Composition and Function of the Interstitial Fluid | 1

ABSTRACT

Body water is divided between intracellular and extracellular fluid, with the great bulk being intracellular fluid and approximately 30% extracellular fluid. The latter is divided between extravascular and intravascular fluid (blood plasma). ¹ Extravascular fluid can be further demarcated into interstitial fluid and specialized fluids found in the cerebrospinal, synovial, and ocular compartments (aqueous humor of the eye). The peritoneal transudate, synovial fluid, and interstitial fluid share some characteristics related to their formation from plasma. There is evidence for connectivity between brain interstitial fluid and cerebrospinal fluid. ² ^– ⁵ Abbott ² presents evidence for brain interstitial flow into the cerebrospinal fluid and the possibility of cerebrospinal fluid flow into the brain. Iliff and coworkers ³ showed that cerebrospinal fluid exchange with brain interstitial fluid provides a mechanism for the removal of brain interstitial solutes. This group ³ ^, ⁴ have designated this exchange between brain interstitial fluid and cerebrospinal fluid as the glymphatic system and suggested that an imbalance in this system could be responsible for brain edema. The term glymphatic is defined as a paravascular fluid exchange pathway that enables brain interstitial and cerebrospinal fluid turnover and is facilitated by glial cells. ⁴ Stukas and coworkers ⁵ have shown that apolipoprotein A-1 passes into the brain via cerebrospinal fluid. These investigators suggest that apolipoprotein A-1 passes from the vascular system into cerebrospinal fluid by specific cellular transport (transcytosis). The exchange between brain interstitial fluid and cerebrospinal fluid would be confined to the brain and associated nervous tissue with little systemic consequence. That said, there is the curious issue of prothrombin expression in the brain ⁶ and the importance of PAR-1 receptors in neurobiology. ⁷ Some characteristics of several human body fluids are shown in Table 1.1. Table 1.1 Protein Content of Various Human Body Fluids and Secretions

