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Human Physiology > Respiratory system > Flashcards

Respiratory system Flashcards

1
Q

What is the main function of the lung? other functions?

A

gas exchange between the blood and external environment (occurs by diffusion at the blood-gas barrier)
- reservoir and filter for blood
- involved in metabolism of some compounds
- providing airflow for speech

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2
Q

How does ventilation work (overview)?

A

Oxygen is inhaled from the air into the lung and diffuses into the blood. The oxygen transported by the blood to tissues is used for chemical reactions within the cells; carbon dioxide is the major end product of those reactions. Carbon dioxide is also transported via the blood and diffuses into the lung from the pulmonary capillaries. Carbon dioxide is exhaled into the air.

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3
Q

What does diffusion occur by?

A

the rate of diffusion of gas across a tissue sheet is described by Fick’s law pf diffusion.
law states that the rate of gas transport across a tissue sheet is proportional to the area of the sheet (A), a diffusion constant, and the difference in partial pressure (P1-P2), and is inversely proportional to the thickness (T)
Vgas = (constant * (P1-P2)) * A/T
Vgas: volume of gas transferred per unit time

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4
Q

Describe the diffusion constant

A

The diffusion constant is proportional to the gas solubility but inversely proportional to the square root of its molecular weight:
1. Size of the molecule -> smaller molecule -> larger constant
2. Solubility of the molecule -> higher solubility -> higher constant

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5
Q

For efficient gas exchange in the lung - what should the conditions be?

A

The blood-gas barrier needs to be very thin and have a large surface area
- diffusion is fast when there is a large pressure gradient, large surface area, and thin surface

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6
Q

What is partial pressure of a gas in a mixture? How do you calculate?

A

partial pressure of a gas is the pressure exerted by any one gas in a mixture of gases
partial pressure of a specific gas equals the total pressure of the gas mixture times the fractional concentration (e.g. in a mixture of 70% O2 and 30% CO2 with a pressure of 200mm Hg: the partial pressure of O2 (PO2) = 0.7*200= 140 mm Hg, and PCO2 = 60 mm Hg)

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7
Q

What is the PO2 of air at sea level and in the lung?

A

sea level = 160 mm Hg
in the lung = 150 mm Hg

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8
Q

What is the partial pressure of a gas in a solution?

A

the partial pressure of a gas in solution is the partial pressure of the gas that is in equilibrium with the solution (regardless of the solubility of the gas in the solution)
- if blood is in equilibrium with a gas with partial pressure of O2 of 100 mm Hg, the PO2 of that blood would be 100 mm Hg. Similarly, if water would be in equilibrium with the same gas, the PO2 of the water would be 100 mm Hg.
*the partial pressure of O2 in arterial blood is referred to as PaO2.

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9
Q

Describe the structure of the lung

A

The function of gas exchange is accomplished at the blood gas barrier - structure is created through a repeated branching structure of airways and alveoli. The lungs are found in the thoracic cavity.
The blood vessels and capillaries provide the blood flow essential for gas transport.

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10
Q

Describe the blood gas barrier - what are the important features? structure

A
  • gas exchange takes place at the blood gas barrier (BGB) in small air sacs called alveoli
  • 2 important features: thin and has large surface area
  • at the air side of the barrier the lung contains a thin layer of fluid that includes surfactant and also contains the very thin type 1 epithelial cell (2 layers - surfactant then type 1).
  • on the capillary side, the surface is lined with endothelial cells
  • small layer called the interstitium is present between the endothelial and epithelial cells
  • although it is composed of these diff layers, its overall thickness is only about 0.5 micrometers - this thickness allows efficient gas-exchange
  • the total surface area of the BGB is estimated to be between 50-100 m^2 - allows for efficient gas exchange.
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11
Q

How do we get such a large surface area for the BGB?

A
  • surface area created by extensive branching of the lung structure to create enormous number (~300 million) of air sacs called alveoli and wrapping the capillaries around them.
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12
Q

What are the airways?

A
  • structural units contributing to generation of the large surface area through repeated branching.
  • inhaled air travels through the airways
  • in the human lungs there are 23 generations of airways starting at the trachea and ending at the alveolar sacs.
    Zone of airway generation -> bronchus -> bronchiole -> terminal bronchiole -> respiratory bronchiole -> alveolar duct -> alveolar sac
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13
Q

What are some important notes regarding the airways and alveoli - structure

A
  • the trachea divides into left and right bronchi, which in turn undergo repeated branching
  • between the trachea and alveolar sacs, the airways divide 23 times
  • airways, bronchi, bronchioles and terminal bronchioles constitute the conducting zone - which is devoid of alveoli and hence do not participate in gas exchange.
  • the terminal bronchioles divide to form respiratory bronchioles (RBL) which have occasional alveoli budding from their walls
  • RBL divides into alveolar ducts, which are completely lined with alveoli; alveolar ducts end on alveolar sacs
  • the last generations of airways (17-23), consisting of respiratory bronchioles, alveolar ducts and alveolar sacs have alveoli and participate in gas exchange, this alveolar region is known as the transitional and respiratory zone
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14
Q

Describe the blood vessels and blood flow around all these branching airway

A
  • like the airways, the pulmonary artery also branches extensively and form a dense network of capillaries which wrap around the alveoli.
  • in the capillaries, blood is exposed to the largest surface area and velocity of blood flow is lowest; this indeed is an efficient arrangement for diffusional exchange of gases.
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15
Q

Describe the physical environment of the lung

A

the lung is located in the thoracic cavity - surrounded by the chest wall and is separated from the abdomen by the diaphragm. The space between the lung and the chest wall is called intrapleural space.

