Physiological Responses to Altitude and Microgravity

Exercise and Environmental Stress

• The lecture covers microgravity and altitude stress.
• Microgravity simulation methods are discussed.
• Physiological and anatomical responses to microgravity are identified; five specific responses are listed.
• Strategies for countering the effects of microgravity are described.
• Short- and long-term physiological adaptations to altitude stress are outlined.
• The subject of altitude sickness is covered.

Microgravity

• Microgravity is defined as conditions where perceived gravity is less than on Earth.
• Examples include locations like the Moon, Mars, and space stations (0 g).
• NASA-sponsored microgravity studies use various approaches: single-flight data analysis, space motion sickness tests, simulating microgravity through parabolic aircraft, and longitudinal studies. These studies explore countermeasures, effects on health risks (e.g., cancer, bone loss), and occupational injuries in space missions.

Gravity

• Gravity is an invisible force of attraction between any two masses.
• Gravitational law states that every particle attracts every other particle with a force directly proportional to the product of their masses and inversely proportional to the square of the distances separating them.
• Weight (W) is the force (F) required to support a mass (m) on Earth (W = mg).

Strategies to Simulate Microgravity

• Researchers use various approaches to simulate microgravity for experiments, including parabolic airplane flights, head-down bed rest, wheelchair confinement, water immersion, and hindlimb suspension (animal models).
• Parabolic flights use parabolic trajectories to create brief periods of near-weightlessness.

Effects of Near-Zero-G During Spaceflight

• Spaceflight can impact stature, increasing it slightly due to removal of gravity's downward force.
• A change in body stature is documented.

Simulating Non-Weight-Bearing Effects

Head-down bed rest subjects are confined to beds in horizontal or head-down positions (e.g., -3° to -12°) for weeks, months, or a year allowing researchers to measure and track effects. Hind-limb suspension involves techniques to elevate the hindquarters or use partial weight-bearing apparatus for animal studies.

Spaceflight Physiology

• In microgravity, hydrostatic gradients and loading on weight-bearing tissues are reduced; affecting the cardiovascular, hematologic, fluid/electrolyte/hormonal, muscle, bone, and neurosensory/vestibular systems.
• Data analysis from single flights and longitudinal studies of multiple missions involving space motion sickness, countermeasures, and long-term impacts on health are examined.

Acute Microgravity Responses

• Blood and fluids shift to a superior thoracic region, causing a 'puffy face', reduction in waist size, eye redness, and skinny legs.
• Blood volume, plasma, and red blood cells decrease.
• Increased venous pooling and a blunted baroreceptor reflex are observed, as well as orthostatic intolerance problems.

Cardiovascular Adaptations

• Total fluid volume decreases, reducing the heart's workload.
• Chronic exposure leads to a decrease in heart size.
• These adaptations are considered an appropriate response without compromising normal cardiovascular function.

Pulmonary Adaptations

• Cellular oxygen demand remains constant during rest and exercise, regardless of the environment; but, adjustments are made in breathing rate and tidal volume in response to external demands.
• Alveolar ventilation keeps up with the demand.

Changes in Pulmonary Diffusing Capacity

• Studies show increased diffusing capacity while in microgravity and post-flight. Changes in Pulmonary Diffusing Capacity are observed at different time points (pre-flight, days 2, 4, 9, and post-flight) associated with changes in lung function.

Changes in Body Fluids

• Plasma volume and red cell mass are measured.
• Total hemoglobin and blood volume changes are monitored over space missions involving spaceflight.

Musculoskeletal Adaptations

• A significant concern is the 1% monthly bone loss experienced in space missions.
• A possible solution is finding crewmembers who have high bone density resistance.
• Bone loss continues even with in-flight exercise.

Skeletal Muscle Adaptations

• Muscle mass and strength decline due to lack of gravity's effect on antigravity muscles.
• This can impact performance in emergencies.
• Training methods (concentric and eccentric strength) in space result in improvements in force-generating capacity and muscular ultrastructure.

Different Duration Spaceflight Effects

• Onboard exercise equipment use improves muscle mass, force-generating capacity, and muscular ultrastructure.
• Studies involving maximal explosive power (MEP) and maximal cycling power (MCP) were done pre- and post-flight, demonstrating that prolonged spaceflight affects muscle function.

Body Composition Changes

• Body composition undergoes changes during space mission due to factors like densitometry and bioelectrical impedance analysis.
• These changes involve fat, extracellular fluids, and the components of lean body mass.
• Declines from 3% to 4% in water, protein, and mineral components in lean body mass are observed.

