THE RESPIRATORY SYSTEM The primary function of the respiratory
system is to allow oxygen from the air to enter the blood and
carbon dioxide from the blood to exit into the air. During
inspiration, or inhalation (breathing in), and expiration, or
exhalation (breathing out), air is conducted toward or away from
the lungs by a series of cavities, tubes, and openings
Slide 3
The respiratory tract extends from the nasal cavities to the
lungs, which are composed of air sacs called alveoli. Gas exchange
occurs between the air in the alveoli and the blood within a
capillary network that surrounds the alveoli. Notice in the blow-up
that the pulmonary arteriole is colored blueit carries O 2 - poor
blood away from the heart to the alveoli. Then carbon dioxide
leaves the blood, and oxygen enters the blood. The pulmonary venule
is colored redit carries O 2 -rich blood from the alveoli toward
the heart.
Slide 4
The respiratory system also works with the cardiovascular
system to accomplish these four respiratory events: 1. breathing,
the entrance and exit of air into and out of lungs; 2. external
respiration, the exchange of gases (oxygen and carbon dioxide)
between air and blood; 3. internal respiration, the exchange of
gases between blood and tissue fluid; 4. transport of gases to and
from the lungs and the tissues.
Slide 5
Cellular respiration, which produces ATP, uses the oxygen and
produces the carbon dioxide that makes gas exchange with the
environment necessary. Without a continuous supply of ATP, the
cells cease to function. The four events listed here allow cellular
respiration to continue.
THE NOSE The nose, a prominent feature of the face, is the only
external portion of the respiratory system. Air enters the nose
through external openings called nostrils. The nose contains two
nasal cavities, which are narrow canals separated from one another
by a septum composed of bone and cartilage Mucous membrane lines
the nasal cavities.
Slide 10
The nasal conchae are bony ridges that project laterally into
the nasal cavity. They increase the surface area for moistening and
warming air during inhalation and for trapping water droplets
during exhalation. Odor receptors are on the cilia of cells located
high in the recesses of the nasal cavities.
Slide 11
THE PHARYNX The pharynx is a funnel-shaped passageway that
connects the nasal and oral cavities to the larynx. Consequently,
the pharynx, commonly referred to as the throat, has three parts:
the nasopharynx, where the nasal cavities open posterior to the
soft palate the oropharynx, where the oral cavity joins the
pharynx; the laryngopharynx, which opens into the larynx.
Slide 12
The soft palate has a soft extension called the uvula that can
be seen projecting into the oropharynx. The tonsils form a
protective ring at the junction of the oral cavity and the pharynx.
The tonsils contain lymphocytes that protect against invasion of
inhaled pathogens. In this way, the respiratory tract assists the
immune system in maintaining homeostasis. In the pharynx, the air
passage and the food passage cross because the larynx, which
receives air, is anterior to the esophagus, which receives food.
The larynx lies at the top of the trachea. The larynx and trachea
are normally open, allowing air to pass, but the esophagus is
normally closed and opens only when a person swallows.
Slide 13
THE LARYNX The larynx is a cartilaginous structure that serves
as a passage- way for air between the pharynx and the trachea. The
larynx is called the voice box because it houses the vocal cords.
The vocal cords are mucosal folds supported by elastic ligaments,
and the slit between the vocal cords is an opening called the
glottis When air is expelled past the vocal cords through the
glottis, the vocal cords vibrate, producing sound. When food is
swallowed, the larynx moves upward against the epiglottis, a flap
of tissue that prevents food from passing through the glottis into
the larynx.
Slide 14
THE TRACHEA commonly called the windpipe, is a tube connecting
the larynx to the primary bronchi. The trachea lies ventral to the
esophagus and is held open by C-shaped cartilaginous rings. The
open part of the C-shaped rings faces the esophagus, and this
allows the esophagus to expand when swallowing. The mucosa that
lines the trachea has a layer of pseudostratified ciliated columnar
epithelium. The cilia that project from the epithelium keep the
lungs clean by sweeping mucus, produced by goblet cells, and debris
toward the pharynx:
Slide 15
THE BRONCHIAL TREE The trachea divides into right and left
primary bronchi which lead into the right and left lungs. The
bronchi branch into a great number of secondary bronchi that
eventually lead to bronchioles. The bronchi resemble the trachea in
structure, but as the bronchial tubes divide and subdivide, their
walls become thinner, and the small rings of cartilage are no
longer present. Each bronchiole leads to an elongated space
enclosed by a multitude of air pockets, or sacs, called alveoli.
The components of the bronchial tree beyond the primary bronchi,
including the alveoli, compose the lungs.
