What is a general anaesthetic?
General anaesthesia involves giving you medication, by injecting it first and then by breathing it in, in order to induce your brain into a state of unawareness. Often, a muscle relaxant is given in order for the anaesthetist to be able to insert a breathing device to help you breathe, and for the surgeon to be able to pull aside muscles that get in the way of surgery. Pain killers are given during surgery even though you are unaware, because your body still responds to pain in the usual way despite you not being aware of it - your heart rate will dramatically increase, blood pressure will shoot up, and pressures inside the brain and other body compartments will increase also. All these can be detrimental to the body, especially if there is pre-existing problems with the heart, the brain, etc.
The unawareness induced by general anaesthetic drugs is most certainly NOT the same as the sleep you have every night, despite the familiar phrase: "I am going to put you to sleep now." uttered by the anaesthetist. Think of it like this: if a surgeon is to try to cut you open with a knife while you are sleeping in the middle of the night, you will most certainly wake up. To ensure that you are not woken up by the painful stimulus of surgery, the brain and nerve functions will need to be depressed to a level much beyond what happens naturally each night in bed. Unfortunately, at this sort of level of depression of brain function, your throat closes up; you stop breathing; your heart and blood vessels don't function well; you stop regulating body temperature; etc, etc. This is why you need a well trained anaesthetist to look after you during surgery.
Is general anaesthesia safe?
There has been significant advances in the medication, breathing machines and anaesthetist training in the last few decades to make general anaesthesia extremely safe for otherwise healthy patients undergoing simple surgery. Risks naturally increase if you have pre-existing medical conditions, such as severe heart or lung diseases. Your anaesthetist is trained to ascertain the likely issues your body might encounter during the particular type of surgery and plan for it in advance, so that even if things do go awry, they are often fixed seamlessly before further deterioration occurs. This means having a full bank of emergency medications on hand and ready to deliver, as well as having a range of techniques and settings available to secure your airway and adjust breathing patterns on the machine. Sometimes, in order to ensure additional safety, your anaesthetist may choose to insert additional monitoring lines into your wrist or neck in order to monitor your blood pressure more closely. This is often necessary for long surgeries in elderly patients, with heart conditions. Patient safety is paramount in surgery.