Countermeasures Strategies to Microgravity

• Countermeasures include in-flight resistance training, including treadmill walking and running, cycle ergometry, and leg rowing. These activities are essential for maintaining physical function and performance in space missions. Diverse exercise equipment is used for resistance training to maintain muscle mass and improve force-generating capacity in space missions.

Exercise Training in Space

• Resistance training and concentric/eccentric strength training are used on in-space exercise equipment. These exercises help increase muscle mass and force-generating capacity, which are crucial in space missions.

Heart Rate Response

• Heart rate responses are collected both during continuous and intermittent treadmill exercise during space missions.

Space Pharmacology

• Space motion sickness is a common problem in space. Different pharmacological treatments, such as anticholinergics, antihistamines, and sympathomimetics, are used to combat space motion sickness.

Time Course of In-Flight Adaptations

• Within 14 days of a mission, about 10% of bodily fluid redistribution occurs.
• Cardiovascular system performance changes are noticeable within 3 weeks. Bone mass decreases by 5% and continues declining to 17% over the first year.

Altitude Stress

• High altitudes on Earth present challenges as decreased oxygen pressure has negative effects.
• World maps show terrain with different elevations. Physiological changes occur with increasing elevation.
• Altitude's effects (e.g., hypoxia) directly influence oxygen transport, leading to changes in bodily fluids and composition (cardiovascular, hematologic).

The Stress of Altitude and Environmental/Physiological Changes

• Altitude's physiological challenge is directly related to the reduction in ambient PO₂, leading to oxygen transport cascade changes.
• The increase in altitude results in a decline in barometric pressure.
• The change in barometric pressure is related to a reduction in oxygen pressure; it impacts various physiological systems, including respiratory, cardiovascular, and fluid/electrolyte balance.
• Altitude sickness, a medical condition resulting from reduced arterial Po₂, has various symptoms.

Oxygen Transport Cascade

• The progression of oxygen transport from the atmosphere to cells within the body.
• There is a decline in Oxygen when going to higher altitudes.

Altitude Stress

• The physiologic effects of altitude and the measures taken to address them.
• Physiologic effects on oxygen consumption and metabolic rate due to hypoxia are mentioned.

Altitude Stress (continued)

• Hypoxia, reduced oxygen delivery to tissues, resulting from reduced barometric pressure and PO₂.
• Dehydration and increased solar radiation are additional challenges.

Hyperventilation

• Increased breathing rate due to reduced arterial PO₂.
• This immediate response results in more pronounced acid-base balance disruption and reduced CO₂.

Increased Cardiovascular Response

• Increased resting heart rates and cardiac output at submaximal exercise levels.
• Increased blood pressure is observed.

Catecholamine Response

• Increased sympathicoadrenal activity in response to altitude-associated factors.
• This response leads to changes in blood pressure and heart rate.

Reduction in VO2 Max at Altitude

• Maximal oxygen consumption (VO2 max) is reduced at higher altitudes. This is directly correlated with changes in altitude.

Relationship Between Ventilation, VO2, and Exercise Intensity

• A close relationship exists between ventilation, VO₂ and increasing exercise intensity during simulated altitude environments. Changes in minute ventilation and oxygen consumption levels are significant factors at elevations.

Arterial Desaturation

• Arterial desaturation results from increases in altitude due to reduced PO₂.
• Corresponding impairment in sensory functions and cognitive function are observed with altitude elevation.

Immediate and Longer-Term Hypoxia Adjustments

• Responses to reduced oxygen levels vary transiently and chronically with time and environment.
• Changes can be measured and tracked across various bodily systems.
• Loss in body weight and lean body mass are consequences of reduced oxygen.

Altitude Stress and Dehydration.

• Dehydration results from the cool, dry air found in high altitudes, as ambient air evaporates body fluids.
• Air temperature change, humidity, and increased physical activity all contribute to dehydration.
• Increased fluid loss leads to dehydration.

Combine Altitude Stay with Low-Altitude Training

• Combining high-altitude stays with lower-altitude training.
• Live high-train low approach for athlete training.

At-home Acclimatization

• Different methods to simulate altitude conditions at ground level.
• The use of hypobaric chambers, changing oxygen percentages, and altitude tents for home acclimatization.

Altitude-Related Medical Problems

• Acute mountain sickness and high-altitude pulmonary edema are medical problems.

Altitude Sickness

• Symptoms (headache, nausea, weakness) and causes (decreased oxygen pressure), with potential progression to severe conditions.
• Removal from the altitude and rest are treatments.

Extreme Effects of High Altitude

• Severe effects associated with prolonged exposure.