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Chapter 17
The Respiratory System: Gas
Exchange and Regulation of Breathing
Overview of the pulmonary circulation 肺循環總論
Diffusion of gases 氣體的擴散
Exchange of oxygen and carbon dioxide 氧氣及二氧化碳的交換
Transport of gases in the blood 氣體在血液的運送
Central regulation of ventilation 中樞調控換氣
Control of ventilation by chemoreceptors 化學接受器調控換氣
Local regulation of ventilation and perfusion 換氣及灌流的局部調控
The respiratory system in acid-base homeostasis
I. Overview of Pulmonary Circulation
 The ratio of the amount of CO2
produced by the body to the amount of
O2 consumed is called the respiratory
quotient 呼吸商
 On average, cells consume 250 ml of
O2 per minute while producing 200 ml
of CO2 under resting conditions, so the
average respiratory quotient at rest is
0.8 (200/250=0.8)
Figure 17.1 Movements of oxygen and
carbon dioxide in pulmonary and systemic
tissues during rest. In pulmonary capillary
beds, oxygen (O2) diffuses from alveoli to
blood, and carbon dioxide (CO2) diffuses
from blood to alveoli. In systemic capillary
beds, O2 diffuses from blood to the cells and
CO2 diffuses from cells to the blood.
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
I. Overview of Pulmonary Circulation
 The movement of O2 and CO2 between
alveolar air and blood occurs by diffusion
down concentration gradients
Copyright © 2008 Pearson
Education, Inc., publishing as
Benjamin Cummings.
 The respiratory membrane provides a large
surface area of very thin membrane,
favoring fast rates of diffusion of O2 and
CO2 between alveolar air and blood
Figure 17.2 The respiratory membrane.
The exchange of O2 and CO2 between
alveoli and blood occurs by simple diffusion
across the respiratory membrane. The
respiratory membrane is composed of a
single layer of type I epithelial cells lining the
alveolus, a single layer of endothelial cells
lining the capillary, and the alveolar and
capillary basement membranes.
II. Diffusion of Gases
Partial pressure of gases
 According to the ideal gas law 理想氣體定律, for any pure gas, PV=nRT,
where P = pressure, V = volume, n = number of moles, R = universal gas
constant, and T = temperature
 If V and T are constant, then the pressure exerted by a gas is directly
proportional to the number of moles of the gas
 A gas (air, for instance) is often a mixture of more than one type of
molecule  the total pressure of such a gas is the sum of the pressures
of individual gases that make up the mixture:
Ptotal = P1 + P2 + P3 + … Pn
n: number of gases
 Dalton’s law 道耳吞定律 states that the pressure exerted by the individual
gases occupying the same volume alone  the pressure exerted by an
individual gas in a mixture is called the partial pressure 分壓 of that gas
Partial pressure of gases
Composition of Air
 79% nitrogen (N2) + 21% oxygen (O2) + trace amounts carbon dioxide (CO2),
helium, argon, etc.
 Water (H2O) can be a factor depending on humidity 溼度
Pair = 760 mm Hg = PN2 + PO2
 PN2 = 0.79 x 760 mm Hg = 600 mm Hg
 PO2 = 0.21 x 760 mm Hg = 160 mm Hg
 Air is only 0.03% carbon dioxide  PCO2 = 0.0003 x 760 mm Hg = 0.23 mm Hg
Pair = 760 mm Hg = PN2 + PO2+ PH2O (when100% humidity)
PN2 = 0.741 x 760 mm Hg = 563 mm Hg
PO2 = 0.196 x 760 mm Hg = 149 mm Hg
PH2O = 0.062 x 760 mm Hg = 47 mm Hg
PCO2 = 0.00027 x 760 mm Hg = 0.21 mm Hg
Solubility of gases in liquids
 The ability of a gas to dissolve in water has physiological significance because
O2 and CO2 are exchanged between air in the alveoli and blood (which is
primarily water) and are then transported by the blood
 The concentration of dissolved gas molecules depends not only on the partial
pressure 分壓 but also on the solubility 溶解度 of the gas in that particular
liquid  Henry’s law 亨利定律
 Henry’s law states that when the temperature 溫度 is constant, the
concentration 濃度 of a gas in a liquid is proportional to its partial pressure 
c = kP  c is the molar concentration 莫爾濃度 of the dissolved gas
(moles/liter), P is the partial pressure 分壓 of the gas, and k is the Henry’s law
constant 常數 at a specific temperature
 Because k is a constant, if the pressure changes, the relationship between the
concentration of dissolved gas 溶解的氣體 at the initial pressure (P1) and at
the subsequent pressure (P2) can be described by c1/P1 = c2/P2
 Therefore, the concentration of a gas in plasma is directly related to the partial
pressure of the gas, and movement of a gas by diffusion occurs from areas of
high partial pressure to areas of low partial pressure
Solubility of gases in liquids
Figure 17.3 Solubilities of oxygen and
carbon dioxide in water. The beakers at
left depict the initial conditions in which
water has just been exposed to air that has
a partial pressure of gas of 100 mmHg; the
breakers at right show the conditions after
equilibration, when the gas has dissolved
in the water until the gas’s partial pressure
in both media are equal. (a) After
equilibration, the concentration of O2 in
water is much less than its concentration in
air, indicating that the solubility of O2 in
water is low. (b) After equilibration, the
concentration of CO2 in water is greater
than that of O2 in water at the same partial
pressure, indicating that CO2 is more
soluble in water than is O2.
