Blood plasma, the pale yellow liquid component of blood, serves as the body’s primary circulating fluid medium, making up about 55% of total blood volume. This fluid is overwhelmingly water (typically 92%), with the remainder being a complex mixture of dissolved substances. A reduction in this liquid, known as low plasma volume or hypovolemia, compromises the body’s ability to maintain adequate pressure for circulation. This condition interferes with the delivery of oxygen and nutrients to tissues throughout the body.
Understanding Blood Plasma and Volume
Plasma’s composition is a sophisticated solution that allows it to perform its diverse roles effectively. The remaining 8% consists of vital plasma proteins like albumin and fibrinogen, along with electrolytes, hormones, nutrients, and waste products. Albumin exerts an osmotic pressure, which keeps fluid inside the blood vessels rather than leaking into surrounding tissues.
The primary function of plasma is the transportation of blood cells, hormones, and nutrients to the organs and tissues. Plasma also plays a significant role in temperature regulation by absorbing and releasing heat as it circulates. When discussing low plasma, the focus is on the overall volume of this fluid, not a deficiency in a specific component. A decrease in circulating volume immediately reduces the pressure necessary to propel blood through the vessels.
Primary Drivers of Reduced Plasma Volume
Reduced plasma volume results from the body losing fluid faster than it can be replaced, or from fluid shifting out of the bloodstream. One major pathway is significant external fluid loss, which occurs rapidly with persistent vomiting or severe diarrhea. Excessive sweating during prolonged physical exertion, or extensive skin damage from severe burns, also leads to substantial loss of water and electrolytes.
Another mechanism is the direct loss of whole blood, typically from trauma, internal bleeding, or major surgical procedures, which removes both plasma and cellular components. Fluid can also shift from the blood vessels into other body spaces, a process known as third-spacing. This occurs in conditions like peritonitis or acute pancreatitis, where inflammation causes vessel walls to become leaky. The resulting fluid accumulation outside the circulatory system is functionally lost to the circulating blood volume.
Recognizing the Physical Signs
When plasma volume drops, the body attempts to compensate to maintain perfusion to the brain and heart. This compensatory effort is often observed as a rapid heart rate, or tachycardia, as the heart works harder to pump a smaller volume of fluid more frequently. A common physical sign is low blood pressure, particularly a drop when transitioning to standing, which can cause dizziness or lightheadedness.
The body also tries to conserve fluid by limiting output and constricting peripheral blood vessels. This leads to reduced urination, which may become dark and concentrated, and the sensation of extreme thirst. Physical examination may reveal dry mucous membranes and a loss of skin elasticity. This is seen when pinched skin remains elevated for a moment instead of snapping back immediately.
Restoring and Maintaining Healthy Plasma Levels
Managing low plasma volume begins with confirming the diagnosis and assessing the severity of the loss. Blood tests are often used, such as checking the hematocrit level. This measure of red blood cells can appear artificially high because the plasma volume component has decreased. Treatment then centers on rapid and appropriate fluid replacement.
For mild cases, oral rehydration solutions containing balanced electrolytes and sugars can be sufficient. In more severe hypovolemia, intravenous fluids are required to restore volume quickly. These often include crystalloid solutions, like normal saline or Lactated Ringer’s solution, which contain water and electrolytes to expand the circulating fluid volume. If the cause is significant blood loss, blood products like packed red blood cells or fresh frozen plasma may be administered.
Addressing the underlying cause is also paramount, whether that means stopping active bleeding or treating the source of severe fluid loss like diarrhea or vomiting. Prevention focuses on maintaining adequate daily fluid intake, especially when there is an increased risk of fluid loss, such as during intense exercise or illness accompanied by fever. Sustained hydration is the most effective measure to ensure plasma volume remains within a healthy range.