42_2 Flashcards
gas exchange
Gas
exchange is the uptake of molecular O2 from the environment and the discharge of CO2 to the environment.
partial pressure
the atmosphere is 21% O2 by volume, the partial pressure of O2 is 0.21*760, or about
160 mm Hg. This value is called the partial pressure of O2 (abbreviated PO2) because it is the part of atmospheric pressure
contributed by O2
partial pressures in dissolved gases
Partial pressures also apply to gases dissolved in a liquid,
such as water. When water is exposed to air, an equilibrium is
reached in which the partial pressure of each gas in the water
equals the partial pressure of that gas in the air.
- However, the concentrations of gas in
the air and water differ substantially because gases is much less
soluble in water than in air
gas exchange w/ air as the respiratory medium
As already noted, O2 is plentiful in air, making
up about 21% of Earth’s atmosphere by volume. Compared
to water, air is much less dense and less viscous, so it is easier
to move and to force through small passageways. As a result,
breathing air is relatively easy and need not be particularly efficient. Humans, for example, extract only about 25% of the
O2 in inhaled air
gas exchange w/ water as the respiratory medium
- Gas exchange with water as the respiratory medium is much more demanding.
- The amount of O2 dissolved in a
given volume of water varies but is always less than in an
equivalent volume of air: the concentration in many marine and freshwater habitats is roughly 40 times less than in air. The warmer
and saltier the water is, the less dissolved O2 it can hold. - Water’s lower O2 content, greater density, and greater viscosity mean that aquatic animals such as fishes and lobsters
must expend considerable energy to carry out gas exchange.
respiratory surfaces
- Like all living cells, the cells that carry
out gas exchange have a plasma membrane that must be in
contact with an aqueous solution. Respiratory surfaces are
therefore always moist and tend to be large and thin. - The general body surface of most animals lacks sufficient
area to exchange gases for the whole organism. The evolutionary solution to this limitation is a respiratory organ that
is extensively folded or branched. Gills, tracheae, and
lungs are three such organs.
movement of gases across respiratory surfaces
- The movement of O2 and CO2 across moist respiratory
surfaces takes place entirely by diffusion. - The rate of diffusion is proportional to the surface area across which it occurs
and inversely proportional to the square of the distance
through which molecules must move. In other words, gas exchange is fast when the area for diffusion is large and the
path for diffusion is short. As a result, respiratory surfaces
tend to be large and thin.
gills
Gills are outfoldings of the body surface that are suspended
in the water. Regardless of their
distribution, gills often have a total surface area much greater
than that of the rest of the body’s exterior
ventilation
- Movement of the respiratory medium over the respiratory surface, a process called ventilation, maintains the
partial pressure gradients of O2 and CO2 across the gill that are necessary for gas exchange. - To promote ventilation,
most gill-bearing animals either move their gills through
the water or move water over their gills.
why are gills are generally unsuitable for an animal living on land?
- An expansive surface of wet membrane exposed directly to air
currents in the environment would lose too much water by
evaporation. - Furthermore, the gills would collapse as their fine
filaments, no longer supported by water, stuck together. - In most
terrestrial animals, respiratory surfaces are enclosed within the
body, exposed to the atmosphere only through narrow tubes.
efficiency of countercurrent exchange mechanisms
- Countercurrent exchange mechanisms are remarkably efficient. In the fish gill, more than 80% of the O2 dissolved in
the water is removed as it passes over the respiratory surface. - In other settings, countercurrent exchange contributes to
temperature regulation and to the functioning of the mammalian kidney.
countercurrent exchange
The arrangement of capillaries in a fish gill allows for
countercurrent exchange, the exchange of a substance
or heat between two fluids flowing in opposite directions. In
a fish gill, this process maximizes gas exchange efficiency.