Copyright © 2008 Pearson Education, Inc.,
publishing as Benjamin Cummings.
III. Exchange of Oxygen and Carbon Dioxide
Gas exchange in the lungs
 The exchange of O2 and CO2 between the alveoli and the blood, and between the
blood and systemic tissues, occurs by the same mechanism  each gas diffuses
down its own partial pressure gradient
 In atmospheric air, the PO2 is 160 mmHg, and the PCO2 is 0.23 mmHg  however,
the PO2 in the alveoli is only 100 mmHg, whereas the PCO2 is 40 mmHg
 Alveolar gas pressure differ from
atmospheric pressures for three reasons:
exchange of gases occurs continually
between alveolar and capillary blood
upon inspiration 吸氣, fresh
atmospheric air mixes with air rich in
CO2 are relatively poor in O2 in the
dead space of the conducting zone
air in the alveoli is saturated 飽和 with
water vapor
Gas exchange in the lungs
 The blood entering the pulmonary
capillaries is deoxygenated blood 去氧血,
with a PO2 of 40 mmHg and PCO2 of 46
mmHg  as this blood passes by the
alveoli, O2 and CO2 diffuse down their
partial pressure gradient  diffusion
results in an equilibrium  the pulmonary
veins 肺靜脈 has a a PO2 of 100 mmHg
and PCO2 of 40 mmHg
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
Figure 17.4 Partial pressure of oxygen
and carbon dioxide in atmospheric air,
alveolar air, and at various sites in the
body. O2 and CO2 diffuse down their
concentration gradients from high partial
pressures to low partial pressures. In
pulmonary capillary beds, O2 diffuses from
the alveoli to blood, and CO2 diffuses from
blood to alveoli. In cells, O2 diffuses from
blood to the cells, and CO2 diffuses from
cells to blood.
Gas exchange in the lungs
 Diffusion is a rapid process  provides a margin
of safety 安全範圍 in that gases can still equilibrate
between capillary blood and alveolar air even if
blood is flowing at a rate up to three times faster
than the resting rate, as can occur during exercise
 The diffusion rate is rapid because of the thinness
of the respiratory membrane 呼吸性的膜很薄 
whenever the respiratory membrane is effectively
thickened 變厚, gas exchange is hampered 阻礙,
such as pulmonary edema 肺水腫
Figure 17.5 Gas exchange as a function of pulmonary
capillary length. In the rapid exchange of gases
between alveolar air and pulmonary capillary blood,
equilibration of the partial pressure of (a) O2 and (b) CO2
occurs within the first 33% of a capillary’s length.
Mixed venous blood refers to the blood in the pulmonary
artery 肺動脈 that contains blood returned to the heart
from the systemic veins 體靜脈.
Copyright © 2008 Pearson
Education, Inc., publishing as
Benjamin Cummings.
