Oxygen and carbon dioxide pass into and out of the blood through the thin cell walls of the alveoli and tiny blood vessels called __________.

The correct answer is capillaries because they are the tiny blood vessels that facilitate the exchange of gases between the alveoli in the lungs and the blood. Capillaries are extremely thin-walled, allowing oxygen to diffuse from the alveoli into the blood and carbon dioxide to diffuse from the blood into the alveoli for exhalation. This gas exchange is essential for respiration and is a crucial part of the respiratory and circulatory systems working together to supply oxygen to the body and remove carbon dioxide.

The other options do not perform this function. Mucous refers to a sticky secretion that helps trap particles and pathogens but is not involved in gas exchange. Veins carry deoxygenated blood back to the heart and do not engage in gas exchange within the lungs. Arteries transport oxygenated blood away from the heart to the body's tissues and also do not facilitate the exchange of gases in the lungs. Thus, capillaries are specifically designed for efficient gas exchange at the alveolar level.