Lecture six
Respiratory membranes diffusing capacity: It is the volume of a gas that will diffuse through the membrane each minute for a partial pressure difference of 1 mmHg. All the factors discussed earlier that affect diffusion through the respiratory membrane can affect this diffusing capacity.Diffusing capacity for oxygen: In the average young man, the diffusing capacity for oxygen under resting conditions averages 21 ml/min/mm Hg. The mean oxygen pressure difference across the respiratory membrane during normal, quiet breathing is about 11 mm Hg. Multiplication (11*21=230).
So 230 ml of oxygen diffusing through the respiratory membrane each minute at rest; this is equal to the rate at which the resting body uses oxygen.
Change in oxygen diffusing capacity during exercise: During strenuous exercise the diffusing capacity for oxygen increases in young men to a maximum of about 65 ml/min/ mm Hg, which is three times the diffusing capacity under resting conditions. This increase is caused by several factors, among which are (1) opening up of many previously inactive pulmonary capillaries or extra dilation of already open capillaries, thereby increasing the surface area of the blood into which the oxygen can diffuse; and (2) a better match between the ventilation of the alveoli and the perfusion of the alveolar capillaries with blood, called the ventilation perfusion ratio (will discuss later).
Diffusing capacity for carbon dioxide: The diffusing capacity for carbon dioxide has never been measured because carbon dioxide diffuses through the respiratory membrane so rapidly and its partial pressure difference is less than 1 mm Hg, technically this difference is too small to be measured. It has been shown the diffusing capacity varies directly with the diffusion coefficient of the particular gas. As the diffusion coefficient of carbon dioxide is slightly more than 20 times that of oxygen, one expects a diffusing capacity for carbon dioxide under resting conditions of about 400 to 450 ml/min/ mm Hg and during exercise of about 1200 to 1300 ml/min/mm Hg.
Diffusing capacity for carbon monoxide: Diffusing capacity for carbon monoxide in young men at rest is 17 ml/min/mm Hg. Physiologists usually measure carbon monoxide diffusing capacity instead of measuring oxygen diffusing capacity because it is so difficult, so inaccurate and not practical to measure oxygen diffusing capacity by a direct procedure. To convert carbon monoxide diffusing capacity to oxygen diffusing capacity, the value is multiplied by a factor of 1.23 because the diffusion coefficient for oxygen is 1.23 times that for carbon monoxide (so diffusing capacity for oxygen 17*1.23=21 ml/min/mm Hg).
Effect of the ventilation- perfusion ratio on alveolar gas concentration
The ventilation-perfusion ratio is expressed as VA/Q, When VA (alveolar ventilation) is normal for a given alveolus (4.2L/min) and Q (blood flow) is also normal for the same alveolus (5.5 L/min), the ventilation-perfusion ratio (VA/Q) is also said to be normal (0.8). The VA/Q is zero when the ventilation (VA) is zero, while yet there is still perfusion (Q) of the alveolus. Or, The VA/Q is infinity when there is adequate ventilation (V) but zero perfusion (Q). At a ratio of either zero or infinity, there is no exchange of gases through the respiratory membrane.Gas exchange and alveolar partial pressures when VA/Q is normal (fig 12)
When there is both normal alveolar ventilation and normal alveolar capillary blood flow, alveolar PO2 is normally at a level of 104 mm Hg and alveolar PCO2 is normally at a level of 40 mm Hg.
Alveolar oxygen and carbon dioxide partial pressures when VA/Q equals zero (fig 12)
When VA/Q is equal to zero, that is without any alveolar ventilation, the air in the alveolus comes to equilibrium with the blood oxygen and carbon dioxide. Because the blood that perfuses the capillaries is venous blood returning to the lungs from the systemic circulation and has a PO2 of 40 mm Hg and a PCO2 of 45 mm Hg and the alveolar gases will have the same partial pressures after equilibrium.