- Because blood flows in the direction opposite to that of water
passing over the gills, at each point in its travel blood is less
saturated with O2 than the water it meets.
countercurrent exchange and partial pressure
- As blood enters a gill capillary, it encounters water that is
completing its passage through the gill. Depleted of much of
its dissolved O2, this water nevertheless has a higher PO2 (partial pressure of O2) than
the incoming blood, and O2 transfer takes place. - As the blood
continues its passage, its PO2 steadily increases, but so does
that of the water it encounters, since each successive position in the blood’s travel corresponds to an earlier position in the
water’s passage over the gills. - Thus, a partial pressure gradient
favoring the diffusion of O2 from water to blood exists along
the entire length of the capillary.
tracheal system
- Although the most familiar respiratory structure among terrestrial animals is the lung, the most common is actually the
tracheal system of insects. - Made up of air tubes that
branch throughout the body, this system is one variation on
the theme of an internal respiratory surface. - The largest tubes, called tracheae, open to the outside.
- The finest branches extend close to the surface of nearly
every cell, where gas is exchanged by diffusion across the
moist epithelium that lines the tips of the tracheal branches. - Because the tracheal system brings air
within a very short distance of virtually every body cell in an
insect, it can transport O2 and CO2 without the participation
of the animal’s open circulatory system.
lungs
- Unlike tracheal systems, which branch throughout the insect body, lungs are localized respiratory organs.
- Representing an infolding of the body surface, they are typically
subdivided into numerous pockets. - Because the respiratory
surface of a lung is not in direct contact with all other parts
of the body, the gap must be bridged by the circulatory system, which transports gases between the lungs and the rest
of the body. - Lungs have evolved in organisms with open circulatory systems, such as spiders and land snails, as well
as in vertebrates.
larynx
When food is swallowed, the larynx (the
upper part of the respiratory tract) moves upward and tips the
epiglottis over the glottis (the opening of the trachea, or
windpipe). This allows food to go down the esophagus to the
stomach (see Figure 41.11). The rest of the time, the glottis is
open, enabling breathing
respiration: air entering
- Air enters through the nostrils and is then filtered by hairs,
warmed, humidified, and sampled for odors as it flows
through a maze of spaces in the nasal cavity. - The nasal cavity
leads to the pharynx, an intersection where the paths for air
and food cross. The glottis is open, and from the larynx, air passes into the trachea. Cartilage reinforcing the walls of both the larynx and the trachea keeps this
part of the airway open.
vocal cords
- Within the larynx of most mammals, exhaled air rushes by a pair of elastic bands of muscle called
vocal folds, or, in humans, vocal cords. - Sounds are produced
when muscles in the larynx are tensed, stretching the cords so
they vibrate. - High-pitched sounds result from tightly
stretched cords vibrating rapidly; low-pitched sounds come
from less tense cords vibrating slowly.
bronchi
The trachea branches into two bronchi (singular,
bronchus), one leading to each lung. Within the lung, the
bronchi branch repeatedly into finer and finer tubes called
bronchioles
- The entire system of air ducts has the appearance of an inverted tree, the trunk being the trachea.
mucus escalator
The epithelium lining the major branches of this respiratory tree is
covered by cilia and a thin film of mucus.
- The mucus traps
dust, pollen, and other particulate contaminants, and
- the
beating cilia move the mucus upward to the pharynx, where
it can be swallowed into the esophagus.
- This process, sometimes referred to as the “mucus escalator,” plays a crucial role
in cleansing the respiratory system
alveoli
- Gas exchange in mammals occurs in alveoli, air sacs clustered at the tips of the
tiniest bronchioles. - Human lungs contain millions of alveoli,
which together have a surface area of about 100 m2, 50 times that of the skin. - Oxygen in the air entering the alveoli dissolves in the moist film lining their inner surfaces and rapidly diffuses across the epithelium into a web of capillaries
that surrounds each alveolus. - Net diffusion of carbon dioxide
occurs in the opposite direction, from the capillaries across
the epithelium of the alveolus and into the air space
alveoli contamination
- Lacking cilia or significant air currents to remove particles
from their surface, alveoli are highly susceptible to contamination. - White blood cells patrol alveoli, engulfing foreign
particles. However, if too much particulate matter reaches the
alveoli, the defenses can be overwhelmed, leading to inflammation and irreversible damage. - For example, particulates
from cigarette smoke that enter alveoli can cause a permanent reduction in lung capacity. - For coal miners, inhalation
of large amounts of coal dust can lead to silicosis, a disabling,
irreversible, and sometimes fatal lung disease.