Fluid	Protein (mg/mL) ^a	Comment	Refs.
Extracellular fluid	N/A	The body fluid can be divided into two major components: the intracellular fluid and the extracellular fluid. Between 60% and 70% of the body fluid is intracellular in nature, while the remainder is extracellular in nature. The extracellular fluid, in turn, consists of two primary components: intravascular fluid (blood plasma; approximately 25%) and extravascular fluid (approximately 75%). The extravascular fluid consists mostly of interstitial fluid with small specialized fluids in various spaces; specialized fluids include cerebrospinal fluid, synovial fluid, and ocular fluid.	1, 2, 3, 4, 5
Blood plasma	78.9 ± 0.5 ^b	A protein-rich fluid defined by being confined within the vascular system and representing one-quarter to one-fifth of the total extracellular fluid. It is in equilibrium with the interstitial fluid, which feeds into the lymphatic system and is returned to the venous system. Other areas of extracellular fluid include the peritoneal fluid, ocular fluid, and cerebrospinal fluid. Plasma is also defined as the protein-rich fluid obtained by the removal of the cellular elements of whole blood collected with an anticoagulant.	6, 7, 8
Blood serum	72.9 ± 0.5 ^b	A protein-rich fluid derived from the clotting of plasma or whole blood. Most frequently collected by the clotting of whole blood collected without the addition of an anticoagulant. The protein concentration of serum is usually less that of corresponding plasma, reflecting the loss of fibrinogen and other plasma proteins. Serum may also contain products secreted by platelets and other cellular elements during the process of coagulation.	6, 7, 8
Interstitial fluid ^c	50.9 ^d	The concentration of protein in interstitial fluid is 40–60% of that in plasma. The volume of interstitial fluid is two to three times larger than plasma volume and the concentration of a given protein in the interstitial fluid depends on the excluded interstitial volume for a specific protein and the size of the protein. Albumin is the most common protein in interstitial fluid, with lower concentrations of larger proteins such as IgG.	9, 10, 11
Interstitial fluid	29.8	Plasma protein concentration was given at 70.0 mg/mL; the interstitial volume was 8.4 L, with an excluded interstitial volume of 2.1 L.	12
Interstitial fluid	27.2 18.3	Wick fluid. ^e Blister technique. ^e	13
Interstitial fluid	37 ^f 24	Perivascular. Peribronchial.	14
Lymph	26–51 ^e	Lymph is derived from plasma via interstitial fluid. There is tissue variability in lymph flow rate and regional composition. In this study, as with others, albumin was the major protein. This study also referred to other proteins as members of the classical globulin fractions.	15
Lymph	17–25 ^f	Protein concentration increased to 44 mg/mL in skin lymph but not muscle lymph after thermal injury.	16
Lymph ^g	42	The lymph/plasma ratio was 0.71 for total protein, 0.70 for albumin, and 0.25 for immunoglobulin.	17
Lymph ^h	25–27	The protein concentration of lymph was slightly less than that of interstitial fluid. The concentration of lactic dehydrogenase was much higher in interstitial fluid than in lymph; the concentration in lymph in turn is much higher than that in plasma.	18
Lymph ⁱ	27	The protein concentration of lymph was slightly less than that obtained for interstitial fluid (25 mg/mL). Albumin is the major protein (18 mg/mL), with smaller quantities of globulin ^j (8.2 mg/mL).	19
Peritoneal fluid	25	Peritoneal fluid is usually obtained only the in the case of ascites; it is labeled a transudate if the protein concentration is less than 25 mg/mL and an exudate if the protein concentration is greater than 25 mg/mL. ^k A transudate can result from increased hydrostatic pressure, while an exudate can result from decreased capillary permeability.	20, 21, 22
Peritoneal fluid	42.2 ± 6	Peritoneal fluid obtained from normal women over the course of a menstrual cycle.	23
Peritoneal fluid	43.2 ± 0.8	The concentration of protein in peritoneal fluid is 68% of that in plasma; the relationship of plasma transcortin and peritoneal fluid is similar (71%). Luteinizing hormone (LH) and follicle-stimulating hormone (FSH) are also found in peritoneal fluid but correlated with plasma concentrations are lower (LH, 42%; FSH, 63%). The concentration of steroid hormones such as 17β-estradiol and androstanedione are equal or higher in peritoneal fluid than in plasma, while the concentration of cortisol is lower in peritoneal fluid than in plasma.	24
Cerebrospinal fluid	0.21 ^b	Protein in cerebrospinal fluid is derived from brain and plasma. Albumin is the major protein in cerebrospinal fluid; the ratio of serum albumin concentration to CSF albumin concentration (Q_Alb) is used as a diagnostic tool for determination of blood–brain barrier integrity. There are tight barriers between blood and cerebrospinal fluid; the brain is suggested to be an immunologically privileged area.	25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35
Cerebrospinal fluid	0.21–2.74 (median, 0.49; n = 62) 0.21–2.6 (median, 0.49; n =62)	Pyrogallol red–molybdate method. Coomassie brilliant blue dye binding. ^l	36
Cerebrospinal fluid	0.78 ^m	Coomassie brilliant blue in the presence of sodium dodecyl sulfate.	37
Cerebrospinal fluid	0.149–1.07	Pediatric population ranging in age from 0.1 to 210 months. Protein concentration changes with age. Albumin is the major protein. It is suggested that 80% of cerebrospinal proteins are derived from blood plasma by a transcellular or paracellular process.	38
Ocular fluid	0.26–0.28 ⁿ 0.15–0.48 ^o	MRI signals from Gd show the presence of Gd in ocular fluid after IV administration. Concentrations of albumin, IgG, and α₁-antitrypsin aqueous humor are similar to those in cerebrospinal fluid and have been suggested to show circadian variation. There are tight barriers to the ocular space and the eye is an immunologically privileged region.	39, 40, 41, 42, 43
