Respiratory System - BLOCK QUIZ ANSWERS
Explain
the mechanics of inspiration and expiration in terms of the properties and
features of the respiratory muscles, lung and pleura.
Inspiration
and expiration work on the basic principle that a change in volume brings about
a change in pressure, which causes movement of air.
Inspiration
is assisted by the inspiratory muscles, which are the diaphragm and the external
intercostal muscles. The diaphragm forms the curved floor of the thoracic cavity
and is innervated bilaterally by the phrenic nerve. The diaphragm has
attachments to the lower ribs and sternum anteriorly and spine posteriorly. The
tensing of this muscle, produces a change in the volume of the thoracic cavity
by pushing the abdominal contents downward. Thus a change in volume of thoracic
cavity will also produce a change in volume of the lung as it is mechanically
coupled to the thoracic cavity via the pleura. Thus a change in volume produces
an ultimate change in pressure and as a result air rushes into the lung from the
atmosphere. Inspiration is complete. Inspiration is also assisted by the
external intercostal muscles and contraction of these muscles lifts the rib cage
superiorly and laterally therefore increasing the dimension of the thoracic
cavity will have
the same effect mentioned above.
Expiration
is a passive process under a steady state. The relaxation of the inspiratory
muscles, means that the volume of the thoracic cavity is changed. The thoracic
cavity decreases in dimension and this produces an increase in the pressure
within the thoracic. As the lung is mechanically coupled to the thoracic cavity
via the pleura, the lung recoils also assisted by its elastic recoil properties.
Thus this produces an increase in intrapulmonary pressure, and hence air rushes
out. Expiration is completed. Active expiration results in the event of exercise
or high ventilation rate required. In this case, the abdominal muscles and the
internal intercostal muscles will contract and therefore pull the rib cage
downward, and medially and also push the abdominal contents upward, ultimately
resulting in the decrease in volume of the thoracic cavity. This will cause an
increase in intrapulmonary pressure and hence movement of air into the
atmosphere occurs.
Major Points: Basic principle of inspiration/expiration & Inspiratory muscles & Diaphragm and external intercostal muscles & attachments and changes in dimensions & change volume & change pressure & mechanical coupling via the pleura & air rushes in & Expiration & passive process – recoil and relaxation of the muscles & change in dimension & air movement & muscles involved in active expiration.
Draw
a labeled diagram depicting the static volumes and capacities of a healthy young
male at rest. Explain how volumes differ from capacities and how static and
dynamic measurements differ.
Volumes
are different to capacities because capacities are made up of two or more
volumes. Volumes are indivisible units.
Volumes
and capacities are measured using spirometry and static measurement methods.
This means that the amount of air inspired/expired/present in lungs is the
principle aim to gain. But in dynamic measurements the amount of air
inspired/expired is measured with respect to time. Therefore, dynamic
measurements measure the flow rate whereas static measurements only are
concerned with volumes, and not the rate of flow.
In
a healthy adult male at rest the following volumes and capacities (static) can
be measured:
Tidal
Volume – the volume of air which is inspired or expired under steady
state conditions and when the person is at rest
Total
Lung Capacity – The total volume of air which can be stored in the lung
taking into account the air breathed in, and the air in the lung.
Inspiratory
Reserve Volume – This is the amount of “extra” air which can
inspired following inspiration at rest
Expiratory
Reserve Volume – This is the amount of “extra” air which can be
expired following expiration at rest
Vital
Capacity – This is the total air which can be forcefully breathed out
at rest and is the sum of IRV + ERV + VT.
Inspiratory
Capacity – This is total amount of air which can forcefully breathed
into the lungs following complete expiration and is the sum of VT + IRV.
Residual
Volume – Following complete expiration (Vital Capacity), there is still
some air present in the lungs & this volume refers to this air
Functional
Residual Volume – This is the sum of the air present in the lungs and
also the expiratory reserve volume. This is important as it represents the total
volume of air in the lungs at rest.
Extra…
There
are also Dynamic measurements which are useful in diagnosis of lung conditions.
