General Structure and Function of the Respiratory SystemWe noted in our discussion of digestion that humans, along with other vertebrates, are heterotrophs, and thus must obtain organic molecules from the environment to supply them with energy. That energy is released by a process of oxidation called cellular respiration, which requires oxygen and produces carbon dioxide. This means that organisms such as humans have a constant need to obtain oxygen and rid themselves of carbon dioxide. Since both of these substances exist as gases under physiological conditions, a system that allows gas exchange must be present. The process of breathing, or respiration, refers to the constant exchange of oxygen and carbon dioxide between the organism and the environment, and is the major function of the respiratory system. The major structures of the respiratory system are, in essence, a series of tubes that ultimately connects with capillaries of the circulatory system (see Figure 16.2). The following discussion refers to the human respiratory system; important differences exist in other vertebrates, especially fish, and will be addressed subsequently.
- Nasal passages, the pharynx, and the trachea: During normal breathing, air enters the body through the nostrils and nasal passages, travels through the pharynx, and enters the trachea, or windpipe. The trachea is a strong tube strengthened by rings of cartilage which maintain its structural integrity. The larynx, or upper region of the trachea, contains the vocal cords, which humans use to make sounds.
- Bronchi and bronchioles: The trachea divides into two bronchi (singular, bronchus) inside the lungs. Each bronchus further subdivides again and again, creating many smaller tubes called bronchioles.
- Alveoli: The smallest bronchioles terminate in tiny air sacs called alveoli (singular, alveolus). Each alveolus is thin-walled and surrounded by capillaries from the cardiovascular system. The barrier is so thin that gases can diffuse freely directly across the alveolar wall into or out of the blood. A humanâ€™s two lungs contain about 300 million alveoli, with an exchange surface area of approximately 75 square meters.
The human respiratory system
The entire inner surface of the respiratory system is composed of epithelial tissue which secretes mucus; it is therefore called a mucous membrane. Since the alveoli have such thin walls, the respiratory system is potentially susceptible to the entry of pathogens, and the mucus secreted by the bronchial tubes and trachea acts to trap any pathogens or foreign particles before they can reach the exchange surfaces. Many of the cells of the mucous membrane also contain cilia, small projections which beat in one direction, causing a current of mucus to move towards the top of the trachea. This flow, known as the ciliary escalator, ensures that material trapped by the mucus is removed from the system, protecting the delicate respiratory surfaces from harm.