Biology 112
Chapter 16: Respiratory System
Introduction
- respiratory system includes tubes that remove
particles from incoming air & transport air to & from lungs and air
sacs where gases (oxygen & carbon dioxide) are exchanged
- respiration is entire process of gas exchange between
atmosphere & body cells
- ventilation (breathing): moving air into & out of lungs
- external respiration: gas exchange between blood & air in
lungs
- transport of gases in blood between lungs & body cells
- internal respiration: gas exchange between blood & tissue
cells
Organs of Respiratory System
- nose: supported by bone (posteriorly) & cartilage
(anteriorly)
- nostrils: openings for air
- nasal cavity: mucosa-lined passageway for air into nasopharynx;
divided into left & right halves by nasal septum
- mucous membrane filters, warms & moistens incoming air
- nasal conchae divide nasal cavity into passageways & increase
surface area to warm & moisten air
- ciliary action carries particles trapped in mucus to pharynx, where they
are generally swallowed
- paranasal sinuses: spaces in bones of the skull lined
by mucous membrane that open into nasal cavity
- located in maxillary, frontal, sphenoid & ethmoid bones
- pharynx: passageway for air & food behind nasal cavity
(nasopharynx), oral cavity (oropharynx) & larynx (laryngopharynx)
- larynx (voice box): conducts air, helps prevent foreign
objects from entering trachea & produces sounds of speech
- composed of muscles & cartilage; lined with mucous membrane
- contains vocal cords, which vibrate from side
to side & produce sounds when air from lungs passes through them
- glottis: opening in larynx that serves as a
passageway for air
- epiglottis: elastic cartilage at top of larynx
that closes off larynx during swallowing
- trachea: extends into thoracic cavity to bronchi entering
lungs; anterior to esophagus
- lined with C-rings of hyaline cartilage that helps keep airway open;
cartilage incomplete at back, allows room for expansion of esophagus during
swallowing
- divides into left & right bronchi that
enter lungs
- bronchial tree: branched air passages that lead from trachea
to air sacs in lungs called alveoli
- alveoli are at the distal end of alveolar ducts,
the narrowest tubes of tree
- lungs: soft, spongy cone-shaped organs on either side
of thoracic cavity
- mediastinum separates left & right lungs; diaphragm & thoracic
cage enclose them
- left lung has 2 lobes & cardiac notch (space for heart); right
lung has 3 lobes
- visceral pleura attaches to surface of lungs; parietal
pleura lines thoracic cavity
- each lobe of lungs composed of alveoli, blood vessels & supporting
tissues
Breathing Mechanism
- inspiration: atmospheric pressure forces air into lungs
- pressure in alveoli decreases due to contraction of
breathing muscles (diaphragm & intercostals) & expansion
of thoracic cavity
- when breathing muscles contract, thoracic cage moves upward & outward,
increasing volume (decreases pressure)
- surface tension aids lung expansion
- expiration: increased pressure in lungs forces air out
of lungs
- elastic recoil of lung tissues & surface tension within alveoli provide
forces of expiration
- when breathing muscles relax. thoracic cage moves downward & inward,
decreasing volume (increases pressure)
- respiratory air volume & capacities
- respiratory cycle: one inspiration followed
by one expiration
- respiratory volumes measured with spirometer
- tidal volume (TV): air volume that moves into & out
of the lungs with each breath (~ 500 ml)
- inspiratory reserve volume (IRV): air volume
that can be forcibly inspired beyond tidal volume (~ 1900-3100 ml)
- expiratory reserve volume (ERV): air volume
that can be forcibly expired beyond tidal volume (~ 700-1200 ml)
- residual volume (RV): air remaining in lungs
after forced exhalation (~ 1200 ml)
- inspiratory capacity (IC): TV + IRV
- functional residual capacity (FRC): RV + ERV
- vital capacity (VC): TV + IRV + ERV
- total lung capacity (TLC): VC + RV
Control of Breathing
- normal breathing is rhythmic & involuntary
- respiratory center in brainstem includes portions
of pons & medulla oblongata
- medulla rhythmicity center includes 2 groups of neurons
- dorsal respiratory group controls basic rhythm of breathing
- ventral respiratory group increases inspiratory & expiratory
respiratory movements during forceful breathing
- pneumotaxic area regulates breathing rate (normally
~ 12 breaths/minute)
- factors affecting breathing: chemicals, stretching of
lung tissues, emotional states
- central chemoreceptors associated
with respiratory center
- blood concentrations of carbon dioxide & hydrogen ions affect
chemoreceptors; stimulation of receptors increases breathing rate
- peripheral chemoreceptors are in walls of certain
large arteries
- these chemoreceptors sense low oxygen levels;
when oxygen levels are low, breathing rate increases
- overstretching lung tissue triggers inflation reflex
- reflex shortens duration of respiratory movements & prevents overinflation
of lungs during forceful breathing
- hyperventilation decreases blood carbon dioxide
levels, but is very dangerous when done before swimming underwater
Alveolar Gas Exchange
- gas exchange between blood & air occurs at alveoli,
tiny air sacs clustered at distal ends of alveolar ducts
- respiratory membrane: consists of alveolar & capillary
walls
- site of gas exchange between blood & alveoli
- diffusion across respiratory membrane
- partial pressure of gas is proportional to
concentration of that gas in a mixture or concentration dissolved in a
liquid
- gases diffuse from regions of higher partial
pressure to regions of lower partial pressure
- oxygen diffuses from alveolar air into blood
- carbon dioxide diffuses from blood into alveolar
air
Gas Transport
- oxygen is primarily transported in red blood
cells bound to hemoglobin protein
(oxygen binds to iron of the heme group of
hemoglobin)
- oxyhemoglobin is unstable & releases oxygen
in regions where partial pressure of oxygen is low
- more oxygen is released as blood levels of carbon dioxide increase,
as blood becomes more acidic, and as blood temperature increases
- carbon dioxide may be carried in blood, bound to hemoglobin,
or as bicarbonate ion in blood
- most carbon dioxide is transported as bicarbonate ion
- enzyme carbonic anhydrase forms carbonic
acid from carbon dioxide & water
- carbonic acid dissociates to release hydrogen ions & bicarbonate ions; buffering
system in blood