletal system may be altered by changes in dietary intake and exercise levels and also space motion sickness. In turn, losses from gravity-bearing tissues contribute to the reduced fitness of astronauts when they return to Earth. The general objective of the five musculoskeletal investigations was to study the changes in protein and calcium metabolism and bone homeostasis that occur in microgravity.
Neurovestibular changes related to equilibrium and body orientation affect the early inflight performance of astronauts probably more than any of the other physiological changes. The awareness of body orientation on Earth is attributed, in part, to the detection of gravity by the otolith organs in the inner ear. Gravity sensors in the joints and touch sensors in the skin also are involved, and the eyes contribute by sensing the body's relationship to other objects. In weightlessness, information sent to the brain from the inner ear and other sense organs no longer corresponds to the cues experienced in Earths environment. These conflicts can cause disorientation or space motion sickness. Although adaptation occurs within a few days and crewmembers are usually able to operate efficiently, investigators are working to better understand and counter these negative effects. The goals of the two neuroscience investigations of SLS-2 were to document the changes that occur in the neurovestibular system, to investigate the mechanisms involved in these changes and to identify countermeasures to alleviate the effects of space motion sickness.
Detailed Supplementary Objectives (DSOs) were also performed on this mission. A DSO is a NASA-sponsored investigation that is performed by Space Shuttle crewmembers, who serve as the test subjects. These studies are designed to require minimal crew time, power and stowage. Biomedical DSOs focus on operational concerns, including space motion sickness, cardiovascular deconditioning, muscle loss, changes in coordination and balance strategies, radiation exposure, pharmacokinetics and changes in the bodys biochemistry.
Many of the SLS-2 experiments were conducted in whole or in part on the SLS-1 mission. Contrary to the consistent repeatability of the physical and chemical sciences, studies of physiological systems involve variations in responses from one individual to another. To reduce the influence of the biological variability and to improve the quality of the experimental results, data are collected on multiple individuals and compared through statistical analysis. The SLS-2 mission not only contributed to the statistical assessment of those portions of the experiments performed on SLS-1 but also extended the data collection to studies and procedures being performed for the first time in the flight environment.