Respiratory System

The respiratory system is a group of organs and tissues that permits breathing. The main parts of this system are the airways, the lungs and linked blood vessels, and the muscles that enable breathing.

Figure A shows the location of the respiratory structures in the body. Figure B is an enlarged image of airways, alveoli, and the capillaries. Figure C shows the location of gas exchange between the capillaries and alveoli. Source: NHLBI.

Other Names

Synonyms

• Pulmonary system
• Respiratory apparatus

Related terms

• Cardiopulmonary system

Description

The respiratory system consists chiefly of the airways and the lungs; however, anatomists and physiologists differ in their definitions and descriptions of the respiratory system.

Anatomist's Respiratory System

Anatomists describe the respiratory system as the tubular and cavernous organs and structures involved in pulmonary ventilation and gas exchange between ambient air and blood.

• Nose
  • Nose
  • Nasal cavity
  • Paranasal sinuses
• Pharynx
  • Nasopharynx
  • Oropharynx
• Larynx
• Trachea
• Bronchi
  • Main bronchi
  • Lobar bronchi
  • Segmental bronchi
• Lung

Physiologist's Respiratory System

The respiratory system of physiologists differs from that of anatomists in two chief ways. First, the organs and structures included by anatomists are divided into two portions: the airways (the conducting portion) and the regions where gas exchange occurs (the respiratory portion). Secondly, physiologists include a third portion, the ventilatory portion, which includes the tissues of the thorax that drive air movement within the respiratory system.

Conducting portion

The airways are pipes that carry oxygen-rich air to the lungs and carbon dioxide, a waste gas, out of the lungs.

Air first enters the body through the nose or mouth, which wets and warms the air (Cold, dry air can irritate the lungs). The air then travels through the voice box and down the windpipe. The windpipe splits into two bronchi that enter the lungs.

A thin flap of tissue called the epiglottis covers the windpipe during swallowing. This prevents food or drink from entering the air passages that lead to the lungs.

Except for the mouth and some parts of the nose, all of the airways have special hairs called cilia that are coated with sticky mucus. The cilia trap germs and other foreign particles that enter the airways when a breath is taken.

These fine hairs then sweep the particles up to the nose or mouth. There, they're swallowed, coughed, or sneezed out of the body. Nose hairs and mouth saliva also trap particles and germs.

The airways include the following:

• Upper Respiratory Tract (upper airway)
  • Nasal cavity
  • Pharynx
• Lower Respiratory Tract (lower airway)
  • Extrapulmonary region
    • Larynx, or voice box
    • Trachea, or windpipe
    • Main (primary) bronchi
  • Intrapulmonary region
    • Lobar (secondary) bronchi
    • Segmental (tertiary) bronchi
    • Bronchioles

Respiratory portion

The respiratory portion consists of the pulmonary structures within the lung where gas exchange occurs:

• Respiratory bronchioles
• Alveolar ducts
• Alveolar sacs
• Alveoli

The lungs and linked blood vessels deliver oxygen to the body and remove carbon dioxide. The lungs lie on either side of the breastbone and fill the inside of the chest cavity.

Within the lungs, the bronchi branch into thousands of smaller, thinner tubes called bronchioles. These tubes end in bunches of tiny round air sacs called alveoli.

Each of these air sacs is covered in a mesh of tiny blood vessels called capillaries. The capillaries connect to a network of arteries and veins that move blood through the body.

The pulmonary artery and its branches deliver blood rich in carbon dioxide (and lacking in oxygen) to the capillaries that surround the air sacs. Inside the air sacs, carbon dioxide moves from the blood into the air. Oxygen moves from the air into the blood in the lungs.

The oxygen-rich blood then travels to the heart through the pulmonary vein and its branches. The heart pumps the oxygen-rich blood out to the body.

Ventilatory portion

The ventilatory portion consists of the skeleton of the thorax and the muscles that help expand and contract (tighten) the lungs to allow breathing:

• Thoracic skeleton (thoracic or rib cage)
  • Ribs
  • Sternum
  • Thoracic vertebrae
• Muscles of respiration
  • Diaphragm
  • Intercostal muscles

The diaphragm is a dome-shaped muscle located below the lungs. It separates the chest cavity from the abdominal cavity. The diaphragm is the main muscle used for breathing.

The intercostal muscles are located between the ribs. They also play a major role in breathing.

Beneath the diaphragm are abdominal muscles. These help breathe out when breathing fast (for example, during physical activity).

Muscles in the neck and collarbone area are used to breathe when other muscles involved in breathing don't work properly, or when lung disease impairs breathing.

Role of the Respiratory System in the Body

The respiratory system works to deliver oxygen to all of the tissues in the body and to release carbon dioxide from tissues in exhaled breath. The respiratory system ensures that breathing is changed when a person's activity level or environment is changed.

How the Respiratory System Works

Inhalation

During inhalation (breathing in), the diaphragm contracts (tightens) and moves downward. This increases the space in the chest cavity, into which the lungs expand. The intercostal muscles between the ribs also help enlarge the chest cavity. They contract to pull the rib cage both upward and outward during inhalation.

As the lungs expand, air is sucked in through the nose or mouth. The air travels down the windpipe and into the lungs. After passing through the bronchial tubes, the air finally reaches and enters the alveoli (air sacs).

