Body Fluids: Cerebrospinal, Amniotic, Cytosol and Interstitial Fluid

Let us make an in-depth study of the body fluids. After reading this article you will learn about 1. Cerebrospinal Fluid 2. Amniotic Fluid 3. Cytosol and 4. Interstitial Fluid.

1. Cerebrospinal Fluid (CSF):

It is a clear bodily fluid that occupies the subarachnoid space and the ventric­ular system around and inside the brain. Essentially, the brain floats in it. More specifically the CSF occu­pies the space between the arachnoid mater (the middle layer of the brain cover, meninges) and the piamater (the layer of the meninges closest to the brain).

Moreover it constitutes the content of all intra-cerebral (inside the brain, cerebrum) ventricles, cisterns and sulci (singular sulcus), as well as of the central canal of the spinal cord. It is an approximately isotonic solution and acts as a ‘cushion’ or buffer for the cortex, providing also a basic mechanical and immunological protection to the brain inside the skull.

It is produced in the brain by modified ependymal cells in the choroid plexus. The cerebrospinal fluid is produced at a rate of 500 ml/day. Since the brain can only contain from 135-150 ml, large amounts are drained primarily into the blood through arachnoid granulations in the superior sagittal sinus. This contin­uous flow into the venous system dilutes the concentration of larger, lipoin-soluble molecules penetrating the brain and CSF.

Biochemical constitutes:

The normal fluid is watery with low viscosity. Its specific gravity is 1.003 to 1.008. CSF pressure ranges from 60-100 mm H₂O or 4.4-7.3 mm Hg, with most variations due to coughing or internal compression of jugular veins in the neck. The CSF contains approximately 0.3% plasma proteins or 15 to 40 mg/dl. The proteins of CSF do not coagulate.

The albumin globulin ratio is 3.1. In diseased condition there is an increase in protein, especially globulin. The protein content of CSF in inflammatory meningitis increases to about 125 mg to 1g/100ml. In various brain diseases like neurosyphilis, encephalitis, abscess, tumour the protein content is elevated to 20-300 mg/100 ml and fibrinogen is completely absent. The glucose content of CSF is 50-85 mg/100 ml which is less than the plasma level. It increases in encephalitis, central nervous system syphilis, abscesses and tumors. It is decreased in purulent meningitis.

The lactic acid content of CSF ranges from 1.8 to 2.4 mg/dl. Measurement of lactic acid in CSF is done to differentiate between bacterial and viral meningitis. The concentration of lactic acid is elevated in conditions causing severe or global brain ischemia and anaerobic glycolysis.

Among the minerals, Ca is 4.1-5.9 mg/100 ml of CSF. Na and CI are higher in CSF than in serum, whereas K and P are less than in serum. Chloride content is decreased in meningitis and unchanged in syphilis, encephalitis, poliomyelitis and other diseases of the central nervous system. Chloride in CSF is decreased in tuberculosis meningitis. Magnesium is about 5 mg/100 ml.

Functions: The cerebrospinal fluid has many putative roles including mechanical protection of the brain, distribution of neuroendocrine factors and prevention of brain ischemia. The prevention of brain ischemia is made by decreasing the amount of cerebrospinal fluid in the limited space inside the skull. This decreases total intracranial pressure and facilitates blood perfusion.

When CSF pressure is elevated, cerebral blood flow may be constricted. When disorders of CSF flow occur, they may affect not only CSF movement, but also the intracranial blood flow with subsequent neuronal and glial vulnerabilities. The venous system is also important in this equation. Infants and patients shunted as small children may have particularly unexpected relationships between pressure and ventricular size, possibly due to venous pressure dynamics. This may have significant treatment implica­tions but the underlying pathophysiology needs to be further explored.

Cerebrospinal fluid can be tested for the diagnosis of a variety of neurological diseases. It is obtained by lumbar puncture, to count the cells in the fluid and estimate protein and glucose. These parameters alone may be extremely beneficial in the diagnosis of subarachnoid hemorrhage and central nervous system infections (such as meningitis).

