When blood drops contact a smooth surface they usually settle into a spherical shape, because of the force of gravity and the surface tension of the fluid. When they strike a non-smooth surface (for example, a gunshot wound) they can produce spatter patterns. These are known as front and back spatters depending on the direction they come from. Front spatters form a thin spot on the surface of the drop, while back spatter forms a trail of smaller satellite stains which radiate from it.
The size of a blood drop and the speed at which it hits the surface affects the appearance of the stain. Ideally, a blood sample should be drawn on clean high-quality slides that have not been touched with any part of the person's body. If the blood is being collected from an arm, applying a warm compress for 2-3 minutes to dilate the veins can make them easier to see. Using a winged needle set that allows multiple tubes to be filled at once is also helpful.
When adding blood to an image in Photoshop, start by creating a new layer and selecting it from the Layer sub-menu. Fill the outlined area with a dark red color and set its blend mode to Color Burn (choose it from the Mode pop-up menu in the Options bar). This increases the contrast between the original image layer and the blood to make it look more convincing. Finally, smudge the outlined shape to create the effect of blood leaking out of a wound.