Session CL: Biofluids: Physiological Feeding

Special functions of valve organs of blood-sucking female mosquitoes

Food-feeding insects usually have valve organs to regulate the sucking flow effectively. Female mosquitoes sucking lots of blood instantaneously have a unique valve system between two pumping organs located in their head. The valve system seems to prevent reverse flow and to grind granule particles such as red blood cells. To understand the functional characteristics of this valve organ in detail, the volumetric flow rate passing through the valves and their interaction with the two-pumps need to be investigated. However, it is very difficult to observe the dynamic behaviors of pumping organs and valve system. In this study, the dynamic motions of valve organs of blood-sucking female mosquitoes were observed under in vivo condition using synchrotron X-ray micro imaging technique. X-ray micro computed tomography was also employed to examine the three-dimensional internal structure of the blood pumping system including valve organs.   

The ultrafast valve of an aquatic carnivorous plant

Aquatic carnivorous bladderworts (Utricularia spp.) are plants that catch prey animals with suction traps. Here we present an experimental study with high speed video analyses of the extremely fast trapping movements, and show that suction is performed in less than a millisecond, much faster than previously thought. We reveal how the convex door morphology is optimized for a fast opening and closure, which we confirm by numerical simulations: the trapdoor is an elastic valve that buckles inside (entailing rapid opening) and then unbuckles (entailing rapid closure). These precise and reproducible motions are coupled with a strong suction swirl causing accelerations of up to 600 g, and leaving little escape chances for prey animals.   

Optimality and universal scaling for osmotically driven translocation of sugars in plants

The growth of plants depends on efficient translocation of sugars. The current belief is that this takes place predominantly through osmotically driven flow, passively generated by differences in sugar concentrations (the so-called M\"{u}nch mechanism). We show that optimization of translocation speed predicts a universal scaling between the width of the conduits (phloem cells), the length of the plant and the length of the ``loading zones'' (the leaves). This unexpected scaling is verified by data from plants over several orders of magnitude is size, from small green plants to large trees.   

On drinking nectar

Many creatures, including bees, birds and bats, feed on floral nectar. It is advantageous for these creatures to ingest energy rapidly due to the threat of predation during feeding. While the sweetest nectar offers the greatest energetic rewards, the exponential increase of viscosity with sugar concentration makes it the most difficult to transport. We here demonstrate that the energy intake rate is maximized at a particular concentration that depends on the mode of nectar feeding. We here rationalize the different optimal concentrations reported for the three principal nectar drinking strategies, capillary suction, active suction and viscous dipping.   

Microscopic filter feeders at an angle to nearby boundaries: Feeding restrictions and strategies

Microscopic sessile filter feeders are an important part of aquatic ecosystems and form a vital link in the transfer of carbon in marine food webs. ~These filter feeders live attached to boundaries, consume bacteria and small detritus, and are in turn eaten by larger organisms. Such filter feeders survive by creating a feeding current that draws fluid towards them, and from which they filter their food of interest. Eddies form near these organisms as a result of fluid forcing near a boundary. ~The extent of these eddies, and their effect on the nutrient uptake of the organism, depend on the angle of fluid forcing relative to the boundary. For a model with perfect nutrient capture efficiency, and in the absence of diffusion, we show that feeding at an angle greatly increases the feeding efficiency of~filter feeders. We also show experimental data that living filter feeders in culture feed at an angle to the substrate. ~We discuss the effects of nutrient diffusion and inefficient nutrient capture on our model, as well as a possible mechanism for filter feeders to change their orientation.   

The Hungry Fly: Hydrodynamics of feeding in the common house fly

A large number of insect species feed primarily on a fluid diet. To do so, they must overcome the numerous challenges that arise in the design of high-efficiency, miniature pumps. Although the morphology of insect feeding structures has been described for decades, their dynamics remain largely unknown even in the most well studied species (e.g. fruit fly). Here, we use invivo imaging and microsurgery to elucidate the design principles of feeding structures of the common house fly. Using high-resolution X-ray microscopy, we record invivo flow of sucrose solutions through the body over many hours during fly feeding. Borrowing from microsurgery techniques common in neurophysiology, we are able to perturb the pump to a stall position and thus evaluate function under load conditions. Furthermore, fluid viscosity-dependent feedback is observed for optimal pump performance. As the gut of the fly starts to fill up, feedback from the stretch receptors in the cuticle dictates the effective flow rate. Finally, via comparative analysis between the house fly, blow fly, fruit fly and bumble bees, we highlight the common design principles and the role of interfacial phenomena in feeding.   

