Pulmonary Circulation & Ventilation/Perfusion Ratio PDF

Pulmonary circulation and Ventilation and perfusion DR. NOOR AZLINA ABU BAKAR Learning Outcomes 1. Discuss the physiology of bronchial circulation. 2. Explain the important of ventilation-perfusion ratio. 3. Explain the basis difference in ventilation-perfusion ratio in apex and base of th...

Pulmonary circulation and Ventilation and perfusion DR. NOOR AZLINA ABU BAKAR Learning Outcomes 1. Discuss the physiology of bronchial circulation. 2. Explain the important of ventilation-perfusion ratio. 3. Explain the basis difference in ventilation-perfusion ratio in apex and base of the lungs and its physiological and clinical importance. 4. Explain the mechanism of filtration across pulmonary capillaries and genesis of pulmonary oedema. Blood circulation The amount of blood ejected into the pulmonary circulation is the same amount into systemic circulation. The capacity of the pulmonary is significantly less than systemic but pulmonary vascular bed can control the cardiac output so that the pressure not high for the same volume. Systemic blood pressure is high due to the resistance of the systemic vascular bed. But pulmonary circulation is low in pressure as it has low pulmonary vessels resistance. Total of pulmonary circulation resistance is 1/10 of the systemic circulation. Blood supply to the lungs *Pulmonary blood vessels with blood = 40% of lungs total weight. Lungs receive blood via two sets of arteries 1. Pulmonary arteries 2. Bronchial arteries.  Bronchial artery Smooth Blood supply to the lungs Branch of muscle pulmonary artery Respiratory bronchiole Branch of pulmonary Vein Capillary bed on alveolus Pulmonary alveolus Fibrous septum between pulmonary lobules Pulmonary alveolus Subpleural connective tissue Blood supply to the lungs Pulmonary artery carries deoxygenated blood pumped from right ventricle to right and left branches of the lungs (pulmonary circulation). After entering the lung, branch of the pulmonary artery capillary plexus (pulmonary capillary) that is in intimate relationship to alveoli for gaseous exchange. Oxygenated blood from the alveoli is carried to left atrium by one pulmonary vein from each side.  Bronchial artery Bronchial artery Branch of pulmonar Smooth muscle y artery Bronchial artery arises from Respiratory descending thoracic aorta supplies bronchiole arterial blood to bronchi, Branch of pulmonary Vein connective tissue and other structures of lung stroma, visceral Capillary bed pleura and pulmonary lymph nodes on alveolus (1-2% of CO). Pulmonar y alveolus Carry oxygenated blood. Supplies the supporting tissue which empties into the pulmonary Fibrous septum veins and enter left atrium. between pulmonary lobules *(Reasons why left ventricular output Pulmonar y alveolus is 1- 2% greater than right ventricular Subpleural connective output). tissue Bronchial artery Venous blood from these structures is drained by two bronchial veins from each side. Right side of bronchial veins drain into azygos vein. Left bronchial veins drain into superior hemiazygos or left superior intercostal veins. However, the blood from distal portion of bronchial circulation is drained directly into the tributaries of pulmonary veins and mixed with oxygenated blood.  Physiological shunt Is a diversion through which the venous blood is mixed with arterial blood. Physiological shunt has TWO components: 1. Flow of deoxygenated blood from bronchial circulation into pulmonary veins without being oxygenated makes up part of normal physiological shunt (1-2% of CO). 2. Flow of deoxygenated blood from Thebesian veins into cardiac chambers directly. Physiological features of Pulmonary Blood Vessels The walls of the pulmonary artery and its large branches are 30% as thick as the wall of the aorta. Small arterial vessels having little muscle in their walls. Walls on postcapillary contains some smooth muscle. The vessels are thin and distensible, gives the pulmonary arterial tree a large compliance to accommodate large stroke volume of right ventricle. Pulmonary capillary are abundant, having large diameter with multiple anastomoses. Each alveolus sits in capillary basket. Function of pulmonary circulation. 1. Gas exchange To pulmonary From pulmonary vein Bring venous blood from systemic artery circulation to contact with alveoli for gaseous exchange. Capillary Alveolar Begins at the main pulmonary artery, membrane which received mixed venous blood Respiratory membrane pumped by the right ventricle. Oxygenated blood collected from capillary bed by pulmonary veins drains into left atrium. Function of pulmonary circulation. 2. Serve as a filter Pulmonary vessels filter thrombi and emboli that originate from venous compartment and also right side of the heart. Endothelial cell of pulmonary vessels release fibrinolytic agent to lyse blood clots (thrombi), thus prevent its entry (thrombi and emboli) into coronary, cerebral and other important vessels. Function of pulmonary circulation. 