Given their tiny diameter (about
0.25 mm), why don’t alveoli collapse under high surface tension?
- The film of liquid that lines alveoli is subject to surface tension, an attractive force that acts to minimize the surface area
of a liquid - Researchers reasoned that alveoli must be coated with
a material that reduces surface tension. In 1955, biophysicist Richard Pattle obtained experimental evidence
for such a material, now called a surfactant, for surfaceactive agent.
RDS
- Pattle proposed that the absence of surfactant might cause respiratory distress syndrome (RDS), a
disease common among preterm infants born 6 weeks or
more before their due dates. - In the late 1950s, Mary Ellen Avery carried out the first experiment linking RDS to a surfactant deficiency
- Subsequent studies revealed that surfactant contains a mixture
of phospholipids and proteins and typically appears in the
lungs after 33 weeks of development. (full-term: 38wk) Artificial surfactants are
now used routinely to treat early preterm infants.
positive pressure breatrhing
- An amphibian such as a frog ventilates its lungs by positive
pressure breathing, inflating the lungs with forced airflow. - During the first stage of inhalation, muscles lower the
floor of an amphibian’s oral cavity, drawing in air through its
nostrils. - Next, with the nostrils and mouth closed, the floor
of the oral cavity rises, forcing air down the trachea. - During
exhalation, air is forced back out by the elastic recoil of the
lungs and by compression of the muscular body wall.
Two features of ventilation in birds make it highly efficient:
First, when birds breathe, they pass air over the gas exchange
surface in only one direction. Second, incoming fresh air does
not mix with air that has already carried out gas exchange.
How a Bird Breathes
- To bring fresh air to their lungs, birds use eight or nine air
sacs situated on either side of the lungs. The air
sacs do not function directly in gas exchange but act as bellows
that keep air flowing through the lungs. - Instead of alveoli,
which are dead ends, the sites of gas exchange in bird lungs are
tiny channels called parabronchi. Passage of air through the entire system—lungs and air sacs—requires two cycles of inhalation and exhalation.
negative pressure breathing
- Unlike amphibians and birds, mammals employ negative
pressure breathing—pulling, rather than pushing, air into
their lungs. - Using muscle contraction to actively expand the thoracic cavity, mammals lower air pressure in their lungs below that of the air outside their body. Because gas flows from a region of higher pressure to a region
of lower pressure, air rushes through the nostrils and mouth
and down the breathing tubes to the alveoli. - During exhalation, the muscles controlling the thoracic cavity relax, and
the volume of the cavity is reduced. The increased air pressure in the alveoli forces air up the breathing tubes and out of
the body. - Thus, inhalation is always active and requires work,
whereas exhalation is usually passive.
diaphragm
Expanding the thoracic cavity during inhalation involves
the animal’s rib muscles and the diaphragm, a sheet of
skeletal muscle that forms the bottom wall of the cavity.
- Contracting the rib muscles expands the rib cage, the front
wall of the thoracic cavity, by pulling the ribs upward and
the sternum outward.
- At the same time, the diaphragm contracts, expanding the thoracic cavity downward.
lungs’ double membrane
- Within the thoracic cavity, a double membrane surrounds
the lungs. - The inner layer of this membrane adheres to the
outside of the lungs, and - the outer layer adheres to the wall
of the thoracic cavity. - A thin space filled with fluid separates
the two layers. Surface tension in the fluid causes the two layers to stick together like two plates of glass separated by a film
of water: The layers can slide smoothly
past each other, but they cannot be
pulled apart easily. - Consequently, the
volume of the thoracic cavity and the
volume of the lungs change in unison.