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16
Q

Describe how the physical environment contributes to pressure and breathing within the lungs
What happens with a pneumothorax?

A
  • the pressure inside the lung (intrapulmonary pressure) is atmospheric.
  • in contrast, the pressure in the intrepleural space is negative (- 5 mm Hg)
  • this negative pressure is created by the elastic properties of the chest wall and those of the lung; the chest wall has a tendency to move outwards whereas the lung’s elastic properties have a tendency to collapse.
  • thus this difference in pressure helps to keep the lungs open for breathing
  • with a pneumothorax (hole in the diaphragm or chest wall) the intrapleural pressure is atmospheric. this causes the chest to move outwards and the lungs to collaspse.
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17
Q

What is ventilation?

A

Process by which air moves in and out of the lung.
inflation of the lung during inspiration, and deflation during expiration brought by changing the volume of the thoracic cavity.

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18
Q

Describe inspiration

A
  • active process (contraction of inspiratory muscles: diaphragm - contracts and external intercostals - pulls ribcage out) - makes more space -> increases pressure gradient
  • diaphragm innervated by phrenic nerve (segments 3, 4, 5)
  • external intercostals innervated by intercostal nerves
  • when the diaphragm contracts, abdominal contents are pushed downward and forward resulting in a major increase in the vertical dimension of the thoracic cavity.
  • when the external intercostals contract, the ribs are pulled upward and forward resulting in an increase in the lateral anteroposterior diameters of the thorax.
  • the increase in volume causes the negative intrapleural pressure to become even more negative and, as a consequence, the lung expands leading to slightly sub-atmospheric intrapulmonary pressure. allows air to flow into the lung = inflation
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19
Q

Describe expiration/ exhalation

A
  • expiration is normally a passive process (does not require muscle contraction)
  • the lung and chest wall are elastic structures and tend to return to their equilibrium position after they are actively expanded during inspiration
  • during exercise and voluntary hyperventilation, expiration becomes an active process
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20
Q

What are the major expiratory muscle for the active process? (e.g. exercise)

A
  1. muscles of the abdominal wall (include rectus abdominus, internal and external oblique muscles and transverse abdominus)
  2. the internal intercostals (connect adjacent ribs)
    - contraction of muscles of the abdominal wall increases intraabdominal pressure and pushes the diaphragm upward causing decrease in thoracic volume
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21
Q

What forces do we need to overcome to inflate the lung? Why are these pressures so small?

A
  • Due to the distensibility of the lung - termed lung/ pulmonary compliance
  • the two properties of the lung that influence compliance are: elasticity of the lung tissue, and surface tension forces of the alveolar lining fluid
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22
Q

What is lung/ pulmonary compliance? how can it be determined?

A
  • defined as volume change per unit pressure
    compliance = change in volume / change in pressure
  • can be determined by creating a pressure-volume curve.
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23
Q

Describe how the pressure-volume curve is obtained

A
  • can be obtained by inflation of an isolated lung in 2cm pressure intervals and determination of the corresponding volumes at each pressure.
  • subsequently the lung is deflated in a similar fashion
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24
Q

What are the important features of the PV curve?

A
  1. the curve is non-linear. at beginning takes a lot less volume to increase pressure - requires more volume to increase pressure later (at higher pressure) - during inflation the lung is more compliant at these higher pressures (higher compliance)
  2. At high pressure (>20 cm H2O) the lung becomes once again less compliant
  3. At the same pressure the lung has more volume during deflation than inflation - called hysteresis - hysteresis is a consequence of the phenomenon of surface tension and surfactant.
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25
Q

How can compliance be measured in humans?

A

indirect technique: subject swallows a small balloon at the end of a catheter; the other end of the catheter is connected to a pressure recorder to measure intraesophageal pressure which approximates intrapleural pressure.
- the subject inspires air (with glottis open) in small amounts at a time and pressure is recorded - repeated
- then subject exhales in small volumes and pressure is recorded
- get inflation and deflation curve

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26
Q

What are the forces involved id determining lung compliance?

