Respiratory Series 4

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PULMONARY CIRCULATION • Pulmonary blood flow at rest: 5-6 L/min (output of left & right ventricle same) • Low pressure & low R system (pulmonary R 1/10 of systemic R)

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↑ CO → pulmonary artery P does not ↑ much because pulmonary vascular R ↓ due to recruitment & distension of pulm. capillaries -

FACTORS AFFECTING DLO2

• Exercise → ↑ CO causes recruitment & distension of pulmonary  Capillaries → ↑ S.A for diffusion  →↑ vel. blood flow → ↑ partial P gradient bet. alveolar air & pulm. capillary blood

↑ DLO2

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↑ CO → pulmonary artery P does not ↑ much because pulmonary vascular R ↓ due to recruitment & distension of pulm. capillaries -

• Regulation of pulmonary blood flow:  PAO2 - most important

A

• Low PAO2 (alveolar hypoxia) → • VasoC of pulm. blood vessels (small arteries, arterioles) . •This is sometimes called hypoxic pulmonary vasoconstriction.

Residents at high altitude

↓ Patm ↓

↓ ↓ PO2 of inspired air (dry) (0.21 x Patm)

↓ ↓ PO2 of tracheal air (0.21 x (Patm -47)

↓ ↓ PAO2 (alveolar hypoxia ) • →VasoC of pulm. blood vessels (small arteries, arterioles) • →↑pulmonary artery pressure • Pulmonary hypertension common 10

REGULATION OF PULMONARY BLOOD FLOW : • PAO2  Most Important • Low PAO2 (alveolar hypoxia)  Vasoconstriction of pulmonary blood vessels o Small arteries o Arterioles • Low PaO2  Vasoconstriction • High PACO2, ↓ pH  Vasoconstriction • Sympathetic stimulation (NA, Adrenaline)  Vasoconstriction • Parasympathetic stimulation, high PAO2, ACh, prostaglandine E, nitrous oxide  Vasodilation

Mechanism or physiological causes of Hypoxemia (↓↓ PaO2)

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Diffusion impairment at the alveolar-capillary membrane  ↓↓ DLO2 ↓ PO2 of inspired air eg. at high altitude  Low PAO2  ↓ PaO2 Alveolar hypoventilation e.g. :  Depression of respiratory centre  Airway obstruction  Low PAO2  ↓ PaO2 Right-to-left shunt eg. :  Septal defect in heart V/Q imbalance/mismatch - most common physiological cause of hypoxaemia

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. (V/Q) VENTILATION-PERFUSION RATIO

(4L/min) (5L/min)

Q = perfusion / blood flow/min

For the lungs as a whole V/ Q = 0.8 (4L/5L) For gas exchange to be efficient, there must be a close match bet. ventilation & perfusion (pulm. capillary blood flow) in individual alveoli (ideally V/Q ~ 1)

PO2 & PCO2 of blood coming to alveolus is 40 mmHg & 46 mmHg respectively

PO2 & PCO2 of capillary blood from alveolus is normal

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V/Q MISMATCH/IMBALANCE/ INEQUALITY

Ventilated alveolus with no blood supply

Ventilated alveolus with inadequate perfusion

V/Q MISMATCH/IMBALANCE/ INEQUALITY

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V/Q MISMATCH/IMBALANCE/ INEQUALITY

Underventilated alveoli normal perfusion

Nonventilated alveoli which are perfused

LOCAL HOMEOSTATIC MECHANISM IN LUNGS TO MINIMISE MISMATCHING OF V&Q

(i)

In alveoli with normal perfusion, ventilation < normal •

Low V/Q (< 0.8)

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↓ PAO2

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Vasocontriction of pulmonary arterioles to alveoli

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Less perfusion to poorly ventilated alveoli

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V/Q ↑ towards normal (0.8) & blood diverted to alveoli with better ventilation

LOCAL HOMEOSTATIC MECHANISM IN LUNGS TO MINIMISE MISMATCHING OF V&Q

(ii) In alveoli with normal ventilation but perfusion < normal •

V/Q high (> 0.8)

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Low PACO2 in alveoli

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Bronchoconstriction

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↓ ventilation to poorly perfuse alveoli

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V/Q ↓ towards 0.8

• In upright position, perfusion ↓ from base to apex in the lungs due to gravity • Ventilation ↓ from base to apex of the lung

Apex

Base

APEX OF THE LUNG

(TRANSPULMONARY P)

BASE OF THE LUNG

At end of quiet expiration/ start of inspiration in upright position: alveoli at apex of lung are larger

Apex

Base

V/Q MISMATCH/IMBALANCE/ INEQUALITY

• Alveolar-arterial O2 difference / alveolar-arterial O2 gradient / alveolar-arterial PO2 difference  (A-aDO2; A-aPO2)  Difference of PAO2 & PaO2

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• Alveolar-arterial O2 difference / alveolar-arterial O2 gradient / alveolar-arterial PO2 difference  (A-aDO2; A-aPO2)  Difference of PAO2 & PaO2

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REASONS FOR NORMAL ALVEOLAR-ARTERIAL O2 DIFFERENCE (b) some inequality or mismatching of ventilation & perfusion V/Q ratio high at apex and low at base of lungs – affect PO2 of blood coming out of alveoli at apex and base of lungs (b) anatomical shunt / physiological shunt - slight mixing of blood with deoxygenated blood: . oxygenated . (i) draining from bronchi & bronchioles (ii) from coronary circulation via Thebesian veins

The airways receive their blood supply thro’ bronchiol circulation. Part of the bronchiol circulation returns to the systemic venous system in normal way, but part drains into the pulmonary veins, ‘contaminating’ the oxygenated blood with deoxygenated blood. This situation constitutes a ‘shunt’