These are as follows:
Forced
Vital Capacity – This is equivalent to vital capacity but in this case
the air is forcefully expired as fast as possible.
Forced
Expiratory Volume – This is the amount of air which is forcefully
expired in one second.
Peak
Expiratory Flow – This is the flow rate measured at the beginning of
the expiration maneuver and it is mainly determined by the diameter of larger
conducting airways.
Forced
Mid Expiratory Flow – This is the flow rate measures during the middle
stages of the expiration maneuver and it is mainly produced by the diameter of
the smaller airways
Restrictive
vs. Obstructive
Restrictive
pulmonary disorders are caused due to lung compliance. If the lung becomes less
compliant – reduction in the ability to stretch – then it will mean that
lower volumes of air can be stored in the lung. Obstructive Pulmonary disorders
are caused by resistance to the expiration process. This means that less air is
able to be expired by the person due to the large airway resistance, which can
be due to mucus accumulations in the conducting airways and also constriction of
the airways.
Thus a low FVC will mean restrictive disorder, while a low FEV1 will mean an obstructive disorder.
Draw
the oxygen dissociation curve for hemoglobin under “standard” conditions.
Explain what happens to the position of the curve in response to a rise in
temperature or a fall in blood PH. What are the functional implications of these
changes, for instance during exercise?
The
oxygen dissociation curve passes through the following points (70% saturation &
40mmHg PO2 at tissues and P50 occurs at PO2 = 25mmHg).
A
rise in temperature will cause a right shift in the oxygen dissociation curve.
This means that the hemoglobin has lesser affinity to the oxygen molecules
therefore oxygen offloading is favored. In the case of a fall in blood pH,
implying increasing acidity of the blood, then the affinity of the hemoglobin
towards oxygen will decrease and therefore a right shift of the curve will
result. This is so because increasing acidity results in more hydrogen ions to
be present. This weakens the bonds (Bohr effect) between the hemoglobin molecule
and the oxygen and therefore oxygen offloading is favored.
In
the case of exercise. Exercise usually means that the body tissues (especially
skeletal muscles) requiring energy in the form of ATP, which is derived from the
aerobic metabolism. Thus a greater supply of oxygen is required for skeletal
muscles during exercise. During exercise a rise in body temperature is evident
due to the increased heat production from all the metabolism occurring. Thus a
rise in body temperature from above will cause a right shift and decrease
affinity of hemoglobin towards oxygen. This is favored because more oxygen
offloading is required and hence assist the functioning of the skeletal muscles.
Also during exercise, the skeletal muscles produce lactic acid which reduces the
blood pH levels. Therefore as explained about a right shift in the curve would
result, meaning that the affinity is lost between hemoglobin and oxygen and
therefore oxygen offloading is favored. This is important in supplying enough
oxygen to the working skeletal muscles.
Extra…
An
increase in partial pressures of carbon dioxide in blood will mean that more
carbon dioxide is produced at the working tissues. This means that more oxygen
supply is required in order to compensate for this accumulation of carbon
dioxide. Thus, the oxygen dissociation curve is shifted right and therefore
affinity is lost towards oxygen and offloading of oxygen is favored.
Also
the body’s red blood cells contain a substance known as
2,3-biphosphoglycertate. This substance binds to the beta polypeptide chains of
the hemoglobin molecule and therefore result in greater offloading of oxygen.
This shifts the oxygen dissociation curve towards the right. Increased altitude
produces an increased level of this substance in the red blood cells.
Also fetal hemoglobin contains a special gamma polypeptide chain as part of the hemoglobin molecule (which is made up of 4 polypeptide chains each contain a haem complex and each can be split up into alpha and beta polypeptide chains) this has a greater affinity towards the oxygen molecule. This shifts the curve to the left.
List
the ways in which carbon dioxide is transported by the blood. Outline the
mechanism involved in the transport of CO2 as bicarbonate.