A
  1. Elasticity
  2. Surface tension
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27
Q

Describe the difference between a
PV curve using air vs fluid

A
  • air-filled shows normal PV curve
  • fluid-filled inflation/deflation shows that it takes less pressure to expand the lung with fluid - also shows no hysteresis
  • the reason for the difference between the two curves is that the fluid inflation/ deflation does not include the forces needed to expand the air-liquid interface that exist at the alveolar surface. does have to overcome elasticity
  • estimated that 1/3 of lung’s pressure need to fill up lung with air is needed for overcoming the elastic forces, and 2/3 is needed to overcome the surface tension properties of the air liquid interface of the lung
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28
Q

Describe the elasticity of the lung tissue

A
  • fibers of elastin and collagen are present on alveolar walls and throughout the lung.
  • the elastin fibers are easily stretched whereas collagen fibers are not
  • specific geometric arrangement of these fibers is responsible for the elasticity of the lung
  • elasticity found within the interstitium layer
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29
Q

What is surface tension?

A
  • surface tension arises from the attractive forces between molecules within a liquid
  • molecules within the bulkface of the liquid are attracted by all molecules surrounding
  • molecules at surface only experience a force into the bulk of the liquid (no attractive forces above) - don’t really like to be at surface
  • surface tension is the force that causes water to form droplets
  • because of effects of surface tension - generally air will flow from smaller bubble to larger bubble
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30
Q

What is the law of laplace?

A

the reason that surface tension forces are important in opening and closing of the lung can be explained by the law of Laplace which states that the pressure across a bubble (alveolus) is equal to 2 times the surface tension divided by the radius.
difference in pressure = 2*surface tension / radius
- the implication of laplace’s law is that when surface tension is high (such as that of water) bubbles (or alveoli) will easily collapse, especially when the radius is small

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31
Q

What would happen if the alveolar fluid lining were water? what are the problems? what actually lines the alveolars?

A
  • the pressure needed to overcome surface tension forces of the lung would be extremely high and lung would have tendency to collapse
  • would also get uneven inflation
  • instead lined by pulmonary surfactant
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32
Q

What is pulmonary surfactant?

A
  • pulomonary surfactant reduces the surface tension by forming a lipid film at the air-liquid interface.
  • the lipid molecules eliminate water molecules from the surface and can interact with the molecules underneath the film (phospholipid), thus reducing the surface tension
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33
Q

How does surfactant help during respiration?

A
  • during respiration surfactant reduces surface tension such that at low lung volumes (small radius) the surface tension is extremely low
  • during inflation surface tension rises, resulting in higher surface tension at higher lung volumes
  • this means that the pressure requirements for inhaling and exhaling are different (different radius and surface tensions - laplace’s law) - specifically, by increasing the surface tension while increasing the radius (inflation) relatively small changes in pressure are required to inflate the lung.
  • this also stabilizes the alveoli at very low lung volumes
  • differences in the surfactant film during inflation and deflation result in the hysteresis seen in the PV curves.
  • surface tension is low at low volumes
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34
Q

What is present in surfactant? what is their role? what produces surfactant?

A
  • lipids
  • surfactant-associated proteins (SP-A, SP-B, SP-C, SP-D)
  • SP-B, and SP-C are responsible for generating and maintaining the film
  • other two proteins (SP-A, SP-D), have roles in the metabolism of surfactant and in protecting the lungs from inhaled pathogens.
  • both surfactant lipids and proteins are synthesized and secreted by alveolar type 2 cells.
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35
Q

What is spirometry?

A

a method to measure lung volumes
- the classical spirometer consists of a upside down bell filled with air hanging in a liquid. the subject breathes the air from the bell and the movement of the bell of the bell (up with exhalation, down with inhalation) is recorded with a pen on a moving chart. person may be instructed to take specific breaths (maximum, etc). or requested to exercise
- nowadays spirometry measurements are performed via a computer set-up
- a number of lung volumes can be determined

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36
Q

What is tidal volume?

A

the volume of air inhaled with each breath

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37
Q

What is vital capacity (VC)?

A

the volume of air that can be forcibly exhaled after a maximal inspiration

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38
Q

What is residual volume (RV)?

A

the volume of air remaining in the lungs after a maximal expiration

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39
Q

What is functional residual capacity (FRC)?

A

the volume of air remaining in the lungs at the end of a normal expiration

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40
Q

What is inspiratory reserve volume (IRV)?

A

the volume of air that can be forcibly inhaled following a normal inspiration

41
Q

What is expiratory reserve volume (ERV)?

A

the volume of air that can be forcibly exhaled following a normal expiration

42
Q

What is total lung capacity (TLC)?

A

the volume of air in the lungs at the end of a maximal inspiration

43
Q

What is minute volume or pulmonary ventilation?

A

the volume of air inhaled per minute;
it is given by tidal volume * frequency of respiration (normal = 500 ml * 12 breaths/min = 6000 ml)
= tidal volume * respiratory rate

44
Q

What is forced expiratory volume - 1 second (FEV-1 second)

A

the fraction of vital capacity expired in one second
- in normal subjects about 80% of the VC is expired in one second (FEV-1sec = 80% of VC).
- in restrictive diseases (pulmonary fibrosis) VC is reduced but FEV-1sec is normal or increased
- in obstructive diseases (bronchial asthma) FEV-1sec is reduced more than VC.

