Respiratory System Notes
Airway
1. General Information:
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The chief function of the respiratory system is to supply body tissues with
oxygen and eliminate carbon dioxide.
The respiratory system consists of the nasal cavities, pharynx, larynx,
trachea, bronchi, and lungs.
The nasal cavities, pharynx, larynx, trachea, and bronchi constitute the
airway.
2. Nose
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The nose has two nasal cavities, which open on the face through the
anterior nasal apertures called the nares. The nasal septum separates the
cavities.
The anterior portion of the septum is composed of hyaline cartilage. The
posterior portion is composed of bone called the vomer and the
perpendicular plate of the ethmoid bone.
Each nasal cavity has three mucosa-covered structures called the superior,
middle, and inferior conchae.
The nasal cavities are lined with hairs that trap dust and foreign particles.
Epithelial cells lining the nasal cavities secrete mucus, which collects
foreign particles which are then moved by cilia toward the pharynx, where
the particles and mucus can be removed by swallowing, sneezing, or
spitting.
The paranasal sinuses, which surround and drain into the nasal cavities,
are located in the frontal, sphenoid, and maxillary bones.
3. Pharynx
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The pharynx is also known as the throat.
Acts as a passageway for air and food. It also functions in speech,
changing shape to allow phonation of vowel sounds.
The entire pharynx is composed of striated muscle and lined with mucous
membrane.
The pharynx has three divisions…
i. Nasopharynx
ii. Oropharynx
iii. Laryngopharynx
4. Larynx
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The larynx is called the voice box. It is a triangular organ in the front of
the neck. It extends from the 4th to the 6th cervical vertebrae, attaching to
the hyoid bone.
The larynx is composed of muscle and numerous cartilages
The true vocal cords are a pair of horizontal folds that project into the
laryngeal cavity. They are separated by a space called the glottis.
The epiglottis, which overhangs the larynx, prevents food from entering
the lungs.
5. Trachea
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The trachea is also called the windpipe.
It is a membranous tube measuring 10 to 12.5 cm long.
On entering the mediastinum, the trachea branches into the right and left
main bronchi at the 5th thoracic vertebrae.
Dorsally, the trachea contacts the esophagus.
The trachea has a series of C-shaped cartilage rings to strengthen the
trachea and prevents it from collapsing during inspiration.
The trachea is lined with ciliated pseudostratified columnar epithelium
containing mucus-secreting goblet cells, which trap and propel inhaled
debris upward to the pharynx for removal through coughing.
6. Bronchi
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The right and left primary bronchi branch from the trachea.
The secondary bronchi are smaller passageways that branch from the
primary bronchi. The right primary bronchi divides into three secondary
bronchi and the left branches into two secondary bronchi.
The secondary bronchi branch into tertiary bronchi which then branch into
smaller bronchioles which branch into progressively smaller tubes until
they branch into alveolar ducts, which terminate in clusters called alveoli.
Alveoli are tiny air sacs lined with thin squamous epithelium. They are
surrounded by capillaries where oxygen and carbon dioxide are
exchanged.
7. Lungs
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The lungs are paired, cone-shaped organs that fill the pleural divisions of
the thoracic cavity. They extend from the root of the neck to the
diaphragm.
The right and left lungs are separated by the heart and other mediastinal
structures.
Each lung is enclosed in a pleura, a protective, double-layered serous
membrane. The parietal pleura lines the wall of the thoracic cavity. The
visceral pleura covers the lung directly. The space between the two
membranes contains fluid, which lubricates the lungs as they expand and
contract.
8. Lobes
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The right lung is divided into three lobes. The left lung, smaller than the
right, is divided into two lobes. It contains a concavity called the cardiac
notch, which is molded to accommodate the heart.
9. Blood Supply
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Blood circulates through the lungs via the pulmonary and systemic
circulatory systems.
In pulmonary circulation, the pulmonary arteries branch profusely into the
pulmonary capillaries which surround the alveoli.
In systemic circulation blood travels directly to the lung tissue. Bronchial
arteries supply the lung tissue with blood.
10. Breathing:
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Inspiration occurs when the diaphragm contracts, moving downward, and
the intercostal muscles expand the size of the chest cavity. The increase in
the size of the chest cavity reduces pressure within it and air moves from
the outside high pressure to the low pressure inside the lungs.
Expiration occurs when the diaphragm and intercostal muscles relax and
therefore reduce the size of the chest cavity. This increases air pressure
and air moves out. The elastic recoil of lung tissue aids in expiration.
11. Lung Capacity
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Tidal volume is the normal breathing volume.
Vital capacity is the maximum amount of air that can be moved out of the
lungs after a maximum inspiration and expiration. Vital capacity volumes
vary according to age, health, gender, and health.
