2.20 - Pulmonary Circulation & It's Determinants

Question 1

What factors effect diffusion of gas across the alveolar respiratory membrane?

Answer 1

Thickness of the membrane Diffusion coefficient of the gas Total surface area of the respiratory membrane Pressure gradients (Affected by CO and Hb concentration)

Question 2

What are two pathological processes that could affect membrane thickness?

Answer 2

lung oedema (interstitial and alveolar fluid) Lung fibrosis (interstitial due to inflammation)

Question 3

Describe the relative diffusion coefficients of CO2 and O2

Answer 3

CO2 has a diffusion rate 23x better than that of O2. Hardly every diffusion problems for CO2 O2 diffusion can easily become limited

Question 4

Describe the changes in total membrane surface area and how it effects gas diffusion

Answer 4

Some loss of area throughout life (non-pathlogical) Can change due to emphysema or lung resections

Question 5

Describe lung perfusion

Answer 5

Globally corresponds to CO A low pressure in lung vessels compared to systemic circulation

Question 6

Describe the vascular resistance (RL) in the lungs

Answer 6

There is low resistance in lung vessels (10x smaller than systemic) 40% of vascular resistance is determined in capillaries Lung volume depends determines vascular resistance in capillaries. In the lung the resistance will be dependent on respiration: o ↑RL at beginning of expiration (because PA ↑) o ↓RL at beginning of inspiration (because PA ↓)

Question 7

Why is perfusion of the lungs best at the base than at the apex?

Answer 7

This is due to hydrostatic pressure. The alveolar pressure is greater than capillary pressure in the apex of the lung, this leads to collapsed capillaries and therefore no/minimal perfusion

Question 8

Describe the ventilation/perfusion relationship

Answer 8

For optimal function, ventilation and perfusion need to be matched. If mistmatched, O2 and CO2 exchange are impaired. If ventilation > perfusion (i.e. V/Q > 1) –> relative hyperventilation If ventilation < perfusion (i.e. V/Q relative hypoventilation

Question 9

Describe how the body changes perfusion or ventilation when one of the factors is altered

Answer 9

If not ventilated, alveolar partial pressures reach those in blood (hypo ventilated). Vessels: vasoconstriction (“shut problem area off”) & Bronchi dilate –> improves ventilation If not perfused, alveolar partial pressure reach those in trachea (hyperventilated). Vessels: vasodilation (“open area up”). Bronchi constrict –> limits ventilation

Question 10

Describe the changes in ventilation and perfusion from the apex to the base of the lung

Answer 10

Ventilation increases from apex to base of lung. Perfusion increases also, but more than ventilation. From apex to base, VA/Q decreases: o At apex relative hyperventilation V/Q>1: ↓RL; ↑RAW o At base, relative hypoventilation V/Q<1: ↑RL; ↓RAW

Question 11

Which of the following statements best describes the relative differences in blood flow among the upper, middle and lower portions of the lung during resting conditions (standing) and during exercise in a running person?

Answer 11