Space Physiology & Microgravity

SECTION TWO

Human Performance in Extreme Environments

CHAPTER 12 | SPACE PHYSIOLOGY AND MICROGRAVITY

Bone Loss, Radiation, Muscle Atrophy, Neurovestibular, Cardiovascular & Motion Sickness — CAsP Unit 12 Objectives 1–44

Human spaceflight removes the physiological anchors of Earth-based life — gravity, mechanical loading, the day-night cycle — and adds the hazard of space radiation. The result is a predictable set of deconditioning syndromes: bone demineralization, muscle atrophy, cardiovascular deconditioning, neurovestibular re-adaptation, and space motion sickness. This chapter covers all 44 CAsP Unit 12 objectives across six topic areas.

SECTION A: BONE LOSS IN MICROGRAVITY (Objectives 12.1–12.9)

• Rate of loss: Approximately 1–2% bone mineral density (BMD) per MONTH in weight-bearing bones. This is 10–12× faster than terrestrial bed rest and comparable to the worst postmenopausal osteoporosis on an annualized basis.
• Most affected regions (Mir data, 4–6 month missions): Femoral trochanter −11%; calcaneus −7%; pelvis −7%; femoral neck −6%; lumbar spine −1% (lumbar losses continued postflight for up to 6 months while other regions recovered).
• Gender: Terrestrial gender differences in bone loss (women > men) are markedly reduced in space — both sexes lose bone at approximately the same rate, confirming mechanical unloading as the dominant driver.
• Recovery: Up to 2 years for BMD recovery; not all subjects fully recover (5/7 subjects in one Mir study). Elevated fracture risk during recovery compounded by muscle weakness and vestibular ataxia.

Even at sub-toxic spacecraft CO₂ levels (≤0.5%), chronically elevated CO₂ drives respiratory acidosis. Bone mineral (hydroxyapatite) acts as a carbonate buffer for the acidosis — osteoclastic resorption increases to release calcium carbonate and neutralize the acid load. This accelerates bone demineralization beyond the mechanical unloading effect alone.

• Hypercalciuria: Released calcium appears in the urine, used as a real-time biomarker of bone resorption during spaceflight. Also increases kidney stone risk — a recognized long-duration spaceflight complication.

• ↓ dietary Ca²⁺ → ↓ serum ionized Ca²⁺.
• Parathyroid glands detect fall → ↑ PTH secretion.
• PTH acts on: (a) Bone — stimulates osteoclasts to resorb bone, releasing Ca²⁺; (b) Kidney — ↑ tubular Ca²⁺ reabsorption + stimulates 1-α-hydroxylase to produce calcitriol (active Vitamin D); (c) Intestine — calcitriol ↑ intestinal Ca²⁺ absorption.
• Serum Ca²⁺ restored → PTH secretion falls (negative feedback).

Spaceflight paradox: Bone resorption (from unloading) chronically elevates serum Ca²⁺ → PTH suppressed → calcitriol ↓ → intestinal Ca²⁺ absorption ↓. High serum Ca²⁺ but poor dietary calcium utilization.

• Absent mechanical loading (most important): Osteocyte mechanosensors detect no strain → osteoblast activity decreases; osteoclast activity continues → net resorption.
• Muscle atrophy: Reduced muscle contractile forces on bone compound the unloading effect.
• Reduced Vitamin D synthesis: No UVB penetrates spacecraft hull → no cutaneous Vit D synthesis → ↓ calcitriol → ↓ intestinal Ca²⁺ absorption.
• Elevated CO₂: Acidosis drives bone dissolution as acid buffer.
• Altered hormonal environment: Fluid shifts, elevated cortisol, circadian disruption impair bone metabolism.
• Sedentary state: Reduced metabolic demand limits mechano-stimulation.

In microgravity, gravitational compressive load = 0. Bones evolved to support 70–80 kg of body weight carry no weight at all. Weight-bearing bones (vertebrae, femur, tibia, calcaneus, pelvis) are most affected. Upper extremity and skull bones, which are not primarily gravity-loaded, are relatively spared. Some astronauts performing extensive EVA work show minimal arm bone loss or slight hypertrophy.