Total lung capacity is about 6,000 mL.
Total lung capacity = vital capacity + residual volume.
Although tidal volume is 500mL, about 150mL of this air never reaches
the alveoli. It fills the upper respiratory passages and is exhaled with the
next breath.
The upper respiratory passages contain that air is called dead space. As a
result of this space, only 350mL of new air enters the alveoli with each
breath and mixes with air already in the lungs. This slow replacement of
air has several advantages.
i. The slow mixture of atmospheric air with alveolar air prevents
wide fluctuations in oxygen and carbon dioxide levels. Wide
fluctuations could produce adverse effects.
ii. Because oxygen and carbon dioxide in the blood are in equilibrium
with their gas forms in the alveolar air, stable concentrations of
alveolar oxygen and carbon dioxide promote stable concentrations
of these gases in arterial blood.
iii.
12. Gas Exchange
 Air exerts a total pressure of 760 mm Hg at sea level.
 Air contains about 21% oxygen. The partial pressure of oxygen is 21% of
the atmospheric pressure 760 mm Hg. So the partial pressure of oxygen is
159.6 mm Hg.
 Gas concentrations in inspired air…
i. Nitrogen = 79%
ii. Oxygen = 21%
iii. Water vapor = .5%
iv. Carbon dioxide = .04%
 Gas concentrations in alveolar air…
i. Nitrogen = 74.9%
ii. Oxygen = 13.6%
iii. Water vapor = 6.2%
iv. Carbon dioxide = 5.3%
 Gas concentrations in expired air…
i. Nitrogen = 74.9%
ii. Oxygen = 15.7%
iii. Water vapor = 6.2%
iv. Carbon dioxide = 3.6%
13. Gas diffusion
 In diffusion substances move from an area of higher concentration to an
area of lower concentration. A gas diffuses from an area with a high
partial pressure of the gas to one with a lower partial pressure.
14. Transport of oxygen
 Oxygen is transported by the blood in two different methods.
i. About 3% of oxygen is dissolved in blood plasma
ii. The remaining 97% is chemically bound with hemoglobin.
Oxygen uptake by hemoglobin is most efficient in the lungs where
the oxygen concentration is high. Oxygen release occurs most
readily in the tissues where the oxygen concentration is low.
iii. Lower blood pH and higher temperatures of actively metabolizing
tissue, which occurs during vigorous exercise, also enhances
oxygen release from hemoglobin.
15. Transport of carbon dioxide
 Carbon dioxide is transported by three different methods.
i. A small amount is dissolved in the blood plasma
ii. Some is loosely combined with amino groups in the hemoglobin
molecule.
iii. Most of the carbon dioxide is converted in erythrocytes to
bicarbonate by the enzyme carbonic anhydrase. Erythrocyte
uptake of carbon dioxide begins in the capillaries.
1. In the erythrocyte, the hemoglobin releases oxygen to
supply the tissues. At the same time, carbon dioxide
diffuses from the tissues into the erythrocytes where
enzymes help turn carbon dioxide and water into carbonic
acid. H2O + CO2  H2CO3.
2. Then carbonic acid dissociates into hydrogen ions (H+) and
bicarbonate ions (HCO3 -). The hemoglobin molecules
take up the hydrogen ions. The remaining bicarbonate ions
accumulate until their concentration in the erythrocyte
exceed that in the plasma, then the excess will diffuse out
into the plasma.
3. Simultaleously, chloride ions diffuse into the erythrocyte to
replace the bicarbonate ions. Then the bicarbonate ions
combine with plasma sodium and form sodium bicarbonate.
4. At the lungs the whole process reverses and carbon dioxide
diffuses into the alveoli and is exhaled.
16. Control of Respiration
 A control center in the brain stem, called the respiratory center, regulates
the rate and depth of respiration.
 This center discharges impulses to neurons that innervate the diaphragm
and intercostal muscles.
 Neurons in the respiratory center are stimulated directly by increased
arterial concentrations of carbon dioxide and hydrogen ions.
 During exercise, more carbon dioxide is produced. This stimulates the
respiratory center to increase respiratory rate and depth.
 Chemoreceptors in the aortic arch and carotid sinus detect levels of carbon
dioxide and convey impulses to the respiratory center.
 Receptors in the lungs respond to stretching as the lungs inflate, sending
impulses to the respiratory center to inhibit further inspiration.
 The cerebral cortex sends impulses to the respiratory center in response to
strong emotions, such as anxiety, fear, and anger.
 Some sensory stimuli, such as irritating vapors may cause reflex inhibition
of respiration.
17. Disorders
a. Emphysema
b. Cystic Fibrosis
c. Pneumonia