Bone microvascular flow differs from skin microvascular flow in response to head-down tilt

Idiosyncratic bone responses to blood flow restriction exercise: new insights and future directions

Applying blood flow restriction (BFR) during low-load exercise induces beneficial adaptations of the myotendinous and neuromuscular systems. Despite the low mechanical tension, BFR exercise facilitates a localized hypoxic environment and increase in metabolic stress, widely regarded as the primary stimulus for tissue adaptations. First evidence indicates that low-load BFR exercise is effective in promoting an osteogenic response in bone, although this has previously been postulated to adapt primarily during high-impact weight-bearing exercise. Besides studies investigating the acute response of bone biomarkers following BFR exercise, first long-term trials demonstrate beneficial adaptations in bone in both healthy and clinical populations. Despite the increasing number of studies, the physiological mechanisms are largely unknown. Moreover, heterogeneity in methodological approaches such as biomarkers of bone metabolism measured, participant and study characteristics, and time course of measurement renders it difficult to formulate accurate conclusions. Furthermore, incongruity in the methods of BFR application (e.g., cuff pressure) limits the comparability of datasets and thus hinders generalizability of study findings. Appropriate use of biomarkers, effective BFR application, and befitting study design have the potential to progress knowledge on the acute and chronic response of bone to BFR exercise and contribute toward the development of a novel strategy to protect or enhance bone health. Therefore, the purpose of the present synthesis review is to 1) evaluate current mechanistic evidence; 2) discuss and offer explanations for similar and contrasting data findings; and 3) create a methodological framework for future mechanistic and applied research.

Contemporary review of dermatologic conditions in space flight and future implications for long-duration exploration missions

BACKGROUND

Future planned exploration missions to outer space will almost surely require the longest periods of continuous space exposure by the human body yet. As the most external organ, the skin seems the most vulnerable to injury. Therefore, discussion of the dermatological implications of such extended-duration missions is critical.

OBJECTIVES

In order to help future missions understand the risks of spaceflight on the human skin, this review aims to consolidate data from the current literature pertaining to the space environment and its physiologic effects on skin, describe all reported dermatologic manifestations in spaceflight, and extrapolate this information to longer-duration mission.

METHODS AND MATERIALS

The authors searched PubMed and Google Scholar using keywords and Mesh terms. The publications that were found to be relevant to the objectives were included and described.

RESULTS

The space environment causes changes in the skin at the cellular level by thinning the epidermis, altering wound healing, and dysregulating the immune system. Clinically, dermatological conditions represented the most common medical issues occurring in spaceflight. We predict that as exploration missions increase in duration, astronauts will experience further physiological changes and an increased rate and severity of adverse events.

CONCLUSION

Maximizing astronaut safety requires a continued knowledge of the human body's response to space, as well as consideration and prediction of future events. Dermatologic effects of space missions comprise the majority of health-related issues arising on missions to outer space, and these issues are likely to become more prominent with increasing time spent in space. Improvements in hygiene may mitigate some of these conditions.

Changes in interstitial fluid flow, mass transport and the bone cell response in microgravity and normogravity

In recent years, our scientific interest in spaceflight has grown exponentially and resulted in a thriving area of research, with hundreds of astronauts spending months of their time in space. A recent shift toward pursuing territories farther afield, aiming at near-Earth asteroids, the Moon, and Mars combined with the anticipated availability of commercial flights to space in the near future, warrants continued understanding of the human physiological processes and response mechanisms when in this extreme environment. Acute skeletal loss, more severe than any bone loss seen on Earth, has significant implications for deep space exploration, and it remains elusive as to why there is such a magnitude of difference between bone loss on Earth and loss in microgravity. The removal of gravity eliminates a critical primary mechano-stimulus, and when combined with exposure to both galactic and solar cosmic radiation, healthy human tissue function can be negatively affected. An additional effect found in microgravity, and one with limited insight, involves changes in dynamic fluid flow. Fluids provide the most fundamental way to transport chemical and biochemical elements within our bodies and apply an essential mechano-stimulus to cells. Furthermore, the cell cytoplasm is not a simple liquid, and fluid transport phenomena together with viscoelastic deformation of the cytoskeleton play key roles in cell function. In microgravity, flow behavior changes drastically, and the impact on cells within the porous system of bone and the influence of an expanding level of adiposity are not well understood. This review explores the role of interstitial fluid motion and solute transport in porous bone under two different conditions: normogravity and microgravity.