• Calcium: 1,200–1,500 mg/day — essential for mineralization.
• Vitamin D: 800–1,200 IU/day — essential for intestinal Ca²⁺ absorption.
• Phosphorus: High intake ↑ PTH → ↑ bone resorption. Keep intake low relative to calcium.
• Protein: Adequate required for collagen matrix; excess increases urinary Ca²⁺ loss.
• Sodium: High intake promotes urinary Ca²⁺ excretion. Low-sodium is bone-protective.
• Vitamin K: Required for osteocalcin carboxylation (calcium binding in bone).
• Alcohol and caffeine: Both increase urinary Ca²⁺ excretion. Detrimental in excess.

Spacecraft materials block all UVB radiation (290–315 nm) → no cutaneous Vitamin D synthesis. Without UVB → ↓ calcitriol → ↓ intestinal Ca²⁺ absorption → PTH rises → bone resorption increases. Oral Vitamin D supplementation (800–1,200 IU/day) is mandatory in spaceflight nutrition protocols.

• Resistive exercise (ARED — most effective): High-force muscular contractions apply compressive and tensile loads to bone, partially replicating terrestrial mechanical stimulation. ISS Advanced Resistive Exercise Device provides up to 600 lbs simulated load via vacuum cylinder. Resistive > aerobic for bone preservation.
• Aerobic exercise (treadmill + harness, cycle ergometer): Some bone loading benefit but insufficient alone (Skylab, Mir data: aerobic exercise did not prevent significant bone loss).
• Vibration (LMHF): Low-magnitude high-frequency vibration under investigation; promising in bed rest models.
• Pharmacological (under investigation): Bisphosphonates (alendronate, risedronate) block osteoclast activity; show promise in bed rest studies.

• Calcium: 1,200–1,500 mg/day.
• Vitamin D: 800–1,200 IU/day.
• Adequate protein (0.8–1.2 g/kg/day); not excessive.
• Low sodium (<2,300 mg/day).
• Adequate Vitamin K.
• Low phosphorus-to-calcium ratio; avoid sodas/processed foods.
• Adequate total calories (anorexia from SMS/workload accelerates muscle/bone loss).

SECTION B: RADIATION HAZARDS IN SPACE (Objectives 12.10–12.19)

Ionizing radiation is electromagnetic or particle radiation with sufficient energy to remove electrons from atoms, creating ions. Ionization of biological molecules (DNA, proteins, lipids) is the mechanism of biological damage. Types: electromagnetic (X-rays, gamma rays) and particulate (alpha particles, beta particles, neutrons, high-energy heavy nuclei).

• X-rays (low LET, sparsely ionizing): Photons deposit energy in widely spaced ion pairs. Multiple photons needed for lethal DNA damage. Effectively shielded by dense material (lead). The terrestrial radiation model.
• HZE particles (high LET, densely ionizing): A single iron nucleus (from GCR) deposits enough energy along its track to sever both DNA strands simultaneously and destroy multiple cellular structures in a linear ‘scar.’ This dense ionization cluster is far more difficult to repair than sparse X-ray ionization. Dense shielding can WORSEN HZE dose by producing secondary radiation showers (neutron fragments).

• Galactic Cosmic Radiation (GCR): Originates outside solar system (supernovae). ~85% protons, 14% alpha, 1% HZE nuclei (iron of greatest biological concern). Omnidirectional; cannot be practically shielded. Highest during solar minimum. Primary deep space concern.
• Solar Particle Events (SPE): Episodic proton/electron bursts from coronal mass ejections. Acute high dose in hours to days. More shieldable than GCR. Primary acute radiation hazard.
• Van Allen Belt trapped radiation: Inner belt (protons; 1,000–5,000 km) + outer belt (electrons; 15,000–25,000 km). ISS orbits below at ~400 km but transits the South Atlantic Anomaly (SAA) where inner belt dips lower.

