Although all of the structures of the respiratory system are in contact with the circulatory system, which respiratory structure is most closely associated with dense network of capillaries?

A. pharynx
B. vocal folds
C. trachea
D. alveoli

Here is a picture of an alveoli

http://www.visualphotos.com/photo/1x3747285/alveolus_in_lung_alveoli_of_the_lungs_colored_scanning_electron_micrograph_sem_of_a_section_th_P4490.jpg

alveoli

The respiratory structure that is most closely associated with a dense network of capillaries is the alveoli. Alveoli are small, air-filled sacs located at the end of the respiratory bronchioles in the lungs. These sacs are the primary site of gas exchange, where oxygen from the inhaled air diffuses into the bloodstream and carbon dioxide from the blood is released into the lungs to be exhaled.

To find the answer to this question, you can start by understanding the functions of the different respiratory structures and how they interact with the circulatory system. Then, you can analyze each option given in the question.

Option A, the pharynx, is a passageway for air and food but does not have a dense network of capillaries.

Option B, the vocal folds, are part of the larynx and are involved in speech production. While they are important for the respiratory system, they do not have a significant association with capillaries.

Option C, the trachea, is a tube connecting the larynx to the bronchi. It contains specialized cells that produce mucus to trap particles and protect the respiratory system, but it does not have a dense network of capillaries.

Option D, the alveoli, are surrounded by an extensive network of capillaries, making them the primary location for gas exchange in the respiratory system.

Therefore, the correct answer is D. alveoli.