Shock is a life-threatening condition defined as a lack of blood flow which results in the body not receiving enough oxygen and nutrients. Any condition which affects the heart, vessels or blood volume can induce shock.  If not recognised and treated immediately it can be fatal.
There are several different types of shock including:
Hypovolemic – The most common type of shock, caused by decreased blood volume (such as with blood loss).
Cardiogenic – Caused by decreased blood circulating the system due to damage to the heart.
Septic – Drop in blood pressure caused by bacterial infection circulating in the blood.
Anaphylactic shock – Drop in blood pressure due to a severe allergic reaction.
If unconscious, check that the airway is open. Clear secretions from the mouth with your fingers.
If bleeding, control by applying direct pressure to the wound.
Place a towel or blanket on your cat to keep him/her warm.
Don’t give anything to eat or drink.
Keep your cat’s head lower than the heart to maintain blood flow to the brain.
Proceed to your vet immediately.
Treatment is aimed at providing supportive care and restoring and maintaining blood flow along with addressing the underlying cause.
Stemming blood loss
Providing external warmth (if suffering from hypothermia)
Shock is extremely serious and can quickly kill if not treated immediately. If your cat has been hit by a car but appears happy and well it is still important to take him to the veterinarian for a check-up because he may be in shock without you knowing.
 Cat Owner’s Home Veterinary Handbook – Delbert G. Carlson