LINEAR ENERGY TRANSFER (LET; keV/μm) is the most important factor. Higher LET = denser ionization along the particle track = more complex clustered DNA damage = greater biological effect per unit energy deposited. High LET produces double-strand DNA breaks in close proximity that overwhelm normal repair pathways.

RBE = (dose of 250 kV X-rays to produce a given effect) / (dose of test radiation to produce the same effect). X-rays: RBE = 1.0 (reference). Alpha particles: RBE ≈20. Fast neutrons: 5–20. HZE particles: up to 30+ for carcinogenesis endpoints. Dose equivalent (Sieverts) = absorbed dose (Grays) × radiation weighting factor (which approximates RBE for protection purposes).

~70% of radiation biological damage is INDIRECT, mediated by free radicals from water radiolysis. Ionizing radiation → ionizes H₂O → produces hydroxyl radical (•OH; most damaging), superoxide (O₂•⁻), and H₂O₂. The hydroxyl radical reacts with DNA (strand breaks, base oxidation), proteins, and membrane lipids within nanometers to micrometers of its formation site. Antioxidants (Vitamins C, E, NAC) partially counteract •OH but are less effective for high-LET direct ionization.

• Carcinogenesis (primary concern): NASA career limit: <3% excess lifetime cancer mortality risk (REID). Cancers: lung, breast, colon, stomach, bladder, leukemia, thyroid.
• Cataracts: Lens is radiosensitive; HZE component particularly cataractogenic. Documented in astronauts.
• CNS effects: HZE particles cause direct neuronal damage, oxidative stress, neuroinflammation → cognitive impairment, reduced neurogenesis (animal data). Primary deep space mission concern.
• Cardiovascular disease: Radiation-associated endothelial damage and atherosclerosis acceleration. Some astronaut mortality analyses suggest elevated cardiovascular risk.
• Hereditary effects: Germline mutations; relatively low risk compared to cancer.

Low-dose radiation priming (≤0.2 Gy) activates DNA repair enzymes (BER, NER, DSBR pathways), antioxidant enzymes, and apoptotic pathways via p53/NF-κB transcription factors, increasing resistance to subsequent higher doses. DNA repair capacity is the key determinant of individual radiation sensitivity. Impaired repair (ATM, BRCA2 mutations) = higher cancer risk. Adaptive response primarily demonstrated for low-LET radiation; less effective for HZE particles whose complex clustered damage exceeds normal repair capacity.

ISS crews at ~400 km altitude receive ~0.5–1.5 mSv/day (150–500 mSv per 6-month mission). Primary sources: GCR (steady background; unshieldable majority of dose) + SAA transits (peak dose spikes) + SPE risk (acute if caught unshielded) + EVA (substantially increased exposure outside the spacecraft hull). EVA timing is managed to avoid known SPE periods.

• Shielding: Water/polyethylene (high H content) for SPE proton protection. Dense metals (aluminum) produce more secondary radiation from HZE particles. Crew storm shelters (water-wall sleeping quarters) provide SPE protection on ISS.
• Mission planning: Schedule deep space missions during solar maximum (lower GCR). Minimize EVA during SPE periods.
• Dose monitoring: Personal dosimeters (TLD, TEPC) for real-time and cumulative dose tracking. Career limits management.
• Pharmacological: Antioxidants (Vit C, E, NAC); G-CSF for post-acute-exposure bone marrow recovery. No approved drug fully prevents HZE carcinogenesis.
• NASA career dose limits: <3% excess lifetime cancer mortality risk (REID); age- and sex-specific.

SECTION C: MUSCLE LOSS IN MICROGRAVITY (Objectives 12.20–12.22)

Muscle atrophy in microgravity preferentially affects anti-gravity postural muscles of the lower extremity and trunk. Studies document 15–20% total volume loss after 6-month ISS missions in:

• Soleus (most affected — highest slow Type I fiber ratio; primary anti-gravity function).
• Gastrocnemius, tibialis anterior.
• Quadriceps (especially vastus intermedius), hamstrings.
• Erector spinae (lumbar extensors).

