My name is Julia Rasmussen. I'm a second year graduate student in physics and am in the Doyle group.
- 1 Final Project: TITLE
- 1.1 A brief tour of the lung
- 1.2 Early assessment of lung form
- 1.3 Role of surfactant in stabilizing lung foam during respiration
- 1.4 Components of lung surfactant
- 1.5 Synthetic lung surfactants
- 1.6 References
Final Project: TITLE
A brief tour of the lung
When a person inhales, air flows through the windpipe, through the bronchi, and into smaller tubes known as the bronchioles. Bronchioles lead to small sacs called alveoli where gas exchange occurs. The figure below shows to the major parts of the lung:
A closer look at the alveoli reveals how the gas exchange process functions. The walls of the alveoli, known as alveolar epithelium, are comprised of type 1 cells with long cytoplasmic extensions. Alveolar epithelium are covered with capillaries with a thin blood gas barrier that permits gas exchange to occur. Before reaching this barrier, gas diffuses through a liquid layer secreted by type 2 cells. Contained in the liquid layer, a phospholipid called lung surfactant stabilizes each alveoli during the respiration cycle. Such a layer develops during a baby’s first few hours of life creating a thin foam essential for proper lung function. A more detailed discussion of lung surfactant’s role in stabilizing the lung from collapse follows in this article. The figure below shows a cross section of an alveoli. Lung surfactant is a thin layer that lines the interior of the alveoli.
Early assessment of lung form
Early examinations of lung foam focused on lung extracted from animal dissections. Scientists found these foams to be unusually stable and resistant to silicon antifoams. Simple agitation of blood serum or oedema fluid (a fluid pools in the lungs during some illnesses) creates less stable foams, indicating that lung tissue is not simply comprised of these materials. In contrast to simple forms, bubbles in lung foam retain their size when placed in water for longer periods of time suggesting that they have zero surface tension. Bubbles in oedema or serum foams quickly disinigrate in water due to excess internal pressure .
Low surface tension between lung capillaries and alveoli is necessary for normal lung function. If the surface tension between these two regions were high, fluid from the capillaries would be sucked into the alveoli preventing normal respiration. An insoluble protein lining on the outside of the alveoli would reduce this surface tension. Estimates of the surfactant monolayer's thickness were made by drying and weighing lung tissue samples and assuming known bubble sizes. The monolayer was found to be about 50 angstroms thick .
Role of surfactant in stabilizing lung foam during respiration
Lung surfactant forms a thin monolayer at the alveoli liquid-air interface, and lowers the surface tension from that of normal water (~70 dyne/cm) to close to zero as a human exhales . Functioning lung surfactant must have a minimum surface tension of 10 dynes/cm when compressed to prevent alveolar collapse . Surface tension in this monolayer increases as the area available area for the monolayer decreases. From experiments, the surface tension T of lung surfactant was found to have the following relationship with area of an alveoli :
<math> T = 10 + 36(A/A_0)</math>
where <math>A_0</math> is the maximal volume.
Laplace’s law for a spherical structure of radius r gives the following relationship between internal pressure P and surface tension T:
<math>P = 2T/r.</math>
Combining these two equations yields the pressure as a function volume for alveolus. The figure on the left below shows this theoretical curve based on an experimentally measured constant surface tension of 46 dynes per centimeter during expansion . During exhalation, the stability of alveoli relies on its ability to maintain its volume as the pressure decreases. The condition for constant volume dP/dV > 0 is equivalent to the product of the compressibility K and surface tension T being less than a dimensionless constant k which depends solely on the geometry of the surface:
<math>KT \leq k</math> where <math>K = (1/A)(\delta A/\delta \gamma)</math>.
For a spherical geometry k=2. The figure on right show the dividing curve for alveoli stability. Combinations of surface tension and compressibility that lie above this curve will not be able to maintain a stable volume. It is only when the surface tension of lung surfactant drops to 11 dynes/cm that the alveoli become unstable .
Lung surfactant must change the surface tension appropriately with changes in air pressure to maintain a consistent radius, preventing a collapse of the alveoli during the respiration cycle.
This figure demonstrates alveoli function with and without proper lung surfactant.
