​​Why Doctors before giving Injection to Patients blow some medicine in air while keeping Injection upward?

The blood circulation in our body is kind of laminar flow (one layer above another layer flow with almost constant and slow velocity) and when doctors fills medicine in injection some air comes along with medicine, so what doctor does, he keep injection vertically upward and injectes the air out and only medicine remains in injection. But why he do so? As I told blood flow in our veins is of laminar flow, if any air bubble enters in veins it will create turbulence in flow and get burst in veins which may cause vein repture. And if the bubble reaches the heart it will burst their it may cause serious injury to heart and can cause death. Hence to avoid this doctors do so.