Upper extremity muscles (deltoid, biceps, forearm) are relatively spared — actively used for ISS operations and EVA activities.

• Reduced caloric intake: SMS, altered food palatability, workload → energy deficit → catabolism.
• Reduced IGF-1 (anabolic): Insulin-like growth factor 1 decreases in microgravity → reduced protein synthesis signaling (mTOR pathway impaired).
• Elevated cortisol (catabolic): Confinement stress, workload, sleep disruption, circadian disruption → elevated cortisol → muscle protein catabolism.
• Oxidative stress: Radiation and mitochondrial dysfunction → ROS accumulation → damages muscle proteins and mitochondria.
• Sleep/circadian disruption: 16 sunrises/day on ISS → circadian disruption → impaired slow-wave sleep → ↓ growth hormone → ↓ protein synthesis.

• High load (≥70–80% 1RM): Required to activate mTOR/IGF-1 hypertrophy signaling and counteract atrophy. Light aerobic exercise is insufficient.
• Multi-joint compound exercises: Squat, deadlift, row equivalents engaging multiple postural muscle groups simultaneously.
• Eccentric loading: Eccentric contractions produce greater mechanical tension per unit metabolic cost → potent protein synthesis stimulus. ARED permits both concentric and eccentric loading.
• Frequency: ISS protocol: 6 days/week combined aerobic + resistive exercise, ~2.5 hours/day.
• Specificity: Must target the most vulnerable muscles (soleus, quadriceps, erector spinae) — upper body exercise does not protect lower body postural muscles.

SECTION D: NEUROVESTIBULAR EFFECTS (Objectives 12.23–12.27)

The primary effect is the REMOVAL of the gravitational reference signal from the otolith organs. On Earth, the constant 1-G force on the otoconia provides the stable gravitational vertical anchor for the CNS orientation model. In microgravity, this signal disappears — the otoliths can no longer define a gravitational vertical, and the CNS’s internal model (calibrated to 1-G over a lifetime) must fundamentally re-adapt to a 0-G reference frame.

The balance control system demonstrates remarkable neuroplasticity in 0-G:

• Early adaptation (days 1–6): SMS most severe. Canal-otolith conflict + visual-vestibular conflict from novel 0-G environment.
• Progressive adaptation (weeks): CNS down-weights otolith signals (unreliable gravity reference) and re-weights canal + visual inputs. Vestibular nuclei and cerebellum recalibrate processing.
• Re-adaptation on return to Earth: Now-space-adapted system encounters 1-G → postural instability, gait ataxia, enhanced tilt perception for days to weeks.

After adaptation to 0-G, the internal model expects no gravitational otolith contribution. On return to Earth:

• Enhanced (exaggerated) tilt perception: The restored gravitational signal is ‘unexpected’ by the 0-G-adapted CNS. Even modest head tilts produce a much larger perceived tilt than actually occurred — the space analog of G-excess effect.
• Impaired lateral acceleration perception: Inconsistent otolith interpretation during translational movement → tilt illusions during lateral movement.
• Postural instability: Vestibulospinal reflexes re-calibrated to 0-G produce inappropriate corrections in 1-G → falls, broad-based gait, ataxia. Astronauts require physical assistance post-landing.

The otolith-VOR is recalibrated to a 0-G baseline during spaceflight. On return to Earth, the restored gravitational component produces inappropriate eye movements (tilt-induced nystagmus, ocular counterrolling) in response to head movements. Dynamic visual acuity (ability to see clearly during head movement) is reduced immediately post-landing. The canal-driven VOR is relatively better preserved. Recovery occurs over days to weeks as VOR recalibrates.

• Gait ataxia: Broad-based, unsteady, heel-to-toe impaired. Combined effect of re-adapted vestibular system (false tilt perceptions) + impaired proprioceptive calibration (re-calibrated to 0-G) + muscle weakness/atrophy.
• Elevated fracture risk: Reduced BMD + muscle weakness + vestibular ataxia = high fall risk during recovery.
• Emergency egress impairment: Self-egress from spacecraft immediately post-landing may be impossible. Astronauts require crew assistance or rescue for initial mobility.
• Recovery: Basic balance recovers in days to weeks; full neuromuscular performance may require 1–6 months.