Perilymph	2.75 ^b	Inner-ear fluid, similar in composition to interstitial fluid. There are connections to cerebrospinal fluid.	25,43,44
Endolymph	0.38 ^b	Inner-ear fluid, thought to be derived from the perilymph.	25
Synovial fluid	10.9 ^p 20–35 ^q 0.718 ± 0.705 ^r	Synovial fluid is a viscous fluid with non-Newtonian behavior found within various joints in the body, such as the knee and temporomandibular joints. Synovial fluid has a lubricating function within the joint. The high concentration of hyaluronic acid is thought to provide the basis of the viscosity as well as providing some of the lubrication characteristics. The interaction of hyaluronic acid with plasma proteins such as fibrinogen may be responsible for the non-Newtonian behavior of synovial fluid. The composition does change with inflammation, with a decrease in viscosity possibly reflecting the degradation of hyaluronic acid. There is some evidence that injection of hyaluronic acid into osteoarthritic joints has a positive therapeutic effect.	45, 46, 47, 48, 49, 50, 51, 52
Gingival crevicular fluid	0.022–0.060 ^s	Gingival crevicular fluid is largely supplied by an ultrafiltrate of plasma entering the oral cavity via the gingival crevice (between the tooth and the epithelial integument). Flow rate is increased with gingival inflammation. Gingival crevicular fluid is the source of most of the plasma proteins found in saliva.	53, 54, 55, 56
Saliva		Saliva is composed of contributions from several glandular sources including the parotid gland, submaxillary (submandibular) gland, and sublingual glands, as well as minor volume contributions from gingival crevicular fluid. Saliva serves a variety of functions including digestive and antibacterial. There are factors in saliva that promote wound healing inside and outside the oral cavity. The determination of protein concentration of saliva is dependent on method and standard.	57, 58, 59, 60, 61, 62
Saliva	1.4–6.4 ^t 1.8–4.2 ^u	Mixed human saliva (also referred to as whole saliva) refers to saliva obtained from all salivary secretions. Mixed saliva represents the combined glandular secretions with gingival crevicular fluid, which is mixed with food during mastication. The collection of mixed saliva is considered a noninvasive process and saliva is receiving increased attention for diagnostic purposes. Unstimulated saliva is collected by “drooling,” ^v while stimulated saliva is collected after some stimulation of the salivary glands, as by chewing. Stimulation influences the quality of the salivary secretion, and so gleeking, the process of forcibly ejecting saliva from the submandibular gland by application of pressure with the tongue, can be used to collect a stimulated salivary sample (see Chapter 2).	63, 64, 65 66, 67, 68, 69
Parotid saliva	2.35 ± 3.87 ^t 1.64 ± 0.51 ^u	Parotid saliva is a serous secretion considerably less viscous than either submandibular or sublingual saliva. There is considerable homology between the parotid gland and the pancreas. Several enzymes show an inverse level of expression in the parotid or pancreas. For example, mouse parotid glands express a high level of DNAse I, while there is a low level of expression in the mouse pancreas.	63,70, 71, 72, 73, 74
Submandibular fluid	1.14 ± 0.58 ^t 0.77 ± 0.36 ^u	Submandibular saliva is a mucous secretion more viscous that parotid saliva. The viscosity is a property of the mucins present in submandibular saliva. Submaxillary secretion composes approximately two-thirds of the total volume of unstimulated human saliva and one-third of the volume of stimulated human saliva. The remaining volume of saliva is primarily provided by parotid saliva, with minor volume contributions from other glandular sources and gingival crevicular fluid. On stimulation, the relative volume contribution of submaxillary secretion decreases, while that of parotid secretion increases.	63,75, 76, 77
Digestive secretions		There are a variety of digestive secretions, including the secretion of pepsinogen by the chief cells in the stomach and a variety of proenzymes, such as chymotrypsinogen and trypsinogen, by the pancreas. The secretion of the proenzymes is exocrine. The pancreas is an example of separation between exocrine function and endocrine function, with insulin produced by the endocrine side and secretion of pancreatic proenzymes and enzymes produced by the exocrine side.	78, 79, 80, 81
Seminal fluid	55	Seminal fluid is secreted by the gonads and other male sexual accessory glands and may contain spermatozoa. It also contains a variety of proteins and other organic compounds.	82, 83, 84, 85, 86, 87, 88
Cervico-vaginal secretions		Vaginal fluid is composed of transudate and contributions from leukocytes and other cellular materials. Proteomic analysis suggests that 50% of the proteins are of plasma origin, although composition does depend on the sampling process. Samples obtained via colposcopy are more complex than those obtained by vaginal fluid lavage. As with other transudates, albumin is a major component of vaginal fluid. There may be contributions from cervical mucus, leading to the use of the terms cervical-vaginal fluid and cervico-vaginal fluid to describe this material. Cervical secretion is thought to be distinct from vaginal secretion. Much of the work used a mixture of fluids obtained from the cervix, vaginal secretion, and other sources such as amniotic fluids. Cervical secretion is much more viscous than the other fluids, reflecting the high concentration of mucins.	89, 90, 91, 92, 93
Tear fluid	5.77 ± 1.32 ^m	Coomassie brilliant blue/bovine serum albumin standard.	94, 95, 96, 97, 98
	11.09 ± 1.94	Coomassie brilliant blue/IgG standard.
	9.59 ± 1.32	Lowry/bovine serum albumin standard.
	7.47 ± 1.28	Lowry/IgG standard.