Patterns in Clam Excurrent Siphon Velocity According to External Environmental Cues

This study attempts to determine the patterns and/or randomness of the excurrent velocity of actively feeding clams, \textit{Mercenaria mercenaria}. We hypothesize that clams alter their feeding current velocity patterns or randomness according to external cues in the environment such as hydrodynamic characteristics, density of the clam patch, and presence of predators in the upstream flow. A PIV system measured vector fields for two-dimensional planes that bisect the clam excurrent siphons, and time records were extracted at the siphon exit position. Fractal and lacunarity analysis of the jet velocity time records revealed that clams alter their jet excurrent velocity unsteadiness according to the horizontal crossflow velocity. The results also reveal that the effect of clam patch density on the feeding activity was dependent on the size of the organism. This size/density dependent relationship suggests that predation by blue crabs dominates the system since larger clams are no longer susceptible to blue crab predation, whereas clams of all sizes are susceptible to whelk predation. Finally, clams increase the randomness of their excurrent jet velocity values when predator cues are located in the upstream flume flow. This suggests that the presence of predators elicits clam behavior that promotes the mixing and dilution of their chemical metabolites.   

Peristaltic pumping in an elastic tube: feeding the hungry python

Biological ducts convey contents like food in the digestive system by peristaltic action, propagating waves of muscular contraction and relaxation. The motion is investigated theoretically by considering a radial force of sinusoidal or Gaussian form moving steadily down a fluid-filled axisymmetric tube. Effects of the prescribed force on the resultant fluid flow and elastic deformation of the tube wall are presented. The flow can induce a rigid object suspended in the fluid to propel in different ways, as demonstrated in numerous examples.   

Micro-Macro Scale Mixing Interactions by Intestinal Villi Enhance Absorption: a 3D Lattice-Boltzmann Model

Muscle-induced villi motions may create a micro-scale flow that couples with a lumen-scale macro flow to enhance nutrient transport and absorption in the intestine. Using a 3D multiscale lattice Boltzmann model of a lid-driven cavity flow with microscale 3-D leaf and finger-like villi in pendular motion at the lower surface, we analyze the coupling between micro and macro-scale nutrient mixing and absorption at the villi surfaces. RESULTS: The villi motions enhance absorption by creating a micro-mixing layer (MML) that pumps low concentration fluid from between villi groups and attracts fluid with high concentration from the macro flow. The MML couples with the macro flow via a diffusion layer. Leaf-like villi create the strongest MML and, consequently, the highest absorption rates. The finger-like villi create a weaker MML due to the existence of flow between villi. The strength of the MML and nutrient absorption increases with villus frequency. The absorption rate also increases with villus length; however the simulations predict an optimal length close to the physiological length of villi in humans. The complex flow structure will be discussed. We conclude that the interaction between micro-scale villi-induced fluid motions and macro-scale motility-induced flow may play a significant role in intestinal absorption. \textit{Supported by NSF Grant CTS-056215.}   

Contraction-driven Mixing and Absorption in the Intestine with a 3D lattice-Boltzmann Model

The primary purpose of the intestines is absorption of nutrient molecules and transport of chyme by specific motility patterns. These are broadly classified as \textit{segmental }(for radial mixing) and \textit{peristaltic} (for axial transport). Our AIM was to identify an optimal patterning of contractions. METHODS: Fluid motions were modeled in a tubular intestine with a 3D lattice Boltzmann algorithm with prescribed motions of the lumen wall, parameterized from MRI data. Nutrient concentration was modeled as a passive scalar. Scalar was zero at the lumen wall to model rapid nutrient uptake. Simulations were initialized with uniform concentration. RESULTS: The mechanics are more complex than commonly believed. Whereas absorption is maximal when fully occluded, there is little differentiation between segmentation and peristalsis at high occlusions. However at lower occlusions absorption is reduced 28{\%} from maximum with segmentation, but as much as 56{\%} with peristalsis. The power requirements are reduced nearly 90{\%} from maximum in segmentation at the lower occlusions! Thus there is a great physiological advantage to choose segmentation at lower occlusion values to enhance mixing and absorption. The MRI measurements indicate that \textit{in vivo} occlusion values are in the low range where the tradeoff between mechanics and energetics is functionally optimal. \textit{Supported by NSF.}