Gas Exchange in Respiring Tissue
 In the interstitial fluid 間質液 surrounding
the capillaries, the PO2 is lower because
respiring cells consume O2 , and the
PCO2 is higher because the cells
produce CO2  O2 diffuses from blood
to cells; CO2 diffuses from cells to blood
 Actual amount of O2 and CO2 that is
exchanged in any given vascular bed
depends on metabolic activity of tissue
  rate of metabolism   exchange
 All venous blood returns to right atrium
右心房 and mixes together before being
pumped by the right ventricle 右心室 into
pulmonary artery 肺動脈  the blood in
the pulmonary artery is called mixed
venous blood 混合的靜脈血
 PO2 = 40 mm Hg & PCO2 = 46 mm Hg
Copyright © 2008 Pearson Education, Inc.,
publishing as Benjamin Cummings.
Determinants of Alveolar PO2 and PCO2
 Factors affecting alveolar partial pressures
 PO2 and PCO2 of inspired air
 Minute alveolar ventilation
 Rates at which respiring tissue consume O2 and produce CO2
 Most critical is rate of alveolar ventilation relative to rate of O2
consume and CO2 production
 Hyperpnea 呼吸增強 = increased ventilation due to increased
demand  minimal changes in arterial PO2 and PCO2
 Hypoventilation 換氣不足 = ventilation does not meet demands
 arterial PO2 decreases & arterial PCO2 increases
 Hyperventilation 換氣過度 = ventilation exceeds demands
 arterial PO2 increases & arterial PCO2 decreases
Table 17.2 Some term used in respiratory physiology
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
IV. Transport of Gases in Blood
Oxygen transport in blood
Transport of O2 in the blood has a special need
 the transport mechanism must be readily 容易地
reversible 可逆的  hemoglobin (Hb) 血紅素 has
a unique structure that allows O2 to do just that
Every liter of arterial blood contains about 200 ml
of O2  O2 not very soluble in plasma  thus
only 3.0 mL/200 ml arterial blood O2 dissolved in
plasma (1.5%) & other 197 mL of O2 (98.5%) is
transported by Hb 以血紅素運送
Hb contains of four subunits  4 globins 球蛋白
(2 alpha & 2 beta)  4 heme 血基質 groups that
contain iron (Fe2+) 鐵離子
O2 binds to heme group  4 heme groups per Hb
 thus each Hb can bind 4 O2
Copyright © 2005 Pearson Education, Inc.,
publishing as Benjamin Cummings.
Oxygen transport in blood
 The complex of Hb and bound O2 is
called oxyhemoglobin 氧合血紅素
& a Hb without any O2 is called
deoxyhemoglobin 去氧血紅素
 For Hb to function in O2 transport, it is
critical that it binds the O2 reversibly 
tightly enough so that it can pick up
large quantities of O2 in the lung, but not
so tightly that it cannot release the O2
into the respiring tissues later on
Figure 17.6 Transport of oxygen by
hemoglobin. (a) Formation of oxyhemoglobin.
Once O2 diffuses from alveolar air 肺泡空氣
to blood in pulmonary capillaries 肺臟微血管,
it diffuses into erythrocytes 紅血球and binds
to hemoglobin for transport 運送in the blood.
Oxygen transport in blood
 Binding of O2 to Hb follows law of mass
action 質量作用定律 : more O2  more
binds to Hb
Hb (deoxyhemoglobin) + O2  Hb.O2 (oxyhemoglobin)
 The binding 結合 or release 釋放 of O2
depends on PO2 in the fluid surrounding
Hb  high PO2 facilities the binding of
O2 with Hb, whereas low PO2 facilitates
release from Hb
Figure 17.6 Transport of oxygen by
hemoglobin. (b) Dissociation of oxygen
from hemoglobin. In systemic capillaries
體循環微血管, hemoglobin in erythrocytes
紅血球 release O2, which then diffuses
擴散 from blood into tissue cells.
Oxygen transport in blood
 When Hb is 100% saturated,
1 g Hb carries 1.34 mL of O2 &
the normal concentration of Hb
in blood is an average 150 g/L
 The O2 -carrying capacity of the
Hb in blood is about 200 mL O2
per liter of blood (1.34x150=200)
 at normal arterial PO2 of 100
mmHg, Hb is at approximately
98% of its O2-carrying capacity
Figure 17.7 Saturation of hemoglobin. (a) In arterial blood the
partial pressure of O2 is 100 mmHg and Hb is 98.5% saturated
 very little O2 is dissolved in the cytosol of the erythrocyte.
