RESPIRATORY SYSTEM
The respiratory system functions to exchange gases between the blood and the external
environment. Using blood as a transporting fluid, the cardiovascular system organs transport
respiratory gases between the lungs and the tissue cells. If either system fails, body cells begin to
die from oxygen starvation and accumulation of carbon dioxide.
Nose – The nasal cavity is divided by the nasal septum and is lined with ciliated mucosa.
 Olfactory receptors for smell are located in the mucosa.
 The internal mucosa lining moistens and warms the inhaled air. Mucus traps inhaled foreign
particles and contains lysozyme enzymes which destroy the inhaled bacteria. Cilia move the
mucus toward the pharynx, where it is swallowed and digested.
 Mucosa-covered projections called conchae increase the surface area exposed to air.
 Paranasal sinuses surround the nasal cavity and act as resonance chambers for speech.
Pharynx (throat) – is lined with mucous membranes and is a muscular pathway for food and air to
enter. Pharynx is divided into the nasopharynx, oropharynx, and laryngopharynx. The Eustachian
tube opens into the nasopharynx. The tonsils are located in the upper region of the pharynx.
Larynx (voice box) – superior part of trachea lined with mucous that routes air and food into proper
channels. It contains vocal cords, fibrous bands, which vibrate with expelled air to produce sound.
 The larynx contains rigid hyaline cartilages and a flap of elastic cartilage called the epiglottis.
The thyroid cartilage is the largest of the hyaline and protrudes forming the “Adam’s apple”.
 The epiglottis protects the superior opening of the larynx. When swallowing, the epiglottis
closes off the trachea so food enters the esophagus.
Trachea (windpipe) – is reinforced with C-shaped rings of hyaline cartilage to support this
passageway for air. The passageway is lined with ciliated mucosa to trap and propel foreign debris.
Bronchi – the trachea branches into the right and left primary bronchi which run obliquely before
entering the medial depression (hilum) of each lung. The right main bronchus is wider, shorter and
straighter than the left which makes it easier for inhaled objects to become lodged.
 Smaller subdivisions of the bronchi are within the lungs and direct air towards the alveoli (air
sacs). These bronchi continue to divide forming a bronchial tree and ending in the smallest
conducting passageway called bronchioles. The terminal bronchioles lead into the respiratory
bronchioles into alveoli ducts which feed air to the alveoli sacs/alveoli.
Alveoli - The alveoli walls contain elastic fibers and simple squamous epithelium. The external
surface is covered with pulmonary capillaries where gas exchange occurs between the alveoli and
the blood.
 Roughly 600 million alveoli are located
in the lungs. They contain alveolar
macrophages to engulf bacteria and
other debris. The cuboidal cells in the
alveolar walls produce surfactant
which coats the surfaces to prevent
the alveoli from collapsing and reduce
surface tension from the air.
Lungs – found in the thoracic cavity. The superior apex is deep to the clavicle and the inferior base
rest on the diaphragm. The diaphragm muscle contracts and relaxes to alter the volume of the chest
cavity. Each lung is divided into fissures --- left lung has 2 lobes and a cardiac notch and the right
lung has 3 lobes.
 The surface of the lung is covered with visceral pleura and the walls of the thoracic cavity are
lined with parietal pleura. Pleural fluid in the pleural space allows the lungs to move during
breathing without friction.
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Atelectasis – lungs collapse resulting from air entering the
pleural space; usually associated with a chest wound
Pleurisy – inflammation of the pleural space.
PHYSIOLOGY – to supply the body with O2 and
dispose of CO2 these four events must occur:
1. Pulmonary Ventilation – air moves into and out of
the lungs in the alveoli (breathing)
2. External Respiration – gas exchange between the
pulmonary capillaries and the alveoli
3. Respiratory Gas Transport - O2 is transported in the
hemoglobin and CO2 is transported in the
bicarbonate ion (HCO3-) in the plasma
4. Internal Respiration – gas exchange between
capillaries and tissue
Mechanisms of Breathing
Inspiration – diaphragm and external intercostals contract increasing the size of the thoracic cavity.
As intrapulmonary volume increase, gases within the lungs spread out to fill the space causing a
decrease in pressure in the lungs. The decrease in pressure produces a partial vacuum which sucks
air into the lungs. Air continues to move into the lungs until the intrapulmonary pressure equals
atmospheric pressure.
Expiration – inspiratory muscles relax, the rib cage descends and the lungs recoil. As the
intrapulmonary volume decreases, the gases inside the lungs are forced more closely together, and
the intrapulmonary pressure rises above atmospheric pressure. This increase in pressure causes
gases to flow out to equalize the pressure inside and outside the lungs.
 The normal pressure within the pleural space is
intrapleural pressure, is always negative, and is
the major factor preventing collapse of the
lungs. If intrapleural pressure becomes equal to
atmospheric pressure, the lungs immediately
recoil completely and collapse.
 Nonrespiratory air movements – reflex activities
such as coughing, sneezing, crying, laughing,
hiccups, yawning
Respiratory Volumes & Capacities
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Tidal volume (TV) – amount of air in a normal breath (500 ml)
Inspiratory reserve volume (IRV) – amount of air that can be taken in forcibly over the TV
Expiratory reserve volume (ERV) – amount of air that can be forcibly exhaled after TV
Residual volume – amount of air that remains in the lungs that can NOT be voluntarily expelled
(1200 ml); Residual air allows gas exchange to go on continuously and keep the alveoli inflated
Vital capacity (4800 ml) – sum of TV+IRV+ ERV ….. Vital capacity varies with age, gender, and
physical condition. Measuring VC gives a device for diagnosis of disorders and judging
effectiveness of treatment.
Dead space volume (150 ml) – air that remains in
conducting pathway of tract & never reaches alveoli
Control of Respiration – activity of diaphragm and
external intercostals is regulated by nerve impulses
transmitted by the phrenic & intercostal nerves. Neural
centers in the pons and medulla oblongata control
respiratory rhythm and depth.
Homeostatic Imbalances
o Asthma – chronically inflamed and hypersensitive
bronchial passages lead to spasms of bronchioles
o Apnea – cessation of respiration
o Dyspnea – labored breathing
o Eupnea – normal breathing rate
o Hyperpnea – breathing more vigorously
o Bronchitis – mucosa become severely inflamed and produce excessive amounts of mucus
o Emphysema – alveoli enlarge and chronic inflammation promotes fibrosis of the lungs. Lungs
lose elasticity and airways collapse.