Just as it is prohibited by Torah law to kill any living being, it is also prohibited by Torah law to cause a loss of blood. After all, “blood is life” (Devarim 12:23). Additionally, with the release of a little blood, there is a localized loss of life. This prohibition applies even when the blood does not exit the body, but simply hemorrhages from blood vessels and accumulates under the skin. This too is viewed as a localized loss of life (SA 316:8; BHL s.v. “ve-haĥovel).
Accordingly, if one strikes another person on Shabbat with intent to injure and causes a hemorrhage or hematoma, he is not only guilty of an interpersonal transgression but also desecrates Shabbat. This is also true of one who angrily strikes an animal and causes a hemorrhage or hematoma. In addition to violating tza’ar ba’alei ĥayim, he transgresses the prohibition of Ĥovel.
One may not do a blood test on Shabbat. Since the goal is to use the blood, taking it is prohibited by Torah law. In cases of danger to life, of course, it is permitted. It is also forbidden to scratch or pick at a scab if doing so will cause it to bleed, or to brush one’s teeth in a way that will cause the gums to bleed (as explained in 14:2).
A Jew may administer a shot to one who is ill, even if not dangerously so, as long as the injection is into flesh, since this will not necessarily cause the sick person to bleed. In contrast, if the shot or infusion is done into a vein, it is prohibited for a Jew to administer it to one who is not dangerously ill, since there will definitely be at least a little blood. However, a non-Jew may be asked to give this injection. If the sick person is dangerously ill, even a Jew may give the injection (below 28:7).