Respiration
Chapter 33
Why Exchange Gases?
• Gas exchange supports cellular
respiration
• Gas exchange in vertebrates
– O2 is inhaled into lungs, deposited in blood,
and transported to body cells
– O2 is used in cellular respiration to convert
the energy in nutrients into ATP, generating
CO2 as a waste product
– Blood transports CO2 from tissues to lungs
– CO2 released from lungs during exhalation
Common Respiratory Features
• All animal respiratory systems share three
features
– (1) Respiratory surface must be moist so
gases can diffuse across cell membranes
– (2) Cells lining respiratory surface are thin to
optimizes gas diffusion
– (3) The respiratory surface area must be
large to allow for adequate gas exchange
Animals in Moist Environments
• Some animals in moist environments lack
specialized respiratory structures
• Gases diffuse short distances in smaller
animals to reach cells
– Gas exchange optimized by long, flat bodies
with greater surface area
– Examples: flatworms
Animals in Moist Environments
• Low energy demands translate into larger
animals that rely on their moist body
surface for gas exchange
– Larger size possible because less O2 needed
by cells
– Example: sea jellies
• Some animals bring the environment
close to all their cells
– Allows greater exposure of cells to O2
– Example: sponges
Animals in Moist Environments
• Other animals combine large skin surface
area with well-developed circulation for
delivery to cells
– Skin has many capillaries that carry O2 to
internal body tissues
– This arrangement sustains a favorable O2
concentration gradient between skin and
blood
– Example: earthworm
Gas Exchange
• Respiratory systems facilitate more
effective exchange of gases between the
environment and an animal’s body
• Respiratory systems alternate bulk flow
of air/water and diffusion of gases
• Bulk Flow: describes when fluids or
gases move through spaces from high
pressure to low pressure
Gas Exchange
• In mammals
– Air or water moves past respiratory surface
by bulk flow (down pressure gradient)
– O2 and CO2 are exchanged by diffusion
– Gases transported to/from tissues by bulk
flow
– Gases exchanged with tissues (cells) by
diffusion
Gas Exchange in Water
• Gills are external projections of the body
that exchange gases
– Most commonly used in aquatic animals
– Can be elaborately folded to maximize their
surface area
– Have many capillaries to bring blood to body
surface for gas exchange
Gas Exchange in Water
• Fish gills are complex structures
– Protected by a bony flap (operculum)
– Fish controls water flow over gills by
swimming with mouth open
– Water flows over gills and out of body through
opercular openings
– Gills are elaborately folded, and cannot
support themselves out of water
Internal Respiratory Structures
• Internal respiratory structures are used by
most terrestrial animals to help keep
respiratory surfaces moist
– Gas exchange is optimized across moist
surfaces
• Two common terrestrial respiratory
structures
– Tracheae (insects)
– Lungs (most terrestrial vertebrates)
Tracheae
Tracheae are elaborately branched internal
tubes that deliver air to body cells
– Used by insects
– Branch into smaller tubes (tracheoles)
– Air enters tracheae though abdominal
openings (spiracles)
– Some insects use abdominal contractions to
enhance air movements
Lungs
• Lungs are internal chambers containing
moist respiratory surfaces
– Used by most terrestrial vertebrates
– Developed to allow ancestral fish to survive in
stagnant, O2-poor water
Lungs
• Lungs have differing levels of complexity
• In amphibians
– Many use gills as larvae and simple, sac-like
lungs as more terrestrial adults
– Many use the skin as a supplemental
respiratory surface
– Example: tadpoles and a bullfrog
Lungs
• In reptiles
– Scales reduce body water loss and allow for
survival in dry environments
– Scales reduce gas exchange through skin
– Lungs have more respiratory surface area
than amphibians
– Example: a mangrove snake
Lungs
• In birds
– Exclusively lung breathers
– Extremely efficient lungs accommodate O2
demands during flight
– Air flows through lungs during inhalation and
exhalation due to coordination of air sac
activity
– Bird lungs filled with thin walled tubes
(parabronchi)
The Human Respiratory System
• The human respiratory system can be
divided into two parts
– The conducting portion
– The gas-exchange portion
The Conducting Portion
• The conducting portion is a series of
passageways that carry air into the gasexchange portion of the lungs
– Warms and moistens air on way to lungs
– Debris in air sticks to mucus that lines
respiratory passages
– Mucus carried to pharynx by cilia lining
respiratory passages
The Conducting Portion
• Nose and mouth
• Nasal cavity and oral cavity
• Pharynx: chamber where nasal and oral
cavities converge
• Larynx: opening called the “voice box”