A cerebrospinal fluid culture examination shows microorganism that has caused the infection. By the detection of the oligoclonal bands, an on-going inflammatory con­dition (for example, multiple sclerosis) can be recognized. A beta-2 transferrin assay is highly specific and sensitive for the detection of cerebrospinal fluid leakage.

Electrophoresis of CSF and its use in diagnosis:

The proteins in the CSF are identified by isoelectric focusing (IEF) and crossed immune-electrophoresis wherein forty distinct bands are seen. But only 22 CSF protein bands are formed after polyacrylamide gel electrophoresis (PAGE). These protein patterns are of great diagnostic importance. There is an abnormal alkaline gamma-globulin region in patients with multiple sclerosis. CSF protein abnormalities are found in patients with spinal muscular atrophy and with mus­cular dystrophy.

Two-dimensional gel electrophoresis is a technique with the capacity to resolve complex mixtures of thousands of proteins. Samples are subjected to IEF, then PAGE, to produce a gel pattern of proteins. The position of the proteins is determined by their isoelectric point (pI) and relative molecular mass (Mr.). The stained density of each polypeptide on the gel is a function of its concentration. A highly sensitive stain like silver stain or Coomassie Brilliant Blue is required to identify the proteins in the gel.

CSF electrophoresis for the detection of oligoclonal bands is performed if there is suspicion of an inflammatory and/or demyelinating condition of the central nervous system. A concomitant serum sample for electrophoresis and protein estimation is mandatory for correct interpretation of the CSF results.

CSF is produced by the choroid plexus. The blood brain barrier acts as a molecular sieve excluding the passage of high molecular weight proteins, including immunoglobulin. Some inflammatory conditions of the central nervous system (CNS) result in increased production of immunoglobulin’s, and thus a raised CSF immunoglobulin level.

These immunoglobulin’s have restricted specificity and thus restricted electrophoretic mobility, producing oligoclonal banding on CSF electrophoresis. Other causes of raised CSF immunoglobulin, with or without increase in other proteins include malignancy (such as lymphoma), hypergammaglobulinaemia (including serum Para proteins) and increased permeability of the blood-brain barrier.

Oligoclonal bands can be detected in up to 90% of those with multiple sclerosis (MS). They can also be found in other inflammatory and demyelinating conditions of the CNS, such as Guillain-Barre syndrome, bacterial meningitis, viral encephalitis, sub-acute sclerosing pan-encephalitis (SSPE), neurosyphilis, chronic progressive myelopathies, optic neuritis and idiopathic polyneuritis.

Repeated testing can aid in differential diagnosis. In the first three conditions, the oligoclonal bands are transient whilst in SSPE new bands may develop. In SSPE, antibodies specific for measles are detectable. In MS and its variants, including some progressive myelopathies and optic neuritis, the pattern tends to remain unchanged over time.

The detection of IgG oligoclonal bands in CSF in the absence of corresponding bands in serum, implies local production of IgG of restricted specificity, highly suggestive of an intra-cerebral inflamma­tory process. The commonest cause is multiple sclerosis (MS).

CSF is first concentrated because the protein levels are much lower than those of serum. An electro-phoretogram (EPG), which separates proteins on the basis of their electrical charge is then performed, preferably in conjunction with a corresponding serum sample. Immuno-fixation with antisera to IgG confirms that the bands are Immunoglobulin G.

Oligoclonal bands are defined as two or more discrete, narrow immunoglobulin bands in the gamma region. They are usually faint unless the CSF immunoglobulin level is markedly elevated. In multiple sclerosis and other inflammatory conditions of the brain, the oligoclonal immunoglobulin’s are synthesised locally in the central nervous system, hence are present in CSF but not in serum.

Very rarely, some patients with MS can also have oligoclonal banding in their serum but those in the serum are usually less prominent and less numerous than those in the CSF. If bands are prominent in both serum and CSF, changes are presumed to be secondary to other systemic conditions such as viral infections, malignancy or immune complex disease.

