Extracellular fluid
In
Extracellular fluid is the internal environment of all
The extracellular fluid, in particular the interstitial fluid, constitutes the body's
The volume of extracellular fluid in a young adult male of 70 kg (154 lbs) is 20% of body weight – about fourteen liters. Eleven liters are interstitial fluid and the remaining three liters are plasma.[7]
Components
The main component of the extracellular fluid (ECF) is the interstitial fluid, or tissue fluid, which surrounds the cells in the body. The other major component of the ECF is the intravascular fluid of the circulatory system called blood plasma. The remaining small percentage of ECF includes the transcellular fluid. These constituents are often called "fluid compartments". The volume of extracellular fluid in a young adult male of 70 kg, is 20% of body weight – about fourteen liters.
Interstitial fluid
Interstitial fluid is essentially comparable to plasma. The interstitial fluid and plasma make up about 97% of the ECF, and a small percentage of this is lymph.
Interstitial fluid is the body fluid between blood vessels and cells,[8] containing nutrients from capillaries by diffusion and holding waste products discharged by cells due to metabolism.[9][10] 11 liters of the ECF are interstitial fluid and the remaining three liters are plasma.[7] Plasma and interstitial fluid are very similar because water, ions, and small solutes are continuously exchanged between them across the walls of capillaries, through pores and capillary clefts.
Interstitial fluid consists of a water solvent containing sugars, salts, fatty acids, amino acids, coenzymes, hormones, neurotransmitters, white blood cells and cell waste-products. This solution accounts for 26% of the water in the human body. The composition of interstitial fluid depends upon the exchanges between the cells in the biological tissue and the blood.[11] This means that tissue fluid has a different composition in different tissues and in different areas of the body.
The plasma that filters through the blood capillaries into the interstitial fluid does not contain red blood cells or platelets as they are too large to pass through but can contain some white blood cells to help the immune system.
Once the extracellular fluid collects into small vessels (lymph capillaries) it is considered to be lymph, and the vessels that carry it back to the blood are called the lymphatic vessels. The lymphatic system returns protein and excess interstitial fluid to the circulation.
The ionic composition of the interstitial fluid and blood plasma vary due to the Gibbs–Donnan effect. This causes a slight difference in the concentration of cations and anions between the two fluid compartments.
Transcellular fluid
Transcellular fluid is formed from the
Function
Extracellular fluid provides the medium for the exchange of substances between the ECF and the cells, and this can take place through dissolving, mixing and transporting in the fluid medium.
Oxygenation
One of the main roles of extracellular fluid is to facilitate the exchange of molecular oxygen from blood to tissue cells and for carbon dioxide, CO2, produced in cell mitochondria, back to the blood. 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.[16]
However, hydrophobic molecular oxygen has very poor water solubility and prefers hydrophobic lipid crystalline structures.[17][18] As a result of this, plasma lipoproteins can carry significantly more O2 than in the surrounding aqueous medium.[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.
The oxygen-carrying capacity of lipoproteins, reduces in
Regulation
The internal environment is stabilised in the process of homeostasis. Complex homeostatic mechanisms operate to regulate and keep the composition of the ECF stable. Individual cells can also regulate their internal composition by various mechanisms.[22]
There is a significant difference between the concentrations of sodium and potassium ions inside and outside the cell. The concentration of sodium ions is considerably higher in the extracellular fluid than in the intracellular fluid.[23] The converse is true of the potassium ion concentrations inside and outside the cell. These differences cause all cell membranes to be electrically charged, with the positive charge on the outside of the cells and the negative charge on the inside. In a resting neuron (not conducting an impulse) the membrane potential is known as the resting potential, and between the two sides of the membrane is about −70 mV.[24]
This potential is created by sodium–potassium pumps in the cell membrane, which pump sodium ions out of the cell, into the ECF, in return for potassium ions which enter the cell from the ECF. The maintenance of this difference in the concentration of ions between the inside of the cell and the outside, is critical to keep normal cell volumes stable, and also to enable some cells to generate action potentials.[25]
In several cell types voltage-gated ion channels in the cell membrane can be temporarily opened under specific circumstances for a few microseconds at a time. This allows a brief inflow of sodium ions into the cell (driven in by the sodium ion concentration gradient that exists between the outside and inside of the cell). 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. When tears are secreted, or saliva is formed, sodium ions are pumped from the ECF into the ducts in which these fluids are formed and collected. The water content of these solutions results from the fact that water follows the sodium ions (and accompanying
.Calcium ions have a great propensity to
The tertiary structure of proteins is also affected by the pH of the bathing solution. 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. A change in the pH of the ECF therefore alters the ionized calcium concentration of the ECF. Since the pH of the ECF is directly dependent on the partial pressure of carbon dioxide in the ECF, hyperventilation, which lowers the partial pressure of carbon dioxide in the ECF, produces symptoms that are almost indistinguishable from low plasma ionized calcium concentrations.[29]
The extracellular fluid is constantly "stirred" by the
Interaction between the blood plasma, interstitial fluid and lymph
The arterial blood plasma, interstitial fluid and lymph interact at the level of the blood
The movement of water out of the capillary at the arteriolar end causes the concentration of the substances that cannot cross the capillary wall to increase as the blood moves to the venular end of the capillary. The most important substances that are confined to the capillary tube are plasma albumin, the plasma globulins and fibrinogen. They, and particularly the plasma albumin, because of its molecular abundance in the plasma, are responsible for the so-called "oncotic" or "colloid" osmotic pressure which draws water back into the capillary, especially at the venular end.[34]
The net effect of all of these processes is that water moves out of and back into the capillary, while the crystalloid substances in the capillary and interstitial fluids equilibrate. Since the capillary fluid is constantly and rapidly renewed by the flow of the blood, its composition dominates the equilibrium concentration that is achieved in the capillary bed. This ensures that the
A small proportion of the solution that leaks out of the capillaries is not drawn back into the capillary by the colloid osmotic forces. This amounts to between 2-4 liters per day for the body as a whole. This water is collected by the lymphatic system and is ultimately discharged into the left subclavian vein, where it mixes with the venous blood coming from the left arm, on its way to the heart.[23] The lymph flows through lymph capillaries to lymph nodes where bacteria and tissue debris are removed from the lymph, while various types of white blood cells (mainly lymphocytes) are added to the fluid. In addition the lymph which drains the small intestine contains fat droplets called chylomicrons after the ingestion of a fatty meal.[28] This lymph is called chyle which has a milky appearance, and imparts the name lacteals (referring to the milky appearance of their contents) to the lymph vessels of the small intestine.[35]
Extracellular fluid may be mechanically guided in this circulation by the vesicles between other structures. Collectively this forms the interstitium, which may be considered a newly identified biological structure in the body.[36] However, there is some debate over whether the interstitium is an organ.[37]
Electrolytic constituents
Main
Main
- Chloride (Cl−) 103–112 mM
- Bicarbonate (HCO3−) 22–28 mM
- Phosphate (HPO42−) 0.8–1.4 mM
See also
References
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- ^ "Interstitial Fluid - What is the Role of Interstitial Fluid". Diabetes Community, Support, Education, Recipes & Resources. 2019-07-22. Retrieved 2019-07-22.
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