英国纽卡斯尔大学生理学课件0012.ppt

1、Respiratory System,Lecture 12,Synopsis,Factors that affect gas exchange - surface area - thickness of the membrane (diffusion distance) - concentration gradient,Gas exchange proportional to : A , 1/T , P nature of the gas,Structural design for gas exchange,Diffusion of O2 in tissue: 1 m 5 ms 0.1 mm5

2、 s 10 cm150 h,500 mio alveoli about 75m2,Daltons law: The total pressure of a gas mixture is equal the sum of partial pressures of the individual gases Example: Pair=PN2+PO2+PCO2 +PH2O 78.6% N2 20.9% O2 760=597+159+0.3+3.5(mm Hg) 0.04% CO2 0.46% H2O Henrys law: The amount of gas dissolved in a liqui

3、d is determined by the partial pressure of the gas and its solubility in the liquid,Equilibrium: partial pressures Time to reach the equilibrium: solubility! Example: SCUB diving/free diving,Henrys law,Partial pressure of various gases ( sea level),Humidification Gas exchange between alveoli and cap

4、illaries Mixing of old and new air,Dry airAlveolar airArterial bloodMixed venous blood PO2PCO2 0.3404046 PH2O0.0474747 PN2 600569573573 Ptotal 760760760706,in mm Hg,Blood PO2 and PCO2 during gas exchange,The equilibrium for O2 and CO2 is established at about the same rate : CO2 is about

5、 20 x more soluble than O2 concentration gradient for O2 is much higher (10 x) releasing CO2 from the blood is relatively slow,CO2 O2 N2 20 x 2x (CO O2 ),Gas exchange at the alveoli and cells,Gas exchange,Matching ventilation and perfusion,Low CO2 tends to cause constriction of the alveolar ducts,al

6、veoli at the apex of the lung are distended and limit the blood flow blood flow at the bottom of the lung is independent of the gas pressure,(cm H2O),20.4 27,2 44.9 44.9 74.8,Pulmonary blood pressure,gravity gas pressure hypoxic pulmonary vasoconstriction nervous control of blood vessel resistance,B

7、lood flow: Normal hydrostatic pressure in the pulmonary system is not sufficient to perfuse the lung in the apical zone. Ventilation: Due to gravity the lung is relatively compressed at the base. Consequently, it expands better on inspiration than the apex.,Ventilation/Perfusion relation,Ventilation

8、/Perfusion relation II,1,3,2,Hypoxic vasoconstriction note that in systemic arterioles low oxygen levels have the opposite effect!,V,=,Alveolar Ventilation,Q,Cardiac Output,Alveoli with V/Q 1:,-,relative excess of ventilation (physiological dead space),-,high alveolar P,O2,-,low alveolar P,CO2,-,ful

9、ly saturated blood, but “waste of air”,Alveoli w,ith V/Q 1,-,0.9:,-,“ideal” alveolus,-,saturated blood, normal P,O2,/ P,CO2,Alveoli with V/Q 0.9:,-,relative excess of perfusion,-,low alveolar P,O2,-,high alveolar P,CO2,-,gases not properly exchanged,Mechanisms to reduce V/Q mismatch:,-,Low alveolar

10、P,O2,causes constriction of pulmonary arterioles,-,Low alveolar P,CO2,causes constriction of alveolar ducts,-,Nervous control,summary,1 This does not affect gas exchange in the alveoli: Diffusion distance (M) Fibrotic lungs(C) Outside temperature (A) Surface area(N),2 Which of the statements is wrong? V always matches Q(L) VQ at apex of the lung (N) QV at base of the lung (A) V=Q at about 1/3 of lung(O),3 Spot the wrong gas pressure: alveolar O2: 100 mmHg(I) venous CO2: 46 mmHg(U) venous O2: 706 mmHg(A