Week 14 Physiology - Resp Phys I

Question 1

What is anatomical dead space?

Answer 1

Anatomical dead space = volume of conducting airways (i.e. not participating in gas exchange) usually 150mL

Question 2

What is physiological dead space?

Answer 2

Physiological dead space = volume of lung which doesn’t eliminate CO2.

**In healthy patients, very closely mirrors anatomical dead space, but in underlying lung disease, VQ mismatch can significantly increase areas of lung not participating in gas exchange

Question 3

What does Bohr’s method state regarding physiological dead space?

Answer 3

All measured CO2 must come from respiratory zone, therefore:

Volume of physiological dead space / tidal volume = (alveolar CO2 - expired CO2) / Alveolar CO2

i.e. dead space in non-conducting areas (or gas exchange participating areas of the lung) where exchange isn’t happening.

**PAO2 = alveolar O2, PaO2 = arterial O2

Question 4

What is an acinus?

Answer 4

The gas exchanging unit of the lungs, comprising portion of the lung distal to the terminal bronchiole (i.e., the last purely conducting airway), which is composed of the respiratory bronchioles, alveolar ducts, alveolar sacs, and alveoli

Question 5

Define tidal volume:

Answer 5

Amount of air moving into the lungs with each inspiration/expiration - 500mL

Question 6

Define inspiratory reserve:

Answer 6

Air inspired with with maximal inspiratory effort in excess of tidal volume - 3000mL

Question 7

Define inspiratory capacity:

Answer 7

Maximum amount of volume that can be inhaled after a normal tidal expiration (TV + IRV)

Question 8

Define expiratory reserve:

Answer 8

Volume expelled by active expiratory effort after passive expiration - 1200mL

Question 9

Define residual volume:

Answer 9

Volume remaining in lungs after maximal expiratory effort -1200mL

Question 10

Define forced vital capacity:

Answer 10

Largest amount of air that can be expired after maximal inspiratory effort - 4700mL

Question 11

Define FEV1:

Answer 11

Fraction of vital capacity expired during the first second of forced expiration

Question 12

What is Fick’s law for diffusion?

**Think, one proptional, one inversely proportional

Answer 12

Rate of transfer of gas across a membrane is proportional to tissue surface area and difference in gas partial pressure between the two sides.

Inversely proportional to the tissue thickness

(0.03 micrometres = usual thickness of alveolar membrane)

Question 13

What is the diffusion constant of a gas mean regarding gas exchange?

Answer 13

Gas exchange proportional to solubility of gas and inversely proptional to molecular weight

i.e. CO2 diffuses 20x more rapidly than O2 as it is much more soluble, despite similar molecular weight

Question 14

Give an example of a lung condition that affects Fick’s law gas diffusion for surface area and membrane thickness?

Answer 14

Emphysema = decreased tissue surface area
ILD = increased membrane thickness

Question 15

What does ‘diffusion limited’ mean regarding gas exchange, and what gas is the prototypical example?

Answer 15

Carbon monoxide.

CO rapidly crosses blood gas barrier from alveoli to blood stream, and then tightly binds to Hb –> meaning it doesn’t really increase the partial pressure of CO in the blood, and so maintains a concentration gradient that favours movement from alveoli to blood.

Therefore, the gas transfer is limited by rate of gas crossing membrane, rather than amount of blood passing at a given time.

Question 16

What does ‘perfusion limited’ mean regarding gas exchange, and what gas(es) are prototypical examples?

Answer 16

NO2, CO2.

Gases not bound by Hb cause increased partial pressure in the blood, which then influences diffusion down concentration gradient. It requires blood to be constantly moving to maintain a concentration gradient that is favourable to gas exchange. (i.e so that alveolar gas conc > arterial)

Question 17

How is O2 a combination of both diffusion and perfusion limited?

Answer 17

Capillary PO2 reaches equivalent of alveolar gas when RBC is 1/3 of way along capillary. This is when gas exchange is perfusion limited (i.e. concentration gradient reaches equilibrium )

Because of Hb, that allows partial pressure of O2 to remain lower than alveolar O2, facilitating diffusion down concentration gradient.

In cases of increased membrane thickness, the Pa02 value doesn’t reach the alveolar value by the endow the capillary, and instead is more diffusion limited.

Question 18

What impact does exercise have on oxygen exchange at the alveolar membrane?

Answer 18

Becomes diffusion limited because HR increases and blood is travelling much faster through capillary, therefore doesn’t have enough time for diffusion to equalise the alveolar and arterial O2

Question 19

Where is most resistance in the airways?

Answer 19

Major site of airway resistance is medium sized bronchi

**Airway resistance progressively increases up to 7th airway generation, then drops –> very small bronchioles contribute very little to resistance

Question 20

What are factors determining airway resistance?

