S2: Mechanics of Breathing

Question 1

Why do we need a respiratory system?

Answer 1

Cells require energy to function and aerobic respiration provides this energy as ATP. This requires O2 and produces CO2.

The atmosphere provides a source of O2 and CO2 can be expelled. Our bodies are too large rely on diffusion of gases from the atmosphere to tissues and evolution of the respiratory system overcomes this problem.

Question 2

Functions of the respiratory system

Answer 2

• Provides and ventilates a specialised surface where gas exchange can take place
• Contributes to acid base balance (e.g. PH of the blood)
• Communication
• Metabolism

Question 3

How does oxygen get from the atmosphere into cells?

Answer 3

1. O2 inhaled from atmosphere into alveoli within the lungs
2. O2 diffuses from alveoli into blood within pulmonary arteries
3. O2 transported in blood mostly bound to haemoglobin
4. O2 diffuses into cells/tissues for use in aerobic respiration
5. CO2 diffuses from respiring tissues to blood to be exchanged at the lungs

Question 4

Why is ventilation of gas exchange structure necessary?

Answer 4

Tissues continually demand O2 and produce CO2. An adequate concentration gradient between alveolar air and blood is required for efficient gas exchange (diffusion).
Therefore fresh air is required from the atmosphere to ensure alveolar oxygen pressure.

Alveolar O2 pressure is high and alveolar CO2 pressure is low relative to blood.

Question 5

How do gases move?

Answer 5

Gases naturally move from (connected) areas of higher pressure to lower pressure until an equilibrium is re-established.

At equal pressure, an equilibrium is established.

Question 6

Explain the Ideal gas law and Boyle’s law

Answer 6

Ideal gas law:

PV= nRT
Pressure = the number of gas molecules within a given volume

Boyle’s law:
p = n/v
If the n remains constant, increase in volume = decrease in pressure

Question 7

What causes the movement of air in and out of lungs?

Answer 7

This is achieved by changing the volume of the thoracic cavity

Question 8

Explain inspiration (active)

Answer 8

1. Respiratory muscles (e.g. diaphragm) contract
2. Volume of thoracic cavity increases
3. Alveolar pressure decreases below atmospheric pressure
4. Air moves down pressure gradient, through airways into alveoli, expanding the lungs

Question 9

Explain expiration (passive)

Answer 9

1. Respiratory muscles (e.g. diaphragm) relax, lungs recoil due to elastic fibres and deflate
2. Volume of thoracic cavity decreases
3. Alveolar pressure increases above atmospheric pressure
4. Air moves down pressure gradient into atmosphere deflating lungs

Question 10

How are the lungs and chest indirectly attached?

Answer 10

Pleural cavity

Question 11

What is the pleural cavity?

Answer 11

The pleural cavity is the fluid filled space between the membranes (pleura) that line the chest wall (parietal pleura) and line each lung (visceral pleura).

Pleurae also provide a frictionless surface to aid movement of the lungs.

Question 12

Explain the negative pressure in pleural cavity and ‘pleural fluid bond’

Answer 12

The opposing elastic recoil of the chest (outwards) and lungs (inwards) generates negative pressure within the pleural cavity.
The pleurae is prevented from seperating by the ‘pleural fluid bond’.

If the recoiling forces of the lungs and chest wall hypothetically disappeared, the molecules within the pleura cavity will occupy a certain volume, generating a certain pressure (equal to the atmosphere).

Question 13

What happens if negative intrapulmonary pleural pressure is lost?

Answer 13

Intrapleural pressure is naturally sub-atmospheric due to the opposing recoil of the chest wall and lungs

• Piercing either of the pleura will cause air to enter the space (pneumothorax) from either the lungs or atmosphere, depending on the injury
• Negative intrapleural pressure is lost and as the chest wall and lung are no longer indirectly attached, they will recoil in opposite directions causing the lungs to collapse (atelectasis)

Question 14

Compare open and closed pneumothorax

Answer 14

Pneumothorax causes affected part of the lung to collapse due to elastic recoil

Open pneumothorax is a hole in the parietal pleura and closed pneumothorax is piercing the visceral pleura.

Question 15

How is pneumothorax treated?

Answer 15

Pneumothorax is treated by removing air from the pleural cavity and re-establishing negative intrapulmonary pleural pressure (Pip).

Question 16

Explain inspiration and expiration using Pip (intrapulmonary pleural pressure) , Palv, air flow and volume change

Answer 16

1. As the thoracic cavity expands during inspiration, the lungs are forced to expand due to the negative Pip and the pleura fluid bond. However, the further the lungs are stretched, the more elastic recoil force generated lowering Pip.
2. As the lungs expand, the increase in volume decreases Palv. As air enters the lungs, the pressure increases once again as the increased concentration of gas molecules compensates for the increased volume (P=n/V).
3. When PAlv < PAtm the pressure gradient causes air to move into the lungs. Where PAlv > PAtm air moves out. The level of airflow is proportional to the size of the pressure gradient.
4. Entry of air into the lungs leads to inflation and increased volume which is reversed during expiration.

Question 17

What is the level of airflow proportional to?

Answer 17

The level of airflow is proportional to the size of the pressure gradient.

Question 18

What is transpulmonary pressure?

What is Hysteresis?

Answer 18

Transpulmonary pressure reflects the difference between Palv and Pip.

Hysteresis is the difference in transpulmonary pressure in expiration and inspiration

Question 19

Describe the air-liquid interface of alveoli

Answer 19

Air-liquid interfaces (e.g. Alveoli) generate surface tension which resist inflation. They are lined with fluid to enable gas exchange as the gas molecules can dissolve into water before diffusing.

Within the bubble formed by the water-air interface, surface tension arises due to the H-bonds between the water molecules, exerting a collapsing force toward the centre of the bubble

Question 20

What is Laplace’s law for alveoli?

Answer 20

The law of laplace describes the pressure generated by the surface tension within a bubble in the alveoli.

The collapsing forces generates pressure, The amount within a specific bubble is described by Laplace’s law:
P = 2T/r
T: Surface Tension
r: radius of bubble

Therefore if T remains constant: P =1/r
The smaller the alveoli, the larger the pressure generated!

Question 21

Why is the smaller the alveoli the harder it is to inspire equally throughout the lungs?

Answer 21

More pressure to collapse is therefore generated within a smaller bubble than a larger bubble.
Pressure gradients would be created between different sized alveoli resulting in smaller alveoli emptying into larger ones. This creates one big bubble making it hard to get inspiration on an equal basis throughout the lungs.

Question 22

What reduces alveoli surface tension?

How?

Answer 22

Alveoli surface tension is reduced by the presence of pulmonary surfactant secreted by type II pneumocytes. This helps equalise pressure between the large and small alveoli.

As alveoli expand, the concentration of surfactant molecules decreases, increasing surface tension. Now larger alveoli tend to collapse into smaller ones helping consistent inflation of the lungs.

Question 23

What is neonatal respirator distress?

Answer 23

Neonatal respiratory distress syndrome is caused by insufficient production of pulmonary surfactant. This can occur due to premature birth, maternal diabetes and congenital developmental issues.

This causes stiff, low compliance lungs with alveolar collapse eventually leading to respiratory failure. THis causes hypoxia which can cause pulmonary vasoconstriction, endothelial damage, acidosis, pulmonary and cerebral harrmorrhage.

Treatment: Artificial surfactant supplementation of infant and maternal gluocorticoid supplementation.