To distinguish raised CSF IgG due to local CNS production from leakage of serum into the CSF, CSF and serum IgG levels are compared with reference to albumin, a value known as the IgG index. A CSF IgG:albumin ratio higher than that of serum (raised IgG index) is indicative of local CNS production of IgG. A serum IgG:albumin ratio very much higher than that of CSF (low IgG index) is suggestive of hypergammaglobulinaemia or low serum albumin ( normal is 0.26-0.70).

The presence of oligoclonal IgG bands in CSF together with a raised IgG index is highly specific for a demyelinating condition such as multiple sclerosis.

2. Amniotic Fluid:

It is the nourishing and protecting liquid contained by the amnion of a pregnant woman. The amnion grows and begins to fill, mainly with water, around two weeks after fertilization. After a further 10 weeks the liquid contains proteins, carbohydrates, lipids, phospholipids, urea and electrolytes, all of which aid in the growth of the fetus.

In the late stages of gestation much of the amniotic fluid consists of fetal urine. The amniotic fluid increases in volume as the fetus grows. The amount of amniotic fluid is greatest at about 34 weeks after conception or 34 weeks ga (gestational age). At 34 weeks ga, the amount of amniotic fluid is about 800 ml. This amount reduces to about 600 ml at 40 weeks ga when the baby is born.

Amniotic fluid is continually being swallowed, ‘inhaled’ and replaced by ‘exhaling’ and through urina­tion by the baby. It is essential that the amniotic fluid be breathed into the lungs by the fetus in order for the lungs to develop normally. Swallowed amniotic fluid contributes to the formation of meconium.

Analysis of amniotic fluid, drawn out of the mother’s abdomen in an amniocentesis procedure, can reveal many aspects of the baby’s genetic health. This is because the fluid also contains fetal cells which can be examined for genetic defects. It has been found that amniotic fluid is also a good source of non- embryonic stem cells. These cells have demonstrated the ability to differentiate into a number of different cell-types, including brain, liver and bone.

Amniotic fluid also protects the developing baby by cushioning against blows to the mother’s abdo­men, allows for easier fetal movement, promotes muscular/skeletal development and helps protect the fetus from heat loss.

The fore waters are released when the amnion ruptures, commonly known as when a woman’s ‘water breaks’. When this occurs during labour at term, it is known as ‘spontaneous rupture of membranes’ (SROM). If the rupture precedes labour at term, however, it is referred to as ‘premature rupture of membranes’ (PROM). The majority of the hind waters remain inside the womb until the baby is born.

Too little amniotic fluid (oligohydramnios) or too much (polyhydramnios or hydramnios) can be a cause or an indicator of problems for the mother and baby. In both cases the majority of pregnancies proceed normally and the baby is born healthy but this isn’t always the case.

Babies with too little amniotic fluid can develop contractures of the limbs, clubbing of the feet and hands, and also develop a life threatening condition called hypo plastic lungs. If a baby is born with hypo plastic lungs, which are small underdeveloped lungs, this condition is potentially fatal and the baby can die shortly after birth.

Preterm premature rupture of membranes (PPROM) is a condition where the amniotic sac leaks fluid before 38 weeks of gestation. This can be caused by a bacterial infection or by a defect in the structure of the amniotic sac, uterus, or cervix. In some cases, the leak can spontaneously heal, but in most cases of PPROM, labor begins within 48 hours of membrane rupture. When this occurs, it is necessary that the mother receives treatment to avoid possible infection in the newborn.

3. Cytosol:

The cytosol or ‘cytoplasm’, (often abbreviated as ICF [intracellular fluid]) which also in­cludes the organelles) is the internal fluid of the cell, and where a portion of cell metabolism occurs. Proteins within the cytosol play an important role in signal transduction pathways and glycolysis. They also act as intracellular receptors and form part of the ribosomes, enabling protein synthesis.