Oxygen transport in blood
 When cardiac output is 5 L/min & 200 mL O2
per liter blood  the blood supplies almost 1000
ml O2 to respiring tissue per minute
 Because respiring tissues need
only about 250ml of O2 per minute
 only 25% of the O2 diffuses into
respiring cells, while means that
75% of the binding sites on Hb are
still occupied (75% saturated)
Figure 17.7 Saturation of
hemoglobin. (b) In mixed venous
blood the partial pressure of O2 is
40 mmHg and Hb is 75% saturated
 three of every four binding sites
are occupied by O2.
 Anemia 貧血 is a decrease in the
O2 carrying capacity of blood 
tissue may not be supplied with O2
they need  tire more easily
The hemoglobin-oxygen dissociation curve
Figure 17.8 Hemoglobin-oxygen
dissociation curve. The binding of O2
to Hb depends on the partial pressure of
O2 in blood. At low partial pressures,
little O2 is bound to Hb. As PO2 increase,
binding increases and then levels off as
saturation approaches 100%.
The binding of O2 to Hb is non-linear
非線性 S-shaped (sigmoidal) 乙狀
The binding of one O2 to Hb
increases the affinity 增加親和力 of
the Hb for O2 and therefore increases
the likelihood that another O2 will bind
with Hb  positive cooperatively
Copyright © 2005 Pearson Education, Inc., publishing as Benjamin Cummings.
Other factors affecting the affinity of hemoglobin for oxygen
Temperature, pH, PCO2, 2,3-DPG
 At least four other factors (temperature, pH, PCO2, and 2,3-DPG) affect the
affinity of Hb for O2
 The first three factors ─ temperature, pH, and PCO2─all work to promote O2
unloading 卸下 from Hb in respiring tissues and O2 loading 裝載 in the lungs,
both of which increase the efficiency of O2 exchange at and transport to the
tissues that need it
 2,3-DPG (2,3-diphosphoglycerate) is produced in erythrocytes from an
intermediate compound 中間物 in anaerobic 無氧呼吸 glycolysis 醣解作用
under conditions of low O2 such as anemia 貧血 and high altitude 高海拔
 Synthesis of 2,3-DPG is inhibited by oxyhemoglobin 氧合血紅素 & 2,3-DPG
decreases affinity 降低親和力 of Hb for O2 enhancing O2 unloading 卸下
 Another factor that affects the binding of O2 to Hb is carbon monoxide (CO)
一氧化碳  CO is toxic because when present it binds to Hb more readily
than O2, which prevents O2 from binding and decreases the O2 -carrying
capacity of blood
Other factors affecting the affinity of hemoglobin for oxygen
Changes of affinity
 Rightward shift 右移 (親和力降低)
 Higher PO2 necessary
to saturate Hb (harder loading)
 Easier unloading of O2 較容易卸下
 Leftward shift 左移 (親和力增加)
 Easier loading 容易裝載
 Harder unloading 不容易卸下
Figure 17.9 Effects of changes in the affinity of hemoglobin for oxygen
on the hemoglobin-oxygen dissociation curve. An increase in affinity of
Hb for O2 causes a leftward shift 左移 in the curve, whereas a decrease in
affinity of Hb for O2 causes a rightward shift 右移 in the curve.
Other factors affecting the affinity of hemoglobin for oxygen
 Temperature 溫度 affects the affinity
of Hb for O2 by altering the structure
of the Hb molecule
 As metabolic of tissues increases,
temperature increases  decrease
Hb affinity for O2  increases O2
unloading in tissue
Figure 17.10 Effects of temperature and pH on the hemoglobin-oxygen
dissociation curve. (a) Increases or decreases in temperature 溫度 from
normal body temperature (370C) cause decreases or increases, respectively,
in the affinity of Hb for O2.
Other factors affecting the affinity of hemoglobin for oxygen
 The effect of pH on Hb-O2 dissociation
curve is known as the Bohr effect
伯爾氏效應  when O2 binds to Hb,
certain amino acids in the protein release
hydrogen ions (H+)
Hb + O2 
Law of
mass action
 Active tissue produces H+ (a decrease
in pH )  push the reaction to left 
decreases affinity (rightward shift) 
increases O2 unloading
Figure 17.10 Effects of temperature and pH on the hemoglobin-oxygen
dissociation curve. (b) Increases or decreases in pH 酸鹼值 from normal
arterial pH (7.4) cause increases or decreases, respectively, in the affinity of
Hb for O2.
