Physiology of Respiration

Question 1

The airways are a conduit for the passage of gases. On which airway factors does the flow of gas depend

Answer 1

1) Pressure gradient between alveoli and atmosphere

2) Compliance and resistance of airways

Question 2

What physiological changes occur to affect airway compliance/resistance

Answer 2

Constriction and dilatation of airways under autonomic control

Question 3

How do the airways protect the lungs

Answer 3

1) Air filtered by nasal hair and mucociliary escalator
2) upper airway warms and humidifies the air
3) vocal folds in the larynx and cough reflex protect the lungs against aspiration

Question 4

What muscles are used in inspiration

Answer 4

Diaphragm (main respiratory muscle)
External intercostals 
Accessory inspiratory muscles
- scalenes
- sternomastoid
- pectoralis
- latissimus dorsi

Question 5

Describe the action of inspiration (passive and forced)

Answer 5

Diaphragm contracts and flattens
External intercostals elevate the rib and sternum to increase the anterior-posterior dimension of thoracic cavity
Both of the above increases the intra-thoracic volume, reduces the intra-thoracic pressure and allows air to flow passively into the lungs
- forced inspiration uses accessory muscles to further increase the intra-thoracic volume and further reduce pressure

Question 6

What muscles are used in expiration

Answer 6

Passive - elastic recoil produces positive intra-alveolar pressure
Forced - abdominal wall muscles and intercostal muscles increase the intra-alveolar pressure and drive the air out

Question 7

Describe the action of expiration

Answer 7

Diaphragm relaxes and domes
Lungs and chest wall recoil
Thoracic volume reduces, intra-thoracic pressure rises and airflows out of the lungs

Question 8

How is airway compliance calculated

Answer 8

compliance = change in lung volume/change in pressure

Question 9

How does lung compliance affect lung volume

Answer 9

High/good compliance means the lungs are easily inflated

Poor/low compliance means the lungs are stiff, difficult to inflate, and therefore don’t reach normal volumes

Question 10

Describe the role of surfactant in respiration

Answer 10

Surfactant is a substance produced by pneumocytes. It counteracts the surface tension that would otherwise make the spherical alveoli collapse. This minimises the work needed to re-inflate the lungs with each inspiration

Question 11

Describe the work of breathing

Answer 11

During inspiration only as expiration is passive

1) work to expand lungs against elastic forces and surface tension
2) work to overcome airway resistance
3) movement of chest wall

Question 12

What factors affect a persons respiratory capacity

Answer 12

Depends on volume of gas moved and respiratory rate at which this occurs

Question 13

What is the respiratory minute volume

Answer 13

Tidal volume x respiratory rate

- amount of air brought into the lungs per minute

Question 14

What is physiological dead space

Answer 14

Anatomical dead space + alveolar dead space

Anatomical dead space - e.g. nose, mouth, pharynx, larynx, trachea and bronchi

Alveolar dead space - volume of diseased lung unable to perform gas exchange

Question 15

How to calculate the alveolar ventilation rate

Answer 15

Alveolar ventilation rate = (tidal volume - dead space) x respiratory rate

Therefore any increase in dead space causes an increase in respiratory minute volume in order to maintain the same alveolar ventilation rate

Question 16

Describe the Bohr equation

Answer 16

Used to calculate physiological dead space

• As atmospheric pCO2 is practically zero, all expired CO2 must come from the communicating alveoli, and none from the dead space
• The Bohr equation compares to pCO2 in expired air with that of pCO2 in communicating alveoli (this is the same pCO2 as the arterial blood and so can be obtained with an ABG).
• This comparison calculates the ‘non-pCO2 containing volume’ a.k.a. the physiological dead space

Question 17

Definitions of the following in spirometry

• TV - tidal volume
• IRV - inspired respiratory volume
• ERV - expiratory reserve volume
• RV - residual volume
• FRC - functional residual capacity
• VC - vital capacity
• TLC - total lung capacity
• FVC
• FEV1

Answer 17

TV - volume of air moved in quiet respiration (0.5L)
IRV - max volume inspirable (3L)
ERV - maximum volume expirable after tidal volume expiration (1L)
RV - residual volume in the lungs after maximum expiration (1.5L)
FRC - ERV + RV = amount left in lungs after tidal volume expiration (2L)
VC - the volume that can be expired after a maximal inspiratory effort (4.5L)
TLC - sum of vital capacity and residual volume (6L)
FVC - gives an idea of vital capacity
FEV1 - forced expiratory volume - volume expired in 1st second of a forced expiration FVC measurement

