Hypoxia, Hypercapnia & Vascular Physiology

Question 1

Define: Hypoxia & Hypercapnia

Answer 1

Hypoxia: deficiency of O2 @ tissue level

Hypercapnia: CO2 retention & increased arterial pCO2

Question 2

What is the most common type of hypoxia?

Answer 2

Hypoxic hypoxia aka hypoxemia: when arterial pO2 is reduced

Question 3

Name four common causes of hypoxia

Answer 3

1. Hypoventilation
   - results in increased arterial pCO2
   - failure to ventilate alveoli properly
   - caused by: muscular weaknedd (MND), obesity & loss of respiratory drive (eg. when brain is prevented from accessing the lungs due to morphine)
2. Diffusion Impairment
   - when alveolar membranes thicken/ SA decrease. This causes bloof pO2 and alveolar pO2 to fail to equilibrate
   - caused by: pulmonary oedema (gaseous diffesion issues cuz of fluid in lungs), anaemia & interstitial fibrosis (btwn alveolus and capillaries, connective tissue thickened)
3. Shunting
   - anatomical abnormality of cadiovascular system that causes mixed venous blood to bypass ventilated alveoli in passing from right to left side of the heart (eg. ventricular septal defect (BSD) - Eisenmenger’s syndrome)
   - an intrapulmonary defect in which mixed venous blood perfuses unventilated alveoli, can happen in bronchial arteries
4. Ventilation-Perfusion Mismatch
   - MOST COMMON CAUSE of hypoxemia
   - occurs in chronic obstructive lung disease & many other lung diseases
   - arterial pCO2 may be normal or increased, depending on how much ventilation is reflexively stimulated
   - can be caused by: pulmonary embolus (blockage of an artery in lung), asthma, pneumonia & pulmonary oedema

Question 4

What are the 2 possible causes of hypercapnia

Answer 4

1. Hypoventilation

2. Ventilation-perfusion mismatch (depending on how much ventilation is reflexively stimulated)

Question 5

Describe Respiratory Failure Type 1

Answer 5

• caused by hypoxia
• pO2 low
• pCO2 low or normal
• Pulmonary embolism (a form of ventilation-perfusion mismatch) most commonly causes it

Question 6

Describe Respiratory Failure Type 2

Answer 6

• pO2 low
• pCO2 high
• caused by hyperventilation

Question 7

Name the two circulations in the lungs

Answer 7

Pulmonary and Bronchial Circulation

Question 8

What are the differences between pulmonary circulation and bronchial circulation

Answer 8

Pulmonary circulation:

• main circulation of lung
• from right ventricle
• receives 100% of cardiac output

Bronchial circulation:

• from left ventricle
• receives 2% of cardiac output

Question 9

What are the differences between pulmonary and systemic circulation (especially their vessels)?

Answer 9

Pulmonary:

• thin vessel walls
• minor smooth muscularisation
• no need for redistribution
• vessels dependent on alveolar pressure & expand by radial traction as lungs expand

Systemic:

• thick vessel walls
• significant smooth muscularisation
• redistribution required
• vessels are embedded in tissues

Question 10

What are the pressure differences between pulmonary and systemic circulation

Answer 10

Pulmonary:

• right atrium 5mmHg
• right ventricle 25mmHg
• pulmonary artery 25/8mmHg

Systemic:

• left atrium 5mmHg
• left ventricle 120mmHg
• aorta: 120/80mmHg