In addition to preventing alveoli collapse, reductions in the surface tension reduce the amount of energy necessary to inflate the lungs, making breathing easier . Lung surfactant deficiencies are associated with a range of lung problems including oxygen starvation, lung collapse and pulmonary edema . Patients with acute respiratory distress syndrome (ARDS) often have lung surfactant that lacks the full range of surface tension modulation upon compression . This disease has a 30-50% mortality rate and is found in children and adults. Although this disease is caused by improperly functioning lung surfactant, patients respond poorly to replacement lung surfactant (RLS) therapies that are successful in premature babies born without lung surfactant . Doctors believe that ARDS in adults may result from surfactant inactivation caused by plasma and inflammatory leaking into alveoli . Such processes dramatically alter the surfactants' response to changes in pressure and reduce lung function. Some research on synthetic lung surfactants also focuses on understanding surfactant inactivation. Treatments that specifically target reducing surfactant inactivation would be more effective in adults than replacement lung surfactant therapies that were developed to provide premature babies with lung surfactant.
Components of lung surfactant
Proper lung surfactant is a complex mixture of roughly 90% lipids and 10% proteins in an aqueous solution. For most animals, DPPC (dipalmitoylphosphatidylcholine) comprises between 40% to 80% of the lipids in weight. The rest are unsaturated phosphoatidylcholines, saturated and unsaturated phophatidynlglycerols, other anionic lipids and cholesterol . Four identified proteins are creatively named SP-A, SP-B, SP-C and SP-D. Only SP-B and SP-C are amiphilic. Replacement lung surfactants often lack water soluble SP-A and SP-D proteins because they do not affect surface activity and are thought to be involved in immune system . Doctors believe that a person's immune system might react to the presence of proteins involved in the immune response of other animals. Although it is not believed to be part of the surface layer, SP-A may also play a role in surfactant transportation and adsorption . In this role, it could regulate the surface layer's composition.
Synthetic lung surfactants
Replacement lung surfactant (RLS) therapies are often refined from either bovine or porcine lungs due to limited human lung surfactant supplies. Since most RLS are generated from biological sources, virus transmission from animals to humans is a risk, and manufacturing costs are high. On going research focuses on developing cheaper, safer synthetic lung surfactants and adding synthetic additives to biological based lung surfactants. Proteins are often removed from animal derived lung surfactants during processing for use in humans for safety concerns. Often this affects a surfactant's properties, and ongoing research also examines how additives may improve an RLS's effectiveness.
Properties of a functioning lung surfactant
Any functioning replacement lung surfactant must have three main properties that ensure proper lung function. It must have a low surface tension on compression to prevent alveoli from collapsing. At the same time, RLS must remain viscous to avoid leaking out of alveoli after exhalation. During inhalation, lung surfactant must quickly adsorb to and re-spread along the interface . Native lung surfactant beautifully balances these key features.
Measuring properties of thin layers
Traditionally the phase behavior of a lung surfactant's thin monolayer was studied using a Langmuir trough method. More recently, experiments have measured expansion and contraction of captive bubbles to probe the same behavior.
A Langmuir trough is often used to measure the surface tension of a thin amphiphillic films. Typically this device consists of a teflon trough partially filled with water with a movable barrier at one end. A thin film is generated by dissolving a known amount of apmhiphillic molecules into a spreading solvent. This mixture is dropped into the trough, and the solvent is permitted to evaporate, leaving a thin film of amphiphillic molecules behind. The figure below shows an example of a Langmuir trough.
Potential for monolayer composition variation over respiration cycle
Wide variation exists among replacement lung surfactants on the market, and even different batches of the same replacement lung surfactant can differ widely . Although the composition of the overall surfactant in the human lung is known from analyzing minced lung samples, the precise composition in the monolayer liquid-air interface may differ from the lungs overall composition . Some researchers have even suggested that monolayer composition may vary over the course of a respiration cycle . Exchange between dissolved surfactant and monolayer surfactant provides one mechanism for local variation in monolayer composition. This makes production of replacement lung surfactants challenging because the ideal composition remains unknown. A new push in this field is to understand the role that each individual lipid and protein play in functioning lung surfactant.
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