Breathing

Question 1

Compliance=…?

Answer 1

Change in volume/change in pressure

Compliance is the ability of the lung to stretch- the opposite of elastance

Question 2

What are the determinants of lung compliance?

Answer 2

Stretchability of tissue

Surface tension

Question 3

Describe surface tension in the alveoli

Answer 3

Arises because if the attractive forces between water molecules being greater than those between liquid and gas
Is actually 10% the theoretical value
Surfactant released by type 2 pneumocyte reduces surface tension and stops alveoli collapsing
Keeps the lungs ‘dry’- prevents surface tension sucking fluid out of the capillaries into the alveoli
Stops small alveoli collapsing into buffer alveoli because of its area-dependent effect- more efficient at reducing surface tension I smaller alveoli

Question 4

What is the intrapleural pressure?

Answer 4

Lungs tend to recoil inwards and the chest wall tends to move out creating a negative intrapleural pressure of 0.5kPa
So the lung distending pressure at rest is 0.5kPa which prevents the lung from collapsing

Question 5

Describe the structure of the airway

Answer 5

Trachea- contains cartilage rings, ligament containing smooth muscle and a ciliates epithelium
Biforcations with alveoli beginning at gen 17
Bronchioles have no cartilage but have smooth muscle and have a less ciliates epithelium- the speed of gas now matches the speed of diffusion- slow

Question 6

Describe air flow

Answer 6

Flow= pressure/resistance

Air flow in the respiratory system= driving pressure/ airway resistance
V= P(A)-P(B)/R(AW)

Question 7

Describe airway resistance

Answer 7

Poiseulle’s law would imply that the smaller airways have the most resistance- R=kx 1/r^4
Actually it is the larger airways that have the most resistance because of the turbulent flow found there and smaller airways have laminar flow

Question 8

Describe determinants of airway resistance

Answer 8

Airway smooth muscle tone- bronchodilation via beta2 receptors- adrenaline>noradrenaline
Bronchoconstriction via M3 receptors- ACh
Histamine and leukotrines are bronchoconstrictors

Question 9

Describe pulmonary stretch receptors

Answer 9

Vagal nerve ending in smooth muscle of the trachea and lower airways, stimulated by the lung inflation- slowly adapting stretch receptors
If the lungs are inflated to above threshold at anytime of inspiration is immediately switched off
Inhibits inspiration and promotes expiration
➡️NTS➡️PRG (suppresses DRG)

Question 10

Describe lung-irritant receptors

Answer 10

Vagal nerve endings between epithelia of trachea and lower airways
Stimulated by noxious gases, smoke, dust, cold air
Rapidly adapting receptors
Bring about- rapid shallow breathing
-long, deep augmented breaths

Question 11

Describe pulmonary C receptors/J receptors

Answer 11

In bronchial walls and alveolar walls close to capillaries
Usually silent- stimulated by mechanical distortions and increase in interstitial fluid
Evoke rapid shallow breathing

Question 12

Describe the nerve control in the nose and upper airways

Answer 12

Mechanical and chemical stimulation
Evoke sneezing, coughing and bronchoconstriction apnoea and bradycardia
Liquid evokes diving reflex

Question 13

Describe proprioreceptors in neural control

Answer 13

In muscle spindles in intercostal muscles
Allow for increased force of inspiration and expiration if movement is impeded
Responsible for the sensation of dyspnoea if large effort is required

Question 14

Describe atrial baroreceptors

Answer 14

Increase in blood pressure leads to reflex hypoventilation

Decrease in blood pressure leads to reflex hyperventilation

Question 15

Describe the muscles used in breathing

Answer 15

Inspiration- diaphragm is the only major inspiratory muscle
External muscles- scalene and sternocleomastoid muscle used in exercise, coughing and vomiting
Expiration- passive diaphragm relaxation
Internal muscles- abdominal and oblique muscle involved only in forced breathing

Question 16

Briefly describe central control of timing breathing and rhythm

Answer 16

PRG in the pons important for timing
DRG and VRG in the medulla important for rhythm generation
DRG- inspiratory neurones, in the nucleus tractus solitarius (NTS)- spontaneous activity synchronised with breathing
VRG- inspiratory/expiratory motor neurones, activity in the DRG excites inspiratory neurones and inhibits expiratory neurones

Question 17

What causes inspiration during quiet breathing?

Answer 17

DRG- self-excitable neurones, repetitively generating action potentials to cause inspiration
➡️C spinal cord ➡️phrenic nerves ➡️diaphragm contracts ➡️inspiration

Question 18

What causes expiration?

Answer 18

Entirely passive

DRG inactive➡️ diaphragm relaxes➡️ expiration

Question 19

What neural control causes forced breathing?

Answer 19

Inspiratory neurones in the VRG via the T spinal cord causes other inspiratory muscles to contract
Expiratory neurones causes expiratory muscles to contract

Question 20

Describe the involvement of higher brain centres in respiration

Answer 20

Hypothalamus- temperature, fight/flight response
Limbic system and cerebral cortex- emotion/pain
Motor cortex- limb receptors, excercise

Question 21

Give a brief overview of CNS in respiratory control

Answer 21

Brainstem- automatic- metabolic

Cortical- voluntary- behavioural

Question 22

Briefly describe peripheral chemoreceptors

Answer 22

Carotid bodies and aortic bodies involved with blood gas and acid-base homeostasis- respond to PO2, PCO2 and pH
Intrinsically sensitive to oxygen
Functional unit is the glomerulus (type 1 or glomus cells)

Question 23

Describe the process of sensing hypoxia

Answer 23

Low oxygen in blood➡ O2 sensor in PM of glomus cell➡ inhibition of K channels– cell depolarisation- voltage-gated calcium channel open- Calcium influx➡ neurosectretion of ATP/ACh/DA➡ afferent action potentials

Question 24

How might hypoxia be sensed in glomus cells

Answer 24

1. Hypoxia inhibits oxidative phosphorylation which increases AMP:ATP ratio which activates AMP kinase which inhibits K channels
2. In hypoxia conditions haem oxygenase 2 (HO-2) is inhibited so it does not generate CO so cystathionine-gamma-lyase (CSE)-regulated H2S generation is not inhibited so H2S closes K channels and the cell becomes depolarised

Question 25

Describe central chemoreceptors

Answer 25

Ventral sites eg. retrotrapezoid nucleus
Dorsal sites eg. solitary tract nucleus
Midline sites
Glial cells?
Generate 80% of the total response to CO2
Inhibited by hypoxia
The non-polar H2O and CO2 diffuse through the BBB and form HCO3 and H which are sensed by central chemoreceptors➡ medullary respiratory neurones➡ adjustment of ventilation

Question 26

What is the relationship between gas velocity pressure and size of airways

Answer 26

Lower pressure in larger airways
Higher velocity in larger airways
Increased gas velocity causes decreasing pressure- Bernoulli
Increased turbulent flow in larger airways
Pressure falls more quickly in turbulent flow- Reynolds

Question 27

What is the equal pressure point?

Answer 27

If the pressure in the airways is less than or equal the P(Pl) the airway will collapse unless it is in large airways with cartilage
(Pressure falls along the airways because of Bernoulli and Reynolds)

Question 28

Describe peak expiratoiry flow?

Answer 28

Greatest at total lung capacity

• surfactant is least effective ie. Large surface tension
• able to generate greatest positive P(Pl) to prevent aoitrway collapse
• airway resistance at lowest