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Articles in PresS, published online ahead of print June 21, 2002
J Appl Physiol, 10.1152/jap.00230.2002
Submitted on March 18, 2002
Accepted on April 30, 2002
1 Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, Texas, USA
2 Departments of Medicine, Physiology and Biophysics, University of Washington School of Medicine, Seattle, Washington, USA
* To whom correspondence should be addressed. E-mail: Robert.Johnson{at}utsouthwestern.edu.
The mammalian diaphragm is composed of two separate muscles (costal and crural) connected by a central tendon that serves as a piston head for drawing air into the lungs. The two muscles are described as having different embryological origins, segmental innervations, and mechanical functions [6, 7]. Based on regional blood flow measurements the two muscles appear to be non-uniformly recruited at rest, but we anticipated that the two muscles would become uniformly recruited at heavy exercise to efficiently support the high energy requirements of ventilation. We used fluorescent microspheres to measure regional blood flow within the two muscles as an index of muscle recruitment from rest to heavy treadmill exercise in well-trained foxhounds. However the heterogeneity of blood flow at rest persisted as exercise workloads were increased. Blood flow / g of muscle remained two fold greater in ventral than dorsal regions of both muscles from rest to heavy exercise. This pattern was matched by a two fold greater regional mechanical advantage in ventral than dorsal regions of the two muscles measured anatomically. Hence blood flow was preferentially and efficiently distributed to those regions capable of generating the greatest inspiratory power independent of muscle mass. The two muscles were recruited from rest to heavy exercise as a single functional unit, not as two muscles under separate control.
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