Mechanisms of bone blood flow regulation in humans

Bone is a highly vascularized tissue. However, despite the importance of appropriate circulation for bone health, regulation of bone blood flow remains poorly understood. Invasive animal studies suggest that sympathetic activity plays an important role in bone flow control. However, it remains unknown if bone vasculature evidences robust vasoconstriction in response to sympathoexcitatory stimuli. Here, we characterized bone blood flow in young healthy individuals [n = 13, (four females)] in response to isometric handgrip exercise (IHE) and cold pressor test (CPT). These provide a strong stimulus for active vasoconstriction in the inactive muscle, and perhaps also in the bone. During sustained IHE to fatigue and CPT, we measured blood pressure, whole leg blood flow, and tibial perfusion using near-infrared spectroscopy. Tibia perfusion was determined as oxy- and deoxyhemoglobin. For both stimuli, tibial metabolism remained constant (i.e., no change in deoxyhemoglobin) and thus tibial arterial perfusion was represented by oxyhemoglobin. During IHE, oxyhemoglobin declined (beginning -0.20 ± 1.04 μM; end -1.13 ± 3.71 μM, both P < 0.01) slower than whole leg blood flow (beginning -0.85 ± 1.02 cm/s; end -2.72 ± 1.64 cm/s, both P < 0.01). However, during CPT, both oxyhemoglobin (beginning -0.46 ± 1.43 μM; end -0.60 ± 1.59 μM, both P < 0.01) and whole leg blood flow (beginning -1.52 ± 1.63 cm/s; end -0.69 ± 1.51 cm/s, both P < 0.01) declined with a similar timecourse, even though the magnitudes of decline were smaller than during IHE. These responses are likely due to the different timecourses of sympathetically mediated vasoconstriction in bone and muscle. These results indicate that sympathetic innervation of the bone vasculature serves a functional role in the control of flow in young healthy individuals.NEW & NOTEWORTHY The current study is the first one to noninvasively investigate control of bone blood perfusion in vivo in humans, on a moment-by-moment basis. Our results indicate that tibial bone vasculature demonstrates active vasoconstriction in response to sympathoexcitatory stimuli in young healthy individuals. Compared with whole leg vasculature, bone vasoconstrictor response seems to be smaller, delayed, and more variable.

Use of near-infrared systems for investigations of hemodynamics in human in vivo bone tissue: A systematic review

A range of technologies using near infrared (NIR) light have shown promise at providing real time measurements of hemodynamic markers in bone tissue in vivo, an exciting prospect given existing difficulties in measuring hemodynamics in bone tissue. This systematic review aimed to evaluate the evidence for this potential use of NIR systems, establishing their potential as a research tool in this field. Major electronic databases including MEDLINE and EMBASE were searched using pre-planned search strategies with broad scope for any in vivo use of NIR technologies in human bone tissue. Following identification of studies by title and abstract screening, full text inclusion was determined by double blind assessment using predefined criteria. Full text studies for inclusion were data extracted using a predesigned proforma and quality assessed. Narrative synthesis was appropriate given the wide heterogeneity of included studies. Eighty-eight full text studies fulfilled the inclusion criteria, 57 addressing laser Doppler flowmetry (56 intra-operatively), 21 near infrared spectroscopy, and 10 photoplethysmography. The heterogeneity of the methodologies included differing hemodynamic markers, measurement protocols, anatomical locations, and research applications, making meaningful direct comparisons impossible. Further, studies were often limited by small sample sizes with potential selection biases, detection biases, and wide variability in results between participants. Despite promising potential in the use of NIR light to interrogate bone circulation, the application of NIR systems in bone requires rigorous assessment of the reproducibility of potential hemodynamic markers and further validation of these markers against alternative physiologically relevant reference standards. © 2018 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:2595-2603, 2018.