The Respiratory Systemmmsalemscienceteacher.weebly.com/uploads/2/3/3/6/... · 2018. 9. 9. · Gas Exchange •Gas exchange is driven by differences in partial pressures •Blood returning

The Respiratory System

1

Gas Exchange

• One of the major physiological challenges facing

all multicellular animals is obtaining sufficient

oxygen and disposing of excess carbon dioxide

• In vertebrates, the gases diffuse into the

aqueous layer covering the epithelial cells that

line the respiratory organs

• Diffusion is passive, driven only by the difference

in O2 and CO2 concentrations on the two sides

of the membranes and their relative solubilities

in the plasma membrane

2

Lungs

• Lungs of mammals are packed with

millions of alveoli (sites of gas exchange)

• Inhaled air passes through the larynx,

glottis, and trachea

• Bifurcates into the right and left bronchi,

which enter each lung and further

subdivide into bronchioles

• Alveoli are surrounded by an extensive

capillary network3

4

Lungs

Nasal cavity

Nostril

Larynx

Right lung Left lung

PharynxGlottis

Diaphragm

Pulmonary venule

Pulmonary arteriole

Blood flowBronchiole

Alveoli

Smooth muscle

Trachea

Left

bronchus

Capillary

network on

surface

of alveoli

Alveolar

sac

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Gas Exchange

• Gas exchange is driven by differences in partial

pressures

• Blood returning from the systemic circulation,

depleted in oxygen, has a partial oxygen

pressure (PO2) of about 40 mm Hg

• By contrast, the PO2 in the alveoli is about

105 mm Hg

• The blood leaving the lungs, as a result of this

gas exchange, normally contains a PO2 of about

100 mm5

6


Lung

Systemic arteriesSystemic veins

Peripheral tissues

Peripheral tissues

CO2

O2

Pulmonary

vein

CO2 O2

Pulmonary

artery

O2

CO2

O2CO2Alveolar gas

P = 105 mm Hg

P = 40 mm Hg

O2

CO2

P = 40 mm Hg

P = 46 mm Hg

O2

CO2

P = 100 mm Hg

P = 40 mm Hg

O2

CO2

P = 40 mm Hg

P = 46 mm Hg

O2

CO2

P = 100 mm Hg

P = 40 mm Hg

O2

CO2

Alveolar gas

P = 105 mm Hg

P = 40 mm Hg

O2

CO2

Lung Structure and Function

• Outside of each lung is covered by the

visceral pleural membrane

• Inner wall of the thoracic cavity is lined by

the parietal pleural membrane

• Space between the two membranes is

called the pleural cavity

– Normally very small and filled with fluid

– Causes 2 membranes to adhere

– Lungs move with thoracic cavity7


• During inhalation, thoracic volume

increases through contraction of two

muscle sets

– Contraction of the external intercostal

muscles expands the rib cage

– Contraction of the diaphragm expands the

volume of thorax and lungs

• Produces negative pressure which draws

air into the lungs

8


• Thorax and lungs have a degree of

elasticity

• Expansion during inhalation puts these

structures under elastic tension

• Tension is released by the relaxation of

the external intercostal muscles and

diaphragm

• This produces unforced exhalation,

allowing thorax and lungs to recoil9

10


a.

Inhalation

Air

Lungs

Sternocleidomastoid

muscles contract

(for forced inhalation)

Muscles

contract

Diaphragm

contracts

Exhalation

b.

Air

Diaphragm

relaxes

Abdominal muscles

contract (for forced

exhalation)

Muscles

relax


• Tidal volume

– Volume of air moving in and out of lungs in a person

at rest

• Vital capacity– Maximum amount of air that can be expired after a

forceful inspiration

• Hypoventilation– Insufficient breathing

– Blood has abnormally high PCO2

• Hyperventilation– Excessive breathing

– Blood has abnormally low PCO212


• Each breath is initiated by neurons in a

respiratory control center in the medulla

oblongata

• Stimulate external intercostal muscles and

diaphragm to contract, causing inhalation

• When neurons stop producing impulses,

respiratory muscles relax, and exhalation occurs

• Muscles of breathing usually controlled

automatically

– Can be voluntarily overridden – hold your breath13


• Neurons are sensitive to blood PCO2 changes

• A rise in PCO2 causes increased production of

carbonic acid (H2CO3), lowering the blood pH

• Stimulates chemosensitive neurons in the aortic

and carotid bodies

• Send impulses to respiratory control center to

increase rate of breathing

• Brain also contains central chemoreceptors that

are sensitive to changes in the pH of

cerebrospinal fluid (CSF)14

15

Reduced HCO3− levels (and

corresponding drop in CSF pH) result

in increased respiration, which

subsequently results in lower arterial

PCO2.

Medulla

oblongataSignal to

respiratory

system

Chemosensitive

neuron

Cerebrospinal

fluid (CSF)

H+ + HCO3–

H2CO3

H2O + CO2

CO2

Capillary

blood

Choroid

plexus of

brain

a.

Stimulus Stimulus

Sensor

Comparator Comparator

Response

Effector

b.