45
Q

What is maximal ventilation (MVV) - also called maximal breathing capacity

A

the volume of air that can be moved into and out of the lungs in one min by voluntary effort
- normal MVV is in the range of 125-170 L per min
- FRC and RV cannot be measured directly with a simple spirometer. These are calculated as follows:
FRC = ERV + RV
RV = FRC - ERV

46
Q

What is the helium dilution method?

A
  • not all lung volumes can be measured by spirometer (RV, FRC, and TLC)
  • helium dilution method can determine these values
  • this method utilizes a “closed circuit” (system does not allow gas to escape)
  • subject is connected to a spirometer at a specific lung volume (e.g. after a normal expiration when lung volume is FRC) containing a known concentration of helium
  • helium does not cross the blood gas barrier
  • during several breaths, the helium is mixed with the gas inside the lung and ultimately it is completely mixed (equilibrium) - since no helium has been lost, the amount of helium present before equilibration equals amount after so:
    C1XV1 = C2X(V1+V2)
    where C1 = initial He concentration, C2 = final He concentration, V1 = spirometer volume, V2 = volume inside the lung at the time of connection to the spirometer
47
Q

What is anatomic dead space?

A

the volume of air that remains in the conductive zone -does not enter the respiratory zone
- in a normal, 70 kg subject, the dead space volume is approximately 150 ml
- consequence of the structure of the lung - does not participate in gas exchange

48
Q

What is alveolar ventilation?

A

the volume of air entering the respiratory zone per minute
- not all volume of gas entering the lung participates in gas exchange
- for each tidal volume entering the lung, a portion will go to the dead space and the remainder will reach the respiratory zone. only the air that reaches the respiratory zone will participate in gas exchange and constitutes alveolar ventilation
alveolar ventilation = pulmonary ventilation - dead space ventilation

49
Q

What is physiological (or total) dead space?

A

total volume of air entering the lung that does not participate in gas exchange
- there are spaces additional to the anatomic dead space that receive air but do not participate in gas exchange - these structures may be deeper in the lung and just not have blood flow e.g.
- in normal subject the anatomic dead space will be similar to the total dead space, but in certain diseases, inhaled air may enter nonfunctional alveoli, and this would contribute to total dead space

50
Q

What is Bohr equation?

A

calculates the volume of total dead space
- this calculation is based on the assumption that all expired CO2 comes from alveolar gas (site of gas exchange) and not from dead space
VD/VT = (PACO2 - PECO2) / PACO2
where VD = dead space volume, VT = tidal volume, PACO2 = PCO2 of alveolar gas, and PECO2 = PCO2 of expired gas
- in normal subjects, PCO2 in alveolar gas (PACO2) and arterial blood (PaCO2) are virtually identical so the equation can be modified by substituting PaCO2 with PACO2

51
Q

What is the importance of dead space?

A

Can be observed when we determine changes in alveolar ventilation that can occur - especially with changes in respiratory rate and tidal volume
e.g. amount of air available for gas exchange (alveolar ventilation) is drastically reduced despite no difference in pulmonary ventilation - but tidal volume was lower

52
Q

What is oxygen consumption (VO2)?

A

volume of oxygen that the body utilizes (while breathing room air)
- can simply be measured by determining the amount of oxygen inhaled minus the amount exhaled
VO2 (1/min) = O2 inhaled (1/min) - O2 exhaled (1/min)
can be rewritten as:
VO2 (1/min) = [(Tidal volumebreaths per minute) * %O2 of inhaled air] - [(Tidal volumebreaths per minute) * O2 of exhaled air]
- this is because inhaled O2 is equivalent to minute volume * percent O2 inhaled, and minute volume is equivalent to tidal volume * frequency

53
Q

How is VO2 measured?

A
  • a subject is connected to a spirometer to measure minute volume while a gas analyzer is utilized to measure the exhaled O2 levels.
  • the % of O2 of the inhaled air will be 21% if breathing room air.
  • VO2 measurements can be taken at rest or during exercise
54
Q

What is VO2 max?

A

defined as the maximum amount of oxygen that the body can utilize per minute (while breathing room air).
- this measurement is generally believed to be one of the best measures of aerobic fitness
- expressed as ml/min/kg bodyweight (average = 30-40)

55
Q

What is the procedure for measuring VO2 max?

A
  • involves a subject performing an aerobic exercise (e.g. treadmill) while VO2 is measured
  • exercise increased in stepwise fashion (initially this will lead to an increased VO2 each time)
  • when an exercise level is reached at which the body cannot consume any more oxygen (VO2 no longer increases) this point is the VO2 max
56
Q

What are the factors that influence VO2max?

