Case 6 - physiology part 2

Question 1

How does compliance change in pneumothorax

Answer 1

A puncture in the thoracic case will cause the lungs to collapse as air enters. The thoracic cage then expands outwards as it undergoes elastic recoil

Question 2

What is surfactant

Answer 2

It is secreted by type 2 alveolar cells, it is a mixture of phospholipids and proteins.

Question 3

Role of surfactant

Answer 3

It reduces the surface tension forces in the alveoli:
• This reduces the work of breathing.
• Helps prevent fluid accumulation in the alveoli.
• Helps keep alveoli uniformly ventilated.

Question 4

Amount of surfactant in the alveoli

Answer 4

There is a higher concentration of surfactant in the smaller alveolus so it has a lower surface tension. The surface tension of water and surfactant is proportional to the surface area. This will improve ventilation making it more uniform between the different alveolus as its normally more difficult to inflate a smaller alveolus

Question 5

What is responsible for elastic recoil?

Answer 5

• Elastic fibres in the lungs= elastin and collagen fibres in the alveolar walls and around vessels and bronchi. These are affected by emphysema and change with age.
• Surface tension= there is a liquid film lining the alveoli, the attractive forces between adjacent liquid molecules are much stronger than those between liquid and gas. This keeps the lungs and chest wall together.

Question 6

What is surface tension

Answer 6

A measure of the forces acting to pull a liquid’s surface molecules together at an air-liquid interface, The surface tension in the alveolus has to be overcome in inspiration

Question 7

What forces do the respiratory muscles have to overcome in inspiration?

Answer 7

• Elastic recoil and thorax, 80% of the work of breathing.
• Resistance to airflow in airways.
• Resistance due to deformation of tissue in the lungs and thorax (tissue resistance).

Question 8

Importance of the intrapleural pressure

Answer 8

This pressure keeps the lungs and chest wall stuck together, the fluid molecules in the intrapleural space are attracted to each other keeping the lungs and chest together.

Question 9

What does the intrapleural pressure vary with:

Answer 9

• Stage in respiration (inspiration vs expiration).
• Position in the lung (apex vs base) when they are standing up.
• Pattern of breathing (rest vs forced breathing), forced inhalation will cause a more negative pressure

Question 10

Intrapleural pressure in the base

Answer 10

End of expiration= -0.4kPa

End of inspiration= -0.8kPa

Question 11

Intrapleural pressure in the apex

Answer 11

End of expiration= -1.0kPa

End of inspiration= -1.4kPa

Question 12

Intrapleural pressure during forced breathing

Answer 12

End of expiration= +3kPa

End of inspiration= -4kPa

Question 13

What is lung compliance

Answer 13

The ease in which a lung can be inflated. A measure of the change in lung volume which occurs with a specific change in pressure

Question 14

Lung compliance equation

Answer 14

Compliance= Change in lung volume / Change in intrapleural pressure

Question 15

Compliance in inspiration and expiration

Answer 15

High pressure causes a higher volume, it then plateaus out. In expiration you will have a higher volume for a given pressure. The lung never fully empties so the volume cant be 0.

Question 16

Specific compliance equation

Answer 16

Specific compliance= Compliance / lung volume (one lung)
As compliance is affected by lung volume, it is difficult to use references as people have different lung volumes. This is why you use specific compliance

Question 17

Compliance in a healthy person

Answer 17

2L/kPa

Question 18

Response to CO2 in hypoxia

Answer 18

The effect of hypoxia and Hypercapnia is more then the sum of hypoxia alone and Hypercapnia alone

Question 19

What part of the lungs offer the most and least resistance

Answer 19

The bronchi have the most resistance as the cross sectional area is the lowest, the resistance is lowest in the bronchioles because they have the highest cross-sectional area

Question 20

Equation for airway resistance

Answer 20

Airway resistance= Driving pressure / gas flow

Question 21

What is driving pressure (resistance)

Answer 21

The difference between the atmospheric pressure (mouth) and the average pressure across all alveoli in the lungs

Question 22

Factors affecting airway resistance- tethering

Answer 22

A normal airway will be held open by the tethering effect of the alveolar wall whilst a ‘floppy’ airway in patients with emphysema will no longer be supported by alveolar tissue. A floppy airway will have increased resistance as the airway collapses when you breathe out, and becomes constricted. The tethering tissue is lost in emphysema

Question 23

Other factors that affect airway resistance

Answer 23

1) A high lung volume causes decreased resistance
2) A foreign body in the airways reduces diameter
3) Contraction of smooth muscle, controlled by ANS reduces diameter
4) Breathing out creates a more positive intrapleural pressure which compresses the airway and increases resistance, this occurs in emphysema. As less air can be removed there will be a bigger starting volume and reduced compliance

Question 24

Local control of airway diameter (produced in the airways)

Answer 24

• Histamine- increases smooth muscle contraction, increases vascular permeability causing airway odema, increases mucus secretion.
• Prostaglandins and thromboxanes- contraction or relaxation of smooth muscle
• Leukotrienes- contraction of smooth muscle, antagonists are used in cancer treatment.

Question 25

What causes Bronchoconstriction

Answer 25

Histamines, leukotrienes, prostaglandin, pulmonary stretch receptors and irritant receptors.

Question 26

What causes Bronchodilation

Answer 26

Sympathetic Beta2 stimulation, adrenaline and increased CO2 tension.

Question 27

What activates the irritant fibres

Answer 27

Mechanical and chemical stimulation

Question 28

What activates the C-fibre receptor

Answer 28

Chemical stimulation

Question 29

Parasympathetic innervation of airways

Answer 29

The afferent branch of the Vagus nerve is a parasympathetic nerve activated by the irritant receptor. It sends signals to the CNS. C-fibre receptors also send signals to the CNS. The CNS sends an efferent nerve to the mucus gland and smooth muscle. They release Ach which binds to Muscarinic Receptors, there is an increase in Ca+2 concentration which causes bronchoconstriction. There is also mucus secretion from goblet cell which decrease luminal diameter and increase resistance

Question 30

Sympathetic innervation of the airways

Answer 30

By adrenaline which binds to Beta2 adrenergic receptors which cause vasodilation by increasing Cyclic AMP. The Alpha-1 adrenergic receptor inhibits. Has little effect

Question 31

What drugs can reduce bronchial muscle tone and airway resistance

Answer 31

Beta 2 agonist
Antihistamines
Muscarinic antagonists
Leucocyte anatgaonists
Steroids
Adrenaline