Phases of Respiration

Most people think of respiration simply as the process by which air moves into and out of the lungs, that is, breathing. By scientific definition, respiration is the process by which oxygen is obtained from the environment and delivered to the cells. Carbon dioxide is transported to the outside in a reverse pathway (Fig. 14-1). Respiration includes three phases:
* Pulmonary ventilation, which is the exchange of air between the atmosphere and the air sacs (alveoli) of the lungs. This is normally accomplished by the inhalation and exhalation of breathing.
* External exchange of gases, which occurs in the lungs as oxygen (O2) diffuses from the air sacs into the blood and carbon dioxide (CO2) diffuses out of the blood to be eliminated.
* Internal exchange of gases, which occurs in the tissues as oxygen diffuses from the blood to the cells, whereas carbon dioxide passes from the cells into the blood.
Gas exchange requires close association of the respiratory system with the circulatory system, as the circulating blood is needed to transport oxygen to the cells and transport carbon dioxide back to the lungs. The term respiration is also used to describe a related process that occurs at the cellular level. In cellular respiration, oxygen is taken into a cell and used in the breakdown of nutrients with the release of energy. Carbon dioxide is the waste product of cellular respiration.

Figure 14-1 Overview of respiration. In ventilation, gases are moved into and out of the lungs. In external exchange, gases move between the air sacs (alveoli) of the lungs and the blood. In internal exchange, gases move between the blood and body cells. The circulation transports gases in the blood.

The Respiratory System

The respiratory system is an intricate arrangement of spaces and passageways that conduct air into the lungs (Fig. 14-2). These spaces include the nasal cavities; the pharynx, which is common to the digestive and respiratory systems; the voice box, or larynx; the windpipe, or trachea; and the lungs themselves, with their conducting tubes and air sacs. The entire system might be thought of as a pathway for air between the atmosphere and the blood.

The Nasal Cavities
Air enters the body through the openings in the nose called the nostrils, or nares (sing. naris). Immediately inside the nostrils, located between the roof of the mouth and the cranium, are the two spaces known as the nasal cavities. These two spaces are separated from each other by a partition, the nasal septum.

The superior portion of the septum is formed by a thin plate of the ethmoid bone that extends downward, and the inferior portion is formed by the vomer. An anterior extension of the septum is made of hyaline cartilage. The septum and the walls of the nasal cavity are covered with mucous membrane.

On the lateral walls of each nasal cavity are three projections called the conchae. The shell-like conchae greatly increase the surface area of the mucous membrane over which air travels on its way through the nasal cavities.
     The mucous membrane lining the nasal cavities contains many blood vessels that deliver heat and moisture. The cells of this membrane secrete a large amount of fluid-up to 1 quart each day. The following changes are produced in the air as it comes in contact with the lining of the nose:
* Foreign bodies, such as dust particles and pathogens, are filtered out by the hairs of the nostrils or caught in the surface mucus.
* Air is warmed by blood in the wellvascularized mucous membrane.
* Air is moistened by the liquid secretion.
     To allow for these protective changes to occur, it is preferable to breathe through the nose rather than through the mouth.
     The sinuses are small cavities lined with mucous membrane in the skull bones. They are resonating chambers for the voice and lessen the weight of the skull. The sinuses communicate with the nasal cavities, and they are highly susceptible to infection.

The Pharynx
The muscular pharynx, or throat, carries air into the respiratory tract and carries foods and liquids into the digestive system (see Fig. 14-2). The superior portion, located immediately behind the nasal cavity, is called the nasopharynx; the middle section, located posterior to the mouth, is called the oropharynx; and the most inferior portion is called the laryngeal pharynx.
     This last section opens into the larynx toward the anterior and into the esophagus toward the posterior.

Figure 14-2 The respiratory system. (A) Overview. (B) Enlarged section of lung tissue showing the relationship between the alveoli (air sacs) of the lungs and the blood capillaries. (C) A transverse section through the lungs.