3. Metabolic function Metabolism of vasoactive hormones. Angiotensin converting enzyme (ACE) of pulmonary endothelial facilitate the conversion of Angiotensin I to II. Inactivate bradykinin, serotonin, prostaglandins E1, E2, E2a and Norepinephrine. Function of pulmonary circulation. 4. Blood reservoir 500 mL circulating blood present in the pulmonary circulation. Volume of the pulmonary capillary approximately equal to the stroke volume of right ventricle. Blood in the lungs can vary from as little as one half normal up to twice its volume. Loss of blood from the systemic circulation by haemorrhage can be partly compensated for by the automatic shift of the blood from the lungs into the systemic vessels. Pulmonary Lymphatics Lymphatic vessels are present in all the supportive tissues of the lung, beginning in the connective tissue space that surround the terminal bronchioles, coursing to the hilum of the lungs, and then mainly into the right thoracic lymph duct. Functions: 1. Particulate matter entering the alveoli. 2. Plasma protein leaking from the lung capillaries (prevent oedema). Pulmonary blood pressure Pulmonary blood vessels are more distensible than systemic blood vessels. Therefor, pulmonary blood pressure are less than systemic blood pressure. Pressure in the pulmonary circulation Pulmonary circulation Systemic circulation Mean pressure (mmHg) Mean pressure (mmHg) Pulmonary artery 15 Systemic artery (Aorta) 100 Capillary stat 12 Capillary start 30 Capillary end 8 Capillary end 10 Left atrium 5 Right atrium 2 Driving pressure 10 Driving pressure 98 Comparison of pressure (mmHg) in the pulmonary and systemic circulations.  During systole, the pressure in the right pulmonary artery is equal to the pressure in the right ventricle. (Refer to diagram) After the pulmonary valve closes at the end of systole, ventricular pressure falls and pulmonary arterial pressure falls more slowly as blood flows through the capillaries of the lungs. Systolic pulmonary arterial pressure : 25 mmHg Diastolic pulmonary arterial pressure : 8 mmHg Mean pulmonary arterial pressure : 15 mmHg Comparison of pressure (mmHg) in the Mean Pulmonary capillary pressure : 7 mmHg pulmonary and systemic circulations. Left atrial and pulmonary venous pressure Mean pressure in the left atrium and major pulmonary veins average 2 mmHg (1 - 5mmHg). Comparison of pressure (mmHg) in the pulmonary and systemic circulations. Pressure different in vessels of the lungs Pressure in the different vessels of the lungs. D: Diastole; M: mean; S: systole; red curve, arterial pulsations. Shift of the blood indicates cardiac pathology Failure of the left side of the heart or increase resistance to blood flow through mitral valve (i.e. mitral stenosis or mitral regurgitation) cause increase in pulmonary vascular pressure. Comparison of pressure (mmHg) in the pulmonary and systemic circulations. Distribution of blood flow through the lungs Capillary bed and Capillary network: Pulmonary capillary bed – ‘sheet of blood’ Pulmonary capillary bed essentially – lower resistance. Blood flow not through “sheet” of blood that flow across alveolar entire site surface (form less resistance to blood flow). Systemic capillary have a small diameter and represent high resistance to blood Systemic network – Individual capillaries very small in diameter – high resistance compare to flow. pulmonary bed. Special feature of pulmonary circulation. Low in pressure: Mean pulmonary arterial pressure = 15 mmHg, low resistance. Pulmonary artery, vein and branches having thin walls and less smooth muscle, therefor, it is more compliance. The lung is required to receive the whole of the cardiac output at all times. It is shorter and wider, can accommodate large volume of blood. Eg: standing to recumbent position, large volume of blood shift from lower limb to the lung. High compliance of lungs accommodate the volume.  500 mL (10%) Pulmonary blood flow 500 mL of blood present in the pulmonary circulation 10% of the total blood volume. Pulmonary arteries : 150 mL Pulmonary veins : 270 mL Pulmonary capillaries : 80 mL Pulmonary circulation : 500 mL Factors affecting pulmonary blood flow Pulmonary blood flow is regulated by the following factors: 1. Cardiac output 2. Vascular resistance 3. Nervous factors 4. Chemical factors 5. Gravity and hydrostatic pressure Factors affecting pulmonary blood flow 1. Cardiac output Pulmonary blood flow is directly proportional to cardiac output. Any factor that can alter cardiac output Force/rate of contraction can affect pulmonary blood flow. 1. Venous return 2. Force of contraction 3. Rate of contraction 4. Peripheral resistance Venous return Factors affecting pulmonary blood flow 1. Cardiac