A
  1. Cardiovascular system: “get the oxygen there”
  2. Muscles: “oxygen utilization”
  3. Respiratory system: “get the oxygen in the blood”
57
Q

Describe oxygen transport in blood

A

oxygen transport in blood occurs by two different ways:
1. as a dissolved gas
2. bound to hemoglobin

58
Q

Describe oxygen transport as a dissolved gas

A
  • O2 transport as a dissolve gas represents a minor component of total O2 transport
  • the amount of gas that is dissolved in a liquid is defined by Henry’s law
  • O2 is not very soluble in blood and this only 0.3 ml O2/ 100 ml blood can be transported this way
  • this amount of O2 would not be adequate to meet tissue demands of O2
59
Q

What is Henry’s law?

A

Concentration of a dissolved gas = pressure* solubility

60
Q

Describe oxygen transport bound to hemoglobin

A
  • binding of O2 to hemoglobin (Hb) represents the major transport of O2 transport in the blood
  • hemoglobin is a protein consisting of 4 subunits - each subunit contains a heme moiety to which one O2 molecule can bind, thus each Hb can bind 4 O2 molecules
  • one gram of hemoglobin can hold a maximum of 1.34 ml of O2 (reached at high PO2 and called 100% Hb saturation)
  • in blood there is approximately 15 g Hb/ 100 ml (so 100 ml of blood could hold a maximum of 20.1 ml of O2)
61
Q

Describe the Oxygen-hemoglobin dissociation curve

A
  • the PO2 is not always high - lung, and peripheral tissue have diff circumstances (diff PO2 values)
  • the dissociation curve can be obtained by performing an experiment in which blood is incubated with oxygen gas at different PO2 levels and measuring the amount of O2 in the blood and bound to hemoglobin.
62
Q

What are the important features of the sigmoidal curve of the oxygen-hemoglobin dissociation curve?

A
  1. the PO2 in the lung is approximately 100 mm Hg, this means that the O2 saturation is about 97.5%
  2. due to the sigmoidal shape of the curve, a small drop in alveolar PO2 will still allow an adequate Hb saturation (for example if alveolar PO2 would fall to 70 mm Hg, Hb saturation would still be 93%)
  3. the steep portion of the dissociation curve allow peripheral tissue extract large amounts of oxygen with a relatively small drop in
    PO2 (PO2 is low in peripheral tissue, allows O2 dissociation)
    - thus the sigmoidal shape of hemoglobin-O2 dissociation curve allows for uploading of O2 in the lung and unloading in the peripheral tissue
    - the sigmoidal shape of the hemoglobin-O2 dissociation curve is a consequence of the binding of hemoglobin to oxygen. binding of the first O2 molecule to a Hb subunit increases the affinity of the other subunits to O2
63
Q

What are the factors that influence the oxygen-hemoglobin dissociation curve?

A

the shape of the curve will remain the same but the curve can shift left and right
- a shift to the right means that, at the same PO2, unloading of O2 will occur
- the following factors can shift the curve to the right:
1. decreased pH
2. increased temp
3. increased PCO2
*think about this in terms of exercise: in an exercising muscle there is decreased ph (acid formation), increased temp, and increased CO2 production, and in response to these changes there is an increased oxygen unloading in the tissue.
4. increased 2,3-diphosphoglycerate in the blood (DPG is an end product of red blood cell metabolism. it increases hypoxia and facilitates more unloading of O2 in peripheral tissues.

64
Q

Describe carbon dioxide transport

A

carbondioxide transport in blood occurs three different ways:
1. as a dissolved gas
2. as bicarbonate
3. bound to Hb

65
Q

Describe carbon dioxide transport as a dissolved gas

A
  • similar tonO2, CO2 is dissolved in blood according to Henry’s law which states that the amount of a gas that is dissolved in a liquid is related to the pressure of the gas and the solubility of the gas.
  • compared to O2, CO2 is ~20 times more soluble in blood
  • dissolved CO2 represents a significant proportion (about 10%) of CO2 transport in blood
66
Q

Describe carbon dioxide transport in bicarbonate form

A
  • the bicarbonate form is used for about 70% of the total CO2 transport
  • bicarbonate is formed in the red blood cell and involves the enzyme carbonic anhydrase
    CO2 + H2O -> H2CO3 -> HCO3 + H
  • after bicarbonate is formed it can diffuse into the plasma, Cl- will diffuse into the red blood cell to maintain electron neutrality
67
Q

Describe carbon dioxide transport bound to hemoglobin

A
  • this accounts for approximately 20% of total CO2 transport
  • within the red blood cell, H+ will bind to deoxygenated hemoglobin to form HHb. The HHb will rapidly bind CO2 to form Carbamino-Hb
  • in peripheral tissues the unloading of oxygen from Hb enhances the ability to transport CO2: this is known as the
    Haldane effect
68
Q

What is the role of carbon dioxide transport in acid base balance?

