Structure and Function of the Respiratory System

The respiratory system brings oxygen into the body through the breathing process. With inspiration, or inhaling air, oxygen is brought into the lungs. With expiration, or exhalation, carbon dioxide is removed from the lungs.

The respiratory system functions in three ways:

There are three processes of respiration:

Both the voluntary and involuntary nervous systems control respiration. The body chemically regulates the rate (how fast the breaths are) and depth (how deep they are). If the concentration of gases in the blood changes, the brain adjusts respiratory rate and depth to counteract the changes to maintain homeostasis (Table 13-1).

Air enters the respiratory system (Fig. 13-1) through the nose (nasal cavity). The nasal cavity is lined with hairs called cilia to help filter out any foreign particles.

It also helps to warm and moisten the air. Air also enters through the mouth (oral cavity), when the nasal cavity is blocked. The tonsils and adenoids at the back of the throat help the body resist infection.

The sinuses are hollow spaces in the bones of the skull that open into the nasal cavity. Sinuses help regulate the temperature of air before it reaches the sensitive lungs. They also humidify and filter the air. The spaces act as chambers for vibration of the air producing “vocal resonance” or the sound quality of the voice.

The air from the nose or mouth is then funneled through the throat (pharynx) and into the windpipe (trachea). The pharynx is divided into three portions: nasopharynx (nose), oropharynx (mouth), and laryngopharynx (larynx).

The trachea or windpipe is lined with rigid cartilage to keep the passageway open. Sometimes an opening is made into the trachea as an alternative method for the exchange of gases (tracheotomy).

The voice box (larynx) is located below the pharynx. Voice sounds are made when air moves through it. A flap of tissue called the epiglottis covers the voice box or larynx during swallowing to prevent food and liquid from entering the bronchi and lungs. The pharynx also contains the opening for tubes through which air reaches the middle ear to adjust for pressure changes (eustachian tubes).

The trachea branches into two tubes called bronchi. Each bronchus enters one of the lungs and then branches into smaller tubes called bronchioles. The bronchi and bronchioles are lined with tiny hairs called cilia and sticky mucus called phlegm to catch dust and germs. The respiratory system produces approximately 125 mL of mucus each day, which is removed by the movement of the cilia.

The bronchioles have small sacs at their ends called alveoli. Capillaries in the walls of the alveoli exchange oxygen and carbon dioxide by the process of diffusion. Each alveolus is lined with a liquid called surfactant through which the diffusion occurs.

The large number of alveoli, nerves, lymph tissues, and capillaries give the lungs a spongelike texture. The lungs are divided into sections called lobes. The right lung has three lobes, and the left lung has two lobes. The space separating the lungs (mediastinum) contains the esophagus, heart, and bronchi.

Each lung is surrounded by a double membrane called pleura. The pleurae separate and lubricate the delicate lung tissues. The pleurae are slippery, allowing a gliding motion of the lungs during respiration. The ribs support and protect the chest cavity (thorax). During breathing, muscles lift and separate the ribs to help the lungs expand.

The diaphragm is a large, flat muscle that separates the thoracic cavity from the abdominal cavity. The diaphragm contracts and moves downward during inhalation. This creates suction, and air is pulled in from outside the body. Exhalation occurs when the diaphragm relaxes (Fig. 13-2).

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