SECTION E: CARDIOVASCULAR CHANGES (Objectives 12.28–12.33)

IMMEDIATE cardiovascular effect: HEADWARD FLUID SHIFT. On Earth, ~500–700 mL of blood pools in lower extremity veins. In 0-G, this pooling ceases → blood redistributes centrally:

• Stroke volume: ↑ 20–40% in first hours (increased central venous pressure → increased preload → Frank-Starling → increased SV).
• Heart size: Cardiac chambers transiently distend from increased preload → atrial stretch → ANP release → diuresis → plasma volume contracts 10–17% over 24–48 hours.
• Appearance: 'Puffy face, bird legs'— facial/conjunctival edema, nasal congestion, thin lower extremities.

After the initial diuresis, plasma volume is chronically 10–17% below preflight. Reduced circulating volume → reduced cardiac preload. Additionally, no gravitational gradient exists against which the heart must pump blood — total cardiac work is substantially reduced. Like skeletal muscle, cardiac muscle is use-dependent: it atrophies proportional to the reduction in workload. Echocardiography has documented left ventricular mass reduction of approximately 8–12% after 6-month missions.

• Plasma volume: DECREASES 10–17% over 24–48 hours (ANP-mediated diuresis). Remains reduced for the mission.
• Hematocrit: Initially INCREASES (pseudo-hemoconcentration as plasma volume falls with initially unchanged RBC mass).
• Erythropoietin: DECREASES — elevated hematocrit signals less RBC production needed. Reduced EPO → decreased erythropoiesis → neocytolysis (selective destruction of young red blood cells) → RBC mass falls ~10–15% → hematocrit normalizes despite contracted plasma volume.

After even 9–14 days of spaceflight, return to Earth reveals significant cardiovascular deconditioning:

• VO₂max: Decreases ~20–25% below preflight. Reduced cardiac output capacity + contracted plasma volume + muscle deconditioning.
• Peak stroke volume: Reduced — lower preload (contracted plasma volume) + beginning cardiac atrophy + gravitational venous pooling in upright exercise.
• Peak cardiac output: Reduced — reflects reduced stroke volume + impaired overall cardiac performance.

The most important contributor is REDUCED PLASMA VOLUME (10–17% below preflight). With less total circulating volume, the cardiovascular system has insufficient reserve to compensate for gravitational lower-extremity venous pooling that immediately occurs on standing after landing. Result: ↓ venous return → ↓ stroke volume → ↓ blood pressure → presyncope/syncope. Affects ~25% of short-mission and up to 80% of long-mission astronauts. Secondary contributors: impaired baroreflex sensitivity, reduced vascular tone, cardiac atrophy.

LBNP applies sub-ambient pressure to the lower body in a sealed chamber, drawing blood into the lower extremities — simulating the orthostatic stress of gravity in 0-G. Daily LBNP sessions exercise the baroreflex by repeatedly challenging the cardiovascular system with orthostatic stress, maintaining baroreflex sensitivity and vascular tone. The Russian Chibis suit is the primary operational LBNP device, used combined with pre-landing fluid loading (salt tablets + water) to expand plasma volume before re-entry.

SECTION F: SPACE MOTION SICKNESS (Objectives 12.34–12.44)

The SENSORY CONFLICT THEORY (neural mismatch hypothesis) is most broadly applicable. Motion sickness results when sensory inputs from visual, vestibular, and somatosensory systems are internally inconsistent OR differ from the brain’s internal model prediction. In space: Canal-otolith conflict (otoliths lose gravitational reference → unexpected response to head movements) + visual-vestibular conflict. Head movements in pitch are the primary SMS triggers because they maximally engage the canal-otolith conflict in 0-G.

• Stomach awareness (epigastric discomfort) — earliest symptom.
• Nausea (may be severe; occurs in ~70% of astronauts early in flight).
• Vomiting (~40–50% of astronauts; acute management critical in microgravity — aspiration hazard in 0-G).
• Pallor, cold diaphoresis.
• Headache.
• Malaise, lethargy, reduced motivation.
• Sopite syndrome: Drowsiness + cognitive impairment without frank nausea (operationally significant).

