Consequences of Increased Movement of Fluid across Pulmonary Capillaries

Question 1

What is meant by Partial Pressures of Gases?

Answer 1

• Pressure (P) can refer to the partial pressure of a gas in a mixture of gases, or when a gas is dissolved in a liquid (ie a concentration PT = P1 + P2 + P3 ….)
• eg partial pressure of O2 in air at sea level is approx 160 mmHg This is a reflection of: – proportion of O2 in air (FO2 approx 0.21) – total atmospheric pressure (760 mmHg at sea level)
• -> PO2 at sea level = 760 x 0.21 = 160 mmHg
• -> PO2 at 18,000 feet = 380 x 0.21 = 80 mmHg
• -> PO2 breathing 10.5% O2 at sea level = 760 x 0.105 = 80 mmHg

Question 2

What are the functions of the respiratory system?

Answer 2

There are 3 main functions of the respiratory system.

- involves:
	- movements of air in and out of the lungs
	- exchange of oxygen and carbon dioxide across the AC membrane
- Pathology that affects mechanics:
	- resistive or elastic WOB --> load on the respiratory muscles
	- oxygen consumption, breathlessness, altered pattern of breathing.

Question 3

What are some Consequences of Abnormalities of the Pulmonary Vessels?

Answer 3

1. Increased movement of fluid out of the pulmonary capillaries
2. Increase Pressure within the pulmonary vessels

Question 4

What are some features of the pulmonary circulation?

Answer 4

• Low pressure, low resistance, mostly at level of heart
• The disadvantage of being a low pressure system is that fluid can cross over the barrier and influence the diffusion system etc.
  –> it is critical that the alveoli are kept dry.
  – Pulmonary artery pressure (PA P) = 25/8 (mean = 15 mmHg)
  – Capillary pressure = 12 - 8 mmHg
• Thin walled vessels & thin right atrium and ventricle
• Receives all of right ventricular output (ie the same as the LV output = 5-20 L/min)
• At rest capillary volume 60-80 ml (ie CO of about 5L/min with blood in capillaries for about 0.75 sec)
• During exercise pulmonary artery pressure does not rise with increased cardiac output due to dilatation & recruitment of pulmonary vessels
• Pulmonary circulation serves as a reservoir for blood eg if lie down, blood drains from legs to lungs
• Pooling of blood also occurs on inspiration
  –> decreased venous return to left atrium
  –> decreased cardiac output
  –> decreased systolic BP on inspiration cf expiration

Question 5

What are some Abnormalities of the Pulmonary Circulation?

Answer 5

• Increased leakage of fluid across the pulmonary capillaries
• Increased pressure in pulmonary arteries

Question 6

What are some causes of pulmonary oedema?

Answer 6

1. Increased Capillary Hydrostatic Pressure
   - -> eg left ventricular dysfunction, mitral stenosis, fluid overload, pulmonary veno-occlusive disease
2. Increased Capillary Permeability
   - -> eg toxins, sepsis, multiple trauma, aspiration of gastric acid ? high altitude, ? heroin, ? neurogeni

• decreased colloid osmotic pressure and decreased lymphatic drainage can be exaggerated factors

Question 7

What is meant by lung water?

Answer 7

• Capillary endothelium is highly permeable to water, ions and small molecules (not protein)
• Alveolar epithelium is not and actively pumps water from the alveoli into the interstitial spaces
• Starling’s equation models the forces that influence fluid movement across the capillaries
• Lymphatics actively pump interstitial fluid at a rate of about 20 ml/hour in normals
• Fluid loss from capillaries in excess of lymphatic drainage will cause interstitial and then alveolar oedema
• Interstitial oedema causes little functional effect but alveolar oedema has a large effect on lung function