Structure and Function of the
Pulmonary System
Pulmonary System
• Made up of two lungs
– Where gas exchange takes place
• Airways
– To get air to lungs
• Blood vessels
– To circulate oxygen and carbon dioxide
• Chest wall
– To move air in and out of lungs through
pressure changes and protect lungs
Airways
• Can divide respiratory system into two
parts:
– Upper respiratory tract : nasal cavity, pharynx
– Lower respiratory tract: larynx, trachea,
bronchi, bronchioles, alveoli
• Upper respiratory tract serves to conduct
air into the body, and filter, warm and
humidify the air.
Nasal cavity
• Hairs and mucus serve to protect the
respiratory system by trapping dust,
organisms and foreign particles. Irritant
receptors in nares remove substances by
sneezing when irritated by chemical or
mechanical irritants.
• Mucous membranes of nasal cavity and
paranasal sinuses warm and humidify the
air. Turbinate bones increase surface area
for this purpose.
• Air then passes into the pharynx, which
has three divisions:
– Nasopharynx – pseudostratified ciliated
columnar epithelium
– Oropharynx - nonkeratinized stratified
squamous epithelium
– Laryngopharynx - nonkeratinized stratified
squamous epithelium
Larynx
• Made up of 9 cartilages:
– Epiglottis
– Thyroid cartilage
– Cricoid cartilage
– Paired cartilages : cuneiform, arytenoid, and
corniculate cartilages
• Two sets of vocal folds:
– Upper – false vocal cords – prevent passage of
foreign substances into trachea during swallowing
– Lower – true vocal cords – for formation of sounds vocalization
Muscles of larynx
• Internal laryngeal muscles control tension
on vocal cords
• External muscles move larynx as a whole
during swallowing.
Trachea
• Held open by 20 “C” shaped cartilages.
• Extends to about level of T5, add
branches into two primary bronchi
• Lined with pseudostratified ciliated
columnar epithelium
• Ridge called carina – mucous membrane
here is most sensitive area for triggering
cough reflex.
Bronchi
• Right primary bronchus is wider, shorter
and more vertical
• Enter lungs at hilus
• Branch into secondary or lobar bronchi,
then into
• Tertiary or segmental bronchi
• Bronchioles – no cartilage, smooth
muscle, supply the alveoli
Lungs
•
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Two lungs
Right lung has 3 lobes and two fissures
Left lung has 2 lobes and one fissure
Blood vessels and bronchi enter at hilus
Alveoli
• Small, thin air sacs surrounded by a “hair
net” of capillaries
• Most of the walls of the alveoli are made
up of simple squamous epithelium, called
Type 1 alveolar cells. Gas exchange takes
place through these cells
Other cells of the alveoli
• Type II alveolar cells produce surfactant
(lipoprotein) to decrease the surface
tension of water in the lungs
– Premature infants – respiratory-distress
syndrome of the newborn
• Alveolar macrophages, or “dust cells”
phagocytize foreign particles and microbes
Respiratory membrane
• Made of alveolar type I cell, endothelium
of capillary and their basement
membranes.
• As thin as possible for gas exchange
which takes place by diffusion from areas
of high partial pressure to low partial
pressure
• Any disorder that thickens the membrane
decreases gas exchange.
• Usually, about one third of the pulmonary
vessels are perfused at one time, so if
right heart output increases does not
increase arterial pressure in lungs, simply
perfuse more vessels.
• Lungs are surrounded by pleural
membranes : parietal pleura and visceral
pleura
• Separate by the mediastinum
• Enclosed in the thoracic cavity.
Pulmonary ventilation (breathing)
• Takes place by decreasing the pressure
inside the thoracic cavity, and allowing
atmospheric pressure to force air into lungs.
• Thoracic cavity is expanded primarily by the
contraction and lowering of the diaphragm,
but is aided by expansion of chest by
contraction of the external intercostal
muscles.
Expiration
• Is normally a passive process due to the
relaxing of the inspiratory muscles and the
elastic recoil of the lungs.
Respiratory volumes
• Amount of air moved in and out of lungs
• Tidal volume is the amount of air moved
with a normal breath
• Minute volume of respiration = tidal
volume X breaths per minute
Control of breathing
• Both voluntary and involuntary
• Respiratory center in the medulla oblongata
controls breathing.
• The dorsal respiratory group sets the basic
rhythm of breathing. (ventral –strenuous )
• Receives input from peripheral
chemoreceptors and from pneumotaxic area
• Hering-Breuer reflex – stretch receptors in lung
activate the pneumotaxic area, which inhibits
insipiration – preventing over inflation of lungs.
Central Chemoreceptors
• Chemical influences:
– Central chemoreceptors in medulla oblongata
– Sensitive to changes in conc. of CO2 and pH
– CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3carbon + water
carbonic hydrogen
bicarbonate dioxide
acid
ion
ion
Peripheral Chemoreceptors
• Carotid bodies and aortic bodies
• Stimulated by oxygen concentration
decrease
• Send impulses to respiratory centers, and
breathing increases
• Not triggered until O2 is very low (50 mm
Hg)
Oxygen transport
• Gases move down partial pressure gradient.
• 98 % of oxygen is bound to the iron in
hemoglobin as oxyhemoglobin (HbO2)
• Rest is dissolved in plasma
• 4 Iron atoms per hemoglobin molecule
• 1 hemoglobin molecule can hold up to 4
molecules of oxygen
• Amount of oxygen on hemoglobin is
determined by partial pressure of oxygen.
Carbon Dioxide Transport
• 7 % is dissolved in plasma
• 23 % combines with the amino groups of
the hemoglobin forming
carbaminohemoglobin
• 70 % is converted to bicarbonate ions
Control of pulmonary circulation
• In most areas of the body, hypoxia causes
vasodilation.
• Low oxygen concentrations in the alveoli of
the lungs cause vasoconstriction.
• This sends blood to areas of the lung that
have higher oxygen concentrations
• Chronic alveolar hypoxia can lead to
permanent pulmonary artery hypertension,
which can lead to right heart failure.
• Acidemia also causes pulmonary artery
constriction