First, let's look at the harm of air to blood vessels. Anyone who has had an injection knows that if air enters the blood vessels during the injection, people will feel uncomfortable and the injection site will start to swell. Our bodies have certain defense mechanisms. When a small amount of air enters blood vessels, blood disperses air during transportation, and finally absorbs and relieves air through breathing through a series of body operation modes. The body may be temporarily unwell, but it will not cause great harm to health.
If a large amount of air suddenly enters the blood vessels, that is another matter. Air can form air embolism in the body, which is life-threatening. These air embolism will block the blood vessels, the blood can't be transported normally, the blood circulation is blocked, and finally the heart and lung function will fail. Air plug is terrible, but this phenomenon would not have happened if a large amount of air was not intentionally injected into the human body. Even if the doctor produces a little air when extracting the liquid medicine, it will not bring much influence to people. They will push the potion before giving the patient an injection, in order to avoid 10 thousand accidents and make the patient feel better. And this simple trick will make them look professional.
In addition to the doctor pushing the potion, there are many places to pay attention to in the injection. For example, we can't eat on an empty stomach before an injection. You can't eat for an hour or two after the injection, because it will hinder the body's absorption of the drug.