Through very thin walls of the alveoli, oxygen from the air passes to the surrounding capillaries (blood vessels). A red blood cell protein called hemoglobin helps move oxygen from the air sacs to the blood. (Oxygen is especially drawn to hemoglobin.)

At the same time, carbon dioxide moves from the capillaries into the air sacs. The gas has traveled in the bloodstream from the right side of the heart through the pulmonary artery.

Oxygen-rich blood from the lungs is carried through a network of capillaries, which become the pulmonary vein. This vein delivers the oxygen-rich blood to the left side of the heart. The left side of the heart pumps the blood to the rest of the body. In the body, the oxygen in the blood moves from blood vessels into surrounding tissues.

Exhalation

During exhalation (breathing out), the diaphragm relaxes and moves upward into the chest cavity. The intercostal muscles between the ribs also relax to make the chest cavity size smaller.

As the chest cavity gets smaller, air rich in carbon dioxide is forced out of the lungs and windpipe, and then out of the nose or mouth.

Breathing out requires no effort from the body unless a person has a lung disease or is doingphysical activity. During physical activity, the abdominal muscles contract and push the diaphragm even more against the lungs. This pushes the air in the lungs out rapidly.

Control of breathing

A respiratory control center at the base of the brain controls breathing. This center sends ongoing signals down the spine and to the nerves of the muscles involved in breathing.

These signals ensure the breathing muscles contract (tighten) and relax regularly. This allows breathing to happen automatically, without awareness.

To a limited degree, people can change their breathing rates, such as by breathing faster or holding the breath. Emotions also can change breathing. For example, being scared or angry can affect the breathing pattern.

Breathing will change depending on how active a person is and the condition of the air. For example, when physically active, a person needs to breathe more often. In contrast, the body needs to restrict how much air is inhaled if the air contains irritants or toxins.

To adjust breathing to changing needs, the body has a number of sensors in the brain, blood vessels, muscles, and lungs.

Sensors in the brain and in two major blood vessels (the carotid artery and the aorta) detect carbon dioxide or oxygen levels in the blood and change the breathing rate as needed.

Sensors in the airways detect lung irritants. The sensors can trigger sneezing or coughing. In people who have asthma, the sensors may cause the muscles around the airways in the lungs to contract. This makes the airways smaller.

Sensors in the alveoli (air sacs) detect a buildup of fluid in the lung tissues. These sensors are thought to trigger rapid, shallow breathing.

Sensors in the joints and muscles detect movement of the arms or legs. These sensors may play a role in increasing the breathing rate during physical activity.

Diseases of the Respiratory System

Several diseases and conditions affect the breathing apparatus. Often mechanical ventilation, such as a ventilator, is needed to assist poor respiration.

Injuries

Persons with spinal cord injuries to the cervical spine (neck region) can suffer paralysis of the muscles that control respiration. These individuals may require a mechanical ventilator in order to breathe.

The lungs and respiratory system can also be damaged by repeated exposure to toxins and particulates, such as in long-term tobacco smoking. Chronic obstructive pulmonary disease (COPD) and pulmonary fibrosis are examples of diseases in which the lungs have been injured by some chronic irritant or toxin.

Conditions

As one of the central systems of the body, many conditions affect the respiratory system. Some conditions are a result of infectious diseases from bacteria and viruses, while others are a result of the host immune system attacking parts of the respiratory system. Below are several conditions that affect the respiratory system:

• Asthma
• Congenital Central Hypoventilation Syndrome (Ondine's curse)
• Guillain-Barre Syndrome
• Polio
• Tuberculosis

See the lungs page for conditions affecting the lungs specifically.

Related Professions

Professionals who specialize in the treatment of conditions affecting the respiratory system include the following:

• Respiratory therapists: These professionals are trained in providing care for the respiratory system including lung percussion, breathing treatments, and ventilator management.

• Pulmonologists: A pulmonologist is a medical doctor who has undergone specialty training in internal medicine and pulmonology.

History

Iron lung ward filled with polio patients, Rancho Los Amigos Hospital, California. 1953. Source: Wikimedia Commons.

Iron lungs were heavily used during the polio outbreaks in the mid-20th century. An iron lung is a device that surrounds a person's body and applies external pressure to the rib cage in a rhythmic pattern in order to stimulate breathing.^[1]

Etymology

The term respiratory comes from Middle English respiracioun and from Latin respirare . The term has been in use since the 15th century.^[2]

Research

Interest is rising in the use of mind-body therapies such as deep breathing and relaxation in controlling bodily functions. In a study of over 21,000 individuals in a large Japanese clinic, researchers evaluated the effects of deep breaths compared to a 30 second rest on blood pressure. Those who took the deep breaths had a significantly larger reduction in blood pressure than those who simply rested.^[3]

References

1. ↑ Wikipedia. Iron Lung.
2. ↑ Merriam-Webster online. Respiratory.
3. ↑ Mori H, Yamamoto H, Kuwashima M, et al. How does deep breathing affect office blood pressure and pulse rate? Hypertens Res. 2005 Jun;28(6):499-504. PMID 16231755

External Links

American Association for Respiratory Care

American Lung Association

National Heart, Lung, and Blood Institute