Slide 16
Slide 17
THE LUNGS The lungs are paired, cone-shaped organs that occupy
the thoracic cavity. The right lung has three lobes, and the left
lung has two lobes. A lobe is further divided into lobules, and
each lobule has a bronchiole serving many alveoli.
Slide 18
Slide 19
THE ALVEOLI With each inhalation, air passes by way of the
bronchial tree to the alveoli. An alveolar sac is made up of simple
squamous epithelium surrounded by blood capillaries. Gas exchange
occurs between the air in the alveoli and the blood in the
capillaries. Oxygen diffuses across the alveolar and capillary
walls to enter the bloodstream, while carbon dioxide diffuses from
the blood across these walls to enter the alveoli. The alveoli must
stay open to receive the inhaled air if gas exchange is to occur.
Gas exchange takes place across moist cellular membranes,
Slide 20
Gas exchange in the lungs. The lungs consist of portions of the
bronchial tree leading to the alveoli, each of which is surrounded
by an extensive capillary network. Notice that the pulmonary artery
and arteriole carry O2-poor blood (colored blue), and the pulmonary
vein and venule carry O2-rich blood (colored red).
Slide 21
MECHANISM OF BREATHING During breathing, air moves into the
lungs from the nose or mouth (called inspiration, or inhalation),
and then moves out of the lungs during expiration, or exhalation. A
free flow of air from the nose or mouth to the lungs and from the
lungs to the nose or mouth is vitally important.
Slide 22
Slide 23
Slide 24
Slide 25
Internal respiration refers to the exchange of gases in the
tissues. Specifically, during internal respiration, gases are
exchanged between the blood in systemic capillaries and the tissue
fluid. Blood that enters the systemic capillaries is a bright red
color because the blood is O2-rich. Tissue fluid, on the other
hand, has a low concentration of O2. Why? Because the cells are
continually consuming O2 during cellular respiration. Therefore, O2
diffuses from the blood into the tissue fluid. Tissue fluid has a
higher concentration of CO2 than does the blood entering the
tissues. Why? Because CO2 is an end product of cellular
respiration. Therefore, CO2 diffuses from the tissue fluid into the
blood.
Slide 26
External and internal respiration. During external respiration
in the lungs, CO2 leaves the blood and O2 enters the blood
passively by diffusion. During internal respiration in the tissues,
O2 leaves the blood and CO2 enters the blood passively by
diffusion.
Slide 27
Slide 28
External Respiration External respiration is the exchange of
gases in the lungs. Specifically, during external respiration,
gases are exchanged between the air in the alveoli and the blood in
the pulmonary capillaries. Blood that enters the pulmonary
capillaries is dark maroon because it is relatively O 2 -poor. Once
inspiration has occurred, the alveoli have a higher concentration
of O 2 than does blood entering the lungs. Therefore, O 2 diffuses
from the alveoli into the blood. The reverse is true of CO 2. The
alveoli have a lower concentration of CO 2 than does blood entering
the lungs. Therefore, CO 2 diffuses out of the blood into the
alveoli. This CO 2 exits the body during expiration. Another way to
explain gas exchange in the lungs is to consider the partial
pressure of the gases involved. Gases exert pressure, and the
amount of pressure each gas exerts is its partial pressure,
symbolized as P O2 and P CO2. Alveolar air has a much higher PO2
than does blood. Therefore, O2 diffuses into the blood from the
alveoli. The pressure pattern is the reverse for CO2. Blood
entering the pulmonary capillaries has a higher PCO2 than the air
in the alveoli. Therefore, CO2 diffuses out of the blood into the
alveoli.
Slide 29
Internal Respiration Internal respiration refers to the
exchange of gases in the tissues. Specifically, during internal
respiration, gases are exchanged between the blood in systemic
capillaries and the tissue fluid. Blood that enters the systemic
capillaries is a bright red color because the blood is O2-rich.
Tissue fluid, on the other hand, has a low concentration of O2.
Why? Because the cells are continually consuming O2 during cellular
respiration. Therefore, O2 diffuses from the blood into the tissue
fluid. Tissue fluid has a higher concentration of CO2 than does the
blood entering the tissues. Why? Because CO2 is an end product of
cellular respiration. Therefore, CO2 diffuses from the tissue fluid
into the blood. We can explain exchange in the tissues by
considering the partial pressure of the gases involved. In this
case, oxygen diffuses out of the blood into the tissues because the
PO2 in tissue fluid is lower than that of the blood. And the carbon
dioxide diffuses into the blood from the tissues because the PCO2
in tissue fluid is higher than that of the blood.
