The walls of the alveoli and capillaries are permeable. What does this mean?

Options for the question have not been provided. They are as follows:

Substances can pass through them

They are made of muscle

They are located throughout the body

They are made of bone

Answer:

Substances can pass through them

Explanation:

When a wall or membrane is permeable, it means that substances can pass through it from one side to another. Alveoli and capillary have permeable walls. Alveoli are found in lungs. They need permeable walls for exchange of gases between the body and the environment. Capillaries are a type of blood vessels. They need permeable walls for exchange of nutrients and waste material between the blood and the tissue.