The heart can beat continuously without being fatigued because it is composed of________

Step 1: Analyze the question The question asks which type of muscle tissue makes up the heart and allows it to beat continuously without tiring.

Step 2: Explain the concept There are three types of muscle tissue in the body:

• Skeletal (striated/voluntary) muscles: attached to bones, under conscious control, and they tire quickly with prolonged work.
• Smooth (unstriated/involuntary) muscles: found in the walls of the gut, blood vessels, and bladder. They contract slowly and rhythmically.
• Cardiac muscles: found only in the heart. They are involuntary, striated, and made of branched, interconnected fibres. They have many mitochondria and a rich blood supply (through the coronary arteries), so they are well supplied with oxygen and energy. They also have a long refractory period, which gives them a built-in "rest" between beats, so they do not fatigue.

Step 3: Work through the options

• A. Skeletal muscles: these fatigue easily and are voluntary, so they are wrong.
• B. Smooth muscles: these are involuntary but are not the muscle of the heart, so they are wrong.
• C. Cardiac muscles: the specialised heart muscle, adapted for continuous, rhythmic contraction. This is correct.
• D. Renal muscles: this is not a type of muscle tissue ("renal" refers to the kidney), so it is wrong.

Step 4: Answer The heart beats continuously without fatigue because it is composed of cardiac muscles, which is option C.

Exam tip: Match the organ to its muscle: cardiac muscle is only in the heart. Remember "Cardiac = Cardio = Heart". Also remember that skeletal muscle = voluntary and tires, while cardiac and smooth muscle = involuntary.