Sensor

( – )

( + )

Impulses sent to

respiratory control center

in medulla oblongata

Diaphragm stimulated

to increase breathing

Central chemoreceptors

stimulated (in the brain)Peripheral chemoreceptors stimulated

(aortic and carotid bodies)

H2O + CO2 H2CO3 H+ + HCO3

–

Decreased

CSF pH

Increased blood CO2concentration (PCO2)

Increased tissue

Metabolism

(i.e., muscle contraction)

Inadequate

breathing

Negative

feedbackStimulus

Decreased blood pH


Respiratory Diseases

• Chronic obstructive pulmonary disease

(COPD)

– Refers to any disorder that obstructs airflow

on a long-term basis

– Asthma

• Allergen triggers the release of histamine, causing

intense constriction of the bronchi and sometimes

suffocation

16


• Chronic obstructive pulmonary disease

(COPD) (cont.)

– Emphysema

• Alveolar walls break down and the lung exhibits

larger but fewer alveoli

• Lungs become less elastic

• People with emphysema become exhausted

because they expend three to four times the

normal amount of energy just to breathe

• Eighty to 90% of emphysema deaths are caused

by cigarette smoking17


• Lung cancer accounts for more deaths than any

other form of cancer

• Caused mainly by cigarette smoking

• Follows or accompanies COPD

• Lung cancer metastasizes (spreads) so rapidly

that it has usually invaded other organs by the

time it is diagnosed

• Chance of recovery from metastasized lung

cancer is poor, with only 3% of patients surviving

for 5 years after diagnosis18

19


a: © Clark Overton/Phototake; b: © Martin Rotker/Phototake

Healthy Lungs Cancerous Lungs

Hemoglobin

• Consists of four polypeptide chains: two a and

two b

• Each chain is associated with a heme group

• Each heme group has a central iron atom that

can bind a molecule of O2

• Hemoglobin loads up with oxygen in the lungs,

forming oxyhemoglobin

• Some molecules lose O2 as blood passes

through capillaries, forming deoxyhemoglobin

20

21

The structure of the adult hemoglobin protein

Hemoglobin

• At a blood PO2 of 100 mm Hg, hemoglobin is

97% saturated

• In a person at rest, the blood that returns to the

lungs has a PO2 about 40 mm Hg less

• Leaves four-fifths of the oxygen in the blood as a

reserve

• This reserve enables the blood to supply body’s

oxygen needs during exertion

• Oxyhemoglobin dissociation curve is a graphic

representation of these changes22

23

Hemoglobin

Oxyhemoglobin dissociation curve


Perc

en

t satu

rati

on

0

20

40

60

80

Arteries

100

0 20 40 60 80 100

Amount of O2 unloaded

to tissues during exercise

Amount of O2 unloaded

to tissues at rest

PO2 (mm Hg)

Veins

(exercised)

Veins

(at rest)

Hemoglobin

• Hemoglobin’s affinity for O2 is affected by

pH and temperature

• The pH effect is known as the Bohr shift

– Increased CO2 in blood increases H+

– Lower pH reduces hemoglobin’s affinity for O2

– Results in a shift of oxyhemoglobin

dissociation curve to the right

– Facilitates oxygen unloading

• Increasing temperature has a similar effect24

25

Hemoglobin

The effect of pH and temperature on the oxyhemoglobin dissociation curve


Perc

en

t o

xyh

em

og

lob

in s

atu

rati

on

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140

Perc

en

t o

xyh

em

og

lob

in s

atu

rati

on

0

10

20

30

40

50

60

70

80

90

100

0 20 40 60 80 100 120 140

pH 7.60

pH 7.20pH 7.40

20% more O2 delivered to the

tissues at the same pressure

b. Temperature shifta. pH shift

20% more O2 delivered to the

tissues at the same pressure

PO2 (mm Hg) PO2 (mm Hg)

43°C

20°C

37°C

Transportation of Carbon Dioxide

• About 8% of the CO2 in blood is dissolved in

plasma

• 20% of the CO2 in blood is bound to hemoglobin

• Remaining 72% diffuses into red blood cells

– Enzyme carbonic anhydrase combines CO2 with H2O

to form H2CO3

– H2CO3 dissociates into H+ and HCO3

–

– H+ binds to deoxyhemoglobin

– HCO3– moves out of the blood and into plasma

– One Cl– exchanged for one HCO3– – “chloride shift”

26

27


a.

b.

CapillaryErythrocyteAlveolar

epithelium

Nucleus of

alveolar cell

Nucleus of capillary

endothelial cell

Capillary

endothelium

Alveoli CO2

Hemoglobin

+ CO2

CO2 dissolved

in plasma

H2CO3

H2CO3

CO2 + H2O

HCO3– + H+

HCO3–

Cl–

H2CO3

H2CO3 H+ + HCO3–

H+ combines

with hemoglobin

Cl– HCO3–

(72%)

CO2 dissolved

in plasma (8%)

CO2 combines with

hemoglobin (20%)

CO2

Tissue cells

Capillary ErythrocyteCapillary

endothelium

Nucleus of capillary

endothelial cell

CO2 + H2O

Transportation of Carbon Dioxide

• When the blood passes through

pulmonary capillaries, these reactions are

reversed

• The result is the production of CO2 gas,

which is exhaled

• Other dissolved gases are also

transported by hemoglobin

– Nitric oxide (NO) and carbon monoxide (CO)

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The Respiratory Systemmmsalemscienceteacher.weebly.com/uploads/2/3/3/6/... · 2018. 9. 9. · Gas Exchange •Gas exchange is driven by differences in partial pressures •Blood returning