Blood is everywhere in Macbeth. The very first scene of the play starts out with a captain speaking of the blood spilled at the battle between the Scots and the Norwegians. Those this is the first instance of blood; blood plays a much bigger part in Macbeth. Blood stands for the guilt that Macbeth and Lady Macbeth feel throughout the play.
While Lady Macbeth is planning the murder of King Duncan, she calls upon the spirits of murder to, "make thick my blood; / Stop up the access and passage to remorse" (1.5.50-51). What she plans to do is poison Macbeth's soul into being able to murder an innocent man. But before she can do this she must "thicken" her blood to stop her from feeling remorse, or guilt.
After Macbeth kills Duncan, he is hit by guilt and regrets it. ""Will all great Neptune's ocean wash this blood / Clean from my hand?" (2.2.78-79). Neptune's Ocean refers to the entire Ocean as a whole. Macbeth says that all the water in the entire world will not be able to wash of the blood (guilt) from his hands. He tells Lady Macbeth this as he returns from King Duncan's room with the daggers that he used. And she laughs at him.
Lady Macbeth serves a huge contrast to Macbeth. Where he feels guilt, she feels none. ""My hands are of your colour; but I shame / To wear a heart so white" (2.2.82-83). Saying that her hands are the same color shows that she can and has done things as evil as he, but when she refers to him as wearing a "heart so white" she means that he is cowardly, and that she isn't ashamed of what she has done.
As the story progresses blood continues to be a huge part of the story in that it still emphasizes the guilt of Macbeth and Lady Macbeth. Macbeth and his wife continue to kill more and more people in order to keep themselves politically safe. Macbeth persists in feeling guilty about what he has done and even Lad Macbeth begins to feel the pain of what she has done.