Carbon
dioxide is transported in blood in three different ways: it is stored in
solution, bound to hemoglobin molecule, or stored as bicarbonate ions in the
plasma. About 5% of the total volume of CO2 is stored in solution, out which 10%
is contributed during alveolar exchange. About 10% is stored as part of
hemoglobin out of which 20-30% is contributed during alveolar exchange. About
80-90% is stored as bicarbonate out of which 70-80% is contributed during
alveolar exchange.
The
main mechanism of carbon dioxide storage is as bicarbonate ions in plasma. As
carbon dioxide production at the tissues means there is a gradient at this
level, CO2 diffuses into the blood capillaries with ease. This is also due to
the high solubility it has in lipids. When it enters the red blood, it is
quickly converted to carbonic acid. This is facilitated by the presence of an
enzyme within the red blood cells called carbonic anhydrase. But, as carbonic
acid unstable, it quickly dissociates into bicarbonate ion and hydrogen ions.
There is a concentration gradient between the plasma and red blood cells with
respect to bicarbonate ions, so this diffuses out of the cell to be stored in
the plasma. Movement of a negatively charged ion out of the cell, means chloride
ions shift inwards representing the chloride shift.
At
the alveolar capillaries, as the oxygen moves into the blood and binds to
hemoglobin. This displaces the hydrogen ion buffers and it combines with
bicarbonate ions in the red blood cells to form carbonic acid, which is
immediately changed into CO2 and H20 in the presence of carbonic anhydrase. As
there is a gradient involved between the blood and alveolar capillaries, CO2
diffuses out of the capillaries and into the alveolus. As bicarbonate ions are
used up in the red blood cells, more is diffused into the blood from the plasma.
Also chloride moves out of the cell to compensate for the electrical
disequilibrium
Major
points: The three storage mechanisms and their percentages &
CO2 produced in tissues &
diffuses into the red blood cell &
converted to carbonic acid (presence of enzyme) &
dissociates into bicarbonate ions &
and diffuses out into the plasma &
chloride shift &
hydrogen ions buffers oxygen offloading (Bohr effect) &
at alveolar capillaries &
O2 diffuses into the cell &
binds to hemoglobin &
H+ ions leave and combine with the bicarbonate to form acid &
formation of CO2 &
diffusion of CO2.
Extra..
Carbon
dioxide is also transported bound to hemoglobin as carbaminohemoglobin (carbamino).
This contributes to about 10% of the storage capacity and about 20-30% to
the alveolar diffusion. The carbon dioxide molecule binds to the globin portions
of the Hb molecule. The binding of this is determined by the haldane effect. The
binding of carbon dioxide is largely determined by the degree of saturation of
the hemoglobin molecule. If the hemoglobin molecule is saturated, then the
degree of loading of carbon dioxide is diminished. This is called the haldane
effect and is important during tissue offloading and loading of oxygen and
carbon dioxide.
Extra..
The reason for the dissociation curve to be sigmoid shaped is due to cooperative binding. This means that the binding of oxygen to the heme portions of the hemoglobin molecule, changes the shape of the hemoglobin molecule and therefore this encourages more oxygen to load as part of hemoglobin. This facilitation is called cooperative binding. The same concept is used when oxygen offloads from the hemoglobin molecule. The offloading of one oxygen molecule means that further off loading is facilitated.
Describe
the role played by the brain stem in establishing a normal respiratory rhythm.
Breathing
is rhythmic activity and this rhythm is provided by the brain stem region. The
medulla of the brain stem contains two groups of neurons which have an effect on
the breathing pattern. These are the dorsal respiratory group and the ventral
respiratory group. The dorsal respiratory group has inspiratory neurons which
fire just before inspiration takes place. They relay their signals to the
inspiratory muscles via the phrenic motoneurons. This enables the contraction of
the inspiratory muscles (mostly the diaphragm) and therefore account for
inspiration. The ventral respiratory group has both inspiratory and expiratory
neurons. The expiratory neurons are excited just prior to expiration where they
stimulate contraction of the expiratory muscles (eg.: abdominal muscles) and
thus account for expiration. They also have an inhibitory effect on the
inspiratory neurons.