Note: Information is for human fluids unless otherwise indicated. a

There are a number methods for protein determination that can yield different results for the same biological fluid. (From Sapan, C.V., et al., Biotechnol. Appl. Biochem. 29, 99–108, 1999.)

Biuret method; the plasma was collected with heparin as the anticoagulant. ⁶

Rat, Folin–Lowry.

Also referred to as extracellular fluid in some publications. The strict definition of extracellular fluid is all fluid outside of the cell, which includes blood plasma and interstitial fluid as the constituents. Movement of protein in the interstitial space depends on protein charge, leading to the concept of excluded interstitial space. (From Gyenge, C.C., et al., J. Physiol. 552, 907–916, 2003.)

Sampling of interstitial fluid is a technical challenge and is discussed in Chapter 6.

Rabbit, protein concentration determined by bicinchoninic acid method.

Rat tail lymph, protein determined by Lowry method.

Rabbit, protein concentration estimated from graphical data.

Rabbit.

Individual protein concentrations estimated from electrophoretic analysis.

Transudates and exudates are also seen with pleural effusions. (From Lee, Y.C., et al., Chest 131, 942–943, 2007; Porcel, J.M., Clin. Chest Med. 34, 27–37, 2013.)

There have been some issues with the use of Coomassie brilliant blue for estimation of total protein for other biological fluids. ⁶¹

Somewhat different values were obtained by the other protein assay methods (Lowry, turbidimetric), which also varied by protein standard used. The value was obtained with Coomassie brilliant blue in the presence of sodium dodecyl sulfate. (From Macart, M. and Gerbaut, L., Clin. Chim. Acta 122, 93–101, 1982.)

Rabbit vitreous humor, determined by Coomassie blue dye binding; value estimated from graphical representation of values determined over a 24-hour cycle. There was no significant circadian variation.

Rabbit aqueous humor, determined by Coomassie blue dye binding; values estimated from graphical representation of values determined over a 24-hour cycle. Unlike the data obtained from the vitreous humor, the aqueous humor showed circadian variation.

Mean value (equine; n = 15) ⁴⁵ determined by Coomassie blue dye binding.

Equine determined by refractometry; shown to increase on injection with pentosan polysulfate/N-acetylglucoseamine. ⁴⁶

Human temporomandibular.

The protein concentration increased with increasing sample time. Electrophoretic analysis also showed that the composition of gingival crevicular fluid increasingly resembled serum with increasing sample time.

Unstimulated saliva, determined by the Lowry method.

Stimulated saliva, determined by the Lowry method.

Unfortunately, no other term exists to describe this process. The aim is to collect the saliva as passively as possible. (From Durdiaková, J., et al., Steroids 78, 1325–1331, 2013.)

w There is variance in the values obtained for tear fluid, dependent on assay method and standard. Similar variance was observed for saliva. ⁶¹