KNES 323: Space Physiology Notes PDF

KNES 323: Integrative Physiology Space Physiology Friday December 1, 2023 Class Outline 1. Background 2. Cardiovascular Effects 3. Musculoskeletal Effects 4. Summary Background  Background As we have been discussing over the past few classes, the human body is pretty amaz...

KNES 323: Integrative Physiology Space Physiology Friday December 1, 2023 Class Outline 1. Background 2. Cardiovascular Effects 3. Musculoskeletal Effects 4. Summary Background  Background As we have been discussing over the past few classes, the human body is pretty amazing! It can adapt to function across a wide variety of conditions, from the depths of the oceans to the top of the Himalayas and as we will discuss today, even can survive in the most extreme environment: SPACE  Background However, space in and of itself poses some very unique medical problems due to prolonged exposure to a combination of stressful stimuli: Acceleration forces Radiation Weightlessness  Background To appreciate the uniqueness of space, one should consider that a static gravitational field and a protective atmosphere are major factors that have made Earth able to sustain life.  Background In space, many physical and biological changes occur that highlight the importance of gravity in the evolutionary course, while coincidentally revealing unexpected links between microgravity, disease onset and aging.  Background Between the space race in the 1960s and the extended stay Skylab missions in the 1970s (increasing durations of 28, 56, and 84 days), each with three astronauts, enough data were collected to establish the foundation of space physiology  Background The astronauts on these early extended duration missions showed signs of negative calcium balance with bone density loss, muscle atrophy, cardiovascular and hematic changes, metabolic, endocrine, and sleep disturbances. All similar to changes within aging populations  Background Since these early days, the majority of human spaceflights have taken place in low Earth orbit, which occurs at an altitude above approximately 200 km and below 2000 km. The International Space Station Most satellites operate in low Earth orbit  Background Objects in low Earth orbit are subjected to about 90% of the Earth’s ground-level gravity, but the speed during orbit effectively counterbalances the force of gravity, creating a free-falling state and apparent weightlessness https://www.youtube.com/watch?v=iQOHRKKNNLQ  Background An additional consideration is the possible impact of radiation exposure when outside of the atmosphere. The atmosphere acts as a natural shield to a lot of the radiation that bombards the earth on a daily basis. Astronauts are exposed to heavy ions and proton fluxes, far higher than those experienced at most locations on Earth Cardiovascular Effects  Cardiovascular Effects As we have been discussing in these special topics, the human body has the capacity to adapt to vastly different environmental and metabolic conditions  Cardiovascular Effects Gravity is a crucial factor in fluid distribution and has played an enormous role in shaping the evolution of the cardiovascular system On Earth, fluids shift based on gravity In an upright posture Mean BP in the head is ~70 mmHg Mean BP at the heart is ~93 mmHg Mean BP at the feet is ~200 mmHg In a supine posture Mean BP for the body is ~93 mmHg  Cardiovascular Effects In space (and low Earth orbit), this gradient does not occur There is a substantial reduction in intrathoracic pressure and blood is redistributed towards the head causes altered responses of baroreceptor, nervous and endocrine systems. Within few minutes of microgravity exposure, astronauts suffer from a syndrome, known as “space motion sickness,” which includes anorexia, vomiting, nausea, headache and fatigue  Cardiovascular Effects These motion sickness symptoms arise because changes in gravity alter the sensory input from the vestibular system, which in turn generates a persistent conflict (i.e., mismatch) between expected and actual sensory vestibular inputs during active movements Largely due to the unloading of the otolith organs in the first few days of exposure to microgravity and are often adjusted by ~5 days into the mission  Cardiovascular Effects In the initial stages hours and days of being exposed to microgravity, this shift in fluids to the head will cause the head and chest to swell as blood is redirected from the lower limbs Change is often referred to as “Puffy Head, Chicken Legs” condition Forehead circumference increased by 7% while tibia circumference decreases by 15%  Cardiovascular Effects Additionally, in a microgravity environment, large contractility of the heart is not required to send blood toward the head against gravity, and to maintain blood pressure Thus, the heart is atrophied by −8 to −10% after 10 days of spaceflight Cardiovascular Effects 70 mmHg 93 mmHg 93 mmHg 93 mmHg 93 93 93 mmHg mmHg mmHg 85 105 93 mmHg mmHg mmHg 200 mmHg 93 mmHg  Cardiovascular Effects As blood is redistributed towards the head during microgravity, these changes are detected by baroreceptors which activate autonomic offloading and blood volume regulating reflexes:  Cardiovascular Effects To compensate for the redistribution of blood in the early space exposure period, there is a reduction to HR, reduced level of thirst and increased urine production in order to compensate for the change in blood volume in the head, neck and torso (location of baroreceptors). After acclimatization, there is a reduced total blood volume (increased hematocrit), while MAP and HR are maintained at pre- flight levels.  Cardiovascular Effects Trudel et al. (2019) AJP Hemo 95(3): 267-73 The Hb reduction reported in astronauts returning from missions 145 days may represent the completed human erythrocytic adaptation; the new “space normal”. Cardiovascular Effects Musculoskeletal Effects  Musculoskeletal Effects Prolonged exposure to microgravity affects the musculoskeletal system, with the loss of bone and muscle mass attributed to both reduced use and perfusion changes Clément (2011) Fund Space Med 181-216 Musculoskeletal Effects Furthermore, the rate of bone mineral density loss occurring during a 6-month stay on ISS is similar on average to that occurring with someone aging from 50 to 60 on Earth Musculoskeletal Effects https://www.youtube.com/watch?v=ht9zTT4qPeI  Musculoskeletal Effects Bone mineral loss was higher in sites supporting the body weight in normal gravity, like lumbar spine, femoral neck, and trochanter, pelvis, calcaneus, and leg, whereas arm bones were largely unaffected  Musculoskeletal Effects Muscles are needed for movement and to counteract gravity, and they must be used in order to maintain the structure and the function. During spaceflight or in a microgravity environment, humans do not need to support their bodies; thus the antigravity muscles become atrophied Lambertz et al. (2001) JAPPL 90: 179-88 Musculoskeletal Effects https://global.jaxa.jp/article/special/kibo/nikawa_e.html  Musculoskeletal Effects Baseline (solid bars) and 2–3 days after return (open bars) The main cause of muscular loss is the disappearance of mechanical constraints and the subsequent decrease in muscular activity Lambertz et al. (2001) JAPPL 90: 179-88  Musculoskeletal Effects It is not surprising that a varying degree of muscle loss results from different duration of microgravity exposure. https://gfycat.com/angryquerulouschinchilla  Musculoskeletal Effects These physiological alterations may have a major impact on rehabilitation after injuries, especially in aging subjects, highlighting the need to focus on those muscle groups more prone to mass loss in response with unloading (sedentary behaviours) Williams et al. (2009) CMAJ 180(3): 1317-23  Musculoskeletal Effects In order to protect and maintain the health and wellness of astronauts, countermeasures designed have been designed to try and simulate Earth-like movements and stresses or reloading of gravity https://www.youtube.com/watch?v=G0AworhlfHU&t=51s  Musculoskeletal Effects One of the most important countermeasures for space flight is exercise although consensus regarding the best type of devices and exercise protocols has not yet been reached Summary Summary Aubert et al. (2005) Acta Cardio 60(2): 129-51 https://www.youtube.com/watch?v=y7rDrAKRqjo&t=148s