heightened activity level and breathing
- Depending on activity level, additional muscles may be recruited to aid
breathing. - The rib muscles and diaphragm are sufficient to change lung
volume when a mammal is at rest. - During exercise, other muscles of the neck,
back, and chest increase the volume of
the thoracic cavity by raising the rib
cage. - The result is a piston-like pumping motion
that pushes and pulls on the diaphragm,
further increasing the volume of air
moved in and out of the lungs.
tidal volume
The volume of air inhaled and exhaled with each breath is
called tidal volume. It averages about 500 mL in resting humans.
vital capacity
The tidal volume during maximal inhalation and exhalation is the vital capacity, which is about 3.4 L and
4.8 L for college-age women and men, respectively.
residual volume
The air
that remains after a forced exhalation is called the residual
volume. As we age, our lungs lose their resilience, and residual volume increases at the expense of vital capacity.
Because the lungs in mammals do not completely empty
with each breath,
Because the lungs in mammals do not completely empty
with each breath, and because inhalation occurs through the
same airways as exhalation, each inhalation mixes fresh air
with oxygen-depleted residual air.
- As a result, the maximum
PO2 in alveoli is always considerably less than in the atmosphere.
- The maximum PO2 in lungs is also less for mammals
than for birds, which renew the air in their lungs with every
exhalation.
- This is one reason mammals function less well
than birds at high altitude.
what part of nervous system controls breathing?
- The neurons mainly responsible for regulating breathing
are in the medulla oblongata, near the base of the brain. - Neural circuits in the medulla form a breathing
control center that establishes the breathing rhythm. - When you
breathe deeply, a negative-feedback mechanism prevents the
lungs from overexpanding: During inhalation, sensors that detect stretching of the lung tissue send nerve impulses to the
control circuits in the medulla, inhibiting further inhalation. - The pons, a part of the brain next to the medulla, also regulates breathing, although its exact role remains an open question. The pons may act in the regulatory circuit with the
medulla or modulate the output of that circuit
pH and regulation of breathing
- In regulating breathing, the medulla uses the pH of the
surrounding tissue fluid as an indicator of blood CO2 concentration. - The reason pH can be used in this way is that blood
CO2 is the main determinant of the pH of cerebrospinal fluid,
the fluid surrounding the brain and spinal cord. - Carbon
dioxide diffuses from the blood to the cerebrospinal fluid,
where it reacts with water and forms carbonic acid (H2CO3).
The H2CO3 can then dissociate into a bicarbonate ion
(HCO3-) and a hydrogen ion (H+):
CO2 + H2O
<=>
H2CO3
<=>
HCO3- + H+
pH and exercise
Increased metabolic activity, such as occurs during exercise, lowers pH by increasing the concentration of CO2 in the blood.
- Sensors in blood vessels and the medulla detect this pH change.
- In response, the medulla’s control circuits increase the depth
and rate of breathing. Both remain high until the excess CO2 is
eliminated in exhaled air and pH returns to a normal value.
gas exchange at alveolar capillaries
- Blood flowing through the alveolar capillaries has a lower PO2
and a higher PCO2 than the air in the alveoli. - As a result, CO2
diffuses down its partial pressure gradient from the blood to
the air in the alveoli. - Meanwhile, O2 in the air dissolves in the
fluid that coats the alveolar epithelium and diffuses into the
blood. - By the time the blood leaves the lungs in the pulmonary veins, its PO2 has been raised and its PCO2 has been
lowered. After returning to the heart, this blood is pumped
through the systemic circuit.
gas exchange at tissue capillaries
- In the tissue capillaries, gradients of partial pressure favor
the diffusion of O2 out of the blood and CO2 into the blood. - These gradients exist because cellular respiration in the mitochondria of cells near each capillary removes O2 from and
adds CO2 to the surrounding interstitial fluid. - After the blood
unloads O2 and loads CO2, it is returned to the heart and
pumped to the lungs again.