The
pons area of the brain stem contains two more groups of neurons which monitor
the activity of the DRG and VRG. These are the pneumotaxic center and the
apneustic center. The pneumotaxic center is located near the upper part of the
pons region and have reciprocal connections with the dorsal respiratory group
neurons. This means, that an increase in activity of the pneumotaxic center
causes decrease in period of activity of the inspiratory neurons and therefore
cause shorter faster breaths. This increases the breathing rate.
The
apneustic center on the other hand has an excitatory effect on the inspiratory
neurons and thus prolong inspiration. This means that the inspiration phase is
long therefore more air is breathed in. This affects the tidal volume.
Major Points: Medulla & DRG & VRG & Inspiratory neurons & when do they fire? & via phrenic motoneurons & contraction of the inspiratory muscles & contains both expiratory and inspiratory neurons & when do they fire? & excite expiratory muscles & inhibit inspiratory neurons & Pons & pneumotaxic center > apneustic center > location in pons > decrease level of activity of inspiratory neurons (reciprocal connections with DRG) > phase switching > increase breathing rate > apneustic center > prolong inspiration by excitatory response to inspiratory neurons > increase tidal volume.
Briefly
compare and contrast the main features of the central chemoreceptors and the
peripheral chemoreceptors.
There
are two types of chemoreceptors evident in the body. These are the central and
peripheral chemoreceptors.
The
central chemoreceptors are located on the ventral surface of the the medulla
oblangata. These are separated from the blood due to the Blood Brain Barrier,
although diffusion of carbon dioxide occurs through this barrier. The level of
carbon dioxide has an effect on the ventilatory response. If the carbon dioxide
levels rise, then there is a rise in hydrogen ions as a result of the
dissociation reaction. Cerebrospinal fluid acts as a bad buffer to hydrogen
ions, and therefore this diffuses into the chemoreceptor cells. This triggers
increased ventilation to bring in more oxygen and cause offloading of carbon
dioxide into the alvelus.
On
the other hand, peripheral chemoreceptors are located within the blood vessels
of the neck and thorax region. The main body is located on the bifurcations of
the common carotid artery and this is called the carotid bodies. This area has a
high vascular supply and is innervated by branches of the glossopharyngeal
nerve. There are two types of receptor cells evident here, the type 1 glomus
cells and the type 2 cells. The type 1 cells contain transmitter substances
which are released in response to decreased PO2 levels or increased PCO2 levels.
Another region where peripheral chemoreceptors are located are on the aortic
arch of the aorta and are called aortic bodies. These are innervated by branches
of the vagus nerve. They play a far less important role in controlling
ventilatory response.
Upon
decrease of PO2, the carotid bodies are stimulated and send ascending signals to
the medullary rhythmicity center. Upon an increase in CO2 levels, the carotid
bodies are stimulated by an increase in hydrogen ion concentration.
Major Points: two types evident > location of central receptors > separate from blood brain barrier > increase in CO2 can act via blood and CSF > less of a buffer to Hydrogen ions > enter the cells > stimulate the cells > signals are sent to the medullary rhythmicity center > Peripheral Bodies > carotid body – location, blood supply and innervation > two types of cells found > aortic bodies > innervation > responses to a decrease in PO2 levels and an increased in PCO2 levels by Hydrogen ions.
Briefly
outline the major structural changes that occur in the airways in passing from
the trachea to the aveolar ducts. How are these structure changes related to
their function.
There
are many structural changes that occur in the lower airway. The trachea is the
largest airway leading into the lung. It passes through the thoracic cavity and
then splits at the carina to form two primary bronchi that feed into the lung at
the hilium. Together follow the pulmonary artery and vein. The trachea is made
up of a pseudostratified ciliated columnar epithelium with goblet cells. It also
is covered with tracheal cartilage rings around its out surface which extend 270
degrees posteriorly and is connected by ligamentous membrane. The amount of
smooth muscle in this structure is relatively low compared to the lower airways.
