Answers:
- A) stomach
- B) intestinal
- C) heart
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Explanation:
The stomach muscle helps digest food. The muscle isn't something a person can control at will. Instead, the body handles it automatically. This is why choice A is one of the answers.
The intestinal muscles work in a similar fashion to help pass nutrients into the blood stream and also pass waste out of the body. All of these processes are done automatically and you don't have to think about them. They just happen. Therefore, choice B is another answer.
Another example of an automatic or involuntary muscle doing its own thing is the heart muscle. It continuously pumps blood to help distribute nutrients and oxygen, and also dump waste byproducts (eg: carbon dioxide) out of the body. So choice C is the third and final answer.
An example of a muscle that's voluntary are the biceps. You can flex these muscles, and move them around at will. For example, when you pick up an item. So this allows us to rule out choice D.
I'm not sure why your teacher listed "brain". It has properties related to muscles such as the idea of practicing a topic to help retain it (similar to exercising to stay in shape); however, the brain itself isn't a muscle on a technical biological level.
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Extra info:
Some more involuntary muscles are:
- The diaphragm muscle to help the lungs expand and contract (to breathe in oxygen or exhale carbon dioxide).
- The muscles that allow your eyes to blink. The technical name for them isn't coming to mind at the moment. These muscles blink at some fairly random yet consistent rate, and also blink whenever something like dust particles enter the eye (or threaten to).
- At some level, you could argue that the biceps are involuntary in some way. What I mean by this is that there's a reflex action to them that is automatically controlled. However, for the most part, the biceps are voluntary muscles.
