Download Chapter 22 – Gas Exchange 22.1 Overview: Gas exchange involves

Chapter 22 – Gas Exchange
22.1 Overview: Gas exchange involves breathing, transport of gases, and exchange with body cells
Gas exchange makes it possible for animals to break down the foods brought in by the digestive system.
Three phases of exchange occur:
 Breathing – oxygen diffuses across cells lining the lungs and into surrounding blood vessels. CO 2
diffuses out of the blood and into the lungs. During exhalation, CO2 is released back to the
environment.
 Transport of gases by the circulatory system. Oxygen attaches to hemoglobin of red blood cells and is
carried from the lungs to the tissues. CO2 is transported from the tissues back to the lungs.
 Body cells take up O2 from the blood and release CO2 The O2 is required for cells to break down food
and the CO2 is a waste product from metabolism.
22.2 Animals exchange O2 and CO2 across moist body surfaces
 The respiratory surface is where gases are exchanged with the environment.
 These surfaces are made up of living cells whose plasma membranes must stay wet to function.
 Usually a single layer of cells lines the respiratory surface. Because it is thin, the layer allows oxygen to
diffuse rapidly into the tissues or circulatory system and allows CO2 to diffuse out.
 Some animals use their entire outer skin as a gas exchange organ. These animals can be referred to as
“skin breathers” and must live in damp places or in water to survive. If they are exclusively skin
breathers, then they lack specialized gas exchange organs and are generally small in size. Ex: earthworms.
 In most animals, the skin cannot provide sufficient oxygen for the entire body. Specialized respiratory
surfaces occur in these animals. Gills are in most aquatic animals. They are extensions of the body surface
that are designed for gas exchange. Oxygen diffuses across the gill surfaces and into capillaries, while CO 2
diffuses out of capillaries and into the environment.
 Lungs or gas exchange tubes, tracheae, have evolved in terrestrial animals. These respiratory surfaces are
folded into the body rather than projecting from it. Lungs are lined with moist epithelial tissue and the
inner surfaces are highly branched. Gases are carried between the lungs and body cells by the circulatory
system.
22.3 Gills are adapted for gas exchange in aquatic environments
 Bodies of water contain oxygen as a dissolved gas. The gills of aquatic animals are adapted to take in this
oxygen. Since respiratory surfaces must be kept wet, then exchanging gases in water is not an issue.
However, the amount of dissolved oxygen in water is considerably less than it is in air and the saltier the
water, the less oxygen it holds.
 Structure of the gills – there are four supporting gill arches on each side of the body. Two rows of
filaments project from each arch. Each filament has structures called lamellae, which serve as the
respiratory surface. One lamella is full of capillaries that are separated from the outside by only one or
few layers of cells. Because of this thin layer, red blood cells come into close contact with the dissolved
oxygen.
 Ventilation – any mechanism that increases the flow of water or air over the respiratory surface. When
swimming, fish open their mouth to allow water to flow on the gill surfaces. Increasing this flow ensures a
constant supply of oxygen.
 Capillary arrangement in the gills enhances gas exchange. Blood flows in the direction opposite of water
flow. This allows oxygen to transfer to blood by countercurrent exchange. Because the fluids are moving
opposite to each other, a diffusion gradient exists that enhances transfer. This transfer is so efficient that
it can remove up to 80% of the dissolved oxygen flowing in water.
22.4 The tracheal system of insects provides direct exchange between the air and body cells
Two big advantages of exchanging gas by breathing air:
 Air has a higher content of oxygen
 Air is lighter and easier to move than water.
 These factors make it easier for terrestrial animals to ventilate and means they use less energy. The
problem faced by these animals is water loss through evaporation.
 The tracheal system of insects is made up of air tubes that branch throughout the body. The largest tube,
the tracheae, opens to the outside and are reinforced by chitin. Enlarged parts of the tracheae form air
sacs near organs that supply a large supply of oxygen. Gas is exchanged with body cells by diffusion
across moist epithelium that lines the tips. For small insects, diffusion through the tracheae brings in
enough oxygen and removes enough CO2 to support cellular respiration. Larger insects may ventilate the
body with rhythmic bodily contractions.
22.5 The evolution of lungs facilitated the movement of tetrapods onto land
 Reptiles, most amphibians, mammals, and birds exchange gases in the lungs. Lungs are restricted to one
area of the body and the circulatory system is responsible for movement of the gases throughout the
body. Amphibians have small lungs and also rely on gas exchange through the skin. Reptiles and
mammals rely solely on lungs to provide gases.
 The lungs of endotherms have greater area of surface exchange than that of ectotherms. Humans have
100m2 of respiratory surface (about the size of a racquetball court).
22.6 In the human respiratory system, branching tubes convey air to lungs located in the chest cavity
 In humans, the lungs are found in the chest cavity and bounded at the bottom by a layer of muscle called
the diaphragm. Air passes to the lungs through a branching system of tubes. Air enters through the
nostrils, where it is warmed, filtered, humidified, and sampled for scents. From the nasal cavity (or
sometimes mouth), it passes through the pharynx into the larynx (voice box).
 Air passes through then into the trachea (wind pipe). Speech occurs when we tense and stretch the
muscles of the voice box voluntarily and make them vibrate.
o Higher sounds occur because the cords are tense.
o Lower sounds happen when the cords relax.