The bronchi split into more bronchi and form a tree structure. When the bronchi
size dimishes in diameter below 1mm, these are called bronchioles. The smallest
bronchioles do not have a pseudostratified epithelium, but their epithelium
begins to a low columnar with cilia present. There are no goblet cells present
at this level, and the ratio of smooth muscle to cartilage is high. As we move
down the bronchial tree the amount of cartilage becomes sparse until we get to
the bronchioles where smooth muscle is greatest with no cartilage. This allows
for constriction and providing airway resistance. The ciliated epithelial cells
are still present at this level. As we move further down the tract, to the
alveolar ducts, these have no cartilage, no ciliated cells, their epithelium is
low cuboidal and they have a high ratio of smooth muscle.
Functionally,
the epithelium is important. The tracheal epithelium has cilia and goblet cells.
The goblet cells secrete mucus which sticks to the tips of the cilia and attract
dust particles. The cilia then stroke to move the duct particles and layer of
mucus towards the upper airway to be excreted through the cough reflex etc. The
cartilage present, gives the trachea its structural integrity. As we move down,
the number of goblet cells decrease as most of the dust particles have be swept
away earlier, but cilia are still present in case there are accumulations of
dust in the lower parts of the air. This way they can sweep them away. As we
move down, the smooth muscle ration increases and the cartilage decreases,
allowing for more flexibility to the passageways for constriction etc. In the
lowest parts of the airways, there is no cilia present and the only way of
removing foreign particles is by macrophages.
Throughout
the bronchial tree there are elastic fibres within the connective tissue holding
all of it together. This accounts for the stretch recoil properties of the lung
during inspiration and expiration.
Major Points: Trachea > cartilage, epithelium, elastic, smooth muscle > talk about how epithelium changes > cilia and goblet cells changes > talk about smooth muscle changes > talk about irregularity of cartilage until none present in bronchioles > elastic fibres > paragraph on functional implications of each of these.
Briefly
discuss the factors that are important in the transfer of oxygen and carbon
dioxide between the alveoli and pulmonary capillary blood. Include features of
the respiratory membrane and the properties of the gases themselves.
The
transfer of oxygen into the pulmonary capillaries and transfer of carbon dioxide
into the alveolus is depended on the maintenance of the concentration gradients
of these two gases between the alveolus and capillaries. This maintenance is
provided by the bulk flow mechanisms of pulmonary ventilation, provided by the
respiratory muscles and the pulmonary perfusion, provided by the right ventricle
of the heart.
When
oxygen is breathed in, its partial pressure (the pressure exerted by it in a
mixture of gases à
Dalton’s Law of Partial Pressures) is about 160mmHg. As it enters the upper
and lower airways it is immediately warmed and humified by the water vapour
pressure. The partial pressure of oxygen decreases as a result of the partial
pressure exerted by water vapour, which is approximately 47mmHg. This partial
pressure of oxygen now is about 150mmHg. As it moves further down to the lower
parts, carbon dioxide increases in concentration and exerts its own partial
pressure, about 40mmHg. Partial pressure of oxygen is now decreased to about
105mmHg.
The
partial pressure of oxygen in mixed venous blood is about 40mmHg. Thus there
exists the steep concentration gradient and thus oxygen tends to diffuse into
the blood passing through the respiratory membrane. The partial pressure of
carbon dioxide in mixed venous blood is about 46mmHg, and it moves into the
alveolus due to the concentration gradient present.
The
passage of these gases is through the respiratory membrane which is made of
three main structures. The capillary endothelium, the alveolus epithelium and
their fused basal laminas. This is an extremely thin structure making passage of
gases very easy. The passage of gases is also influenced by the huge surface
area of the respiratory membranes. The edema of this membranes will hinder
passage of gases through it therefore causing breathing problems.
The
constant replenishment of oxygen in the lung is essential for maintaining the
concentration gradient at the respiratory membrane thus aiding diffusion of
gases.
Major
Points: Maintenance of concentration gradient >
provided by bulk flow of blood and air > respiratory muscles and right
ventricle of heart > what happens when air is breathed in? > water vapour
> carbon dioxide > mmHg of O2 in mixed venous blood > conc. Gradient
established > what happens with CO2 > diffusion occurs where >
structure of respiratory membrane > surface area factors.