respiratory pigments
- The low solubility of O2 in water (and thus in blood) poses a
problem for animals that rely on the circulatory system to deliver O2 - Respiratory pigments circulate with the blood or hemolymph and are often
contained within specialized cells - greatly increase the amount of O2 that can be carried in the circulatory
fluid - distinct color; protein bound to metal
hemocyanin
respiratory pigment
One example is the blue pigment
hemocyanin, which has copper as its oxygen-binding component and is found in arthropods and many molluscs.
hemoglobin
The
respiratory pigment of almost all vertebrates and many invertebrates is hemoglobin. In vertebrates, it is contained in the
erythrocytes.
hemoglobin structure
- Vertebrate hemoglobin consists of four subunits (polypeptide chains), each with a cofactor called a heme group that has an iron atom at its center. - Each iron atom binds one molecule of O2; hence, a single hemoglobin molecule can carry four molecules of O2.
hemoglobin reversible binding
- Like all respiratory pigments, hemoglobin binds O2 reversibly, loading O2 in the lungs or gills
and unloading it in other parts of the body. - This process depends on cooperativity between the hemoglobin subunits.
- When O2 binds to one subunit, the others
change shape slightly, increasing their affinity for O2. - When
four O2 molecules are bound and one subunit unloads its O2,
the other three subunits more readily unload O2, as an associated shape change lowers their affinity for O2.
dissociation curve for hemoglobin
- Cooperativity in O2 binding and release is evident in the
dissociation curve for hemoglobin. - Over the
range of PO2 where the dissociation curve has a steep slope,
even a slight change in PO2 causes hemoglobin to load or unload a substantial amount of O2. - Notice that the steep part of
the curve corresponds to the range of PO2 found in body tissues. - When cells in a particular location begin working
harder—during exercise, for instance—PO2 dips in their vicinity as the O2 is consumed in cellular respiration. Because of the
effect of subunit cooperativity, a slight drop in PO2 causes a relatively large increase in the amount of O2 the blood unloads.
bohr shift
- The production of CO2 during cellular respiration promotes the unloading of O2 by hemoglobin in active tissues.
- As we have seen, CO2 reacts with water, forming carbonic
acid, which lowers the pH of its surroundings. Low pH, in
turn, decreases the affinity of hemoglobin for O2, an effect
called the Bohr shift. - Thus, where CO2
production is greater, hemoglobin releases more O2, which
can then be used to support more cellular respiration.
hemoglobin and CO2
- hemoglobin helps transport CO2 and assists in buffering the blood—that is, preventing harmful changes in pH.
- Only about 7% of the CO2 released by respiring cells is transported
in solution in blood plasma. - Another
23% binds to the amino ends of the hemoglobin polypeptide chains, and - about 70% is transported in the blood in the
form of bicarbonate ions (HCO3-).
how is bicarbonate formed in the blood
- carbon
dioxide from respiring cells diffuses into
the blood plasma and then into erythrocytes. There the CO2 reacts with water
(assisted by the enzyme carbonic anhydrase) and forms H2CO3, which dissociates into H+ and HCO3-. - Most of the H+
binds to hemoglobin and other proteins,
minimizing the change in blood pH. - The HCO3- diffuses into the plasma.
conversion of bicarbonate to CO2
- When blood flows through the lungs, the relative partial pressures of CO2 favor the diffusion of CO2 out of the blood. - As CO2 diffuses into alveoli, the amount of CO2 in the blood decreases. This decrease shifts the chemical equilibrium in favor of the conversion of HCO3- to CO2, enabling further net diffusion of CO2 into alveoli. - Overall, the PCO2 gradient is sufficient to reduce PCO2 by roughly 15% during passage of blood through the lungs.
adaptation of diving mammals
ability to store large amounts of O2.
- Compared with humans, the Weddell seal can store about 2x as much O2 and 2x the blood per kilogram of body mass
myoglobin
Diving mammals also have a high concentration
of an oxygen-storing protein called myoglobin in their
muscles. The Weddell seal can store about 25% of its O2 in
muscle, compared with only 13% in humans
