Desiccation of a blood sessile drop on a glass surface leads to the formation of interesting cracking patterns. These desiccation patterns have been identified to have three characteristic regions, i.e., peripheral, coronal and central regions. Driving forces responsible for the formation of cracking patterns are the redistribution of colloidal materials driven by a "coffee ring" effect and the time- and location-dependent development of internal stresses caused by water evaporation and progressive gelation from the drop edge to its center. Since the concentrations of colloidal materials, i.e., cellular components, protein macromolecules and other constituents (glucose, bilirubin and lipids) in blood, influence the cracking patterns, an understanding of these patterns can potentially reveal clues for the evaluation of health conditions and offer a low-cost diagnostic tool for human diseases. This study presents a mechanistic analysis of the pattern formation in desiccating blood sessile drops. We focus on the build-up and release of internal stresses by examining the cracking process. Optical and scanning electron microscopes (SEM) were used to capture the initiation, propagation and directions of cracks in different regions. The propagation and widening of orthoradial and radial cracks in relation to the adhesion and cohesion of the blood sessile drops were observed and characterized. New microscopic insights into internal stress releasing processes provide a new understanding of physical events occurring underneath the gelled film of the blood sessile drop and differences in the distribution of strain energy in different regions, which will aid our understanding of different cracking patterns in those regions.