output cont. The extra flow of blood accommodated in the lungs in three ways: 1. Increase number of open capillaries ~ 3 folds 2. Distending all the capillaries and increase the flow rate ~2 folds 3. Increase pulmonary arterial pressure Factors affecting pulmonary blood flow 2. Vascular resistance Pulmonary blood flow is inversely proportional to the PVR. PVR is altered in different phases of respiration. During inspiration, pulmonary blood vessels is distended (due to low intrathoracic pressure), thus increases blood flow. Therefor, reduces PVR and vice versa. During exercise, PVR decreases thus increased blood flow (due to exercise induced hypoxia and hypercapnia. Effect of exercise on blood flow Blood flow in all parts of the lung increases during exercise. The pulmonary vascular pressure rise during exercise to convert the lung apices from zone 2 to zone 3. Factors affecting pulmonary blood flow Pulmonary vascular resistance. Unique features: Pulmonary vascular resistance (PVR) falls with increased pulmonary arterial pressure (when increase in cardiac output). This is due to TWO mechanism: 1. Capillary recruitment. Normally many capillaries closed in the upper part of the lungs due to low perfusion pressure. When blood flow increases, collapsed vessels opened which decreases the pulmonary resistance. Factors affecting pulmonary blood flow Pulmonary vascular resistance. 2. Capillary distension: Dilation of vessels walls to reduce resistance and pulmonary pressure. This occurs as pulmonary capillary exceedingly thin. With distension, Pulmonary circulation can accommodate a large increase in cardiac output with small increase in pulmonary arterial pressure Factors affecting pulmonary blood flow Physiological significant of low PVR. Its benefits: Decreased resistance causes decreased in velocity, provide adequate time for gaseous exchange. Capillary distension that decreases the pulmonary resistance increases the surface area which facilitate gaseous exchange. In normal situation: High capillary pressure can cause pulmonary oedema which impaired gaseous exchange. I.e, vigorous exercise, causes increase in cardiac output which increase blood flow and pressure in pulmonary circulation, BUT decrease in pulmonary resistance reduces the right ventricular load and lower down the capillary pressure to prevent oedema. Minimized load on the right side of the heart. Factors affecting pulmonary blood flow 3. Nervous factor. Stimulation of parasympathetic nerve increases PVR via vasoconstriction. Sympathetic stimulation decreases PVR via vasodilation. Schematic diagram showing decreased PVR Factors affecting pulmonary blood flow 4. Chemical factor. When the concentration of oxygen in the air of the alveoli decreases below normal (hypoxia), the adjacent blood vessels constrict, Thus increases the PVR. This is to distributes blood flow where it is most effective in the lungs (most aeriated area). Thus, blood is directed to the alveoli of neighbouring area where gaseous exchange occurs. This is opposite to the effect observed in systemic vessels, which dilate rather than constrict in response to low oxygen. In Pulmonary circulation, PO2, Vasoconstriction In systemic circulation, PO2, Vasodilation Factors affecting pulmonary blood flow 4. Chemical factors cont. Hormones: *Potent vasoconstrictor – Serotonin, norepinephrine, histamine, thromboxane A2 and leukotrienes thus increases pulmonary vascular resistance. Adenosine, acetylcholine, prostacyclin (PG-I2) and Isoproterenol – relaxes smooth muscle thus decrease the pulmonary resistance. Factors affecting pulmonary blood flow 5. Gravity and hydrostatic pressure. (Pulmonary vascular pressure varies in different parts of the lungs). Zone I: Apex: Large ventilated alveoli, blood flow is less Zone II: Middle: Pulmonary arterial and capillary exceed alveolar pressure. Pressure in venule is low. The flow is depend on pulmonary artery-alveolar pressure. Constriction cause blood flow “waterfall effect” Zone III: Base: Less ventilated alveoli. The flow is depend on arteriolar-venous pressure difference. Factors affecting pulmonary blood flow 5. Gravity and hydrostatic pressure. (Pulmonary vascular pressure varies in different parts of the lungs). – 0.74 mmHg In upright position: 10 cm above = – 7.4 mmHg Blood flow is low at the apex and = 14 – 7.4 = 6.6 mmHg high at the base of lungs. This is due to the effect of the gravity. Apex located above the heart level = 14 mmHg and base remains below heart level. 