Dynamics of Capillary Exchange of Fluid in the Lungs and Pulmonary Interstitial Fluid
Capillary exchange in the lungs involves the movement of fluids, gases, and solutes between the pulmonary capillaries and the interstitial fluid surrounding alveoli. This process is governed by two primary forces: hydrostatic pressure and osmotic pressure, which together determine the net filtration pressure (NFP).
- Hydrostatic Pressure (Pcap):
- Hydrostatic pressure is the force exerted by blood within pulmonary capillaries. It drives fluid out of the capillaries into the interstitial space.
- In healthy lungs, pulmonary capillary hydrostatic pressure is relatively low (approximately 8-12 mm Hg) to prevent excessive fluid leakage into the interstitium.
- Osmotic Pressure (Colloid Osmotic Pressure or BCOP):
- Osmotic pressure is created by plasma proteins (e.g., albumin) that remain in the blood and pull water back into capillaries from the interstitial space.
- The colloid osmotic pressure in pulmonary capillaries is typically around 28 mm Hg, which counterbalances hydrostatic forces to maintain fluid homeostasis.
- Net Filtration Pressure (NFP):
- The balance between hydrostatic and osmotic pressures determines whether fluid moves out of or into capillaries: NFP = Pcap − BCOP. Under normal conditions, this balance ensures minimal fluid accumulation in lung interstitium.
- Lymphatic System Role:
- Excess fluid filtered into the interstitial space is removed by lymphatic vessels at a rate of approximately 10-20 mL/hour under normal conditions. This prevents fluid buildup in lung tissues.
Interrelation Between Interstitial Fluid Pressure and Other Pressures in the Lung
The dynamics of lung interstitial fluid are influenced by interactions between several pressures:
- Pulmonary Capillary Hydrostatic Pressure (Pcap):
- Elevated Pcap increases filtration of fluid into the interstitium, as seen in left-sided heart failure or volume overload.
- Interstitial Fluid Hydrostatic Pressure (Pis):
- Normally slightly negative due to lymphatic drainage, Pis opposes further filtration from capillaries.
- When Pis becomes positive due to excessive fluid accumulation, it can lead to compression of alveoli and impaired gas exchange.
- Colloid Osmotic Pressures:
- Blood colloid osmotic pressure (BCOP) pulls water back into capillaries, while interstitial colloid osmotic pressure promotes outward movement.
- A disruption in this balance—such as reduced plasma protein levels—can exacerbate fluid leakage into lung tissue.
- Alveolar-Capillary Membrane Integrity:
- Damage to this barrier (e.g., due to inflammation or toxins) increases permeability, allowing proteins and fluids to flood into both interstitial spaces and alveoli.
Definition and Pathophysiological Mechanisms of Pulmonary Edema
Pulmonary edema refers to an abnormal accumulation of fluid within the lung’s interstitial spaces or alveoli, impairing gas exchange.
Pathophysiological Mechanisms:
- Increased Pulmonary Capillary Hydrostatic Pressure:
- Seen in cardiogenic pulmonary edema caused by left ventricular failure or mitral stenosis.
- Elevated left atrial pressure raises pulmonary venous pressures, leading to increased filtration across capillary walls.
- Decreased Plasma Oncotic Pressure:
- Conditions like hypoalbuminemia reduce BCOP, diminishing reabsorption capacity and promoting edema formation.
- Increased Capillary Permeability:
- Non-cardiogenic causes such as acute respiratory distress syndrome (ARDS), sepsis, or inhalation injuries damage alveolar-capillary membranes.
- This allows proteins and fluids to leak freely into both interstitium and alveoli.
- Impaired Lymphatic Drainage:
- Obstruction or dysfunction of lymphatics can prevent adequate removal of excess interstitial fluid.
Definition and Causing Factors of Pleural Effusion
Pleural effusion is defined as an abnormal accumulation of excess fluid within the pleural cavity—the space between visceral and parietal pleurae surrounding each lung.
Causing Factors:
- Increased Hydrostatic Pressure:
- Commonly caused by congestive heart failure where elevated systemic venous pressures lead to transudative effusions.
- Decreased Oncotic Pressure:
- Hypoproteinemia due to nephrotic syndrome or liver cirrhosis reduces plasma oncotic pressure, causing transudative effusions.
- Increased Vascular Permeability:
- Inflammatory conditions such as pneumonia or malignancy increase vascular permeability, leading to exudative effusions rich in proteins and inflammatory cells.
- Obstructed Lymphatic Drainage:
- Malignancies involving lymph nodes can block lymphatic return from pleural spaces, resulting in effusion formation.
- Other Causes:
- Trauma causing hemothorax
- Infection leading to empyema .
