When muscles contract:

Options

A. sarcomeres increase in size
B. myosin slides past actin
C. H zone disappears
D. calcium is taken up by calcium storage sacs

Explanation

**Step-by-Step Solution:**

1. **Understanding Muscle Contraction**: Muscle contraction occurs through a process known as the sliding filament theory. This theory explains how muscles shorten and generate force.

2. **Role of Sarcomeres**: Sarcomeres are the basic functional units of muscle fibers. During contraction, sarcomeres do not increase in size; instead, they shorten.

3. **Interaction of Myofilaments**: In muscle fibers, there are two types of myofilaments: thick filaments (myosin) and thin filaments (actin). During contraction, the myosin filaments slide past the actin filaments.

4. **H Zone Dynamics**: The H zone is the region of the sarcomere that contains only thick filaments (myosin). During contraction, as the actin filaments slide inward, the H zone narrows and can even disappear entirely.

5. **Calcium Ions**: Calcium ions play a crucial role in muscle contraction. They bind to troponin on the actin filaments, allowing myosin heads to attach to actin. However, during contraction, calcium is not taken up by calcium storage sacs (sarcoplasmic reticulum); instead, it is released to initiate contraction.

6. **Conclusion**: The correct statement regarding muscle contraction is that the H zone disappears as the actin and myosin filaments slide past each other.

**Final Answer**: When muscles contract, the H zone disappears.

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