Chapter 7 The pulmonary circulation: Bringing blood and gas together

Transcription

1 Chapter 7 The pulmonary circulation: Bringing blood and gas together Department of Pathophysiology, the School of Medicine, Shandong University Zhongrui Yuan, Ph.D. zhongruiyuan@sdu.edu.cn

2 肺脏有两组血液循环系统, 一是肺循环 ( 是气体交换的功能血管 ), 一是体循环的支气管循环 ( 是气道和胸膜等的营养血管 )

3 Pulmonary circulation resin cast of pulmonary arteries resin cast of pulmonary veins

4 The anatomy of the pulmonary circulation Characteristic: high-flow: 5L/min (at rest); 25L/min (in exercise) to make sufficient gas exchange low-pressure: 25/10 versus 120/70 mmhg to avoid leakage low-resistance:

5 the right ventricle: 1/3 of the left

6 Pulmonary blood vessels 1) larger vessels 2) much thinner wall: less smooth muscle in pulmonary vessels -- readily influenced by the air pressure in the alveoli 3)much more distensible: to maintain low pressure during systole and in the face of increases in CO: via distension (R: /r 4 )and recruitment: Increase in perfusion pressure [pulmonary artery pressure] results in distension & recruitment decreasing pulmonary vascular resistance.

7 4) extremely high density of capillary network : a alveolus: almost 1000 capillaries very little space between the blood and air efficient gas exchange 5) pulmonary P A-V gradient is much less: driving pressure

8 Comparison of Vascular Pressures in the Systemic & Pulmonary Circulations 10 fold difference in mean arterial pressure structural basis: less smooth muscle in pulmonary vessels - greater distensibility + greater compressibility major drop in pressure in the pulmonary circulation is through capillaries major drop in pressure in the systemic circulation is through the arterioles

9 Gas Exchange in the Lungs Takes Place at the Respiratory Zone of the Airways [Airways with Alveoli] gas exchange: across small pulmonary arterial vessels [histologically not capillariesfunctionally capillaries] & pulmonary capillaries there are about 280 billion pulmonary capillaries for about 300 million alveoli resulting in a gas exchange surface of about m 2

10 Matching ventilation and perfusion V/Q 300 million alveoli. Different alveoli may have widely differing amounts of ventilation and of perfusion. Key for normal gas exchange is to have matching of ventilation and perfusion for each alveolar unit Alveoli with increased perfusion also have increased ventilation Alveoli with decreased perfusion also have decreased ventilation V/Q ratio = 1.0

11 Basic Structure of the Lung Anatomic dead space (V D =150 ml) VD DEAD SPACE Area where gas exchange cannot occur V A =350 ml VA A thin walled Sac = Alveolus GAS EXCHANGE OCCURS HERE Blood Vessels Formula: Tidal Volume = Dead Space + Alveolar Space V T = V D + V A 11

12 1) Gravity: Regional Distribution of Pulmonary Blood Flow

13 Regional Distribution of Pulmonary Blood Flow 2) Hypoxic pulmonary vasoconstriction Mechanim: Kv closes deploarization opening of Ca 2+ channel Significance: benefit: to maintain the matching of V/Q harm : COPD long-term hypoxia a widespread pulmonary vasoconstriction a high blood Pre. of pulmonary circulation the failure of the right ventricle (cor pulmonale) 3) Innervation of the pulmonary vessels: There is sparse sympathetic & arasympathetic innervation of the pulmonary vasculature and the effect of stimulation of these nerves is ontroversial.

14 Matching of Ventilation & Perfusion at Alveolar Level Affects Gas Exchange Q = 4-6 L/min V= 4-6 L/min V/Q = The consequence of V/Q matching at alveolar level is important to gas exchange. To appreciate the importance of V/Q matching at alveolar level, consider a scenario where there is perfusion to only the L- lung & ventilation to only the R-lung. What would the V/Q be?

15 Two Lungs, Not One Suppose the left lung is ventilated but not perfused (dead space). Suppose the right lung is perfused but not ventilated (shunt). Total V/Q = 1, but there is no gas exchange (V/Q must be matched at level of alveolar unit).

16 Ventilation-perfusion mismatch under physiological conditions Gravity 3 > 3 note the greater gradient for blood flow relative to ventilation

17 Ventilation/perfusion mismatching under pathological conditions and its effect on blood PaO 2 and PaCO 2 Ventilation/perfusion mismatching: decreased gas exchange V/Q : hypoventilation of partial alveoli V/Q : Low perfusion of partial alveoli True shunt PaO 2,PaCO 2

18 Mixed Blood What is the PO 2 of a mixture of two volumes of blood with different initial PO 2? Determined by interaction of oxygen with hemoglobin. the oxyhemoglobin dissociation curve.

19 1. Hypoventilation of partial alveoli: asthma, chronic bronchitis V functional shunt Q have not accordingly venous admixture even due to inflammation O 2 CO 2 V / Q V 血 N: 3% 30% A 血 venous blood flowing through these units have not been totally arterialized and mixes into arterial blood

20 2. Low perfusion of partial alveoli (V/Q ) dead-space-like ventilation Disorders of pulmonary vessel (vessel inflammation, occlusion, spasm) alveoli at diseased region have low perfusion,while ventilation is not decreased, alveolar ventilation can not be fully used.

21 Functional dead space = dead-space-like ventilation + Anatomic dead space pulmonary embolism, pulmonary arteritis, DIC: 60~70% N: <30% O 2 CO 2 V blood A blood

22 3. Anatomic shunt (true shunt) (1) Under physiological condition bronchia vein venous blood pulmonary vein N: 2%-3% A-V communicating branch true shunt: there are totally no gas exchange in the blood flowing through anatomic shunt, so anatomic shunt also called as true shunt

23 ( 2) Under pathological condition Bronchiectasis( 支气管扩张症 ) flow of bronchial vein flow of A-V shunt true shunt Atelectasis( 肺不张 ) Consolidation of lung( 肺实变 ) true shunt Intracardiac shunt:

24 Intracardiac shunt Congenital heart disease 1 肺动脉流出道狭窄 ; 2 室间隔膜部巨大缺损 3 主动脉右移, 骑跨于室间隔缺损上方 4 右心室高度肥大及扩张 tetralogy of Fallot 法洛四联症 发绀

25 Consolidation of lung 25

26 Anatomic shunt (true shunt) PaO 2 Inhaling O 2 is effective for functional shunt Inhaling O 2 is not effective for true shunt 26

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28 A-a gradient, the efficiency of the gas exchange Shunt Mismatch Diffusion impairment

29 Summary: Causes of Abnormal Oxygenation Hypoventilation V/Q mismatch Shunt Diffusion impairment

30 Internal Respiration All cells require oxygen for metabolism All cells require means to remove carbon dioxide Gas exchange at cellular level Factors affecting the internal respiration: Distance between the cells and the capillary Rate of metabolic rate Speed of the blood flow in capillary

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32 Homework Case of Pulmonary embolus: Causes, the diagnosis and treatment