 The trachea forks into two bronchi, one leading into each lung. They branch repeatedly to form
bronchioles. At the end of each bronchiole is a dead-end cluster of alveoli. Each of these sacs is lined with
epithelial cells forming the respiratory surface. Oxygen and CO2 dissolve across these alveoli. Oxygen
comes into the capillaries that connect here, and CO2 leaves from the capillaries. The trachea and major
branches of the tract are lined with cilia covered epithelium and a thin film of mucus. These serve as
cleaning elements, trapping contaminants and moving them into the pharynx to be swallowed.
Respiratory Problems
 Alveoli are small and have secretions called surfactants that keep them from sticking shut. Alveoli are
susceptible to contaminants, and continuous irritation from smoking or pollution can lead to chronic
obstructive pulmonary diseases (COPD) like emphysema.
22.7 Warning: Cigarette smoke is hazardous to your health
Tobacco smoke irritates the cells lining the bronchi, inhibiting or destroying their cilia. Smoke also kills
macrophages, defensive cells in the respiratory tract that engulf particles and microorganisms. Smoking can
lead to emphysema, where the walls of the alveoli lose elasticity and deteriorate.
22.8 Negative pressure breathing ventilates the lungs
 Breathing is the alternation of inhalation and exhalation. This maintains high oxygen content and low
CO2 content. During inhalation, the rib cage and chest cavity expand and then the lungs do. Ribs move
upward as muscles between the ribs contract. The diaphragm moves downward and expands the chest
cavity as it does. This increase in volume lowers the pressure in the alveoli and air rushes into them. This
is referred to as negative pressure breathing.
 During exhalation, the diaphragm and rib muscles relax and the diaphragm curves upward. This
decreases the volume of the rib cage, forcing air out. Normal volume of air during relaxed breathing is
about 500mL. The maximum volume of air inhaled or exhaled during a forced breath is called vital
capacity, and averages about 3.4L in females and 4.8L in males.
22.9 Breathing is automatically controlled
 Automatic control centers in the brain regulate our breathing movements. This control ensures
coordination between the respiratory and circulatory systems as well as the body’s metabolic need for
gas exchange. Breathing control centers are found in the pons and medulla oblongata of the brain.
 Nerves from the medulla signal the diaphragm and rib muscles to contract, making us inhale (about 1014 times per minute). Between inhalations, the muscles relax and we exhale. The pons smooths out the
basic rhythm of the medulla. The medulla monitors levels in the blood and regulates breathing in
response.



Cues about CO2 concentration come from the pH of the blood. When pH starts to drop, then CO2 levels
have increased. CO2 is a waste product that enters the blood and mixes with water to make carbonic acid.
When the medulla senses this drop it increases breathing rate and depth to eliminate the excess and
return pH to normal.
Hyperventilation occurs when deep rapid breathing purges so much CO2 that the control center briefly
stops sending signals to the rib muscles and diaphragm. Breathing stops until the levels increase enough
to switch the breathing center back on
Control centers respond directly to CO2 levels, but not to oxygen levels. But the same process that deals
with excess levels also allows more oxygen to be brought in. Secondary control of breathing is achieved
by sensors in the aorta and carotid arteries. When oxygen levels drop, the sensors signal the control
center to increase breathing rate.
22.10 Blood transports respiratory gases
 Oxygen is transport by blood throughout the body. One side of the heart handles oxygen-poor blood,
while the other handles oxygen rich blood. The heart pumps oxygen poor blood to the alveolar capillaries
of the lungs. Gases are exchanged here and the oxygen-rich blood returns to the heart and is pumped to
body tissues.
 The exchange of gases between capillaries and the cells around them occurs by diffusion. Each gas in a
mixture accounts for only part of the pressure, this is termed partial pressure. Molecules will diffuse in
relation to their own partial pressure (moving from higher to lower pressure).
22.11 Hemoglobin carries O2 and helps transport CO2 and buffers the blood
 Oxygen is not soluble in water, so most of it is carried by hemoglobin in the red blood cells. Hemoglobin is
made of 4 polypeptide chains of two types. Attached to each chain is a heme group with an iron atom in
the center. Each iron carries one O2 molecule, so each hemoglobin can carry up to 4 oxygens.
 Hemoglobin picks up oxygen in the lungs and takes it to body tissues. It also transports CO2 and assists in
buffering the blood to prevent harmful pH changes. When CO2 leaves the tissues, it diffuses across the
wall of the capillary and into blood fluid (plasma). Most of it enters the red blood cells and combines with
hemoglobin. The rest reacts with water and makes carbonic acid which is later broken down by the
blood.
22.12 The human fetus exchanges gases with the mother’s blood
 The fetus swims in a protective amniotic fluid. Its lungs are filled with the fluid and nonfunctional. Gas
exchange is accomplished by the placenta.
 The placenta includes tissues from both mother and offspring. A net of capillaries goes into the placenta
from blood vessels in the umbilical cord. Fetal capillaries exchange gases with maternal blood that
circulates in the placenta. The maternal circulatory system takes the gases to and from the mother’s
lungs. Fetal hemoglobin takes up oxygen. It is more strongly attracted to the oxygen than adult
hemoglobin.
 At birth, placental gas exchange stops and the baby’s lungs begin to work. Carbon dioxide in fetal blood
acts as a signal. When it stops diffusing from fetus to placenta, the rise in fetal CO2 triggers a drop in pH
which stimulates the centers in the brain to start breathing .