This dissertation characterizes the geometry, kinematics, and physical properties of insect internal structures that make up the respiratory and circulatory systems. This characterization is necessary to better understand how these systems function to transport fluids at the microscale, and ultimately, how we might computationally model this flow. Chapter 2 describes the geometry of the insect tracheal system, specifically testing if Murray's law applies to this system using three-dimensional imaging of tracheal tubes. Chapter 3 begins to characterize the physical properties of insect hemolymph, specifically the viscosity and density of hemolymph, using experimental measurements. Because insects are strongly affected by environmental temperature, this chapter also explores how hemolymph viscosity may be affected by temperature. Chapter 4 builds on the results of Chapter 3, exploring the effects of developmental responses to temperature on hemolymph viscosity and properties, as well as performance of the insect using experimental measurements. Finally, Chapter 5 presents a kinematic and structural characterization of the insect heart using a variety of imaging techniques and analyses. / Doctor of Philosophy / Insect physiology and morphology has long been studied by biologists and entomologists, with many of the basic features understood and characterized. The insect circulatory and respiratory systems differ greatly from those of many other organisms. Physically, these systems transport fluids through microscale environments which include a variety of pumps, networks, and other structures that facilitate flow. Functionally, the circulatory and respiratory systems are largely decoupled, unlike in vertebrates. The respiratory system transports air directly to deliver oxygen to tissues, whereas the circulatory system transports various nutrients and other chemicals via hemolymph. With these unique differences, investigation of these major biological transport systems in insects is essential to fully understand their structure and function. This dissertation addresses many of the basic structural and physical properties of the insect respiratory and circulatory systems that are still unknown, despite growing engineering analysis. First, I measured specific geometric features of the insect tracheal network and determined if Murray’s law applies to this system. Second, I quantified the viscosity of insect hemolymph, including in response to temperature. To expand upon this relationship further, I measured hemolymph viscosity, hemolymph composition, and insect performance after temperature acclimation during development. Last, I investigated the morphology and kinematics of the insect heart, using many methods of imaging and analysis to measure structural features of the heart wall, including during function. Hemolymph properties and heart morphology provide the physical basis of flow production within the circulatory system. Understanding flow production within the circulatory system, as well as design features of the respiratory system, are crucial in the construction of mathematical models of both hemolymph and air flow within the insect.

Cerebrovascular diseases are by far the largest causes of death in the UK, as well as one of the leading causes of adult disability. The brain's healthy function depends on a steady supply of oxygen, delivered through the microvasculature. Cerebrovascular diseases, such as stroke and dementia, can interrupt the transport of blood (and hence oxygen) rapidly, or over a prolonged period of time. An interruption in flow can lead to ischaemia, with prolonged interruptions leading to tissue death and eventual brain damage. The microvasculature plays a key role in the transport of oxygen and nutrients to brain tissue; however, its role in diseases such as dementia is poorly understood, primarily due to the inability of current clinical imaging techniques to resolve microvessels, and due to the complexity of the underlying microvasculature. Therefore, in order to understand cerebrovascular diseases, it is necessary to be able to resolve and understand the microvasculature. In particular, generating large-scale models of the human microvasculature that can be linked back to contemporary clinical imaging is important in helping plug the current imaging gap that exists. A novel statistical model is proposed here that generates such large-scale models efficiently. Homogenization theory is used to generate a porous continuum capillary bed (characterised by its permeability) that allows for the efficient scaling up of the microvasculature. A novel order-based density-filling algorithm is then developed which generates morphologically accurate penetrating arterioles and venules, also demonstrating that the topology of the vessels only has a minor influence on CBF compared to diameter. Finally, the capillary bed and penetrating vessels are coupled into a large voxel-sized model of the microvasculature from which pressure and flux variations through the voxel can be analysed. A decoupling of the pressure and flux, as well as a layering of flow, was observed within the voxel, driven by the topology of the penetrating vessels. Micro-infarctions were also simulated, demonstrating the large local effects they have on the pressure and flux, whilst only causing a minor drop in CBF within the voxel.