In this paper we have explored the linked series of structures that collectively comprise the respiratory system. In examining each of these structural resistors, some seem to be primarily fixed, for example, the trachea, while others must be primarily variable or adaptable, for example, the cardiovascular system. Those structures that are truly variable will not be maintained with structural capacity in excess of their functional demand. As a consequence, these structures are the ones that may most often appear to be limiting O2 uptake. However, we question under which in vivo circumstances the most plastic steps in the cascade of resistances will impart the single-step limitation to O2 uptake. When reviewed in this context, available experimental evidence suggests that among the most athletic animals (those with the greatest weight-specific VO2), the respiratory resistors are likely tuned rather than dominated by a single-step limitation. Skeletal muscle must set the demand for O2 in exercising animals; hence, the relationship between total skeletal muscle mitochondria and maximum O2 consumption is quantitatively consistent, spanning broad differences in body size and aerobic capacity. Those respiratory structures that are primarily nonadaptable must be built with enough "excess structure" to accommodate potential adaptation in an animal's aerobic capacity during its lifetime. Consequently, the least aerobic animals will always appear to experience a limitation to VO2max in one of the most plastic or adaptable structures. We suggest that the adaptable structures upstream to the muscle mitochondria are built and maintained at a cost-benefit maximum ("structural efficiency") in all species. This differs from the concept of optimal structural design or symmorphosis.

AABB hosted the Blood Center Executive Summit on 20 October 2019 during the AABB Annual Meeting in San Antonio, Texas. The session was sponsored by the Commonwealth Transfusion Foundation, a nonprofit, private foundation whose mission is to inspire and champion research and education that optimizes clinical outcomes in transfusion medicine and ensures a safe and sustainable blood supply for the United States. The Summit focused on the intersection of blood centers and plasma centers. Presenters and attendees explored existing and needed data, regulatory requirements, risks and benefits of different donor models, and future direction of the plasma community and blood centers. The Summit also identified priority issues that warrant further investigation and provide opportunities to drive progress. Introductory remarks provided context for the Summit presentations. Debra BenAvram, FASAE, CAE, Chief Executive Officer, AABB (Bethesda, Maryland), noted that during the past year, she and other AABB staff have had many discussions with blood center executives on key issues and challenges. In these talks, many executives requested that AABB provide programming specifically for this member segment. The Summit is a direct result of those requests, and the AABB supports a fruitful discussion as well as important and actionable next steps. Kevin Belanger, DHS, MS, MT(ASCP)SBB, President and Chief Executive Officer of the Shepeard Community Blood Center (Evans, Georgia), observed that he and his colleagues have seen a decrease in the donor base and, at the same time, an increase in plasma centers. He also noted that the resulting discussions about competition and donor compensation have been muted. The Summit provides a forum for a broad, open discussion that can be the start of something important. As chair of the Summit planning committee, he thanked both panelists and audience members for participating. Bob Carden, Chief Executive Officer of the Commonwealth Transfusion Foundation (Richmond, Virginia), who moderated the Summit, joined BenAvram and Belanger in welcoming participants to the day's presentations. He emphasized the need for data and noted that one outcome of the day would be a list of potential research projects that could be pursued and considered for funding.