Journal of Hematology & Thromboembolic Diseases
Open Access

Hematological Health of Astronauts and the Challenges of Long-Duration Missions

Beverley Makris^{* *}Correspondence: Beverley Makris, Department of Experimental Haematology, University of Cairo, Cairo, Egypt, Email:

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Description

Space exploration has captured human imagination for decades, and the advancements in space missions have led to a deeper understanding of the human body's response to spaceflight. As astronauts venture into space, they are subjected to unique conditions that alter various physiological systems, including the hematological system. The absence of gravity, exposure to cosmic radiation, and the confined environment aboard spacecraft can lead to significant changes in blood composition and function. These hematological alterations are optimistic to understanding the health risks astronauts face during long-duration missions and to developing countermeasures to mitigate these effects. This article explores the hematological changes observed in astronauts during spaceflight, focusing on alterations in Red Blood Cells (RBCs), White Blood Cells (WBCs), platelets, and coagulation factors, as well as the mechanisms behind these changes. By understanding how spaceflight impacts the hematological system, we can better prepare for the health challenges astronauts will face during future space missions, including prolonged stays on the International Space Station (ISS) or eventual missions to Mars.

RBCs and hemoglobin in Space

One of the most prominent hematological changes during spaceflight is the alteration in RBC mass and hemoglobin levels. Studies have shown that astronauts experience a decrease in total RBC count and a reduction in hemoglobin concentration during and after space missions. These changes are primarily due to a combination of factors related to fluid shifts, bone marrow suppression, and changes in erythropoiesis.

Decreased RBC mass: During spaceflight, the redistribution of body fluids due to the absence of gravity can lead to a transient increase in plasma volume, which results in a dilution of RBCs and hemoglobin (a condition known as "space anemia"). This process, also known as hemodilution, can cause a temporary reduction in hematocrit levels. The decrease in RBC mass may be partially due to decreased erythropoiesis (the production of RBC) as well as an increase in RBC destruction.

Suppressed erythropoiesis: The production of RBCs is regulated by Erythropoietin (EPO), a hormone primarily produced in the kidneys in response to low oxygen levels (hypoxia). On Earth, gravity and the body's typical daily activities can stimulate EPO production to maintain appropriate oxygen-carrying capacity in the blood. In space, however, the absence of gravity and altered environmental conditions may lead to changes in the EPO response. Studies have shown that astronauts experience a reduced EPO response to hypoxia during spaceflight, possibly due to a lack of physical exertion or the altered regulatory mechanisms of EPO.

Increased RBC destruction: In space, the increased production of free radicals due to cosmic radiation and other stressors can contribute to oxidative damage to red blood cells, leading to their premature destruction. This process, known as hemolysis, may further contribute to the observed decline in RBC mass.

WBCs and Immunity in Space

The immune system undergoes significant alterations during spaceflight, and changes in WBC count and function are central to these shifts. The reduction in WBCs, particularly neutrophils and lymphocytes, is commonly observed in astronauts, which may affect the body’s ability to respond to infections and stress.

Leukopenia and lymphocytopenia: Leukopenia (a reduction in the total number of WBC) is a common observation during spaceflight. The number of circulating lymphocytes, particularly T-cells and B-cells, is significantly reduced during space missions. This decline in immune cell numbers may be attributed to several factors, including stress-induced apoptosis (cell death) of immune cells, alterations in bone marrow function, and changes in the production and release of cytokines. Additionally, astronauts experience a suppression of immune responses due to the absence of regular exposure to pathogens and altered stress hormone levels during spaceflight.

Altered immune function: The reduction in WBC count during space missions does not necessarily correlate with an immediate increase in infection rates, but it does affect the overall immune system's functionality. Spaceflight-induced changes in immune cell distribution and function may increase astronauts' susceptibility to certain infections, particularly reactivation of latent viral infections (e.g., herpes simplex virus). The stress response caused by microgravity, radiation exposure, and isolation can lead to alterations in immune signaling pathways, resulting in immune dysregulation.

Increased inflammation: While immune cell numbers may decrease, some studies suggest that spaceflight can also lead to an increase in systemic inflammation. Elevated levels of pro-inflammatory cytokines, such as Interleukin-6 (IL-6) and C-Reactive Protein (CRP), have been detected in astronauts. This inflammatory response, although typically mild, may play a role in the long-term health effects of space travel and contribute to the higher incidence of autoimmune-like responses and other health complications.

Platelets and coagulation in space

Spaceflight also induces significant changes in platelet function and coagulation pathways, which can affect the risk of thrombosis or bleeding.

Thrombocytopenia: Thrombocytopenia (a reduction in platelet count) has been observed in astronauts during and after space missions. Like other blood cell types, platelets may be affected by fluid shifts and alterations in bone marrow function. However, the mechanism behind spaceflight-induced thrombocytopenia is not yet fully understood. The decrease in platelet count is usually transient and does not typically lead to clinically significant bleeding.

Platelet activation: Despite the reduction in platelet count, there is evidence that platelets in space may become more activated. This phenomenon may be due to the increased presence of oxidative stress and the exposure to microgravity, both of which have been shown to influence platelet activation. Platelets in space exhibit increased levels of thromboxane A₂, a molecule involved in platelet aggregation and vasoconstriction, which may predispose astronauts to a higher risk of thrombosis upon their return to Earth.

Coagulation factors: Spaceflight has been associated with changes in various coagulation factors, including fibrinogen and clotting factors such as Factor VII. These changes are likely related to the overall inflammatory response in space and can influence the clotting ability of astronauts' blood. The microgravity environment may also impact the activity of anticoagulant proteins, such as antithrombin III, and increase the risk of both thrombosis and bleeding during long-duration missions.

Mechanisms behind hematological changes in space

The exact mechanisms behind these hematological alterations are not understood, but several factors are thought to contribute to the changes observed in astronauts' blood profiles during spaceflight.

Microgravity: The absence of gravity in space leads to fluid shifts within the body, with blood and other fluids being redistributed toward the upper body and head. This can affect the volume of plasma and blood cells, leading to dilution effects on hemoglobin and RBC mass. Furthermore, microgravity alters the function of the bone marrow, where blood cells are produced, potentially reducing erythropoiesis and leukopoiesis.

Cosmic radiation: Space radiation, particularly cosmic rays and solar radiation, contributes to oxidative stress, which can damage blood cells, including RBCs and WBCs. This radiation-induced damage may increase hemolysis, leukopenia, and platelet dysfunction. Cosmic radiation also impacts DNA integrity, which could affect the production of blood cells and their function.

Psychological stress: The psychological stress associated with spaceflight, including confinement, isolation, and the challenges of maintaining physical and mental health in a foreign environment, can have an impact on immune function and hematopoiesis. Stress-induced changes in cortisol and other hormones can modulate immune cell function, potentially contributing to immune dysregulation and hematological alterations.

Altered diet and physical activity: Nutritional changes and reduced physical activity during space missions may also play a role in hematological changes. Astronauts' diet may not always meet their nutritional needs, leading to deficiencies in essential vitamins and minerals that support hematopoiesis, such as iron, folate, and vitamin B₁₂. Reduced physical activity due to microgravity may also contribute to decreased erythropoiesis.

Conclusion

Hematological changes in astronauts during spaceflight are complex and multifactorial, involving alterations in RBCs, WBCs, platelets, and coagulation factors. These changes are primarily caused by the unique conditions of space, including microgravity, radiation exposure, psychological stress, and altered physical activity. While most of these changes are transient and do not lead to clinically significant health issues, understanding and mitigating the risks associated with hematological alterations are essential for the health and performance of astronauts on long-duration missions.