Mechanics of Breathing, Gas Exchange and Transport

Question 1

What is airway resistance?

Why do we breath deeper when there’s more resistance?

Effect of asthma on airflow?

What is transpulmonary pressure?

Answer 1

• This occurs when air comes into contact with the airways, leading to changes in airflow. ↓Airway radius = ↑resistance = ↓air flow.
• When resistance is high, the pressure gradient can be increased to compensate. This can be done by deeper inspiration, decreasing alveolar pressure as much as possible.
• A sudden obstruction of the airways will lead to turbulent flow = wheezing noises.
• The difference between the alveolar and intrapleural pressures.

Question 2

What is lung compliance? How is it calculated?

How do some respiratory diseases affect lung compliance?

Answer 2

• The lung’s ability to stretch and expand. Calculated by Volume difference/Pressure difference, using lungs and transpulmonary.

↑Lung compliance = ↓elastic recoil. Less force needed to inflate lungs.
↓Lung compliance = ↑elastic recoil. More force needed to inflate lungs.

• Scoliosis, muscular dystrophy, obesity, COPD, fibrosis etc. cause a change in the chest wall mechanics, alveolar surface tension, and elastin fibres. ↓elastin fibres = ↑Lung compliance, while fibrosis/scarring stiffens the lungs = ↓Lung compliance.

Question 3

What are pulmonary surfactants and what are their roles in respiration?

What disease affects surfactant production?

Answer 3

• They are secreted by type 2 pneumocytes and work to ↓alveolar surface tension.

As the alveoli expand, the conc. of surfactants decreases to increase surface tension. This allows LARGER alveoli to collapse into the SMALLER alveoli - aid in lung inflation.

• NRDS (Neonatal respiratory distress syndrome) leads to insufficient surfactant production. This leads to stiff lungs, alveolar collapse, and oedema → hypoxia + resp. failure. Ultimately leading to acidosis, pulmonary/cerebral haemorrhage.

Question 4

What is the Ventilation (V):Perfusion (Q) ratio?

What is the affect of V:Q inequality?

Answer 4

V:Q < 1 = hypoventilation
V:Q > 1 = hyperventilation

It affects O2 and CO2 exchange. Physiologic dead space is ventilation without perfusion = ↑V:Q - due to heart failure, PE, emphysema etc. Shunt occurs when there’s perfusion without ventilation = ↓V:Q - due to pneumonia, cardiac shunts.

Question 5

What are the factors that affect the rate of gas exchange and what diseases change them?

Answer 5

1. Ventilation: ↓ in hypoventilation, COPD
2. Perfusion: ↓ in PE, COPD, heart failure
3. V:Q ratio: Inequality occurs in PE, pneumonia
4. Alveolar surface area: ↓ in emphysema
5. Diffusion distance: ↓ In oedema, fibrosis, ARDS

Question 6

Purpose of Hb

What are the factors that affect oxygen-Hb affinity? What are their effects?

What conditions are caused low and insufficient oxygen supplies?

Answer 6

Hb increases the blood’s carrying capacity of O2.

CO2, Temperature, DPG, PH all affect the affinity for O2:
↓pCO2, temp, DPG, and ↑PH shifts curve left = ↑affinity
↑pCO2, temp, DPG, and ↓PH shifts curve right = ↓affinity (Bohr Effect)

Cyanosis - blue/purple discolouration of skin due to poor oxygen transport
Central cyanosis - discolouration of skin, mucous membranes, lips, tongue - due to low blood pO2
Peripheral cyanosis - discolouration of extremities - due to poor O2 supply to tissues

Question 7

What is Anaemia and how does it affect O2 transport?

How does CO poisoning affect O2 transport?

Answer 7

• Low number of RBC’s and Hb. There’ll be ↓O2 supply to tissues (hypoxia). Due to iron deficiency or haemorrhage.
• Hb has a higher affinity for CO than O2 = Lower pO2. Hb-CO gives cherry-red colour and hypoxia occurs.

Question 8

How is CO2 transported in the blood?

What is the Haldane effect? what can go wrong?

Why is oxygen therapy bad for COPD patients?

What is the Bohr effect?

Answer 8

• Dissolved in plasma as bicarbonate and CO2, and some is bound to Hb.
• Where O2 displaces CO2 on Hb, to increase CO2 release. If excess CO2 can’t be released (e.g. with COPD) from the body, it accumulates as carbonic acid, leading to ACIDOSIS.
• They hypoventilate, leading to CO2 build up. So, when lots of oxygen is given suddenly, lots of CO2 is displaced from Hb = acidosis.
• When the conc. of CO2 affects Hb affinity for O2.

Question 9

How do the lungs and kidneys work to maintain blood PH?

Answer 9

To maintain PH, a carbonic acid-bicarbonate buffer system is used.

Hypoventilation = ↑CO2 = ↑H2CO3 = ↑HCO3-/H+ = Acidosis.
Hyperventilation = ↓CO2 = ↓H2CO3 = ↓HCO3-/H+ = Alkalosis.

↓HCO3- excretion = ↑blood HCO3- = Alkalosis
↑HCO3- excretion = ↓blood HCO3- = Acidosis