– Contain vocal cords: bands of elastic tissue
controlled by muscles; vibrate as exhaled air
passes over them
The Conducting Portion
• Larynx covered by epiglottis: flap of
tissue that prevents food from entering
larynx when swallowing
• If food lodges in larynx, choking can occur
– Heimlich maneuver can dislodge food
The Conducting Portion
• Trachea: flexible tube reinforced with
cartilage
• Bronchi: splitting of trachea into two
branches; each leading to a lung
• Bronchioles: repeated branchings of
bronchi
– Lined with smooth muscle that can constrict
or dilate passageway
The Gas Exchange Portion
• Alveoli: tiny air sacs where gas exchange
occurs
– 300 million alveoli in both lungs
– Arranged in grape-like clusters
– Surrounded by dense capillary networks
• Where gases are exchanged with the
blood
• Occurs in the alveoli in the lungs
The Gas Exchange Portion
• Alveoli in lungs are well adapted for gas
exchange
– Extensive collective surface area
– Are enmeshed in capillaries
– Made of a single thin layer of endothelial cells
that form the innermost portion of the
respiratory membrane, across which gas
exchange occurs
The Gas Exchange Portion
• The respiratory membrane
– Consists of the alveolar epithelium and the
layer of endothelial cells that forms the
innermost wall of each capillary
– The alveolar and capillary walls are only one
cell thick, minimizing diffusion distance for
gases between the blood and the air
The Gas Exchange Portion
• O2 diffuses from lung air into blood
– O2 in freshly inhaled air has higher O2
concentration than O2-poor blood
– O2 diffuses down concentration gradient into
capillary blood
– Oxygenated blood transported to heart, then
the rest of body
The Gas Exchange Portion
• CO2 diffuses from lung blood into alveoli
– Metabolically active tissues release CO2 into
blood, which transports it to alveolar
capillaries
– Alveolar capillaries have a higher CO2
concentration than that of alveolar air
– CO2 diffuses down concentration gradient into
alveolar air, which is then exhaled
The Gas Exchange Portion
• Surfactant
– Oily secretion lining alveolar walls
– Reduces surface tension of alveolar walls,
preventing collapse during exhalation
Oxygen Transport
• O2 binds reversibly to hemoglobin
molecules in red blood cells
• Hemoglobin: iron-containing protein that
can bind to four O2 molecules
– When bound to O2: cherry-red color
– When not bound to O2: maroon-red color
Carbon Dioxide Transport
• CO2 is transported in the blood in three
ways
– (1) As bicarbonate ions
– (2) Bound to hemoglobin
– (3) Dissolved in plasma as CO2
Carbon Dioxide Transport
• CO2 transport as bicarbonate ions (HCO3-)
– 70% of CO2 reacts with water to form HCO3-,
which is then transported in the plasma
• CO2 transported bound to hemoglobin
– 20% of CO2 binds to and is carried by
hemoglobin
• CO2 transported dissolved in plasma
– 10% of CO2 is transported this way
Inhalation and Exhalation
• Breathing occurs due to volume changes
in the airtight chest cavity
– Located within rib cage
– Bottom of chest cavity defined by domeshaped diaphragm muscle
Inhalation and Exhalation
• Breathing occurs in two stages
– Inhalation
– Exhalation
• Air is inhaled actively and exhaled
passively
Inhalation and Exhalation
• Inhalation: when air is drawn into lungs
– Chest cavity enlarges when diaphragm and
rib muscles contract
– Lungs expand with chest cavity, creating a
partial vacuum that draws air into lungs
Inhalation and Exhalation
• Exhalation: when air is passively
expelled out of lungs
– Chest cavity size decreases when diaphragm
and rib muscles relax
– Decreasing chest cavity size forces air out of
lungs
– Additional air can be expelled by actively
contracting the abdominal muscles
The Respiratory Center
• The respiratory center is a cluster of
nerve cells located in the medulla of the
brain
– Generate cyclic bursts of impulses that cause
contraction of respiratory muscles
– Sets baseline breathing rate
The Respiratory Center
• Breathing rate can be modified by
– Blood CO2 levels
– Blood O2 levels
– Activity levels
The Respiratory Center
• Breathing rate can be modified by blood
CO2 levels
– Chemoreceptors in medulla detect elevated
CO2 levels and stimulate the respiratory
center
– Respiratory center causes an increase in
breathing rate and depth
The Respiratory Center
• Breathing rate can be modified by blood
O2 levels
– Chemoreceptors in aorta and carotid arteries
detect drastically low O2 levels and stimulate
the respiratory center
– Respiratory center causes an increase in
breathing rate and depth
– Little influence on normal breathing
The Respiratory Center
• Breathing rate can be modified by
physical activity
– During exercise, higher brain centers activate
muscles and stimulate the respiratory center
– Causes increased breathing rate and depth
– Occurs in advance of significant changes in
blood CO2 and O2 concentrations