Third spacing describes a medical situation where fluid moves from the blood vessels into a “third space” within the body. When fluid accumulates in these areas, it becomes functionally lost from the circulating blood volume. This phenomenon represents a significant shift in the body’s fluid distribution, which can affect several bodily systems.
Normal Fluid Balance
The human body maintains a precise balance of fluids distributed across distinct compartments. The intracellular compartment holds fluid inside cells, while the extracellular compartment is outside cells. The extracellular fluid is further divided into interstitial fluid, which surrounds cells, and intravascular fluid, found within blood vessels. Movement between these compartments is regulated by opposing forces.
Hydrostatic pressure pushes fluid out of capillaries into the interstitial space. Counteracting this is oncotic pressure, created by proteins like albumin, which pulls fluid back into the capillaries. These forces work together to ensure a continuous and controlled exchange of water and small solutes. This dynamic equilibrium maintains proper tissue function and blood volume.
Physiological Shifts
Third spacing primarily occurs due to disruptions in the delicate balance of fluid exchange at the capillary level.
Increased Capillary Permeability
The walls of tiny blood vessels become more porous, allowing water and larger molecules, such as proteins, to leak out of the bloodstream and into the interstitial space.
Decreased Plasma Oncotic Pressure
A reduction in blood protein concentration, particularly albumin, lessens the reabsorption force that draws fluid back into the capillaries. This causes more fluid to remain in the interstitial space.
Elevated Capillary Hydrostatic Pressure
Increased pressure within blood vessels pushes more fluid forcefully out into the surrounding tissues. This leads to the abnormal accumulation of fluid in areas where it does not typically reside.
Lymphatic Obstruction
The lymphatic system acts as a drainage network, collecting excess fluid, proteins, and waste products from tissues and returning them to circulation. When this system is blocked or damaged, the collected fluid has no pathway back into the blood vessels, resulting in fluid buildup in the interstitial space.
Underlying Conditions
Various medical conditions can trigger the physiological shifts that lead to third spacing.
Increased Capillary Permeability
Severe inflammatory responses, such as those seen in sepsis, extensive burns, or acute pancreatitis, often cause increased capillary permeability. Allergic reactions, particularly anaphylaxis, can also lead to widespread capillary leakage, allowing fluid and proteins to escape from the bloodstream.
Decreased Plasma Oncotic Pressure
Conditions that reduce the amount of protein in the blood can lead to decreased plasma oncotic pressure. Liver failure, for example, impairs the liver’s ability to produce albumin, a crucial blood protein. Severe malnutrition can also result in insufficient protein intake, while certain kidney diseases, like nephrotic syndrome, cause excessive protein loss in the urine. All these scenarios diminish the blood’s capacity to retain fluid within the vessels.
Increased Capillary Hydrostatic Pressure
Cardiovascular issues like heart failure, where the heart cannot effectively pump blood, lead to blood backing up and increasing pressure in the capillaries, particularly in the lungs and lower extremities. Localized conditions like deep vein thrombosis, a blood clot in a deep vein, can also increase pressure in the affected limb, forcing fluid out of the vessels. These conditions create an outward push on fluids.
Lymphatic Obstruction
Finally, lymphatic obstruction, preventing proper fluid drainage, can be caused by various factors. Certain cancers, especially those affecting lymph nodes, can physically block lymphatic vessels. Surgical procedures involving lymph node removal, often done for cancer treatment, can disrupt the lymphatic network. Tropical parasitic infections like filariasis can also severely damage lymphatic vessels, leading to chronic fluid accumulation.
Impact on the Body
The accumulation of fluid in the third space has significant consequences for the body, primarily by reducing the effective circulating blood volume. This reduction means there is less fluid available to transport oxygen and nutrients throughout the body. A decrease in circulating blood volume can lead to hypotension, a drop in blood pressure, as the heart has less fluid to pump. This can compromise blood flow to vital organs.
Reduced organ perfusion, meaning less blood reaching organs like the kidneys and brain, can impair their function. The kidneys may struggle to filter waste, and brain function can be affected due to inadequate oxygen supply. Furthermore, fluid shifts can disrupt the balance of electrolytes in the blood, such as sodium and potassium, which are essential for nerve and muscle function. These imbalances can lead to further complications.
Visible signs of third spacing often include edema, swelling, especially in dependent areas like the ankles and feet. Ascites, the build-up of fluid in the abdominal cavity, is another common manifestation. These external signs reflect the internal redistribution of fluid, highlighting the body’s struggle to maintain normal fluid balance. The overall effect is a decrease in the body’s ability to maintain stable internal conditions.