1 cm increase in height above heart, hydrostatic pressure fall by 0.74 mmHg and vice versa + 0.74 mmHg 5 cm below = + 3.7 mmHg = 14 + 3.7 = 17.7 mmHg Factors affecting pulmonary blood flow Apex: Zone 1 Alveolar pressure is high, compress the capillary, thus reduced the blood flow at the upper region. In pathology condition: i.e. circulatory shock, decrease further the lungs blood flow which affect at zone 2. Factors affecting pulmonary blood flow Middle: Zone 2 Try to force the blood to the capillary. Function intermittently. i.e: during inhalation and exhalation. Factors affecting pulmonary blood flow Base: Zone 3 Pressure in vascular system is high-ensure continuing blood flow. Factors affecting pulmonary blood flow Ventilation/ Perfusion ratio Due to the effect of gravity, it varies in different part of the lungs. Average V/Q at rest = 0.8 (4.2mL/5.5mL) If ventilation > perfusion, PO2 in alveoli will increase and PCO2 will decrease. (> 1) If perfusion > Ventilation, PCO2 in alveoli will increase and PO2 will decrease.(< 0.8) Factors affecting pulmonary blood flow What about supine position? Whole lungs become zone 3: Blood flow to all parts of the lungs. Factors affecting pulmonary blood flow 6. Arterial to venous pressure gradient The pressure different between the arterial and vein determine the rate of blood flow which are affected by alveolar pressure. Abnormalities of pulmonary circulation When left atrial pressure rises as a result of left-side heart failure, Small changes in pressure have virtually no effect on pulmonary circulation as pulmonary venules expand and open up which allow continuous blood flow. When left side of heart fails, blood begins to dam up in the left atrium ~ increase in arterial pressure (greater than 7-8mmHg) ~ development of pulmonary edema. Capillary exchange of fluid and pulmonary interstitial fluid dynamics. The dynamics fluid exchange across lungs capillary: 1. Pulmonary capillary pressure is low 1 4 (7 mmHg) compared to systemic capillary (17 mmHg). 2 2. Pulmonary Interstitial pressure is more negative than at the peripheral tissue. 3. Pulmonary capillary relatively leaky to 3 proteins, Thus increases the interstitial oncotic pressure. 4. Alveolar wall relatively thin and weak, can easily be ruptured by positive pressure of the interstitial which allow dumping of the alveoli. Pulmonary exchange of fluid Negative interstitial pressure and mechanism for keeping the alveoli dry. Small opening between the alveolar epithelial cells allow large protein molecules and large quantities of water and electrolyte to pass through. Then what are the mechanism that keeps the lung DRY? Pulmonary capillaries and lymphatic system maintain a slight negative pressure in the interstitial spaces in which excess fluid is either carried away through pulmonary lymphatics or is absorbed into the pulmonary capillaries The principle of lungs being kept dry The lymphatic drainage – It is increased in the lungs interstitium to a level that enable it to remove fluid until the excessive fluid is being 8 folds over normal. If this occurs, oedema will develop. High interstitial compliance – means the resistance of the interstitium to return to its original dimension, so the fluid may accumulate in the interstitium without rising its hydrostatic pressure. Decrease interstitial oncotic pressure, as the lymph flow will drain any leaked albumin from the pulmonary capillaries. Pulmonary Oedema Developed when excess or free fluid accumulated in the interstitial spaces or alveoli. Causes: 1. Increase capillary hydrostatic pressure (left- sided heart failure or mitral valve disease). 2. Increase alveolar space tension 3. Decrease oncotic pressure 4. Increase capillary permeability 1. Oxidant damage (oxygen therapy, ozone toxicity) 2. Inflammatory reactions (endotoxins) 3. Neurogenic shock (head injury) Physiological basis of treatment of Pulmonary oedema Aim is to reduce pulmonary capillary hydrostatic pressure: Diuretics Digitalis Vasodilators Drowning Fresh water drowning Salt water drowning Death does not occur due to Aspirated water is hypertonic due to pulmonary oedema but due to high content of Na+ and Cl-. ventricular fibrillation. Thus causes pulmonary oedema. Aspirated water enters alveoli and Death due to asphyxia. then into pulmonary capillary due to low hydrostatic pressure and high oncotic pressure. Causes haemolysis of red blood cells which later causes hyperkalaemia and hyponatremia which trigger ventricular fibrillation. Negative Pressure in pleural fluid Is required to keep the lungs expended. The factors that keep the negative pressure (~-4mmHg) is the pumping activity of lymphatic drainage. Keeps the normal lungs pulled against the parietal pleura of chest cavity, thin layer of mucoid fluid acts as lubricant. Pleural Effusion Is collection of large amounts of free fluid in pleural space ~oedema Or “Oedema of pleural cavity”. causes: 1. Blockage of lymphatic drainage 2. Cardiac failure- high peripheral/pulmonary capillary pressure 3. Greatly reduced of plasma oncotic pressure 4. Infection/ inflammation, breakdown the capillary membranes Thank you. If you woke up breathing, congratulations! You have another chance. – Andrea Boydston