Other factors affecting the affinity of hemoglobin for oxygen
 The PCO2 affects the affinity of Hb for O2 because carbon dioxide
(CO2) reacts reversibly with certain amino groups in the Hb to form
carbaminohemoglobin (HbCO2) 碳醯胺基血紅素
Hb + CO2  HbCO2
 Increased metabolic activity  increases CO2 ( PCO2) pushes
the reaction to the right  HbCO2 changes Hb’s conformation and
decreases its affinity for O2  increases O2 unloading in active
tissue carbamino effect 碳醯胺基效應
Carbon dioxide transport in the blood
 Of all the CO2 in the blood, 5-6% is dissolved, 5-8% is bound to Hb as HbCO2,
and 86-90% is dissolved in the blood as bicarbonate ions (HCO3-) 重碳酸鹽
 HCO3- is formed from CO2 within erythrocytes (RBC) 紅血球 which contain
carbonic anhydrase (CA) 碳酸酐脫水酶, an enzyme catalyzes the reversible
reaction that converts CO2 and water (H2O) to carbonic acid (H2CO3) 碳酸
 H2CO3 continues with reversible dissociation of bicarbonate ion (HCO3-)
CO2 + H2O
H+ + HCO3-
Law of Mass Action 質量作用定律
 an increase in CO2 causes an
increase in HCO3- and H+
 In addition to being important in the transport and exchange of CO2, this reaction
is also important in acid-base balance   PCO2,  acidity of the blood
Carbon dioxide exchange and transport in systemic
capillaries and veins
 The CO2 produced in respiring cells
diffuses, based on its partial pressure
gradient, first into interstitial fluid,
plasma and then into the RBCs
 Although some CO2 remains dissolved
in the blood, and some binds to Hb,
most of the CO2 is converted to HCO3and H+ by actions of carbonic
anhydrase in the RBCs
 As HCO3- levels in RBC increases,
HCO3- are transported into the plasma
in exchange to chloride ions (Cl-) via a
transport protein in the RBC plasma
membrane  H+ is buffered by binding
to Hb
 The couple movement of Cl- into the
RBC and HCO3- into the plasma is
called chloride shift 氯移轉
Figure 17.11 Carbon dioxide
P492-493 exchange and transport in blood.
Carbon dioxide exchange and transport in pulmonary
capillaries and veins
 In the lungs, CO2 diffuses down
its pressure gradient from blood
to alveolar air to be exhaled 
decreasing the PCO2 in blood
 As PCO2 in the RBC decreases,
HCO3- enters the RBCs coupled
with Cl- enters the plasma, and
H+ are released from Hb
 The HCO3 and H+ are then
converted by carbonic
anhydrase (CA) to CO2 , which
diffuses into the alveoli to be
Figure 17.11 Carbon dioxide
exchange and transport in blood.
Effect of oxygen on carbon dioxide transport
 Increased PO2 in blood decreases the affinity of Hb for CO2, which
decreases the ability of the blood to transport CO2
 At a given PCO2 (point A), the CO2 content of the blood falls as the PO2 rises
(compared points B and C), a phenomenon known as the Haldane effect
 In respiring tissues, where PO2 is low and
PCO2 is high, the Haldane effect
promotes the loading of CO2 onto Hb
while both the Bohr effect (effect of pH
on Hb’s affinity for O2) and the carbamino
effect (effect of CO2 levels on Hb’s affinity
for O2) work to promote O2 unloading
 In the lungs, where PO2 is high and PCO2
is low, the Haldane effect promotes
unloading of CO2 while the Bohr effect
and the carbamino effect promote O2
Figure 17.12 Effect of PO2
on carbon dioxide
Summary of oxygen and carbon dioxide
loading/unloading in tissue & in lungs
Figure 17.12 Effects of PO2 and PCO2 on carbon dioxide and oxygen loading
and unloading transport. (a) In systemic tissues, as PO2 decreases and PCO2
increases, CO2 loading and O2 unloading occur. (b) In the lungs, as PO2 increases
and PCO2 decreases, CO2 unloading and O2 loading occur.
