Extracellular fluid

Your body has water outside of its cells.

This water helps your cells work well. Much of this water is in your blood.

Other water sits between your cells. This fluid carries food to the cells. It also carries away waste. Some of this water is called lymph. The water helps move the air you breathe. It also keeps things like salt in balance. This helps your body stay healthy.

Your body is full of water. Most of it stays inside your cells. But some water stays outside. We call this extracellular fluid.

This fluid is like a bath for your cells. It helps them work the right way. Most of this fluid is interstitial fluid. This is the liquid that surrounds your cells.

Another big part is blood plasma. Plasma is the liquid part of your blood. Together, they make up 97% of the fluid.

A tiny bit of fluid is called lymph. It moves through small vessels. This fluid also helps carry oxygen. It carries oxygen from your blood to your cells. It also carries away waste like carbon dioxide.

Your body uses homeostasis to keep this fluid steady. Homeostasis is a way to keep things balanced. It keeps the right amount of salt and sugar. It also keeps the acid levels just right. This balance is very important for your health.

Your body is mostly made of water. In a healthy adult, water makes up about 50 to 60 percent of total body weight. Most of this water stays inside your cells. However, some water stays outside the cells. We call this the extracellular fluid, or ECF.

This fluid is very important for life. It acts as the internal environment for all multicellular animals. It provides a medium where things can mix and move. This allows nutrients and gases to reach every part of your body.

There are different parts to this fluid. The largest part is called interstitial fluid. This liquid sits between your blood vessels and your cells. It bathes the cells and helps them exchange materials. Another major part is blood plasma. Plasma is the liquid part of your blood. Together, plasma and interstitial fluid make up at least 97 percent of the ECF.

A small amount of fluid is called lymph. Lymph is what happens when interstitial fluid enters tiny lymph capillaries. The lymphatic system then carries this fluid back to your blood.

There is also a very small amount of transcellular fluid. This makes up about 2.5 percent of the ECF. This fluid stays inside special lined spaces in the body. You can find it in many different places. It is in the cerebrospinal fluid around your brain. It is also in the fluid in your eyes. You can find it in your joints and your inner ear. Because it is in so many places, its makeup changes depending on where it is.

For a young man weighing 70 kg, the ECF is about 20 percent of his body weight. This equals about fourteen liters of fluid. Of those fourteen liters, eleven liters are interstitial fluid. The remaining three liters are blood plasma. The ECF helps move oxygen from the blood to the cells. It also carries carbon dioxide away from the cells. Some molecules, like oxygen, use special carriers called lipoproteins to move through the fluid.

Your body uses a process called homeostasis to keep this fluid steady. Homeostasis is a way to maintain a stable internal environment. It regulates the levels of sodium, potassium, and calcium. It also keeps the levels of blood glucose and oxygen just right. This balance is vital for your cells to work. For example, sodium-potassium pumps in cell membranes help create electrical charges. These charges allow your nerves and muscles to send signals. Without this careful balance, your body could not function properly.

Extracellular fluid, often called ECF, is the total body fluid found outside the cells of any multicellular organism. It serves as the vital internal environment for these animals. In organisms with a circulatory system, a portion of this fluid is blood plasma. The ECF provides a medium for the exchange of essential substances. This exchange happens through dissolving, mixing, and transporting materials within the fluid. Without this medium, cells could not receive the nutrients or gases they need to survive.

The ECF is organized into distinct fluid compartments. The largest component is interstitial fluid, which surrounds the cells. The second major component is blood plasma, the liquid part of the circulatory system. Together, plasma and interstitial fluid make up at least 97% of the ECF. A small amount of the ECF is composed of lymph. Lymph is formed when interstitial fluid enters small lymph capillaries. The lymphatic system then returns this fluid, along with proteins, to the circulation.

A third, much smaller component is known as transcellular fluid. This makes up about 2.5% of the ECF. This fluid is contained within spaces lined by epithelial cells. Because it is found in many different locations, its chemical makeup changes dramatically from place to place. Examples include cerebrospinal fluid in the brain and aqueous humor in the eye. It is also found as joint fluid and the fluids within the inner ear.

To understand the scale of these fluids, consider a young adult male weighing 70 kg. For this individual, the ECF volume is about 20% of his total body weight. This equals approximately fourteen liters of fluid. Within that volume, eleven liters are interstitial fluid and three liters are plasma. The interstitial fluid acts as a bridge between blood vessels and cells. It contains nutrients from capillaries via diffusion and holds waste products from cell metabolism.

Oxygenation is one of the most critical roles of the ECF. The fluid facilitates the exchange of molecular oxygen from the blood to the tissue cells. It also carries carbon dioxide, or CO2, back to the blood. While CO2 dissolves easily in water, oxygen has very poor water solubility. To solve this, plasma lipoproteins act as carriers for oxygen in the ECF. If these lipoproteins are damaged by inflammation or aging, it can lead to tissue hypoxia, which is a lack of oxygen in the tissues.

The body maintains the ECF through a process called homeostasis. This involves complex negative feedback mechanisms to keep the internal environment stable. Homeostasis regulates the pH and the concentrations of electrolytes like sodium, potassium, and calcium. It also maintains levels of blood glucose, oxygen, and carbon dioxide. The ECF is constantly "stirred" by the circulatory system. This ensures the watery environment is virtually identical throughout the entire body.

Electrolytes in the ECF are essential for electrical signaling in the body. There is a significant difference in ion concentrations between the inside and outside of a cell. Sodium ions are much higher in the ECF, while potassium ions are higher inside the cell. This difference is maintained by sodium-potassium pumps in the cell membrane. These pumps move sodium out of the cell and pull potassium in. This process creates a resting potential, which is an electrical charge across the membrane.

Calcium ions in the ECF also play a vital role in protein function. Calcium helps determine the three-dimensional structure of many extracellular proteins. If calcium levels change, it can affect blood clotting factors and nerve function. For example, low calcium levels, called hypocalcemia, can cause muscle spasms known as tetany. Conversely, high calcium levels, or hypercalcemia, can cause muscle weakness and lethargy. Even the pH of the ECF is important, as it affects how much calcium is available to bind to proteins.