Extracellular fluid

The ECF can also be seen as having two components – plasma and lymph as a delivery system, and interstitial fluid for water and solute exchange with the cells.

The ECF composition is therefore crucial for their normal functions, and is maintained by a number of homeostatic mechanisms involving negative feedback.

The volume of body fluid, blood glucose, oxygen, and carbon dioxide levels are also tightly homeostatically maintained.

The volume of extracellular fluid in a young adult male of 70 kg (154 lbs) is 20% of body weight – about fourteen liters.

The other major component of the ECF is the intravascular fluid of the circulatory system called blood plasma.

The volume of extracellular fluid in a young adult male of 70 kg, is 20% of body weight – about fourteen liters.

The composition of interstitial fluid depends upon the exchanges between the cells in the biological tissue and the blood.

The ionic composition of the interstitial fluid and blood plasma vary due to the Gibbs–Donnan effect.

[13] Substances in the ECF include dissolved gases, nutrients, and electrolytes, all needed to maintain life.

Since carbon dioxide is about 20 times more soluble in water than oxygen, it can relatively easily diffuse in the aqueous fluid between cells and blood.

[19][20] If hemoglobin in erythrocytes is the main transporter of oxygen in the blood, plasma lipoproteins may be its only carrier in the ECF.

This causes the cell membrane to temporarily depolarize (lose its electrical charge) forming the basis of action potentials.

The sodium ions in the ECF also play an important role in the movement of water from one body compartment to the other.

[23][28] The voltage gated sodium ion channels in the cell membranes of nerves and muscle have an even greater sensitivity to changes in the ECF ionized calcium concentration.

[31] Relatively small decreases in the plasma ionized calcium levels (hypocalcemia) cause these channels to leak sodium into the nerve cells or axons, making them hyper-excitable, thus causing spontaneous muscle spasms (tetany) and paraesthesia (the sensation of "pins and needles") of the extremities and round the mouth.

In addition, the pH of the ECF affects the proportion of the total amount of calcium in the plasma which occurs in the free, or ionized form, as opposed to the fraction that is bound to protein and phosphate ions.

This means that nutrients can be secreted into the ECF in one place (e.g. the gut, liver, or fat cells) and will, within about a minute, be evenly distributed throughout the body.

Oxygen taken up by the lungs from the alveolar air is also evenly distributed at the correct partial pressure to all the cells of the body.

Waste products are also uniformly spread to the whole of the ECF, and are removed from this general circulation at specific points (or organs), once again ensuring that there is generally no localized accumulation of unwanted compounds or excesses of otherwise essential substances (e.g. sodium ions, or any of the other constituents of the ECF).

However, this plasma is confined within the waterproof walls of the venous tubes, and therefore does not affect the interstitial fluid in which the body's cells live.

From the left atrium onward, to every organ in the body, the normal, homeostatically regulated values of all of the ECF's components are therefore restored.

In addition the lymph which drains the small intestine contains fat droplets called chylomicrons after the ingestion of a fatty meal.

The distribution of the total body water in mammals between the intracellular compartment and the extracellular compartment, which is, in turn, subdivided into interstitial fluid and smaller components, such as the blood plasma , the cerebrospinal fluid and lymph
Cell membrane details between extracellular and intracellular fluid
Sodium–potassium pump and the diffusion between extracellular fluid and intracellular fluid
Differences in the concentrations of ions giving the membrane potential
Formation of interstitial fluid from blood
Diagram showing the formation of lymph from interstitial fluid (labeled here as "tissue fluid"). The tissue fluid is entering the blind ends of lymph capillaries (shown as deep green arrows).