V. Central regulation of ventilation
 The job of the respiratory system is to deliver O2 and remove
CO2 from cells at a rate sufficient to keep up with metabolic
 The signals that indicate whether the respiratory system is
doing this job adequately are the partial pressures of O2 and
CO2 in systemic arterial blood
 To maintain normal arterial partial pressures, the body must
regulate minute alveolar ventilation (VA) 每分鐘肺泡的換氣量
 alveolar ventilation depends on the frequency (RR 呼吸速率)
and volume of breaths (VT 潮氣容積)
Neural control of breathing by motor
 Breathing 呼吸 is a cycle of inspiration 吸氣 followed by
expiration 呼氣  inspiration is an active process that
requires contraction of the inspiratory muscles 吸氣肌,
and expiration is a passive process in which no muscle
contraction is required
 Because the muscles of respiration are skeletal
muscles 骨骼肌, they are stimulated to contract by neural
input from somatic motor neuron 體運動神經  the
phrenic nerve 橫膈神經 innervates 支配 the diaphragm
橫隔, whereas the internal and external intercostal
nerves 內肋間肌及外肋間肌神經 innervates the
intercostal muscles 肋間肌
Neural control of breathing by motor neurons
 Ventilation involves cyclical
changes in neural stimulation of
respiratory muscles, which cause
cyclical changes in lung volume
 In quiet breathing 平靜呼吸,
expiration is a passive process,
and thus no neural or muscle
activity of the expiratory muscles
is present
 During active ventilation 主動換氣,
inspiratory neurons and muscles
become more active, and
expiratory neurons and muscles
become active
Figure 17.14 A comparison of quiet breathing and active ventilation. P494-495
Generation of breathing rhythm in the brainstem
 Breathing is under both voluntary 隨意
and involuntary 不隨意 control  central
control of respiration is not fully understood,
but research indicates that respiratory
control regions 呼吸控制區 are present in
the medulla 延腦 and pons 橋腦 of the
brainstem 腦幹
 Two respiratory control centers are
located on each side of the medulla 
a ventrally located ventral respiratory
group (VRG) 腹側呼吸群 and a more
dorsally located dorsal respiratory group
(DRG) 背側呼吸群
 The respiratory center of the pons 橋腦,
called the pontine respiratory group 橋腦
呼吸群 (PRG; pneumotaxic center 呼吸
調節中心)  may facilitate the transition
轉換 between in inspiration and expiration
Copyright © 2008Pearson Education, Inc.,
publishing as Benjamin Cummings.
Figure 17.15 Brainstem
centers of respiratory
Generation of breathing rhythm in the brainstem
 The central pattern generator (CPG)
is a network of neurons that
generates a regular, repeating
pattern of neural activity called the
respiratory rhythm 呼吸節律
 The location of the CPG and its
mechanism of action are unknown
 one hypothesis suggests that certain
neurons in the CPG have pacemaker
節拍器 activity
 The figure shows a simplified model for
quiet breathing in which the breathing
rhythm is produced by the CPG
Figure 17.16 Activity of inspiratory neurons. Inspiratory neurons show a
ramp 傾斜 increase in the frequency of action potentials during inspiration,
followed by a sudden termination of all activity at the end of inspiration and
beginning of expiration.
 In this model, respiratory control areas of the medulla are primarily
responsible for controlling breathing
 But breathing is also affected by activity in other brain regions, including the
pons, cerebral cortex 大腦皮質, cerebellum 小腦, limbic system 邊緣系統,
hypothalamus 下視丘, and medullary cardiovascular regulatory areas
 Several types of sensory input 感學訊息傳入 can alter respiration,
presumably through indirect communication with the CPG
Figure 17.17 Model of respiratory control during quiet breathing.