Sopite syndrome is a motion sickness variant characterized by drowsiness, fatigue, impaired motivation, and subtle cognitive degradation rather than frank nausea. In space, it may be more common in later mission phases when full vestibular adaptation has reduced overt nausea but subtle sensory conflict effects persist. Operationally, affected crew perform normally on self-paced routine tasks but show disproportionate degradation on sustained vigilance and emergency response tasks. They typically do not self-report impairment and do not appear classically sick — making the syndrome invisible to mission controllers without active screening.

Research consistently shows a performance dissociation during motion sickness: sustained vigilance and monitoring tasks are severely degraded (autonomic arousal, subjective distress, and cognitive narrowing impair continuous attentional effort). Emergency and discrete high-urgency response tasks are substantially BETTER preserved — the acute stress arousal of an emergency provides a countervailing sympathetic performance enhancement. A crew member actively vomiting may still respond accurately and quickly to an alarm requiring a single decisive action.

Anticholinergic drugs (primarily scopolamine) are the most effective single agents for motion sickness prophylaxis. Mechanism: muscarinic receptor blockade (M1/M2) in vestibular nuclei, cerebellum, and brainstem vomiting center — reducing the gain of emetic pathways for vestibular conflict signals. Side effects: drowsiness (central M1), dry mouth (salivary M3), mydriasis (pupil dilation; M3 ciliary ganglion), cycloplegia (impaired accommodation; M3 ciliary muscle), urinary retention, ↓ sweating, tachycardia (M2 SA node blockade). All side effects are dose-dependent and incompatible with aviation flying duties.

P6 acupressure (Neiguan point) is located on the palmar wrist ~3 finger-widths proximal to the wrist crease between the tendons of flexor carpi radialis and palmaris longus. Bilateral pressure via elastic bands or electronic wristbands (ReliefBand) has been studied as a non-pharmacological motion sickness intervention. Proposed mechanism: median nerve stimulation modulates brainstem nausea/vomiting centers. Evidence is mixed — some trials show modest nausea reduction; others show no benefit beyond placebo. Advantages: non-pharmacological, no side effects, no performance impairment, compatible with flying, can be combined with other countermeasures.

• Onset: Within 30 minutes to hours of entering orbit.
• Peak severity: Days 1–3. ~67–70% experience nausea; ~40–50% vomit.
• Resolution: Majority resolve by days 3–6. Some take up to 7–10 days.
• Recurrence on return: Re-entry and return to 1-G can trigger a second SMS episode as the space-adapted vestibular system re-encounters gravity.

Pre-flight motion adaptation training has shown LIMITED effectiveness for SMS prevention. The 0-G sensory conflict is sufficiently novel and specific that no ground-based motion environment can fully simulate it for pre-adaptation. Parabolic flight (KC-135/C-9) provides only ~20-second 0-G intervals — insufficient duration for meaningful pre-adaptation. Exception: Space crew members represent a specific exception to the 'adapt rather than medicate' principle — because ground pre-adaptation cannot be reliably achieved, pharmacological SMS prophylaxis for re-entry IS appropriate for space crews (unlike pilots who should adapt through repeated exposure).

Table 12.1. Scopolamine Side Effects by Organ System

Scopolamine blocks muscarinic receptors in the cerebellar-vestibular pathways that process the sensory conflict signal required to drive CNS re-calibration. Habituation to a motion environment (long-term SMS prevention) requires the CNS to detect, quantify, and learn from sensory conflict through these cholinergic pathways. Scopolamine attenuates the conflict signal — the CNS receives a pharmacologically muted version and adapts more slowly. When the drug is discontinued, the unadapted conflict returns. For space crews who must ultimately work in the 0-G environment without medication, scopolamine provides acute relief at the cost of prolonged adaptation timeline.

Table 12.2. Drugs Used to Treat Space Motion Sickness