Answer 20

Lung volume
Bronchial calibre/bronchial smooth muscle activity
Gas density
Dynamic compression of airways during forced expiration (intrapleural pressure > alveolar pressure)

**Airway resistance increases with nasal breathing (i.e. halving size of the tube)

Question 21

Summarise Poiseuille’s Law:

Answer 21

Resistance is proportional to length and viscosity, and inversely proportional to to radius

Question 22

What is a formulaic definition of compliance?

Answer 22

Compliance = volume change / pressure change

With a balloon analogy, the bigger the change in volume for the same pressure generated by blowing in, the greater the compliance of the balloon

Question 23

What 3 elements regulate breathing?

Answer 23

1. Sensors (i.e. chemoreceptors)
2. Central controller (Midbrain)
3. Effectors (respiratory muscles)

Question 24

What are the 3 main groups of neurons responsible for respiration?

Answer 24

Medullary respiration centre
Apneustic centre
Pneumotaxic centre

**Cortex, which can override function of brainstem (within certain limits)

Question 25

What is the role of the medullary respiratory centre?

Answer 25

Located in ventrolateral region of medulla:
1. Dorsal respiratory group: associated with inspiration
2. Ventral respiratory group: associated with expiration

Pacemaker neurons for intrinsic rate of periodic firing, responsible for generation of respiratory rhythm

Question 26

Describe the “ramp” process of a breath?

Answer 26

After latent period between breaths, action potentials being to appear, increasing in crescendo over the next few seconds to cause a coordinated contraction of respiratory muscles to generate negative intra-thoracic pressure –> drawing air into lungs. Followed by cessation of action potentials –> relaxation and passive recoil of lungs and expiration

Question 27

When is the expiratory area of respiratory centre quiescent?

Answer 27

During normal quiet breathing, there is no need for forced expiration

Question 28

With cortical override, what parameters limit hypo and hyperventilation?

Answer 28

Hypoventilation limited by PCO2 and PO2
Hyperventilation limited by respiratory muscle fatigue

Question 29

Where are central chemoreceptors located?

Answer 29

Near ventral surface of medulla (near exit of 9th and 10th cranial nerves)

Question 30

What is the ion that dictates activity of central chemoreceptors?

Answer 30

H+

Increased = increased ventilation
Decreased = decreased ventilation

Question 31

Explain how increased CO2 leads to increased H+ in CSF:

Answer 31

Central chemoreceptors are bathed in CSF, which is impermeable to H+ ions from plasma.

However, CO2 is freely permeable, and in CSF forms carbonic acid –> H+ and HCO3-

Increasing levels of CO2 diffusion will increase H+ content of CSF and cause increased activity of central chemoreceptors

Question 32

Where are peripheral chemoreceptors located?

Answer 32

Located in Carotid bodies - between bifurcation of the common carotid arteries (CN IX) and aortic bodies - above and below aortic arch (CN X)

Question 33

What factors influence activity of peripheral chemoreceptors?

Answer 33

Decreases in arterial PO2
Increases in arterial PCO2
Decreases in pH (in carotid bodies only)

Question 34

What are pulmonary stretch receptors?

Answer 34

Located in airway smooth muscle, discharge in response to lung distension (via vagus nerve) which acts to inhibit activation of other respiratory muscles. Largely inactive in adults except for exercise.

Question 35

What are irritant receptors?

Answer 35

Located between airway epithelial cells, stimulated by noxious gases, inhaled dust/smoke/cold air

Impulses transmitted via vagus nerve, causing reflex bronchoconstriction and hyperpnoea

Question 36

What are juxtacapillary receptors?

Answer 36

Non-myelinated C fibres located in alveolar walls close to capillaries, send impulses via vagus nerve in response to engorgement of capillaries/increased interstitial fluid, causing rapid, shallow breathing

Question 36

What physiological changes occur at altitude?

Answer 36

Hyperventilation (due to decreased FiO2) –> driven by hypoxic stimulation of peripheral chemoreceptors

Polycythaemia (increased EPO)

Hypoxic vasoconstriction

Question 36

What does pulmonary vasoconstriction occur in response to?

Answer 36

Hypoxia in alveoli –> results ins increased pulmonary arterial pressure and R) sided heart strain (which can be exacerbated by polycythaemia)

Question 37

What occurs to the O2 dissociation curve at altitude?

Answer 37

Initially a RIGHT shift (decreased Hb affinity for O2) due to increased 2-3 DPG (a metabolic breakdown product in RBCs).

At extreme elevation, extreme alkalosis can shift curve to the LEFT. (Due to decreased CO2 levels in the setting of hyperventilation)

Question 38

What is the syndrome of acute mountain sickness?

Answer 38

Headache, fatigue, palpitations, insomnia, High Altitude Pulmonary Oedema (secondary to pulmonary hypertension)