In this week's Parsha, the Torah states "ומכה אביו ואמו מות יומת," "and if someone hits his father or his mother, he shall surely be put to death" (שמות כ"א:ט"ו). Rashi refers us to the Mishnah in Sanhedrin (דף פ"ה:) which explains that one is liable for death in this instance only if in the course of hitting the parent, one causes a חבורה, that is, he opens up a wound and draws blood. A few Pesukim later, we are taught that if one person hits another and incapacitates him, he is required to pay a monetary amount to compensate for his loss of work time and his medical bills (שם פסוקים י"ח-י"ט). Later in the Torah, we read of the monetary payment required of one who damages and wounds another person (ויקרא כ"ד:י"ט-כ). In these cases where one hits and even draws blood from another person, no death penalty is imposed; it is only by hitting and drawing blood from a parent that one incurs the death penalty. The Sefer HaChinuch (מצוה מ"ח) explains that this harsh punishment is meted out for hitting one's parents because by so doing, one is raising one's hands against the people who brought him into the world and did so much for him.
The Gemara in Bava Kamma (דף פ"ו.) states that if one hits one's parent in the ear and causes him or her to go deaf, he is to be put to death even though no blood has actually been drawn. This is because it is impossible to cause someone to become deaf without causing some kind of internal bleeding, even if no blood is actually seen. The Shulchan Aruch (יורה דעה, סימן רמ"א סעיף ב') accepts this ruling. We may deduce from here that if one hits one's parent and causes a "black and blue mark," he will also be liable for the death sentence, since a black and blue mark indicates a bruise where blood under the skin has been drawn, even though no blood is actually seen.
In deriving this rule that one is put to death for hitting a parent only if he caused a wound which drew blood, the Gemara in Sanhedrin (דף פ"ד:) draws a comparison between hitting a person (specifically, hitting one's parent) and hitting an animal. Because the two are linked in the Torah (ויקרא כ"ד:כ"א), the Gemara says that just as one is obligated to pay damages for hitting an animal only if he opens a wound, so too one is liable for the death penalty for hitting one's parent only if he opens a wound. A few lines later, the Gemara extends this comparison even further. The Gemara states that one is exempt from any monetary payment if he draws blood from an animal for the purpose of effecting a cure to an illness; for example, a veterinarian trying to heal a sick animal does not have to pay damages if he draws blood in the course of his work. Similarly, the Gemara says, if a child is a doctor and is trying to cure a medical problem which his parent has, and, in doing so, draws blood from the parent, he is of course not to be put to death. A child may thus remove a splinter, for example, from his parent even though blood may be drawn.
Despite the logic of this comparison, however, the Gemara (שם) quotes the practice of certain Amoraim who would not allow their children to do anything, like extracting a thorn, which may cause blood to be drawn, even for medical reasons. In the Shulchan Aruch (יו"ד שם סעיף ג'), the Mechaber accepts this, writing that one should not assist one's parent with a medical procedure since it may involve drawing blood. The Ramo (שם) adds that this is of course only if there is someone else available to do what the parent needs done, but if there is no other qualified physician around and the parent is in danger or suffering, the child must do whatever has to be done even if he will draw blood, since it is being done for the purpose of a cure, not to injure the parent.
Rav Tzvi Pesach Frank (שו"ת הר צבי חלק יו"ד סימן קצ"ז) was once asked an interesting question by a physician which relates to this issue. If the child, who is a doctor, wishes to comply with the above ruling of the Mechaber and send his ill parent to another physician for his treatment, but the other physician will not render his services for free, may the child then do the treatment for free? If not, who has to pay this other physician, the father or the child? Based on the conclusion of the Gemara in Kiddushin (דף ל"ב.), the Shulchan Aruch (יו"ד סימן ר"מ סעיף ה') rules that if a child, in the course of providing the services required by the Mitzvah of Kibbud Av Va'em, incurs any monetary expenses, the parent must pay for the expenses. It would thus seem that here, since the Halacha is that the child should not perform the operation but should send the parent to another doctor, the parent should pay his own medical bill. Rav Frank suggests, however, that this may not be so. The rule that the parent must pay the expenses may only be when the expenses relate directly to the Mitzvah of Kibbud Av Va'em alone. Here, however, the child is fundamentally obligated to cure his parent, as the Ramo says; the Halacha just urges him not to do so in order for him not to violate the prohibition of drawing blood from the parent. By sending the parent to another doctor, the child is thus protecting himself. The parent is not required to pay for this.