Respiratory Gas Exchange

Front 1

the partial pressures of O2 and CO2 in systemic arterial blood represent a balance between what?

Back 1

the supply of O2 and its utilization in oxidative metabolism, and the production of CO2 during oxidative metabolism and its elimination via alveolar ventialtion

Front 2

the opposing sides of the respiratory surface are defined as what?

Back 2

ventialtion (V) and pulmonary arterial perfusion (Q) and the efficiency with which the lung is able to supply O2 to the blood and remove CO2 from it (how well ventialtion and perfusion are matches)

Front 3

when V/Q does not equal 1, what is the result?

Back 3

the efficiency of cardio-pulmonary function is impaired, and mismatching of ventialtion and perfusion is frequently a hallmark of respiratory disease

Front 4

what is the pressure of a gas?

Back 4

pressure relates the number of molecules in a closed space and the number of collisions these molecules make with the walls of the space. Number of collisions is essentially the temperatuer, or how fast the molecules move

Front 5

what is the pressure-molecule relationship of a gas?

Back 5

PV = nRT (where P = pressure, V = volume, n = number of molecules, R = 0.821 L*atm/mol*K, T = temperature (K)

Front 6

what is the functional pressure-molecule relationship of a gas in respiration?

Back 6

since lung volume and temperature reamin essentially constant, P = nC (gas pressures are essentially due ot the number of molecules present

Front 7

how do partial pressures affect gas transfer between alveolar space and blood?

Back 7

transfer of molecules of any kind between physiological compartments (across cell membranes, between blood and interstitial space, etc) is drive by the concentration gradient between the compartment

Front 8

what is Henry's law in terms of gas transfer between alveolar space and blood?

Back 8

the concentration of a gas in solution is proportional to it's partial pressure

Front 9

what is the transfer of gases across the respiratory surface driven by?

Back 9

the partial pressure gradient (which is equivalent to a concentration gradient)

Front 10

how is oxygen and carbon dioxide exchanged between alveolus and pulmonary capillary bed?

Back 10

across the air-blood barrier via diffusion

Front 11

what are the layer of the air-blood barrier?

Back 11

(1) fluid lining alveolus, containing surfactatnt (2) alveolar epithlil cells (3) epithelial basement membrane (4) thin interstitial space between alveolar epithlium and capillary membrane (5) capillary basement membrane fused with alveolar epithelial basement membrane in many places (6) capillary endothelial membrane (7) red blood cells

Front 12

what is diffusion through air-blood barrier governed by?

Back 12

Fick's Law

Front 13

illustrate the layers of the air blood barrier?

Front 14

what is Fick's Law as a calculation?

Back 14

volume of gas ~(Area/Thickness) * D (P1-P2) where (P1-P2) = difference in partial pressure of gas and D = diffusion constant; depends on gas and tissue type

Front 15

how is the diffusion constatnt for Fick's Law calculated?

Back 15

D ~ solubility of gas/ sqrt(molecular weight)

Front 16

how do O2 and CO2 compare in terms of solubility in tissue and diffusion rate?

Back 16

M.W.: O2 = CO2, solubility in tissue: CO2 > > O2, diffusion rate: 20 CO2 = 1 O2

Front 17

what is the primary mechansim of gas exchange?

Back 17

diffusion: air diffuses along a gas concentration gradient (from higher to lower partial pressure)

Front 18

what maintains the oscygen concentration gradient in gas exhange, allowing oxygen to move across the alveolar membrane?

Back 18

(1) difference in oxygen tension between alveolus and capillary blood (2) oxygen partial pressure in alveoli controlled by rate of entry new oxygen into lungs (ventialtion) and rate of absorption of oxygen into blood (3) oxygen that moves into capillaries associates rapidly with hemoglobin which maintains pO2 concentration gradient

Front 19

what establishes the CO2 concentration gradient, allowing CO2 to move across the alveolar membrane?

Back 19

(1) carbon dioxide continuously being formed in body as a product of aerobic metabolism (2) highly soluble, so lots dissolved in capillary blood (3) elimination by ventialtion maintains tension difference across respiratory surface

Front 20

what are the requirements for gas exchange in the lungs?

Back 20

(1) V, Ventilation, movement of air to and from the air-blood barrier (2) diffusion of gas across the respiratory barrier (3) Q, Perfusion, movement of blood to and from the barrier (4) V/Q, matching of ventilation and perfusion

Front 21

what are the factors determining pO2 & pCO2 in the alveoli?

Back 21

(1) rate of alveolar ventiation (2) rate of oxygen and carbon dioxide tranfer through the respiratory membrane

Front 22

what are the five etiologies of hypoxemia?

Back 22

(1) decreased inspired oxygen concentration (2) hypoventilation (3) diffusion impairment at the respiratory membrane (4) anatomic right to left shunt (5) ventilation:perfusion abnormalities

Front 23

what is hypoxemia?

Back 23

abnormally low paO2

Front 24

what causes decreased inspired oxygen concentration?

Back 24

high altitiude, anesthesia, or other breathing circuit O2 supply malfunction

Front 25

how does hypoxemia occur with decreased inspired oxygen concentration?

Back 25

as pAO2 decreases, so paO2 decreases so the partial pressure gradient between the alveolar and blood compartments becomes smaller, so there is less potential energy driving the transfer of O2 from alveoli to blood

Front 26

what is hypoventilation?

