The space flight environment with its combination of stresses offers a unique challenge to the body’s biochemical control mechanisms. Astronauts returning from Gemini and Apollo flights demonstrated biochemical changes of sufficient magnitude and complexity to warrant detailed studies of the endocrine and metabolic systems. The Bio-Assay of Body Fluids (M073) experiment was designed to study homeostasis in the areas of (1) fluid and electrolyte balance, (2) regulation of calcium metabolism, (3) adrenal function, and (4) carbohydrate, fat, and protein utilization. The study was divided into two broad categories. One category included routine blood and urine studies similar to those used in clinical medical practice. The second category included a more thorough analyses to investigate the metabolic/endocrine responses to the space flight environment, including fluid and electrolyte control mechanisms.
The nine astronauts who flew on Skylab 2, Skylab 3, and Skylab 4 all participated as test subjects. Metabolic monitoring of the astronauts began at least 21 days prior to each flight, continued throughout each flight, and proceeded for at least 17 days after flight. Following an overnight fast, blood samples were drawn. The blood volume required for preflight and postflight analyses was 25 ml and the inflight plasma averaged 3 ml. Sodium ethylenediaminetetraacetic acid (EDTA) was used as an anticoagulant for blood samples. Radioassay, fluorometric and gas chromatographic techniques were used for most hormonal analyses. Radioactive body compartment studies were conducted preflight and postflight. These included dilution studies of body water (tritium), extracellular fluid (35 sulfate), plasma volume (125 I-protein) and exchangeable potassium (42 K and 43 K) both preflight and postflight. Urine was collected on a void-by-void basis before and after each mission. Twenty-four hour urine collections on each subject were made throughout the flights as part of the Mineral Balance (M071) experiment. Because of limitations in return storage weight and volume, a 120 ml aliquot of each day's urine collection was collected, frozen, and returned to Earth. A measured quantity of lithium chloride, added to each 24-hour collection bag prior to flight, permitted urine volumes to be calculated based on lithium dilution. In addition, crewmembers measured the filled collection bag with a gage to provide an estimate of daily urine output. Data samples and results were shared among the three related investigations. Data on food and water consumption, body mass, and urine volume were telemetered nightly to the investigators.
Analysis of postflight blood samples showed elevations in calcium and phosphorous were present throughout the three missions and remained higher than preflight control values for several days following flight. Cortisol and Angiotensin I were generally elevated, though not always significantly. Potassium and creatinine increased inflight and remained high immediately postflight. Plasma aldosterone, total protein, carbon dioxide, thyroid stimulating hormone, and thyroxine increased postflight. Inflight and postflight plasma measurements showed a decrease in sodium, chloride, osmolality, and adrenocorticotropic hormone (ACTH). Glucose, insulin, and aldosterone decreased inflight, but increased postflight. Postflight decreases were also seen in cholesterol, uric acid, magnesium, lactic dehydrogenase, and total bilirubin. Blood urea nitrogen (BUN) and albumin were not changed at recovery, but were decreased on the third and fourteenth postflight days. All electrolytes in the 24-hour urine samples inflight and postflight were increased along with aldosterone, cortisol, and total 17-ketosteroids. Postflight increases were seen in epinephrine, norepinephrine, aldosterone, and cortisol. The data also showed a trend towards inflight decreases in antidiuretic hormone (ADH), epinephrine, norepinephrine and uric acid. The inflight norepinephrine levels were probably the reflection of the high levels of physical exercise undertaken by each crew member during the flight. Significant decreases in sodium, potassium, chloride, osmolality, phosphate, magnesium, uric acid, ADH, and total 17-hydroxycorticosteroids were observed postflight. This experiment examined the biochemical reactions of the body to the stress of space flight. The experiment was the first comprehensive and integrated study of endocrinology and metabolism during prolonged space flight. Significant biochemical changes were observed, but all disappeared shortly after returning to Earth. These changes for the most part indicated a successful adaptation of the body to the combined stresses of weightlessness. The transient nature of some changes, particularly in fluid and electrolyte metabolism, supported the conclusion that a new and stable condition of homeostasis had been achieved. In other areas, particularly those concerned with the metabolism of bone mineral, protein, and carbohydrates, unstable states appeared to persist. At the time, it was unclear in what form these changes would manifest themselves during flights of much longer duration.