Alveolar oxygen and carbon dioxide partial pressures when VA/Q equals infinity (fig 12)
In this case there is no capillary blood flow to carry oxygen away or to bring carbon dioxide to the alveoli. Here the alveolar air becomes equal to the humidified inspired air. And because normal humidified air has a PO2 of 149 mm Hg and a PCO2 of 0 mm Hg, these will be the partial pressures of these two gases in the alveoli.
Concept of physiologic shunt (When VA/Q is below normal)
Whenever VA/Q is below normal, there is inadequate ventilation to provide the oxygen needed to fully oxygenate the blood flowing through the alveolar capillaries. Therefore, a certain fraction of the venous blood passing through the pulmonary capillaries does not become oxygenated. This fraction is called shunted blood. In addition to 2% of the cardiac output from bronchial vessels also called shunted blood. The total quantitative amount of shunted blood per minute is called the physiologic shunt. This physiologic shunt is measured in clinical pulmonary function laboratories. The greater the physiologic shunt, the greater the amount of blood that fails to be oxygenated as it passes through the lungs.
Concept of the physiologic dead space (When VA/Q is greater than normal)
When ventilation of some of the alveoli is great but alveolar blood flow is low, there is an amount of oxygen in the alveoli that cannot be transported away from the alveoli by the flowing blood. Thus, the ventilation of these alveoli is said to be wasted. The ventilation of the anatomical dead space areas of the respiratory passageways is also wasted. The sum of these two types of wasted ventilation is called the physiologic dead space. This is measured in the clinical pulmonary function laboratory. When the physiologic dead space is great, much of the work of ventilation is wasted effort because so much of the ventilating air never reaches the blood.Abnormal VA/Q in the upper and lower normal lung:
In a normal person in the upright position in the upper part of the lung blood flow is less considerably than ventilation. Therefore, at the top of the lung VA/Q is as much as 2.5 times as great as the ideal value, which causes a moderate degree of physiologic dead space in this area of the lung.In the bottom of the lung, there is slightly too little ventilation in relation to blood flow, with VA/Q as low as 0.6 times the ideal value. In this area, a small fraction of the blood fails to become normally oxygenated, and this represents a physiologic shunt.
Abnormal VA/Q in chronic obstructive lung disease
Most people who smoke for many years develop various degrees of bronchial obstruction like emphysema leading to that small bronchioles are obstructed, the alveoli beyond the obstructions are unventilated exhibit serious physiologic shunt and also in those areas of the lung where the alveolar walls have been mainly destroyed but there is still alveolar ventilation, exhibit serious physiologic dead space.Transport of oxygen from the lungs to the body tissues (fig 13)
The PO2 of the gaseous oxygen in the alveolus averages 104 mm Hg, whereas the PO2 of the venous blood entering the pulmonary capillary at its arterial end averages only 40 mm Hg, the pressure difference is 104 40, or 64 mm Hg.Uptake of oxygen by the pulmonary blood during exercise
During strenuous exercise, (1) a persons body may require as much as 20 times the normal amount of oxygen. Also, (2) because of increased cardiac output during exercise, the time that the blood remains in the pulmonary capillary may be reduced to less than one half normal. But until now the blood still becomes almost saturated with oxygen by the time it leaves the pulmonary capillaries because of the great safety factor for diffusion of oxygen through the pulmonary membrane, This explained by:First, the diffusing capacity for oxygen increases almost threefold during exercise due to opening of other capillaries leading to better VA/Q ratio.
Second, on rest conditions, the blood becomes almost saturated with oxygen by the time it has passed through one third of the pulmonary capillary, and little additional oxygen normally enters the blood during the latter two thirds of its transit. That is, the blood normally stays in the lung capillaries about three times as long as necessary to cause full oxygenation. Therefore, during exercise, even with a shortened time of exposure in the capillaries, the blood can still become fully oxygenated, or nearly so.
Fig 12 Normal PO2-PCO2, VA/Q diagram