A
  • due to formation of bicarbonate and associated H+, CO2 transport can have major effect on blood pH
  • the bicarbonate system associated with ventilation is actually one of the mechanisms the body uses to maintain blood pH near 7.4 (buffer system)
  • altered ventilation may change pH - these changes happen very rapidly
  • the role of the kidneys on acid/base balance is much slower than the lungs
69
Q

Describe respiratory alkalosis

A
  • increased ventilation will decrease PaCO2 and drive reaction to the left
  • decreases H+ concentration and leads to more basic pH
  • can occur at high altitude e.g. where there is lower PCO2 in the air
70
Q

Describe respiratory acidosis

A
  • decreased ventilation which will increase PaCO2
  • increase in CO2 drives reaction to the right and increases H+ concentration
  • blood is more acidic
  • e.g. diseases such as chronic obstructive pulmonary disorder or after the use of certain drugs
71
Q

Values of O2 and CO2 in blood?

A
  1. Arterial blood
    O2 content: 19.8 ml/100ml Dissolved gas: 0.3ml Bound to Hb: 19.5ml Hb saturation 97%
    CO2 content: 49ml/100ml Dissolved gas: 2.6ml Bicarbonate: 43.8ml Bound to Hb: 2.6ml
  2. Tissue

….. section 4 end

72
Q

Describe the origin of respiration

A
  • spontaneous respiration is produced by rhythmic discharges from the motor neurons that innervate the respiratory muscles
  • these discharges are totally dependent on nerve impulses from the brain
  • breathing stops if spinal cord is transected above the origin of the phrenic nerves (phrenic nerves originate from cervical segments 3, 4, and 5)
  • the motor neurons supply the expiratory muscles are inhibited when those supplying inspiratory muscles are active and vice versa
73
Q

Describe the voluntary system control of respiration

A
  • respiration can be controlled voluntary (ie. hyperventilate, hypoventilate - to extent that derangement in blood pH, etc occur)
  • neural pathway for voluntary control passes directly from the cerebral cortex downward through the corticospinal tract to the spinal neurons that drive respiratory muscles
  • voluntary system can influence ventilation within limits, but most of the regulation is controlled my autonomic
74
Q

Describe the automatic system control of respiration - what are the components?

A
  • ensures that blood PO2 and PCO2 are maintained in the normal range even during strenuous exercise or other types of respiratory stress
  • there are 3 components contributing to this autonomic regulation of respiration:
    1. receptors which collect information of the current status of blood gases and other factors
    2. central controller which coordinates and responds to the information provided by the receptors
    3. respiratory muscles which receive impulses from the controller to ultimately affect ventilation
75
Q

What are the different parts of the central controller?

A

the central controller consist of neurons located in four respiratory centers:
1. inspiratory area
2. pneumotaxic area
3. apneustic center
4. expiratory center

76
Q

Describe the inspiratory area of the central controller

A
  • represented by a dorsal group of neurons in the medulla
  • neurons in this area have inherent rhythmic excitability (important)
  • this neuronal activity initiates the inspiratory nerve signals to the diaphragm and other inspiratory muscles (rhythmic inspiratory drive)
  • inspiratory signals lasts for a few seconds, then halts - neurons remain dormant for about 3 seconds and then the cycle repeats
  • fundamental role in control of respiration (critical)
77
Q

Describe the pneumotoxic center of the central controller

A
  • represented by a group of neurons in the pons
  • transmits impulses to the inspiratory area to limit the duration of the inspiratory drive - helps to turn off inspiratory signal (prevents excessive inflation of the lung)
  • causes increase in respiratory rate
  • fine-tuning of respiratory but not essential
  • control of inspiratory volume and respiratory rate
78
Q

Describe the apneustic center of the central controller

A
  • represented by a group of neurons in the lower pons
  • transmits signals to the inspiratory area to prevent termination of the inspiratory drive (attempts to prolong the inspiratory drive)
  • as long as the pneumotaxic center is active, it overrides signals from the apneustic center
  • role in normal breathing is not clear, but if pneumotaxic center is damaged the apneustic center becomes active and prolongs inspiratory drive leading to maximal inflation of the lungs, with only occasional expiratory gasps
79
Q

Describe the expiratory area of the central controller

A
  • represented by a ventral group of neurons in the medulla
  • remain dormant during normal quiet breathing (expiration is passive during quiet breathing)
  • when respiratory drive for increased pulmonary ventilation becomes greater, signals from the inspiratory area spill over to the expiratory area and then the expiratory area neurons become active and transmit signals to expiratory muscles
  • contraction of the expiratory muscles promote active expiration
80
Q

What are the four groups of receptors that are responsible for ventilation?

