Blood is composed of cells (red and white) suspended in a liquid component known as plasma. Plasma makes up approximately 55% of the blood’s total volume, providing the fluid environment necessary for the survival and movement of all blood cells. This pale yellow fluid serves as the body’s internal river, enabling continuous circulation and linking every organ system together.
The Makeup of Plasma Water
Plasma is predominantly water, accounting for about 90% to 92% of its volume. This high water content allows it to function as an effective solvent for thousands of different substances. The remaining 8% consists of dissolved solutes, including proteins, electrolytes, nutrients, and waste products. Proteins, such as albumin, globulins, and fibrinogen, are the most abundant solutes. Electrolytes like sodium, potassium, and chloride are also dissolved, contributing to the plasma’s overall concentration.
Primary Roles in Circulation and Transport
The primary function of plasma is to act as the massive transport system connecting the entire organism. It serves as the vehicle for moving blood cells throughout the circulatory network, ensuring oxygen carriers and immune cells reach the tissues. Plasma carries absorbed nutrients, such as glucose and lipids, from the digestive system to cells for energy and growth. It also transports chemical messengers like hormones from endocrine glands to their distant target organs. Furthermore, plasma collects metabolic byproducts, such as urea and lactic acid, carrying them to the liver and kidneys for eventual excretion. The high water content also helps distribute heat generated by metabolism, assisting in the regulation of internal temperature.
Regulating Water and Solute Balance
The body maintains a highly specific concentration of water and solutes in the plasma through homeostatic mechanisms. This precise balance is known as osmolarity, and it is controlled by the interaction of the kidneys and specific hormones. Within the circulatory system, plasma proteins, particularly albumin, maintain an osmotic pressure that pulls water into the bloodstream from the surrounding tissues. This pressure keeps the plasma volume stable and preventing excessive fluid from leaking out of the capillaries. The kidneys play a major regulatory role by filtering plasma and then selectively reabsorbing water and electrolytes back into the blood. Hormones like Antidiuretic Hormone (ADH) respond to increases in plasma osmolarity or decreases in blood volume. ADH prompts the kidneys to increase water reabsorption, which conserves fluid and dilutes the plasma. Aldosterone acts on the kidney tubules to increase sodium reabsorption; because water follows sodium to maintain osmotic balance, this action helps restore plasma volume.
Health Consequences of Imbalance
Disruption of the plasma water balance can lead to significant health issues, categorized as volume loss or excess. A decrease in plasma volume, or hypovolemia, often resulting from dehydration or blood loss, impairs the delivery of oxygen and nutrients to tissues. This loss can lead to symptoms like dizziness and fatigue, and in severe cases, shock due to insufficient blood pressure. Conversely, an excess of fluid, known as hypervolemia, leads to swelling called edema. This often occurs when the plasma protein concentration, particularly albumin, is too low due to liver or kidney disease. With fewer proteins to exert osmotic pressure, water shifts out of the circulation and accumulates in the tissues. Kidney failure to excrete sufficient water and sodium also contributes to hypervolemia, potentially leading to high blood pressure and increased risk of heart complications.