Slide 30
Gas Transport The mode of transport of oxygen and carbon
dioxide in the blood differs, although red blood cells are involved
in transporting both of these gases. Oxygen Transport After O2
enters the blood in the lungs, it enters red blood cells and
combines with the iron portion of hemoglobin, the pigment in red
blood cells. Hemoglobin is remarkably suited to the task of
transporting oxygen because it both combines with and releases
oxygen. The higher concentration of oxygen in the alveoli, plus the
slightly higher pH and slightly cooler temperature, causes
hemoglobin to take up oxygen and become oxyhemoglobin (HbO2). The
lower concentration of oxygen in the tissues, plus the slightly
lower pH and slightly warmer temperature in the tissues, causes
hemoglobin to release oxygen and become eoxyhemoglobin (Hb). This
equation summarizes our discussion of oxygen transport:
Slide 31
Carbon Dioxide Transport Transport of CO 2 to the lungs
involves a number of steps. After CO 2 diffuses into the blood at
the tissues, it enters the red blood cells, where: 1. A small
amount is taken up by hemoglobin, forming carbaminohemoglobin. 2.
Most of the CO 2 combines with water, forming carbonic acid (H 2 CO
3 ). The carbonic acid dissociates to hydrogen ions (H) and
bicarbonate ions (HCO 3 ). The release of these hydrogen ions
explains why the blood in systemic capillaries has a lower pH than
the blood in pulmonary capillaries. 3. The difference in pH is
slight because the globin portion of hemoglobin combines with
excess hydrogen ions and becomes reduced hemoglobin (HHb).
Bicarbonate ions are carried in the plasma because they diffuse out
of red blood cells and go into the plasma. Most of the carbon
dioxide in blood is carried as HCO 3, the bicarbonate ion. As
bicarbonate ions diffuse out of red blood cells, chloride ions (Cl)
diffuse into them. This so-called chloride shift maintains the
electrical balance between the plasma and the red blood cells. In
pulmonary capillaries, a reverse reaction occurs. Bicarbonate
combines with hydrogen ions to form carbonic acid, which this time
splits into CO 2 and H 2 O, and the CO 2 diffuses out of the blood
into the alveoli. The following equation summarizes our discussion
of carbon dioxide transport:
Slide 32
Regulation of respiration: Since the rates of O2 uptake and CO2
production by body cells vary widely with changing metabolic
demand, respiration has to be controlled so as to maintain
appropriate levels of O2 and CO2 (and H+) in the tissues. This
depends on the regulation of ventilation through the interaction of
neurological and chemical control mechanisms so that mean alveolar
gas pressures remain constant. Since pulmonary blood normally
equilibrates with alveolar conditions before entering the systemic
circulation, systemic arterial blood gases can also be controlled
by changes in ventilation.
Slide 33
NEUROLOGICAL CONTROL FROM RESPIRATORY CENTERS IN THE BRAIN :
ALTHOUGH VENTILATION MAY BE CONSCIOUSLY CONTROLLED IT IS NORMALLY
REGULATED VIA INVOLUNTARY NERVOUS MECHANISMS.BREATHING IS REGULAR
AND CYCLICAL, INSPIRATION ALTERNATING WITH EXPIRATION, AND THIS
RHYTHMICAL ACTIVITY OWTHMICAL ACTIVITY IS DEPENDENT ON THE ACTIVITY
OF NEURONES WITHIN THEBRAINSTEM. THE NEURONES WHICH ACTIVATE THE
INSPIRATORY AND EXPIRATORY MUSCLES ARE LOCATED IN THE MEDULLA
OBLONGATA BUT ARE INFLUENCED BY CENTRES IN THE PONS,WHICH MODIFY
THEIR ELECTRICAL ACTIVITY, AND THUS ALTER THE PATTERN OF
VENTILATION.
Slide 34
- Inspiratory respiratory neurones in the medulla oblongata
demonstrate spontaneous, rhythmical activiTy firing regular bursts
of action potentials separated by periods of inactvity. These
neurones stimulate the motoneurones which pass out from the spinal
cord to the diaphragm and external intercostal muscles and so
activate contraction in these inspiratory muscles.During the pauses
in inspiratory neurone activity, the inspiratory muscles relax and
expiration occurs passively. in quiet respiration, therefore, it is
the inspiratory centre which plays the major role in sumulating
ventilation. Expiratory respiratory neurones in the medula are
normally quiescent and only become active during episodes of
increased ventilation involving active, or forced, expiration.