A
  1. central chemoreceptors
  2. peripheral chemoreceptors
  3. lung receptors
  4. other receptors
81
Q

Describe central chemoreceptors

A
  • most important receptors involved in regulation of respiration
  • defined as a receptor that responds to change in the chemical composition of the fluid around it
  • the central chemoreceptors are located in the medulla where they are surrounded by brain extracellular fluid
  • receptors respond to changes in H+ concentration - H+ concentration of brain extracellular fluid affected by the blood CO2 concentration but not directly by H+ blood concentration (H+ and HCO3 do not diffuse into the brain extracellular fluid - while CO2 does and may form these compounds)
  • thus respond to changes in blood PCO2 levels
82
Q

Describe peripheral chemoreceptors

A
  • present in two locations: carotid bodies at the bifurcation of the common carotid arteries
    (most important) and in aortic bodies above and below the aortic arch
  • receive arterial blood and respond to changes in PO2 - when levels decrease to values below 60 mm Hg, the peripheral chemoreceptors will send signals to the central controller resulting in an increase in ventilation (increase respiratory rate, increase tidal volume) which in turn increases PO2 values
  • can also be stimulated by increase in blood PCO2 or fall in blood pH (however both these factors exert their effects on ventilation primarily through the central chemoreceptors)
83
Q

Describe lung receptors

A
  • number of receptors located in the lung that can affect ventilation
  • two examples: pulmonary stretch receptors and irritant receptors
  • pulmonary stretch receptors are activated by (over) distention of the lung and slow down respiratory rate (relevance under normal circumstances is unknown)
  • irritant receptors can be stimulated by dust particles, cigarette smoke, etc.
84
Q

Describe the other receptors of ventilation

A
  • ventilation can be influenced by several other factors
  • receptors not clearly defined
  • examples: receptors in the nose (+ upper airways) respond to chemical and mechanical stimulation, joint and muscle receptors that affect ventilation during exercise and pain and temp receptors
85
Q

Describe the response to changes in PaCO2

A
  • changes in arterial PCO2 represents the most important factor in the control of ventilation
  • increase in PaCO2 will stimulate ventilation whereas a decrease in PaCO2 will reduce the ventilatory drive
  • responses to change are rapid and sensitive
  • normally, ventilation response to PaCO2 variation during day is less than 3 mm Hg (but magnitude of response can be quite large
  • in some conditions (clinical and experimental) a chronic increase in PaCO2 occurs - in this situation there is an adaptation response and subject will have reduced ventilatory response
  • response largely due to central chemoreceptors - peripheral chemoreceptors also respond to a lesser extent
86
Q

Describe the response to changes in PaO2

A
  • ventilation response to changes in PaO2 are less marked than those to PaCO2
  • decrease in PaO2 to about 50 mm Hg (normal > 120 mm Hg) will initiate an increase in ventilation and the maximum increase is approximately 2-fold (to increase PaO2)
  • in normal subjects, response to PaO2 in daily control of respiration is small (but at high altitude + certain conditions PaO2 response becomes important)
87
Q

Describe the combined effects of PaCO2 and PaO2

A
  • the combined effects can exceed the sum of the individual responses
  • e.g. increase in PaCO2 (35 to 40 mm Hg) at PaO2 of 100 mm Hg may increase pulmonary ventilation 2-fold but an increases of PaCO2 (35 to 40) anf decrease of PaO2 (100 to 50) may cause a change in ventilation about 10-fold
88
Q

Describe the response to changes in pH

A
  • generally, reduction in pH will stimulate ventilation through stimulation of the peripheral chemoreceptors
  • note that arterial pH is closely related to PaCO2 levels such that it is difficult to determine specific pH response
89
Q

Describe the response to exercise

A
  • exercise can induce marled increase in ventilation. pulmonary ventilation during exercise can be 15-fold that of resting levels
  • response is rapid however exact mechanisms largely unknown
  • during exercise arterial PCO2, PO2 and pH remain relatively constant so responses not due to changes of these
  • joint and muscle receptors may be responsible
  • may be neurogenic; the cerebral cortex, while transmitting impulses to contracting muscles, may also transmit impulses into the respiratory center to excite respiratory center thus causing stimulation of ventilation
  • additional factors thought to be involved (unknown mechanisms)
90
Q

Describe obstruction of a bronchiole

A
  • in order for gas exchange to be effective it is important that both air and blood reach the blood gas barrier
  • can happen that either a bronchiole or capillary become obstructed (in these situations the lung and blood vessels compensate)
  • if a bronchus or bronchiole becomes obstructed, hypoxia develops in the alveoli beyond the point of obstruction
  • CO2 accumulates in the hypoxic area leading to decrease in pH
  • in response there is vasoconstriction and blood flow is diverted away from the hypoxic area as no purpose is served by perfusing under-ventilated alveoli
91
Q

Describe obstruction of blood flow

A
  • when blood flow to a bronchiole is reduced there will be no increase in PCO2 in those alveoli
  • the lung will respond with bronchoconstriction
  • note: compensatory mechanisms to match ventilation/ perfusion manage to maintain normal gas exchange when relative minor changes occur
  • however, in several diseases, mismatch of ventilation and perfusion is one of the main reasons for inadequate gas exchange
92
Q

Describe what occurs at high altitude

A
  • at high altitude the pressure is a lot lower than at sea level and the fractional concentration of oxygen is still 21% therefore the PO2 of inspired air is much lower (e.g. normal around 150 mm Hg, at high altitude around 70 mm Hg)
  • respiration is more difficult with such a low PO2
  • the first response is hyperventilation as a result of the low PO2 activating peripheral chemoreceptors
  • hyperventilation will cause decrease in PaCO2 resulting in a signal from the central chemoreceptors to reduce ventilation however after a period of time (one day) this response is reduced (adapted?)
  • second response is an increase in red blood cell concentration - results in an increased O2 carrying capacity of the blood
93
Q