In prokaryotes, all chemical reactions take place in the cytosol. In eukaryotes, the cytosol surrounds the cell organelles; this is collectively called the cytoplasm. The portion of cytosol in the nucleus is called nucleohyaloplasm.

The cytosol also surrounds the cytoskeleton which is made of fibrous proteins (ex. microfilaments, microtubules and intermediate filaments). In many organisms the cytoskeleton main­tains the shape of the cell, anchors organelles and controls internal movement of structures (e.g. transport vesicles). The cytosol is composed of free-floating particles, but is highly organized on the molecular level. As the concentration of soluble molecules increases within the cytosol, an osmotic gradient builds up towards the outside of the cell. Water flows into the cell, making the cell bigger.

To prevent the cell from bursting apart, molecular pumps in the plasma membrane, the cytoskeleton, the tonoplast or the cell wall (if present) are used to counteract the osmotic pressure. Cytosol consists mostly of water, dissolved ions, small molecules and large water-soluble molecules (such as protein). Cytosol has a high concentration of K⁺ ions and a low concentration of Na⁺ ions. Normal human cytosolic pH is (roughly) 7.0 (i.e. neutral), whereas the pH of the extracellular fluid is 7.4.

4. Interstitial Fluid:

Interstitial fluid (or tissue fluid, or intercellular fluid) is a solution which bathes and surrounds the cells of multicellular animals. It is the main component of the extracellular fluid, which also includes plasma and trans-cellular fluid. On average, a person has about 11 litres of interstitial fluid providing the cells of the body with nutrients and a means of waste removal. Plasma and interstitial fluid are very similar. Plasma, the major component in blood, communicates freely with interstitial fluid through pores and intercellular clefts in capillary endothelium.

Hydrostatic pressure is generated by the pumping force of the heart. It pushes water out of the capillaries. The water potential is created due to the inability of large solutes to pass through the capillary walls, This build-up of solutes induces osmosis. The water passes from a high concentration (of water) outside of the vessels to a low concentration inside of the vessels, in an attempt to reach equilibrium.

The osmotic pressure drives water back into the vessels. Because the blood in the capillaries is constantly flowing, equilibrium is never reached. The balance between the two forces is different at different points in the capillaries. At the arterial end of the vessel, the hydrostatic pressure is greater than the osmotic pressure, so the net movement favors water and other solutes being passed into the tissue fluid.

At the venous end, the osmotic pressure is greater, so the net movement favours substances being passed back into the capillary. This difference is created by the direction of the flow of blood and the imbalance in solutes created by the net movement of water favoring the tissue fluid. To prevent a build-up of tissue fluid surrounding the cells in the tissue, the lymphatic system plays a part in the transport of tissue fluid.

Tissue fluid can pass into the surrounding lymph vessels and eventually end up re-joining the blood. Sometimes the removal of tissue fluid does not function correctly and there is a build-up. This causes swelling and can often be seen around the feet and ankles, ex. Elephantiasis. The position of swelling is due to the effects of gravity.

Composition:

Interstitial fluid consists of a water solvent containing amino acids, sugars, fatty acids, coenzymes, hormones, neurotransmitters, salts, as well as waste products from the cells. The composition of tissue fluid depends upon the exchanges between the cells in the tissue and the blood. This means that tissue fluid has a different composition in different tissues and in different areas of the body.

Not all of the contents of the blood pass into the tissue, which means that tissue fluid and blood are not the same. Red blood cells, platelets and plasma proteins cannot pass through the walls of the capillaries. The resulting mixture that does pass through is essentially blood plasma without the plasma proteins.

Tissue fluid also contains some types of white blood cell, which help combat infection. Lymph is considered a part of the interstitial fluid. The lymphatic system returns protein and excess interstitial fluid to the circulation. Interstitial fluid bathes the cells of the tissues. This provides a means of delivering materials to the cells, intercellular communication, as well as removal of metabolic waste.