Lecture -4 Interrelations between interstitial fluid pressure and other pressures in the lung
Forces tending to cause movement of fluid outward from the capillaries and into the pulmonary interstitium are: (fig-8)
Capillary pressure (7 mm) + Interstitial fluid colloid osmotic pressure (14) + Negative interstitial fluid pressure (8) = total outward force (29 mm Hg)
Forces tending to cause absorption of fluid into the capillaries:
Plasma colloid osmotic pressure (28) so total inward force (28 mm Hg)Thus, the normal outward forces are slightly greater than the inward forces, providing a mean filtration pressure at the pulmonary capillary membrane; this can be calculated as follows:
(Total outward force +29 mm Hg) (Total inward force 28 mm Hg) = mean filtration pressure +1.
This filtration pressure causes a slight continual flow of fluid from the pulmonary capillaries into the interstitial spaces, and except for a small amount that evaporates in the alveoli; this fluid is pumped back to the circulation through the pulmonary lymphatic system.
Negative Pulmonary Interstitial Pressure and the Mechanism for Keeping the Alveoli Dry
The pulmonary capillaries and the pulmonary lymphatic system normally maintain a slight negative pressure in the interstitial spaces, whenever extra fluid appears in the alveoli, it will simply be sucked mechanically into the lung interstitium through the small openings between the alveolar epithelial cells, so normally keeping the alveoli dry. Then the excess fluid is either carried away through the pulmonary lymphatics or absorbed into the pulmonary capillaries.●Pulmonary edema occur due to any factor causes the pulmonary interstitial fluid pressure to rise from the negative range into the positive range will cause rapid filling of the pulmonary interstitial spaces and alveoli with large amounts of free fluid. The most common causes of pulmonary edema are as follows:
1. Left-sided heart failure or mitral valve disease, with consequent great increases in pulmonary venous pressure and pulmonary capillary pressure and flooding of the interstitial spaces and alveoli.
2. Damage to the pulmonary blood capillary membranes caused by infections such as pneumonia or by breathing noxious substances such as chlorine gas or sulfur dioxide gas. Each of these causes rapid leakage of both plasma proteins and fluid out of the capillaries and into both the lung interstitial spaces and the alveoli.
●Pleural Effusion pleural effusion means the collection of large amounts of free fluid in the pleural space. The causes of the effusion are:
(1) Blockage of lymphatic drainage from the pleural cavity.
(2) Cardiac failure, which causes excessively high peripheral and pulmonary capillary pressures, leading to excessive transudation of fluid into the pleural cavity;
(3) Greatly reduced plasma colloid osmotic pressure, thus allowing excessive transudation of fluid.
(4) Infection or any other cause of inflammation of the surfaces of the pleural cavity, which breaks down the capillary membranes and allows rapid dumping of both plasma proteins and fluid into the cavity.
Physical principles of gas exchange through the respiratory membrane
After the alveoli are ventilated with fresh air, the next step in the respiratory process is diffusion of O2 from alveoli into the pulmonary blood & CO2 in the opposite direction.
Diffusion: It is the process of moving the simple respiratory gases molecules (as O2, CO2) among one another. The net diffusion of the gas will occur from the high-concentration area toward the low-concentration area.
Partial pressures of individual Gases: it is the pressures of gas in a mixture of gases and the pressure is directly proportional to the concentration of the gas molecules. The rate of diffusion of each of these gases is directly proportional to the partial pressure of that gas. For e.g the total pressure of air is 760 mm Hg which is the sum of the individual partial pressures of O2, CO2, N2, H2O and H. The partial pressure of nitrogen in 760 mm Hg mixture is 600 mm Hg (79%), and the “partial pressure” of oxygen is 160 mm Hg (21%).
The partial pressures of individual gases in a mixture are designated by the symbols PO2, PCO2, PN2, PH2O, PHe, and so forth.
●The gases dissolved in water or in body tissues also exert pressure called partial pressure of that gas.
●Factors that determine the partial pressure of a gas dissolved in a fluid according Henry’s Law are:
Concentration of dissolved gas
Partial pressure=Solubility coefficient
1-Concentration of gas has positive relation with partial pressure.
2-solubility coefficient of the gas which has inverse relation with partial pressureThe solubility coefficients for important respiratory gases at body temperature are the following:
Oxygen 0.024, Carbon dioxide 0.57, Carbon monoxide 0.018, Nitrogen 0.012, Helium 0.008
From this one can see that carbon dioxide is more than 20 times as soluble as oxygen. Therefore, the partial pressure of carbon dioxide (for a given concentration) is less than one twentieth that exerted by oxygen.
● Diffusion of gases between the gas phase in the alveoli and the dissolved phase in the pulmonary blood is determined by the difference between the two partial pressures of both gas phase and dissolved phase of the gas.
● the water immediately evaporates from the surfaces of respiratory passages to humidify the air. At normal body temperature, 37C, this vapour pressure is 47 mm Hg.
●The net rate of gas diffusion in fluids is affected by many factors
Factors that have positive relationship with the diffusion rate are:1) The partial pressure difference between the two ends of the diffusion pathway.
2) The solubility of the gas in the fluid.
3) The cross-sectional area of the fluid.
Factors that have negative relationship with the diffusion rate are:
1) The distance through which the gas must diffuse.2) The molecular weight of the gas.
●The temperature of the fluid also affect diffusion rate but the temperature remains reasonably constant and usually need not be considered.
Diffusion of gases through tissues: The gases that are of respiratory importance are all highly soluble in lipids and so in cell membranes. Because of this, the major limitation to the movement of gases in tissues is the rate at which the gases can diffuse through the tissue water instead of through the cell membranes and so it is affected by the same factors discuss earlier.