Describe what happens with inhaled particles and polluted atmosphere

A
  • lung exposed to numerous aerosol particles present in air (smaller in size, further it will travel in lung)
  • variety of mechanisms to remove from lung, but two main:
    1. cleared by mucus that is moved up the airways through ciliary action
    2. taken up and cleared by alveolar macrophages
  • in addition to particles, there are other substances inhaled such as toxic gases from pollution or smoking and these pollutants can have various effects ranging from causing lung cancer, causing inflammation or worsening other respiratory problems
  • nitrogen oxide = “yellow” smog, can cause inflammation of airways
94
Q

Describe effects of carbon monoxide inhalation

A
  • colourless, odourless gas from incomplete burning of fuels (easily inhale without noticing)
  • danger is that it binds hemoglobin with an affinity 210 times that of O2
  • when exposed, has dramatic effect on oxygen transport
  • also affects the hemoglobin-O2 dissociation curve, making it harder to supply tissue with O2 (shifts curve to left)
  • initial symptoms are headache, fatigue, and shortness of breath
  • higher/ prolonged exposure results in death
95
Q

Describe effects of smoking and emphysema

A
  • large number of negative effects due to the large number of chemicals present in cigarette smoke
  • increases risk of lung cancer
  • presence of CO means smoker may have 5% of their hemoglobin bound to CO
  • emphysema is a condition largely due to smoking (may also be genetic)
  • condition is characterized by destruction of alveolar lung tissue leading to enlargement of air spaces
  • specifically, cigarette smoke induces proteolytic enzymes that degrade the alveolar walls and as a consequence the alveolar surface of the lung is decreased (+ loss of elasticity) and some of the lower airway collapses
  • leads to severe impairment in gas exchange (CO2 retention)
  • diagnosis: abnormal pulmonary function test, chronic cough, shortness of breath (w/ exercise), diagnosis often too late due to years of smoking
  • no effective, specific pharmacological therapy for emphysema and patient is treated with only supportive therapy (make comfortable)
96
Q

Describe neonatal respiratory distress syndrome

A
  • lung is last major organ to develop during gestation
  • premature birth thus can result in significant respiratory problems - disease called neonatal respiratory distress syndrome (NRDS)
  • primary lung dysfunction in NRDS is due to deficiency or absence of pulmonary surfactant
  • premature lungs are collapsed, and baby ventilated using mechanical ventilation
  • used to be major cause of infant mortality but decreased significantly since introduction of surfactant therapy (administration of a liquid containing pulmonary surfactant directly into baby’s lungs) - surfactant taken from cow lungs
97
Q

Describe acute respiratory distress syndrome (ARDS)

A
  • can affect people of all ages and has mortality rate ~ 40%
  • no specific cause, rather induced by a variety of diff insults (near drowning, smoke inhalation, infection or trauma)
  • insults lead to inflammation, impairment of surfactant system and some fluid leaking from blood into alveoli (edema)
  • physiological impact = impaired lung compliance and decreased gas exchange
  • definition based on the PaO2 value being below 200 mm Hg when breathing 100% O2 (low PaO2 values - caused by decreasing lung compliance)
  • patients require mechanical ventilation to maintain oxygenation (avoid further lung damage + supportive management)
  • many diff therapies have been tested but no effective pharmacological intervention discovered yet
  • liquid ventilation (perfluorcarbon, high O2 solubility, avoid surface tension forces - problems tho)
98
Q

What are obstructive and restrictive lung diseases?

A
  1. obstructive lung disease: characterized by difficulty to exhale the air from lungs. patients have reduced forced expiratory volume 1 sec, and normal or reduced vital capacity and total lung capacity (e.g. athsma)
  2. restrictive lung disease: associated with difficulty to expand the lungs. in these conditions pulmonary function test results show that these patients have reduced vital capacity and total lung capacity. forced expiratory volume 1sec is normal or decreased (e.g. pulmonary fibrosis and obesity)
99
Q

Describe athsma

A
  • most common lung disease + incidence rising
  • development includes environmental, and genetic components
  • basic impairment = bronchoconstriction - caused by a hyper-reactivity/ inflammation of airways to a number of substances such as allergen, environmental pollutants and cigarette smoke
  • athsma attacks may also be induced by exercise, especially in cold, dry air
  • diagnosis can be made on a variety of criteria including chest x-ray, physical examination, allergy test, pulmonary function test (spirometer) - due to constricted airways, the FEV-1sec is decreased during an asthma attack
  • treatment involves two strategies: to reduce airway inflammation by glucocorticoids (anti-inflammatory), and treatment of bronchoconstriction via inhaled bronchodilators

Human Physiology (13 decks)

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