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Gas exchange Airway flow CO 2 O2O2 no gas exchange.

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Presentation on theme: "Gas exchange Airway flow CO 2 O2O2 no gas exchange."— Presentation transcript:

1 Gas exchange Airway flow CO 2 O2O2 no gas exchange

2 Anatomical dead space V D Anat = 1 ml/lb body wt. airway start inspend insp end exp room air No gas exchange (white) in anatomical dead space flow CO 2 O2O2

3 Minute Ventilation: V E = breathing frequency (f) x tidal volume (V T ) 5 L/min = 10/min x 500 ml Correction for V D Anat : V E = f x (V T – V D anat ) 3.5 L/min = 10/min x (500 – 150) ml

4 Estimating anatomical dead space V T x F E CO 2 = V A x F A CO 2 + V D X F D CO 2 V olume x F raction = Mass of Gas Mass of Gas of V T = sum of gas masses of V A and V D end exp VTVT VDVD VAVA Since F D CO 2 = 0 V A = (V T x F E CO 2 ) / ( F A CO 2 ) Since V A = V T - V D V D = (F A CO 2 - F E CO 2 ) F A CO 2 VTVT

5 Partial pressure of a gas P gas = P B x F gas Daltons Law: in a mixture of gases, partial pressure (P gas ) of each gas contributes additively to total pressure (P B ) in proportion to the gass fractional volume (F gas )

6 dry room airinspired air alveolar gas P O P CO P H2O (37 o C) P N Total (mmHg) N2 O2 21% H2O 14% CO2 Alveolar P O 2 is < Inspired P O 2

7 Henrys Law: at equilibrium, gas pressure above a liquid equals gas pressure in the liquid P O 2, gas = 100 mmHg P O 2, blood = 100

8 Alveolar gas partial pressures determine blood gas tensions flow CO 2 O2O2 P CO 2, blood = 40 P CO 2, alv = 40 P O 2, alv = 100 P O 2, blood = 100 alveolus

9 Blood gas tension determines blood gas content Concentration of gas dissolved per liter of blood (C) depends on gas solubility ( ) and P gas in blood C O2 = x P O 2, blood C O2 = 0.03 x 100 = 3 ml O 2 /liter of blood = 0.3 ml O 2 /100ml of blood, Solubility coefficient (ml O2/[liter.mmHg]) Dissolved gas

10 The oxyhemoglobin dissociation curve P O 2, blood HbO2 saturation (%)

11 The O 2 carrying capacity of blood = Hgb-bound O 2 + dissolved O 2 P O 2, alv O 2 content in 1 liter of blood at P O 2, blood of 100 mmHg = (HgbO 2 sat [%] x 1.34 x [Hgb]) + (.03 x P O 2, blood ) = (0.98 x 1.34 x 150 [g/l]) + 3 = 200 ml O 2 /liter of blood HgbO 2 saturation= 1.34 ml at 100% P O 2, blood dissolved O 2 in blood

12 How tissues get O 2 flow O2O2 P O 2, alv = 100 alveolus O2O2 P O 2, tissue 10 P O 2, blood = 100 P O 2, blood tissue alveolus tissue ~30% P O 2, blood HgbO 2 saturation (%)

13 Low pH unloads O 2 better flow O2O2 P O 2, alv = 100 alveolus O2O2 P O 2, tissue 10 P O 2, blood = 100 P O 2, blood tissue alveolus ~50% tissue P O 2, blood HbO2 saturation (%) pH

14 The oxyhemoglobin dissociation curve P O 2, blood HbO2 saturation (%) pH

15 The pulmonary circulation tissue CO 2 O2O2 Pulmonary artery RV LA Pulmonary vein Pulmonary capillaries

16 PVR = (Ppa – Pla) / CO = 2 units tissue CO 2 O2O2 Ppa = 20 RV LA Pulmonary capillaries Note: pressures are mean and in cmH 2 O. CO is 5 L/min. Pla = 10 Pao = 200 SVR = (Pao – Pra) / CO = 40 units Pra = 0 CO = 5 PVR is <<< SVR

17 The lung has low vascular resistance

18 The lung has low vascular tone pressure flow kidney lung flow autoregulation time

19 Systemic capillaries

20 Alveolar wall Lung capillaries

21 Alveolus

22 The lungs low vascular resistance is due to 1. Low vascular tone 2. Large capillary compliance

23 PA enters mid lung height 30 cm PA

24 Gravity determines highest blood flow at lung base 0 cm End expiration +10 cm -10 cm PpaPla (cmH 2 O) Palv = 0

25 Hypoxic pulmonary vasoconstriction PO 2 = 100 mmHg PO 2 hypoxia

26 Capillary filtration determines lung water content alveolus lymphatic capillary

27 The Starling equation describes capillary filtration FR = Lp x S [ (Pc – Pi) – ( c – i) ] FRfiltration rate Scapillary surface area Pccapillary pressure Piinterstitial pressure reflection coefficient cplasma colloid osmotic pressure iinterstitial colloid osmotic pressure

28 PACapillary bed LA ( c – i) =.8 (30 – 12) ( c – i) Pressure (cmH 2 O) Keeping the alveoli dry: Large capillary pressure drop

29 Keeping the alveoli dry: Perivascular cuff formation

30 Normal lung Early pulmonary edema Perivascular cuffs in early pulmonary edema cuff

31 The ultimate insult: alveolar flooding

32 alveolar space interstitium NaCl NaK Cl Keeping the alveoli dry: active transport removes alveolar liquid active liquid transport Na-K pump NaCl transporter

33 SUMMARY Features of the pulmonary circulation designed for efficient gas exchange: 1. Accommodate the cardiac output * low vascular tone * high capillary compliance

34 SUMMARY Features of the pulmonary circulation designed for efficient gas exchange: 2. Keep filtration low near alveoli * low Pc * vascular interstitial sump

35 SUMMARY Features of the pulmonary circulation designed for efficient gas exchange: 3. Keep liquid out of the alveoli * active transport * high resistance epithelium

36 Control of Breathing Central neurons determine minute ventilation (V E ) by regulating tidal volume (V T ) and breathing frequency (f). V E = V T x f

37 DRG VRG pons medulla Neural Control of Breathing – Respiratory neurons

38 Neural Control of Breathing – The efferent pathway Phrenic n. muscle supply autonomic

39 CO 2, H+ Control of Breathing by Central chemoreceptors major regulators of breathing CO 2, H+ Central chemoreceptors Resp neurons VEVE

40 Carotid body O2O2 Chemical Control of Breathing – Peripheral chemoreceptors IXth n. CO 2 pH ~10% contribution to breathing

41 CO 2 tension in blood (mm Hg) VEVE 5 50 (L/min) quiet breathing hypercapnia CO 2 drives ventilation

42 CO 2 tension in blood (mm Hg) VEVE 5 50 (L/min) Hypoxia is a weak ventilatory stimulus blood pO 2 = 100 mmHg blood pO 2 = 50 mmHg

43 Airway Receptors: Slowly adapting (stretch - ends inspiration) Rapidly adapting (irritants - cough) Bronchial c-fiber (vascular congestion - bronchoconstriction) Parenchymal c-fiber (irritants - bronchoconstriction ) Xth n. Reflex Control of Breathing – Neural receptors afferents


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