Question 18

What is the function of the FEV1:FVC ratio

Answer 18

FVC is reduced in restrictive lung disease
FEV1 is reduced in obstructive lung disease as gas can’t be forced out quickly
FEV1/FVC ratio
- normal > 0.7
- obstructive <0.7
- restrictive >0.7 (normal ratio, but overall reduced FVC)

Question 19

Describe the neurological control of breathing

Answer 19

Neurones controlling respiration are found in the medulla
Respiratory centres are located near the pons
- inspiratory neurones have spontaneous rhythmical activity
- expiratory neurones are inactive unless expiration is forced

Question 20

Describe the action of central chemoreceptors in respiration

Answer 20

These detect chemical changes in the blood due to changes in partial pressure of oxygen or local hydrogen ion concentration in the blood
- central receptors detect pH changes - this is because CO2 crosses the blood brain barrier and dissolves in the CSF. Receptors detect increase in CSF H+ ion concentration

Question 21

Describe the action of peripheral chemoreceptors in respiration

Answer 21

These detect chemical changes in the blood due to changes in partial pressure of oxygen or local hydrogen ion concentration in the blood

Location

• carotid bodies supplied by CN IX (glossopharyngeal)
• aortic bodies supplied by CN X (vagus)

Peripheral chemoreceptors detect changes to pO2 levels in the blood

Question 22

Describe the action of stretch receptors in respiration

Answer 22

Negative feedback from lung stretch receptors as the lung inflates to cause termination of inspiration
Inhibitory impulses pass through the vagus nerve to prevent over-inflation

Question 23

Physiology of gas exchange in the lungs

Answer 23

Simple diffusion across the alveolar-capillary interface

- driven by partial pressures and solubility of gases involved (CO2 more soluble than O2)

Question 24

How to calculate the V/Q ratio

Answer 24

V/Q ratio = alveolar ventilation rate/pulmonary blood flow

Question 25

What does a decrease in the V/Q ratio imply

Answer 25

Occurs when significant regions of the lungs are perfused, but not ventilated - decrease in O2 and CO2 exchange, so there is a gradual decrease in O2 and a gradual increase in CO2 in the arterial blood

Question 26

What does an increase in the V/Q ratio imply

Answer 26

Occurs when significant regions of the lungs are ventilated but not perfused e.g. PE - causes an increase in physiological dead space in the lungs

Question 27

How is oxygen carried in the blood

Answer 27

Small amount is dissolved in plasma - known as PaO2 Most oxygen is carried by haemoglobin - Hb has four binding sites for oxygen

Question 28

Describe how Hb's affinity for oxygen changes as it circulates through the body

Answer 28

Hb readily binds oxygen at the capillary-alveaolar interface. As each haem binds an oxygen molecule, it increases the binding capacity of the other three sites (cooperativity) When all sites are full, the Hb is fully saturated When Hb reaches the capillary-tissue interface, it releases the oxygen. Dissociation of one oxygen molecule here makes it easier for the remaining molecules to dissociate

Question 29

What does the oxyhaemoglobin dissociation curve represent

Answer 29

The relationship between the oxygen saturation of haemoglobin (SaO2) and the partial pressure of arterial oxygen (PaO2)

Question 30

What is the Bohr effect

Answer 30

Factors that alter the position of the oxyhaemoglobin dissociation curve In the lungs, CO2 diffuses from blood to the alveoli and the hydrogen ion concentration in the blood falls - this pushes the curve to the left to increase the quantity of oxygen that can bind with haemoglobin When blood reaches the tissues it absorbed the products of metabolism (CO2) and hydrogen ion concentration in the blood increases - this pushes the curve to the right, to favour the release of oxygen molecules and delivery of oxygen to the tissues

Question 31

What factors push the oxyhaemoglobin dissociation curve to the left and to the right

Answer 31

Pushing left - decrease in PaCO2, decrease in hydrogen ion concentration, lower temperature, increased foetal haemoglobin, increased carboxyhaemoglobin and decreased 2,3 - DPG Pushing right - increase in PaCO2, increase in hydrogen ion concentration, increased temperature and increased 2,3-DPG.

Question 32

How is carbon dioxide transported in the blood

Answer 32

1) free CO2 in plasma - roughly 10% 2) reaction with deoxyhaemoglobin to form carbamino-haemoglobin - roughly 30% 3) reaction with water in plasma to form hydrogen ions and bicarbonate - this reaction is catalysed within red blood cells by carbonic anhydrase - increase red blood cells, hydrogen ions bind to haemoglobin (protein buffer) and bicarbonate ions are diffused out in exchange for chloride ions

Question 33

What is the Haldene effect

Answer 33

As partial pressure of oxygen increases, the amount of CO2 carried by the blood falls Because venous deoxyhaemoglobin is a weaker acid than oxyhaemoglobin