When the human fingertip is pressed against a surface or bent, the hemodynamic state of the fingertip is altered due to mechanical interactions between the fingernail and bone. Normal force, shear force, and finger extension/flexion all result in different patterns of blood volume beneath the fingernail. This phenomenon has been exploited in order to detect finger forces and finger posture by creating a photoplethysmograph "fingernail sensor," which measures the two-dimensional pattern of blood volume beneath the fingernail. In this paper, the anatomical structure of the fingertip is investigated in order to understand the various ways in which the bone and nail interact to alter the hemodynamic state of the fingertip. A qualitative nail-bone interaction model is created and used to explain the different blood volume patterns that result from each stimulus. The model is verified using experimental data from the fingernail sensor. The impact of this study on potential performance and application of the fingernail sensors is discussed.