Peripheral Input to Respiratory Centers
 Several types of sensory input can alter respiration  particularly important
are signals from central and peripheral chemoreceptors 中樞及週邊的化學
接受器 (chemically sensitive receptor cells)
 Additional sensory inputs that affect breathing come from a variety of receptors
— Pulmonary stretch receptors 肺臟的牽張接受器 in the smooth muscle of
pulmonary airways  are excited by inflation 膨脹 of the lung and do not
appear to play a significant role in regulating breathing in humans
— Irritant receptors 刺激接受器 in the lining of the respiratory tract  are
stimulated by inhaled particulates such as smoke or dust  triggers
coughing 咳嗽 (irritant receptors in trachea) and sneezing 打噴嚏 (irritant
receptors in nose)
— Propriceptors 本體接受器 in muscles and joints which detect movement of
the body  plays a role in stimulating the increase in ventilation that occurs
during exercise
— Arterial baroreceptors 動脈的感壓接受器 which detect changes in blood
— Nociceptors 傷痛接受器 and thermoreceptors 體溫接受器 located throughout
the body
VI. Control of Ventilation by
 Changes in chemical concentrations in the blood are detected by
chemoreceptors located in major arteries and in the brain, which relay
signals to the respiratory control center via afferent neurons 感覺神經
 Chemoreceptors detect blood levels of O2 and CO2  relay 轉播 this
information to the respiratory control centers
 Chemoreceptors are classified as either peripheral or central,
depending on their location
 Peripheral chemoreceptors 週邊化學接受器 are located in the carotid
bodies 頸動脈體 near the carotid sinus 頸動脈竇
 The central chemoreceptors 中樞化學接受器 are located in the
medulla oblongata 延腦
Peripheral chemoreceptors
 Peripheral and central chemoreceptors
differ not only in their location but also in
their structures and chemical sensitivities
 Peripheral chemoreceptors are
– specialized chemically sensitive cells that
are in direct contact with arterial blood
– communicate with afferent neurons
projecting to medullary respiratory
control regions
– respond to changes in PO2, PCO2or pH
(which changes when PCO2 changes)
Figure 17.18 Location of peripheral
chemoreceptors in the carotid bodies. Afferents
from the chemoreceptors ascend to the medulla, but
not directly to the respiratory centers.
Copyright © 2008 Pearson
Education, Inc., publishing as
Benjamin Cummings.
Peripheral chemoreceptors
 Decreases in PO2 can directly activate
peripheral chemoreceptors, but only
when the PO2 drops to less than 60 mmHg
 O2 is usually not a primary factor in peripheral
chemoreceptor activation
 In fact, changes in H+ concentration are the
primary stimulus for peripheral chemoreceptors
 the main source of H+ is the reaction of CO2
with H2O  PCO2 can also activate peripheral
chemoreceptors, but most indirectly
Figure 17.19 Respiratory control by chemoreceptors.
(a) Declining arterial PO2 has little effect on minute
ventilation until the PO2 drops to less than 60 mmHg.
(b) Increasing arterial PCO2 has large effects on minute
ventilation as PCO2 increases above or decreases below
normal. At PCO2 greater than 90 mmHg, coma 昏迷 and
then death can occur.
Central chemoreceptors
 Central chemoreceptors are
neurons in the medulla that
respond directly to changes
in H+ concentration in the CSF
腦脊髓液 surrounding this area
 H+ cannot cross the blood-brain
barrier (BBB) 血腦障壁, but CO2
can  CO2 does not effect the
central chemoreceptors
directly, but instead is converted
to H+ and HCO3- by carbonic
anhydrase (CA) in the CSF
 Central chemoreceptors, unlike
the peripheral chemoreceptors,
are not sensitive to changes in
Respond to [H+] in CSF
[H+] depends on [CO2]
Not directly responsive
to [CO2]
Not responsive to [O2]
Figure 17.20 Activation of central
chemoreceptors in the medulla
oblongata. Central chemoreceptors
respond best to changes in pH in the
cerebrospinal fluid (CSF). However,
hydrogen ions (H+) cannot cross the
blood-brain barrier. Instead, CO2 in the
blood diffuses into the CSF, where
carbonic anhydrase (CA) catalyzes the
conversion of CO2 and H2O to carbonic
acid (H2CO3), which dissociates to
bicarbonate (HCO3-) and H+. The H+ can
then activate the central chemoreceptors.
Chemoreceptor reflex
 A decrease in arterial PO2 to less than
60 mmHg activates the peripheral
chemoreceptors but has no effect on
the central chemoreceptors
 An increase in arterial PCO2 activates
the peripheral chemoreceptors
directly, and both the peripheral and
central chemoreceptors after CO2 is
converted to H+ and HCO3 A decrease in arterial blood pH (from
CO2 or lactic acid produced by
cellular metabolism) activates the
peripheral chemoreceptors
 When activate, chemoreceptors
stimulate an increase in ventilation,
which provides negative feedback to
the initial stimulus
Figure 17.21 Chemoreceptor reflex:
the effects of changes in arterial PO2,
PCO2, and pH on ventilation.
