Is it a law that if you get hurt on a job that the company you work for is responsible for your medical expenses?

It depends on the cause of the injury. If, for example you slip on a just-mopped, wet floor and bust your behind, and there was no warning that it was wet, then the company might be liable for your medical expenses because of their negligence. But, if you are lifting heavy objects as part of your job and you hurt your back, that would be covered under Workman's Compensation.