Under these conditions, bursts of expiratory neurone activity
coinciding with the lulls I n inspiratory neurone activity may be
recorded. These stimulate motoneurones causing the internal
intercostal and abdominal muscles to contract (Section 4.1).
Slide 35
.Respiratory cellsin the porns are not essential for
respiration but can modify the pattern of breathing.Stimulating the
pneumotaxic centre tends to inhibit the inspiratory neurones and so
shortens inspiration.Damage to this pneumotaxic area, by comparison
can lead to apneuss, in which inspiration is prolonged and is only
interrupted by short, expiratory gasps. chemical factors modifying
respiratory center activity: Arterial blood gases and pH: Chemical
control by the gases in arterial blood, as monitored by the
respiratory chemoreceptors, is the dominant factor in the
regulation of ventilation. Arterial Pco2 is most important with PH
playing a secondary role allowing respiratory compensation for
metabolic acid-base disturbances surprisingly arterial Po2 plays
little or no role in the normal regulation of ventilation, although
abnormally low levels can powerfully stimulate ventilation.
Slide 36
Elevated arterial Pco2. Any elevation of arterial Pco2
stimulates ventilatio leading to compensatory reduction.in alveolar
pco2 as excess CO2 is blown off. This is the primary mechanism
responsible for the regulation of breathing, and ventilation is
adjusted to keep the arterial pco2 close to 5.3 kPa (40 mmHg) Close
regulation of CO2 levels is important since any rise in Pco2
increases the CO2 content of the blood (see ue co dissocation
curve, fig 72) and promotes acidosis (Eq. 25) Depressed arteria
ph.Any,fall in arterial pH (increase in[ H+] leads to an increase n
ventilation. This reduces alveolar and arterial Pao, and so
elevates the systemic ph by driving the carbonic acid dissociation
reactions towards the left, removing H+ (protons) from the
extracellular fluid (Eq. 25). Respiratory changes can compensate
for an acidosis (low pH) caused by a non respiratory problem (a
metabolic acidosis) in this way (Section 5.9). Similarly,
ventilation may decrease in response to a metabolic alkalosis,
favouring Co2 accumulation and a reduction of the pH back toward
normal.
Slide 37
Depressed arterial Po2.ventilation is also stimulated by low
leves of arterial po2.This only happens, however, if the Po2 drops
well below the normal value (13 kpa,98 mmHg): increased respiratory
drive is only significant when the Po2 is less than about 8 kPa (60
mmH) The shape of the O2 dissociation curve removes any need to
regulate Po2 more tightly than this since haemoglobin remains 85%
saturated with o2 even at this relatively low pressure (Fig. 68B).
Oxygen delivery tissues is not greatly compromised unless arterial
po2 falls below these levels and only then is ventilation
stimulated. Oxygen is not directy involved, therefore, the normal
regulation of ventilation. Nevertheless, the arterial po2 remains
relatively constant under physiological conditions, as a secondary
consequenceof the close Control of Pco2 levels.
Slide 38
Chemoreceptors: The changes in arterial Co2, O2 and H+ levels
which the respiratory changes described above are detected y
respiratory chemoreceptors which regulate ventilation through their
connections with the respiratory centres. These receptors are
divided into two main groups based on their anatomical location and
each has its own pattern of sensitivity to changes in arterial
blood gases and pH levels.
Slide 39
.Central chemoreceptors. These are located within the CNS
itself, dose othe respiratory centre in the medulal These receptors
are particularly sensitive to changes in the arterial Pco, and are
less affected by changes in arterial pH or Po Experiments on the
mechanism of the CO2-stimulatory effect suggest that the
chemoreceptor cells are actually sensitive to H+. Carbon dioxide
rapidly diffuses from the blood into the brain where it reacts with
water to produce H+ (Eq. 25), and it is the resulting drop in pH
that directly stimulates the central chemoreceptors. An acidosis of
the arterial blood itself, however,has little immediate effect on
central chemoreceptors, because H+ cannot easily cross the
blood-brain barrier.
Slide 40
Peripheral chemoreceptors. These are located within the carotid
bodies, close to the bifurcation of the common carotid arteries,
and in the aortic bodies along the aortic arch. These receptors are
less important than central chemoreceptors in the responses to an
increase in pco2 but they are sensitive to changes in arterial pH)
stimulating or inhibiting ventilation in response to arterial
acidosis and alkalosis, respectively.peripheral chemoreceptors play
a further role as part of the fail-safe response to very Low o2
level. They are only activated when the Po2 falls well below the
physiological range (ie at or below 8 kpa;60 mmHg), however, and
this mechanism is not important an in normal ventilatory.