Since vlood is a connective tissue and it connects all the parts of body. So syringes contain medicines which ate needed to be transported to all parts of body or specific parts for which they are made to act. If not given in arteries or espically veins then it will be difficult for medicine to reach its targeted organ where it has to work and even for doctor. For eg syringes given for jaundice cannot be directly injected into liver because it may damage other parts but injecting in blood will be easier as it will be in passive in blood and will only become active in liver. So they are injected in arteries or veins
If you still want answer then injections are given by doctors in veins and arteries so, that medicine present in injection would be mixed in our blood so, that we should be fit and fine
Doctor inject syringe in nerves because it is directlh connnected by the body cell or organ in which organ utilise without digestion and relax rapidly
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