L8 - Mechanisms of breathing

Question 1

Boyle’s Law

Answer 1

• P1V1 = P2V2
• Increase in volume = decrease in pressure
• Intra-alveolar pressure less than atmospheric pressure => air flows into lungs
• Intra-alveolar pressure more than atmospheric pressure => air flows out of lungs

Question 2

What happens during inspiration?

Answer 2

1. Diaphragm contracts + flattens + external intercostal muscles contract
2. Increase of volume of thorax + lungs
3. Decrease of pressure inside the lungs to below atmospheric pressure (-3mmHg)
4. Air flows into lungs
5. Lungs expand more as more air enters in – negative pressure between lungs and thorax (achieved by pleural membranes)

Question 3

What happens during expiration?

Answer 3

1. Diaphragm relaxes
2. Lungs recoil
3. Volume of thorax + lungs decreases
4. Pressure rises to +3mmHg inside lungs
5. Airs moves out of lungs - down pressure gradient

Question 4

What are pleural membranes?

Answer 4

• A double membrane fluid filled bag that surround the lung tissues
• Creates a vacuum round the lungs

Question 5

What happens to pleural membranes during inspiration?

Answer 5

1. Thorax expands + diaphragm contracts
2. Intrapleural pressure – sub-atmospheric
3. Transpulmonary pressure increases
4. Lungs expand
5. Alveolar pressure – sub-atmospheric
6. Air flows in

Question 6

Neuronal control of breathing

Answer 6

• Normal breathing is involuntary
• Regulated by neurons in medulla oblongata + pons
• Medulla oblongata:
• I neurons controlling inspiration
• E neurons controlling expiration
• Neurons - firing rhythmically + sending impulses down the nerve to diaphragm — generation of basic rhythm of breathing

Question 7

Medulla oblongata

Answer 7

• Dorsal respiratory group = inspiratory centre
• Ventral reparatory group = inspiratory + expiratory centre

—> signals from these centre sent to intercostal muscles + diaphragm

Question 8

Pons

Answer 8

• Apneustic centre - sends impulses to I neurons to keep inspiration going
• Pneumotaxic centre - sends impulses to Dorsal respiratory Group that switches off I neurons - limiting time of inspiration

Question 9

What are mechanoreceptors?

Answer 9

• Found in the airways, trachea, lung, and pulmonary vessels
• Provide sensory information to the respiratory centre in the brain
• Sensitive to changes in mechanics/movement – lung volume, airway stretch, vascular congestion

Question 10

What are proprioceptors & Hering-Breur reflex?

Answer 10

• Stretch receptors in the smooth muscle of bronchioles
• Activated by stretch in lung tissues - then send signal to medulla oblongata through vagus nerve to inhibit I neurons —-> Hering-Breuer reflex
• Hering-Breuer reflex - prevents over-inflation of lungs + leads to expiration

Question 11

What are peripheral chemoreceptors?

Answer 11

• Found in glomus cells in aortic + carotid bodies
• Detect changes in arterial blood oxygen - increased response to low oxygen level
• Initiate reflexes that are important for maintaining homeostasis during hypoxemia (low levels of oxygen in your blood)

Question 12

What are central chemoreceptors?

Answer 12

• Found near respiratory centres of medulla oblongata + on surface of brain ventricle
• Sensitive to pH of their environment - respond to H+ concentration of the cerebrospinal fluid (CSF) — not in contact with blood (so blood pH does not fire the receptors)
• CO2 can diffuse into CSF - increasing H+ => lower pH

Question 13

How are central chemoreceptors activated?

Answer 13

1. Blood pCO2 rises
2. CO2 diffuses into CSF
3. CO2 converted to carbonic acid (carbonic anhydrase)
4. carbonic acid dissociates to give bicarbonate ions + H+ ions
5. Increase in H+ — triggers response from chemoreceptors via impulses sent to respiratory centres