Pulmonary Gas Exchange Flashcards
Explain ventilation perfusion matching?
at the extremes some alveoli may?
as we push towards the two extremes what will happen in the bloodstream
explain the distribution of the ventilation/perfusion ratio in the upright lung?
Explain the vertical distribution of PO2 and PCO2 in the lungs?
summarize V/Q matching?
Explain Pulmonary diffusion?
Air is delivered to the gas exchange sites of the lung by bulk flow as a result of contractions of respiratory muscles. Gases are delivered to and transported from lung alveoli by pulmonary blood flow, which depends on cardiac muscle contraction. However, the movement of gases between the alveoli and capillary blood is by simple diffusion, a process requiring no metabolic energy. Gas diffusion in the lung, as elsewhere in the body, is a passive process whereby gases or other molecules move from a region of higher to lower concentration. In other words, diffusion is the passive movement of molecules in an attempt to eliminate concentration differences. The design of the alveolar capillary bed is nearly ideal for optimal gas diffusion. No artificial device can approach the efficiency of the lungs for gas exchange. The factors that govern the rate of exchange or diffusion for the lung are described in Fick’s equation for diffusion.
Explain Fick’s equation for the determinants of diffusion?
Fick’s equation for diffusion relates the variables that govern the diffusion rate of molecules such as O2 and CO2. Fick’s difusion equation states that the rate of diffusion is directly proportional to a diffusion coefficient (k), the area available for diffusion (A), and the concentration difference of the molecules across the membrane (dC), and inversely related to the length of the diffusion pathway (dl). Therefore, when the concentration difference in the molecules across the membrane increases, diffusion is increased. However, as the length of the diffusion pathway increases, the rate of difusion decreases.
Explain the principle of diffusion?
Oxygen diffuses from the alveoli into the pulmonary capillary blood because the oxygen pressure (PO2) in the alveoli is greater than the PO2 in the pulmonary blood.
What are the determinants of diffusion?
The diffusion of gases through the respiratory membranes follows the physics outlined below. The distance through which the gas must diffuse is ~5 microns and the area available for diffusion is 70 sq meters.
Explain membrane phase diffusion?
As mixed venous blood from the pulmonary artery flows into the pulmonary capillaries, it becomes exposed to an alveolar gas tension with a higher PO2 and a lower PCO2. Thus, net O2 diffusion is from alveoli to the pulmonary capillary, whereas CO2 moves in the opposite direction, as dictated by their respective concentration gradients. Normally, gas exchange between alveoli and pulmonary capillary blood is rapid by necessity because the red blood cell spends less than a second in the alveolar capillary. At rest, an RBC is estimated to pass through the alveolar capillary in about 0.75 sec. However, only about 0.25 sec is normally required for O2 and CO2 to completely equilibrate between alveoli and capillary blood. CO2 diffuses about 24 times faster than O2 in a liquid medium (membrane or blood) because CO2 is about 24-fold more soluble in body fluids than O2. As a result, CO2 dissolves in fluids more rapidly than O2 to hasten the establishment of a concentration or diffusion gradient. However, the greater rate of CO2 diffusion is offset to some extent by a smaller partial pressure difference for CO2 (6 Torr) than O2 (60 Torr) between the alveoli and blood. Overall, CO2 diffuses about 20 times faster than O2 between the alveolus and capillary. If diffusion is impaired by lung diseases or edema, the RBC may traverse the alveolar capillary before alveolar and capillary blood O2 and CO2 completely equilibrate (figure, abnormal). Blood then enters the pulmonary vein without being completely equilibrated with alveolar gases. As a consequence, the PO2 would be lower and the PCO2 higher than normal in the systemic arteries.
Explain pulmonary diffusion capacity (conductance)?
Clinically it is often useful to appraise the effectiveness or transfer rate of gases around the alveolarcapillary membrane. This can be accomplished by measuring the lung diffusing capacity (DL). The DL is a measure of the lung conductance (reciprocal of resistance) of gas movement across the alveolar-capillary membranes and chemical attachment to Hb. DL is measured in units of ml/min/Torr. The DL takes into account the effect of x x V Q A , capillary blood flow and the velocity of Hb-O2 reactions. The equations to calculate DL are presented on the right. However, it is difficult to measure the diffusing capacity of O2 (DLO2) because it is technically impossible and impractical to sample pulmonary capillary blood to measure the PO2. Thus, the PcapO2 must be derived using complex mathematical derivatives. Other gases, like carbon monxoide, are frequently used instead ofO2 to measure DL.
During strenous exercise, the diffusing capacity for oxygen increases from approximately 21 ml per min per mm Hg to 65 ml/min/mm Hg. This increase is caused by several different factors, including (1) RECRUITMENT - opening of a number of previously dormant pulmonary capillaries, thereby increasing the surface area of the blood into which the oxygen can diffuse. (2) DISTENSION - dilation of all the pulmonary capillaries that were already open, thereby further increasing surface area. Therefore during exercise, oxygenation of the blood is increased not only by increased alveolar ventilation but also by a greater capacity of the respiratory membrane for transmitting oxygen into the blood.
how do we measure pulmonary diffusing capacity?
Carbon monoxide (CO) is often used in place of O2 to gauge the diffusing capacity (DLCO) of the lung. In this test, the subject inhales a small fraction of CO and the alveolar PCO is measured. Because CO has an extremely high affinity for blood hemoglobin (200 fold greater than O2) and only a small amount is inhaled, it is assumed that all the CO entering the capillary is immediately bound to Hb. Thus the PcapCO is assumed to be essentially 0. With these simplifying assumptions, the diffusing capacity for CO (DLCO) can be estimated according to the equations in the figure.
Perfusion versus difusion limitation in general?
One word of caution: if you want to keep diffusion limitation straight, then think in terms of partial pressure, not content, of oxygen. When we consider diffusion of a gas, we’re really talking about the movement of that gas along a partial pressure gradient. For diffusion limitation to occur, it doesn’t matter how many total gas molecules get moved from one place to another. Instead, what matters is whether or not the partial pressure comes to equilibrium so that no gradient remains across the membrane.
When blood enters a pulmonary capillary of an alveolus containing a variety of gases (some of which may be foreign to the body) how fast does the partial pressure rise in the capillary? The following figures show the difference in response to a gas that is perfusion limited in its uptake versus a gas that is diffusion limited in uptake. The following figures show the uptake of carbon monoxide, nitrous oxide, and O2 along the pulmonary capillary. Note that the blood partial pressure of nitrous oxide virtually reaches that of alveolar gas very early in the capillary so that the transfer of this gas is perfusion limited. By contrast, the partial pressure of carbon monoxide in the blood is almost unchanged so that its transfer is diffusion limited. O2 transfer can be perfusion limited or partly diffusionlimited, depending on the conditions.
Perfusion versus diffusion for NO?
Perfusion vs Diffusion for CO2?
Perfusion vs diffusion for O2?
how do fibrotic lungs, Pulmonary edema, and emphysema affect pulmonary gas exchange?