Back 26

strictly defines as p paCO2 > 45 mm Hg (determined by arterial blood gas analysis)

Front 27

what are causes of hypoventilation?

Back 27

(1) airway obstruction: congenital abnormalities ie brachycephalic dogs, acquired anatomical defects ie elongated soft palette and extra-thoracic tracheal collapse, inflammatory diseases, aspiration (2) neuromuscular weakness or dysfunction: neuromuscular blocking drugs ie NM antagonists, peripheral neuropathy, ypocalcemia, myasthenia gravis (3) decreased central chemoreceptor sensitivity: secondary to opiods, barbiturates, inhalation anesthetics

Front 28

how does hypoxemia occur with hypoventilation?

Back 28

inverse relationship between decreasing paO2 and increasing paCO2 as represented by the alveolar gas equation: pAO2 = plO2 - (pACO2/R) where plO2 is the partial pressure of inspired oxygen and R is the respiratory quotient or respiratory exchange ratio (refers to teh molecular ratio of CO2 production/O2consumption, normal value = 0.8, so according to the equation, when R = 0.8 (normal) the fall in alveolar pO2 is slightly greater than the rise in pCO2 during hypoventilation

Front 29

how does hypoxemia occur with diffusion impairment at the respiratory membrane?

Back 29

only apparent during exercise at high altitude when the partial pressure gradient favoring O2 diffusion from alveolar space to blood is reduced

Front 30

what causes anatomic right-to-left shunt?

Back 30

(1) congenital defects in which exist a true anatomic route for blood flow from the venous (deoxygenated, right heart) circulation to the arterial (oxygenated, left heart) circulation (2) tetralogy of Fallot (3) Eisenmenger's physiology - refers to reversal of left-to-right shunts secondary to vascular hypertrophy, eg. reversed PDA, reversed VSD

Front 31

how does hypoxemia occur with anatomic right-to-left shunt?

Back 31

variable fraction of cardiac output never actually goes to the lung, so can not become oxygenated

Front 32

what causes ventilation:perfusion abnormalities?

Back 32

(1) alveolar dead space, V/Q > 1 (2) physiologic right-to-left shunt (aka:venous admixture), V/Q < 1 (3) Gas exchanging units and gas exchange in the entire lung (4) hypoxic pulmonary vasoconstriction

Front 33

what causes alveolar dead space (V/Q > 1)?

Back 33

this V/Q mismatch occurs as a result of decreased blood flow to a particular lung region, with no change in regional ventilation caused by (1) any low cardiac output state (2) gravitational affects (3) vascular obstruction - emboli (heartworms, thrombi, air, fat), positive pressure ventilation

Front 34

what causes physiologic right-to-left shunt (aka: venous admixture, V/Q < 1)?

Back 34

(1) pneumonia (2) pneumothorax (3) space occupying masses of the lung parenchyma (4) atelectasis of any etiology, including absorption atelectasis from 100% O2 breathing (5) diaphragmatic hernia (6) gravitational affects

Front 35

how does hypoxemia occur with physiologic right-to-left shunt (aka: venous admisture), V/Q < 1?

Back 35

(1) distinguished from a true anatomic shunt by the fact that the cardiovascular anatomy is normal (2) blood actually does go to the lung via the pulmonary circulation (3) characterized by conditions wherein regional ventialtion is imparied with no change in blood flow, so blood goes to a region where the pAO2 is low, and O2 is not added to the portion of cardiac output exposed to those alveoli

Front 36

what is ideal in gas exchanging units and gas exchange in the entire lung?

Back 36

(1) acinus - a single gas exchanging unit consisting of an alveolus and its associated pulmonary capillary (2) perfect lung - all alveoli are ventilated equally, bood flow through all pulmonary capillaries is the same for all alveoli, and the V/Q = 1 for all acini

Front 37

what is reality in gas exchanging units and gas exchange in the entire lung?

Back 37

some acini are underventilated relative to perfusion (V/Q < 1) and some are underperfused relative to ventilation (V/Q > 1) so the normal lung functions as with a V/Q = 0.8 and the global distribution of ventilation and perfusion is such that when an individual assumes normal posture, the uppermost acini operate at V/Q > 1 and the lowermost acini operate at V/Q < 1 simply because of gravitational affect on both regional V and Q, which can be altered by altering body positioning (a homogenous lung is more efficient at adding O2 to blood than one having regional V/Q mismatching, but regional mismatching is the everyday reality) and patients with V/Q abnormalities generally present with normal paCO2 because the pa CO2 drives global ventilation, and the paCO2 rises and falls linearly with changes in ventilation whereas paO2 does not

Front 38

what occurs in hypoxic pulmonary vasoconstriction that causes ventilation:perfusion abnormalities?

Back 38

(1) pulmonary vessels constrict in regions where the pAO2 is low (2) passive routing of pulmonary blood flow away from low O2 alveoli, toward alveoli where pO2 is higher (3) intrinsic negative feedback reflex to minimize contribution of V/Q < 1 on gas exchange efficiency of teh entire lung

Front 39

what inhibits hypoxic pulmonary basoconstriction?

Back 39

inhalation anesthetics, but not IV anesthetics or other drugs