The proximal airway of the bat Tadarida brasiliensis: a minimum entropy production design

Adjustments of the oxygen diffusing capacity to energetic demands during the development of the quail (Coturnix coturnix japonica)

One of the hypotheses that attempt to explain physiological limitations of energy budgets is the symmorphosis hypothesis, which proposes that if matching structures to functional needs were combined with the strict economy of energy and materials, the result would be an optimal organ design for the specific function it serves. Evidence in favor of symmorphosis in adults is as abundant as evidence against it, but the plasticity of some morphological traits may be dependent on the ontogenetic stage at which acclimation acts. Thus, here we studied the adjustment of structure and function in lungs at different stages of development in the quail Coturnix coturnix japonica under two thermal regimes. Our main results show that i) resting metabolic rate, maximum thermogenic oxygen consumption and oxygen diffusion capacity did not exhibit developmental plasticity for two thermal environments; and ii) oxygen diffusion capacity fully adjusted to resting metabolic rate and maximum oxygen consumption during development. C. coturnix has a low safety factor close to 1 which is consistent with the symmorphosis hypothesis.

Macroscopic anatomy of the lower respiratory system in a nocturnal burrowing rodent: African giant pouched rat (Cricetomys gambianus, Waterhouse 1840)

Cricetomys gambianus is a rat that lives principally in burrows, coming out at night in search of food. The design and structure of the lower respiratory system reflects its oxygen and metabolic demand which can be attributed to its habitat. A morphological and morphometric investigation of its lower respiratory system was undertaken to document the normal anatomical features and assess its morpho-functional paradigm. Specifically, an anatomical detail of the lungs and conductive airway was described, the structures being elucidated by dissection and radiography. Evaluation of dissected specimens showed that tracheal cartilages ranged from 21 to 33 rings with an average of 25.5. They exhibited a random pattern of anastomoses between adjacent rings. Transverse diameters of the principal bronchus in the male and female rats were 3.767 and 3.759 mm respectively. The right lung consisted of four lobes while the left lung was not lobed. Bronchogram revealed that lung lobation corresponded with bronchial tree division. Inter-lobar fissures were absent on the right lung except for the ventral boarder separating the cranial lobe from the caudal lobe. The entire lungs provided stability to the heart in situ, through the cardiac notch. This study also included correlation analysis of the dimensions, weights and volumes of the lower respiratory organs with the nose-rump length and body weight of 18 African giant pouched rats of both sexes. The relationship of the anatomy of the lower respiratory system of the rodent to the oxygen tension in their burrow and to their energy utilization is discussed.

Expiration rate drives human airway design

Analyses of human airway architecture based on calculations of airflow resistance or energy dissipation suggest that the branching pattern is not optimized for minimizing energy loss by flow dissipation during respiration. Airway flow dissipates only a few percent of the total body work during normal breathing, so branching patterns deviate from minimum energy loss to also optimize other physiological needs. Studies of airway performance often record some measure of expiration, such as FEV1 (Forced Expiratory Volume in 1s), because airway constriction during expiration limits the rate of rapid respiration. We posit that lung structure is optimized for the rate of expiration as well as minimum energy loss. By increasing the daughter-to-parent airway diameter ratio (h) from 0.794 (corresponding to the energy minimum for symmetrically branching airways) to 0.85 (the observed value in humans) luminal pressures at airway generations 4-15 were substantially increased during exercise (a 4.5 and 15 cmH2O increase during moderate and heavy exercise, respectively). Values of h somewhat larger than 0.794 help airways remain open during expiration by increasing both viscous pressure drop and convective acceleration pressure drop. Asymmetric bifurcations also exhibit higher proximal airway pressures than symmetric ones, but the improvement was not large.