42_2

Question 1

gas exchange

Answer 1

Gas

exchange is the uptake of molecular O2 from the environment and the discharge of CO2 to the environment.

Question 2

partial pressure

Answer 2

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

Question 3

partial pressures in dissolved gases

Answer 3

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

Question 4

gas exchange w/ air as the respiratory medium

Answer 4

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

Question 5

gas exchange w/ water as the respiratory medium

Answer 5

• 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.

Question 6

respiratory surfaces

Answer 6

• 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.

Question 7

movement of gases across respiratory surfaces

Answer 7

• 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.

Question 8

gills

Answer 8

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

Question 9

ventilation

Answer 9

• 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.

Question 10

why are gills are generally unsuitable for an animal living on land?

Answer 10

• 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.

Question 11

efficiency of countercurrent exchange mechanisms

Answer 11

• 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.

Question 12

countercurrent exchange

Answer 12

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.

Question 13

countercurrent exchange and partial pressure

Answer 13

• 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.

Question 14

tracheal system

Answer 14

• 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.

Question 15

lungs

Answer 15

• 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.

Question 16

larynx

Answer 16

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

Question 17

respiration: air entering

Answer 17

• 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.

Question 18

vocal cords

Answer 18

• 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.

Question 19

bronchi

Answer 19

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.

Question 20

mucus escalator

Answer 20

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

Question 21

alveoli

Answer 21

• 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

Question 22

alveoli contamination

Answer 22

• 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.

Question 23

Given their tiny diameter (about

0.25 mm), why don’t alveoli collapse under high surface tension?

Answer 23

• 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.

Question 24

RDS

Answer 24

• 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.

Question 25

positive pressure breatrhing

Answer 25

• 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.

Question 26

Two features of ventilation in birds make it highly efficient:

Answer 26

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.

Question 27

How a Bird Breathes

Answer 27

• 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.

Question 28

negative pressure breathing

Answer 28

• 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.

Question 29

diaphragm

Answer 29

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.

Question 30

lungs’ double membrane

Answer 30

• 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.

Question 31

heightened activity level and breathing

Answer 31

• 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.

Question 32

tidal volume

Answer 32

The volume of air inhaled and exhaled with each breath is

called tidal volume. It averages about 500 mL in resting humans.

Question 33

vital capacity

Answer 33

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.

Question 34

residual volume

Answer 34

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.

Question 35

Because the lungs in mammals do not completely empty

with each breath,

Answer 35

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.

Question 36

what part of nervous system controls breathing?

Answer 36

• 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

Question 37

pH and regulation of breathing

Answer 37

• 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+

Question 38

pH and exercise

Answer 38

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.

Question 39

gas exchange at alveolar capillaries

Answer 39

• 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.

Question 40

gas exchange at tissue capillaries

Answer 40

• 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.

Question 41

respiratory pigments

Answer 41

• 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

Question 42

hemocyanin

Answer 42

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.

Question 43

hemoglobin

Answer 43

The
respiratory pigment of almost all vertebrates and many invertebrates is hemoglobin. In vertebrates, it is contained in the
erythrocytes.

Question 44

hemoglobin structure

Answer 44

- 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.

Question 45

hemoglobin reversible binding

Answer 45

• 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.

Question 46

dissociation curve for hemoglobin

Answer 46

• 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.

Question 47

bohr shift

Answer 47

• 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.

Question 48

hemoglobin and CO2

Answer 48

• 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-).

Question 49

how is bicarbonate formed in the blood

Answer 49

• 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.

Question 50

conversion of bicarbonate to CO2

Answer 50

- 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.

Question 51

adaptation of diving mammals

Answer 51

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

Question 52

myoglobin

Answer 52

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