33. The characteristics of the lung circulation, the perfusion of the lungs, gas diffusion in the lungs

Question 1

What should be mentioned?

Answer 1

The characteristics of the lung circulation
Ratio of ventilation and perfusion
Influence gravity have on lung circulation
Regulation of the perfusion
Hypoxia in the lungs

Question 2

Characteristics of the lung circulation

Answer 2

The lung has a dual blood supply:
a pulmonalis transfers the used blood that needs to be refreshed from the right ventricle to the lung. From the aorta the a.bronchiales transfers fresh blood to the lung tissue.

a.pulmonalis (functional circulation)
• 99% of pulmonary blood supply is from a pulmonalis (venous) flowing into alveolar capillaries
• 99% leaving the lung through vv.pulmonales (oxygenated), arriving to the left atrium.

a.bronchiales (nutritive circulation)
• 1% is from aa brochiales (oxygenated), which transport nutrients towards the pulmonary tissue (which does not participate in gas exchange)
• Venous blood contaminates the refreshed blood through v bronchiales and through the coronaries. Part of the blood goes to the pulmonary veins, thus mixing the local fresh blood with venous blood.

• In the functional areas of the lung the walls of the blood-vessels are much thinner than that of the corresponding systemic vessels. Therefore lung vessels can take up much larger quantities of blood at lower pressure values than systemic vessels.
• The perfusion of poorly ventilated lung areas automatically decreases, thus redirecting the blood to well-ventilated lung territories.
• Reason behind that is oxygen deficiency (hypoxia). This cause a prompt vasoconstriction in those parts of the lungs, to which there is no sufficient oxygen supply. With this reflex from the poorly ventilated lung areas the blood is transferred to the well-ventilated areas.
• Two venous systems lead away blood from the lungs. This decreases the oxygen concentration of the refreshed (“arterialized”) blood coming from the alveolar territory and increases the carbon dioxide concentration slightly.
• A smaller portion of the blood goes directly to the right atrium via the azygous vein (systemic circulation).

Question 3

Ratio of ventilation and perfusion (picture)

Answer 3

The respiratory process is effective in case the airflow (Va, ventilation, liter air/min) is proportional to the perfusion of blood (Q- liter blood/min)

1. In normal condition the Va/Q=1 the blood gas will be balanced with the alveolar air so the blood flows away arterialized.
2. If alveolus is plugged: then Va=0 so Va/Q=0 so the arriving blood flows away unchanged (not refreshed).
3. If the blood flow stops (capillary plug): Q=0 so Va/Q=infinite, no blood flow, therefore the alveolar gas pressure will be the same as the atmospheric.

Question 4

Influence gravity have on lung circulation

Answer 4

• On the dorsal areas of the lung the gravitational effects are lower. The pressure in the alveoli is higher than the blood pressure. The alveolar pressure compresses the blood vessels, the perfusion decreases. (Perfusion is absent)
• At the level of the right atrium alveolar capillaries are open during systole, but they usually close during diastole. (perfusion is sporadic)
• In the lower lung areas, the gravitational force increases the blood pressure considerably. It is higher than the alveolar pressure, so the perfusion is increased
• perfusion is continuous: the alveolar pressure cannot compress capillaries either during systole or diastole. (Perfusion is constant)
• Artificial respiration the lung is inflated with positive pressure, capillaries may completely be compressed.

Question 5

Regulation of the perfusion

Answer 5

• Parasympathetic innervation enlarge the pulmonary vessels through n.vagus (acetylcholine release).
• Sympathetic, noradrenergic fibers (through alpha-receptors) cause vasoconstriction (there are very few B-receptors in the pulmonary region, b-receptors is vasodialators.)
• Adrenalin (through B-receptors) creates intensive alveolar dialation, thus increasing the ventilation (it is crucial in anaphylactic shock, adrenalin may effectively reduce the reaction).

Question 6

Hypoxia in the lungs

Answer 6

It affects the diameter of the blood vessels the opposite way to that in the systemic circulation.
• Local hypoxia causes local stenosis, which results in blood redistribution. Therefore the blood gets redistributed from the lesser ventilated part to the better ventilated area.
• Effect of blood pressure increase on the lung circulation: result in a decrease of the vascular tone (the state of contractile tension in the vessel walls.) This plays an important role in the lung´s ability to achive an extremely high minute volume in the case of the increased physical activity without any change in the pressure conditions.
• F.eks high altitude pulmonary edema