Chemoreceptor reflex
Figure 17.22 The effects of hypoventilation
and hyperventilation on minute ventilation.
VII. Local Regulation of Ventilation and
Ventilation-perfusion ratios
 In the normal lung, the rate of air
flow to the alveoli (ventilation, VA)
is matched to the rate of blood flow
(perfusion, Q); the dots over the
abbreviations indicate that these are
 The relationship of ventilation to
perfusion is called the ventilationperfusion ratio and is abbreviated
VA/Q  in the normal lung, VA/Q is
approximately 1  the PO2 and PCO2
of the alveoli are at the normal
values of 100 and 40 mmHg
Figure 17.23 Ventilationperfusion ratio.
Ventilation-perfusion ratios
 When airways are obstructed, for example, VA in certain alveoli
decreases, and the blood traveling in the capillaries to those alveoli does
not undergo adequate gas exchange  VA/Q<1
 When pulmonary capillaries are damaged, perfusion is obstructed,
causing a decrease in Q  VA/Q>1
Figure 17.23 Ventilation-perfusion ratio. (b) If ventilation decreases and
perfusion is normal, then VA/Q<1  arterial PO2 , PCO2 . (c) If perfusion
decreases and ventilation is normal, then VA/Q>1  arterial PO2 , PCO2 .
Local control of ventilation and perfusion
 O2 acts primarily on the pulmonary arterioles 肺臟小動脈 a low PO2
causes a vasoconstriction 血管收縮 ( Q)
 CO2 acts primarily on the bronchioles 細支氣管  a high PCO2 causes
bronchodilation 支氣管擴張 ( VA)
• If ventilation to certain alveoli decreases:
 PCO2 and  PO2 in blood and air
 PCO2 in bronchioles  bronchodilation
 PO2 in P. Arterioles  vasoconstriction
• If perfusion to certain alveoli decreases:
 PO2 and  PCO2 in blood and air
 PO2 in P. Arterioles  vasodilation
 PCO2 in bronchioles  bronchoconstriction
Factors Affecting Air Flow & Perfusion of Individual Alveoli
Copyright © 2005 Pearson
Education, Inc., publishing as
Benjamin Cummings.
VIII. The Respiratory System in
Acid-Base Homeostasis
Acid-base disturbances in blood
 Although the primary function of the respiratory system is to
control the O2 and CO2 content of arterial blood, it also plays an
important role in regulating the blood’s pH
 Blood pH is tightly regulated by both respiratory system and
kidneys  the normal pH range of arterial blood is 7.38~7.42
 A change in pH alters the distribution of electrical charge in
protein  alters protein’s shape and interferes their normal
– acidosis 酸中毒 = blood pH < 7.35  CNS depression
– alkalosis 鹼中毒 = blood pH > 7.45  CNS over-excitation
The role of the respiratory system in
acid-base balance
 Hb is a buffer緩衝 because it can bind or release H+
Hb + O2
Hb + H+
 This is important because the tissues are producing CO2, which is quickly
converted to HCO3- and H+  some of these H+ can be buffered by Hb
 Bicarbonate (HCO3-) is another major buffering system in blood  if the
H+ concentration increases, H+ bind to HCO3- to form CO2  if CO2 levels in
blood are allowed to increase, acidosis will result
HCO3- + H+
 The relationship between CO2 and
acidity can be described using
Henderson-Hasselbalch equation
 to maintain normal arterial pH =
7.4  [HCO3-] : [CO2] must be 20:1
CO2 + H2O
pH = 6.1 + log
The role of the respiratory system in
acid-base balance
 The lungs regulate the concentration of CO2, whereas the kidneys regulate
the concentration of HCO3-
 Respiratory acidosis 呼吸性酸中毒 is an increase in the acidity of the
blood due to increased CO2, which occurs, for example, during
 Respiratory alkalosis 呼吸性鹼中毒 is decrease in the acidity of the blood
due to decreased CO2, which occurs, for example, during hyperventilation
or at high altitudes 高海拔
• Henderson-Hasselbalch equation, which is based on the equilibrium for the
dissociation of an acid (HA) into a free hydrogen ion (H+) and a base (A-)
H+ + A-
[H+ ][A-]
[H+ ]=
pH = pK + log
Copyright © 2005 Pearson
Education, Inc., publishing as
Benjamin Cummings.