Pulmonary Circulation

Pulmonary circulation Lec5 Respiratory Physiology Dr Suroor Mohammed

Objectives List the features of the pulmonary circulation. List the Factors affecting pulmonary blood flow Explain the Differences in ventilation & perfusion in different parts of the lung Transport forms of o2 & co2 . whats mean by oxy-Hb curve

The primary function of the pulmonary circulation is to allow the exchange of oxygen and carbon dioxide between the blood in the pulmonary capillaries and air in the alveoli. Oxygen is taken up into the blood whilst co2 is released from the blood into the alveoli. Mixed-venous blood is pumped from the right ventricle through the pulmonary arteries and then through the pulmonary capillary network. The pulmonary capillary network is in contact with the respiratory surface and provides a huge gas-exchange area. Gaseous exchange occurs (co2 given up by the blood, oxygen taken up by the blood) and the oxygenated blood returns through the pulmonary veins to the left atrium. The lungs are drained by the pulmonary veins( large veins) carry oxygenated blood from the lungs into the left atrium of the heart


Pulmonary CirculationAt normal P02 arterial blood is about 100 mm Hg.● P02 level in the systemic veins is about 40 mm Hg.■PC02 is 46 mm Hg in the systemic veins.■Provides a good index of lung function● Rate of blood flow through the pulmonary circulation is = flow rate through the systemic circulation. ◦ Driving pressure is about 10 mm Hg. ● Pulmonary vascular resistance is low. ◦ Low pressure pathway produces less net filtration than produced in the systemic capillaries.& Avoids pulmonary edema.● Autoregulation: ◦ Pulmonary arterioles constrict when alveolar P02 decreases(hypoxia) & ◦ Matches ventilation/perfusion ratio.In a fetus: ◦ Pulmonary circulation has a higher vascular resistance, because the lungs are partially collapsed.● After birth, vascular resistance decreases: ◦ Opening the vessels as a result of subatmospheric intrapulmonary pressure.◦ Physical stretching of the lungs.◦ Dilation of pulmonary arterioles in response to increased alveolar P0

PULMONARY CIRCULATION Features of pulmonary circulation 1) Lung is the only organ receiving the entire CO 2) Less affected by gravitational forces compared with systemic vessels. 3) Pulmonary blood vessels:
Pul arteries: thin walled (30% as thick as the wall of the aorta) little smooth muscle and elastic tissue and have larger diameterPul capillaries: larger than systemic capillaries and denser with multiple anastomoses (each alveolus seems to sit in a capillary basket)Pul veins: highly dispensable and act a blood reservoir. Lying down →↑pul blood volume (400mL) →↓ VC & orthopnea in HF

Pulmonary artery

Bronchial arteries
Pulmonary circulation
Systemic circulation
Deoxygenated blood
Oxygenated blood
Gas exchange
To the respiratory tree up to the terminal bronchioles


11) Physiologic shunt Shunt: blood that has not been oxygenated in the lungs is added to systemic circulation Lung: Bronchopulmonary anastomosis. Some bronchial venous blood (de-oxygenated blood) enters pulmonary veins (oxygenated blood) bypassing the right ventricle and returns to left side of heart. This constitutes 2% of blood in systemic circulation. PO2 of pulmonary vein (95 mmHg) is different from that of pulmonary capillary (104 mmHg) due to anatomic shunt

Regulation of pulmonary blood flow:1) Cardiac out put: ↑CO → ↑ pulmonary blood flow.2) Pulmonary vascular resistance:Pulmonary perfusion is inversely proportional to pulmonary vascular resistance.3) Nervous factors:Sympathetic → pulmonary vasoconstriction → ↓pulmonary blood flow by (30% Parasympathetic→ vasodilatation → ↑pulmonary perfusion.4) Chemical factors:Hypoxia, hypercapnia, and acidosis → vasoconstriction →↑ pulmonary arterial pressure (pul hypertension) In all others areas other than lung, hypoxia produces vasodilatation COPD→ hypoxia → vasoconstriction → pulmonary hypertension → RHF

5) Effects of gravity: Remarkable effect on pulmonary circulationIn the erect posture (Apex of lung above the level of heart, base below) → linear ↑ in pulmonary blood flow from the apex to the base of the lung.Alveoli at apex are underperfused (overventilated).◦ Alveoli at the base are underventilated (overperfused) 6) Hormonal factors:Pulmonary arteriolar vasoconstriction (angiotensin II, epinephrine, norepinephrine, PGF2α)Vasodilator (Ach, NO)7) Phases of respiration: Inspiration →pulmonary vasodilatation →↑ pulmonary perfusionExpiration → vasoconstriction → ↓ pulmonary perfusion.

