Ventilation-Perfusion Ratio; Consequences on Arteriole pO2

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Definitions


Ventilation (Breathing): the movement of air (& thus gases) between the environment and alveoli. 12-20 breaths/min @ Rest

Perfusion* (Blood Flow): the process of nutritive delivery of arterial blood to a capillary bed. Pressure ~25/8 mm Hg, therefore Low Resistance; High Flow System

Ventilation-Perfusion Ratio: a measurement used to assess the efficiency and adequacy of the matching of two variables: ventilation & perfusion.


 * Opposite in pulmonary circulation.

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Ventilation
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Gas Laws: Dalton’s Law PTotal = PA + PB + PC…



Gases (singly/mixture) move from areas of HIGH to LOW Pressure (i.e. P)

Boyle’s Law P1V1 = P2V2

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Perfusion
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10% of Total Blood Volume in Pulmonary Circulation (~0.5L @ CO ≈ 5L/min) HIGH Flow; LOW Resistance System F is (Proportional) to the (Change) P/R [= (Change)Pr4/Ln * (pie/8)] Receives the ENTIRE CO per minute R (Proportional) Ln/r4 LOW because of SMALL L(ength) & LARGE cross-sectional Area 90% of R due to Trachea & Bronchi (rigid structures with small cross-sxn area)

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Ventilation-Perfusion Ratio
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Main determinant of blood concentration* of O2 Variables V = Ventilation (expressed as O2 mass reaching alveoli per minute) V = g / min Q = Perfusion (flow of blood in the lungs per minute) Q = L / min Therefore: (V/Q) = [(g/min)/(L/min)] => g / L*


 * Usually, however, the density of the gas (in g/l) can be used to convert the V/Q into a dimensionless ratio, and this is how it is most often expressed.

Physiologic Shunt (V/Q below norm): measure of the amount of (shunted) blood that is not oxygenated passing through lungs. (PICS 3 & 4; ppg 8)

QPS=physio. shunt blood flow per min QT=CO per min CiO2= arterial PO2 w/ ideal V/Q ratio. CaO2 = measured PO2 in arterial blood. CvO2 =arterial PO2 in mixed venous blood.

Physiologic Dead Space (V/Q above norm): measure of the amount of (wasted) ventilation. (PICS 5 & 6; ppg 9)

VDphys=physiologic dead space VT=tidal volume PaCO2=arterial PCO2 w/ ideal V/Q ratio. PECO2=average PCO2 in expired air.

1° - By Regulating the diameters of Arterioles & Bronchioles Arterioles Pulmonary Capillary Properties (*Collapsible) @ Rest: Apical Cap. Beds = Collapsed (V/Q ≈ 2.5x Ideal) Base Cap. Beds = Perfused (gravity-induced increase in hydrostatic pressure; V/Q ≈ 0.6x Ideal) During Exercise: Apical Cap Beds = Perfused (CO-induced increase in B.P.) Local Factors (Resistance is 1° regulated by [PO2] in surrounding interstitial fluid.) PO2 PCO2 Bronchioles (Diameter is 1° regulated by [PCO2] in exhaled air)

Arterioles

(Decrease) Pulmonary Arteriole PO2 (surrounding tissue)→ * Pulmonary Arteriole Vasoconstriction → (Decrease) Pulmonary Venous PO2 → (Decrease) Systemic Arteriole PO2


 * Note: vasoconstriction in Pulmonary arterioles is in response to low PO2; opposite to Systemic arterioles.

Bronchioles

(Increase) Alveolar PCO2 (exhaled air) → (Increase) Bronchodilation → (Increase) Alveolar PO2 (inspired air) → (Increase) Systemic Arteriole PO2


 * PSNS = bronchocontriction; SNS ≈ NOT significant, BUT respond to Epinephrine (Adrenal Medulla; β2 Recepto




 * Note: Responses in parentheses indicate weak responses.

Summary

1. Matching ventilation to alveolar sacs with blood flow past those alveoli, is a two-part process involving both air- & blood-flow. 2. V/Q is a measure of efficiency and adequacy of, ultimately, gas exchange/delivery. 3. Bronchioles & Arterioles are subject to reflex control via NS & hormones. HOWEVER: 4. Bronchioles & Arterioles are controlled on a minute-to-minute basis via paracrines.

References

Dee U. Silverthorn (2004). Texas: Human Physiology, (3rd Ed.) Daryl Fox Arthur C. Guyton & John E. Hall (2006). Mississippi: Textbook of Medical Physiology, (11th Ed.) Elseiver Inc. http://www.wikipedia.org/