All living beings including human beings have different bodily fluids. Each fluid is there to perform several functions or make the execution of several function possible. Generally, a fluid contains a mixtures of compounds, including many proteins. Our bodies make sure that our various bodily fluids remain separate. For example, blood, CSF do not mix.
When studying a disease, researchers often study the proteins involved in the disease or disorder. In a diagnostic experiment, the choice of body fluid is crucial. For example, CSF is a better choice than blood plasma for many brain diseases since blood and neurons are separated by the blood brain barrier.
There are hundreds of different fluids present in our body. We cannot cover all of them in this book. We will only be looking at the most common and most important ones.
Plasma is the liquid portion of the blood. It contains blood cells and other compounds. Plasma is about 55% to 60% of the blood content. To collect blood plasma, a syringe containing suitable anti-coagulant is used. Anti-coagulant is added to prevent blood from clotting. Plasma contain red blood cell (RBC), white blood cells (WBC), platelets, fibrinogens, lipids, salts, urea, antibodies, etc. Plasma serves as a transport medium for nutrients, waste, cells. In proteomics, plasma is fractionated before being examined since it contains to many substances and too many proteins.
To collect serum, blood is allowed to clot. Once the clot is removed, we are left with serum. Serum contain water, electrolytes, albumin, antibodies, etc. It does not contain red blood cell (RBC), white blood cells (WBC), platelets, fibrinogens.
CSF is a clear body fluid that is found in the subarachnoid space in the brain. Subarachnoid space is a between the skull and the cerebral cortex. It cushions and buffers the cortex. CSF is collected by a lumbar puncture. It is used in diagnosing cerebrospinal and neurological diseases such as Jacob Creutzfeldt disease.
In animals, urine is produced by filtering blood through kidney. It is then collected in the bladder and excreted through the urethra or penis. Urine contains excess compounds and undesirable substances that are not needed by the body or harmful to the body.
The major interest in the analysis of urine is to discriminate between glomerular and tubular diseases. In glomerular diseases, additional high molecular weight plasma proteins may be detected in the urine due to alteration of the glomerus. In contrast, tubular diseases show only additional low molecular weight proteins.
Several other fluids are also routinely used in medicine, diagnositics and research but we would not talking about them in this book.