Ventilation perfusion ration(V/Q):differences in V & Q in different parts of the lung: Is the ratio between alveolar ventilation in one min and pulmonary perfusion in one min For proper O2 and CO2 exchange in the lungs, ventilation and perfusion must be matched.

Various lung intravascular and extravascular pressures influence pulmonary blood flow and its distribution in the lung. Pressure in different vascular segments (arteries, capillaries and veins), extravascular pressures (intrathoracic or intrapleural), and transmural pressure can vary considerably during both the cardiac and respiratory cycles. Because these pressures can influence the distribution of blood flow and vascular resistance, they can affect how well blood flow is matched to ventilation.Gravity dependent reduction in perfusion is more marked at the apex than reduction for ventilation → V/Q is highest at the apex and lowest at the base in upright posture.

The pulmonary circulation receives the entire output of the right heart, but vascular pressures are considerably lower than in systemic vessels. Blood flow in the upright lung is distributed preferentially to the lung base because of the influence of gravity. However, the base also receives a greater proportion of the ventilation than does the apex. This imbalance in ventilation to perfusion in the upright lung can lead to a higher VA/Q at the apex than the base This is reflected by a higher alveolar PO2 and lower PCO2 in alveoli at the apex than at lung base.

Composition of Alveolar Air is Kept Constant (Po2 = 100mHg, Pco2 =40mHg)O2 continuously diffuses out of the alveoli into the blood stream and CO2 continuously diffuses into the alveoli from bloodInspired air mixes with alveolar air, replacing the O2 and diluting the CO2. The blood transports O2 and CO2 between the lungs and other tissues throughout the body. These gases are carried in several different forms:Dissolved in plasmaChemically combined with Hb.Converted into a different moleculeOxygen Transport: * 98. % combines with hemoglobin (oxy –Hb )in RBC (Hb increases the O2 carrying capacity of blood 70-fold). * 2 % dissolved in plasma Transport of CO2 in the blood while CO2 is transported in 3 forms:- In the form of HCO3 carbonic acid …. 70% - In dissolved in plasma …………... 7%With Hb proteins as carbamino compounds (23%): CO2 combines reversibly with Hb to form carbamino Hb CO2 + Hb  Hb-CO2CO2 does not bind to iron, as oxygen does, but to amino groups on the polypeptide chains of Hb & of plasma proteins .

Oxyhemoglobin Dissociation Curve It is an S - shaped curve with a steep slope between 10 and 60 mm Hg P02 and a flat portion between70 and 100 mm Hg P02 . At a P02 of 60 mm Hg, 90% of the total Hb is combined with 02. From this point on, a further increase in P02 produces only a small increase in 02 binding
* 60mmg=90%


The position of the curve can be defined by the Po2 at which 50% of Hb is bound to O2 (P50).At normal body temp (37°C) arterial blood with a pH of 7.4, a Pco2 of 40 mmHg, P50 27 mmHg.The higher the P50, the lower the affinity of Hb for O2.when the affinity of Hb for O2  (P50 )  the curve is shifted to the right  unloading of O2 to the tissues

Oxyhemoglobin Dissociation Curve Graphic illustration of the % oxy hemoglobin saturation at different values of P02 .The curve of the relationship between blood PO2 and the percent of Hb saturated with O2. It is an S – shaped curve with a steep slope between 10 and 60 mm Hg PO2 and a flat portion between 60 and 100 mm Hg PO2.Loading and unloading of 02 is influenced by the affinity of hemoglobin for 02.● Affinity is decreased when pH is decreased , Increased temperature and 2,3-DPG that Shift the curve to the right(Affinity of hemoglobin for 02 decreases)& Greater unloading of 02 Steep portion of the sigmoidal curve, small changes in P02 produce large differences in % saturation (unload more 02).Decreased blood temperature The factors that shift the curve to the left, which means that at any given pO2, Hb has more affinity for O2 (lower P50), are:
The factors that displace the curve to the right, which means that at any given PO2, Hb has less affinity for O2 (higher P50), are
1- Decrease in [H+] with pH increasing from 7.4 to 7.6. 2- Decreased 2,3-diphosphoglycerate (2,3-DPG) concentration. 3 -Decreased CO2 4-The presence of large amount of Hb-F 5-Decreased blood temperature
1- Increased [H+] with pH decreasing from concentration from 7.4 to 7.2. 2- Increased CO2 *3 -Increase 2 ,3- DPG a is ,2 ,3 diphosphoglycerate [phosphate compound normally present in the blood. 4 - Increased blood temperature


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