What Are Body Fluids Made Of?

A look at saliva, sweat, cerebrospinal fluid, and more

Sweaty dude
Geber86/iStockphoto

You may be surprised to learn that the composition of our body fluids is quite complex. With respect to body fluids, form follows function. Our body synthesizes these fluids to meet our physical, emotional, and metabolic needs.

Sweat

Sweating is a means of thermoregulation—a way that we cool ourselves. Sweat evaporates off the surface of our skin and cools our bodies.

Why don't you sweat? Why do you sweat too much?

There is variability in how much people sweat. Some people sweat less and some people sweat more. Factors that can affect how much you sweat include genetics, gender, environment, and fitness level.

Here are some general facts about sweating:

  • Men sweat more on average than women.
  • People who are out of shape sweat more profusely than people who are at a higher fitness level.
  • Hydration status can affect how much sweat you produce.
  • Heavier people sweat more than lighter people because they have a greater body mass to cool.

Hyperhidrosis is a medical condition in which a person can sweat excessively, even during rest or when it’s cold. Hyperhidrosis can arise secondary to other conditions, such as hyperthyroidism, heart disease, cancer, and carcinoid syndrome. It is an uncomfortable and sometimes embarrassing condition. If you suspect that you have hyperhidrosis, please meet with your physician. There are treatment options available, such as antiperspirants, medications, Botox, and surgery to remove excess sweat glands.

The composition of sweat depends on many factors, including fluid intake, ambient temperature, humidity, and hormonal activity as well as the type of sweat gland (eccrine or apocrine). In general terms, sweat contains the following:

  • Water
  • Sodium chloride (salt)
  • Urea (waste product)
  • Albumin (protein)
  • Electrolytes (sodium, potassium, magnesium, and calcium)

    Sweat produced by the eccrine glands, which are more superficial, has a faint smell. However, sweat produced by the deeper and larger apocrine sweat glands located in the armpit (axilla) and groin is smellier because it contains organic material derived from the decomposition of bacteria. The salts in sweat give it a salty taste. The pH of sweat ranges between 4.5 and 7.5.

    Interestingly, research suggests that diet can affect sweat composition, too. People who consume more sodium have a higher concentration of sodium in their sweat. Conversely, people who consume less sodium produce sweat that contains less sodium.

    Cerebrospinal Fluid

    Cerebrospinal fluid (CSF), which bathes the brain and spinal cord, is a clear and colorless fluid. CSF has numerous functions. First, it provides nutrients to the brain and spinal cord. Second, it eliminates waste products from the central nervous system. And third, it cushions and protects the central nervous system.

    CSF is produced by the choroid plexus. The choroid plexus is a network of cells located in the brain  ventricles and rich in blood vessels.

    A small amount of CSF is derived from the blood-brain barrier. CSF is made up of several vitamins, ions (aka salts), and proteins including the following:

    • Sodium
    • Chloride
    • Bicarbonate
    • Potassium (lesser amounts)
    • Calcium (lesser amounts)
    • Magnesium (lesser amounts)
    • Ascorbic acid (vitamin)
    • Folate (vitamin)
    • Thiamine and pyridoxal monophosphates (vitamins)
    • Leptin (protein from blood)
    • Transthyretin (protein produced by the choroid plexus)
    • Insulin-like growth factor or IGF (produced by the choroid plexus)
    • Brain-derived neutrotrophic factor or BDNF (produced by the choroid plexus)

    Blood

    Blood is a fluid that circulates through the heart and blood vessels (think arteries and veins). It carries nutrition and oxygen throughout the body. It consists of:

    • Plasma: a pale yellow liquid that forms the fluid phase of blood
    • Leukocytes: white blood cells with immune functions
    • Erythrocytes: red blood cells
    • Platelets: cells without a nucleus that are involved in clotting

    White blood cells, red blood cells, and erythrocytes all originate from the bone marrow.

    Plasma is by and large made of water. Total body water is divided into three fluid compartments: plasma, extravascular interstitial fluid (think lymph) and intracellular fluid (inside cells). Plasma is made of ions or salts (mostly sodium, chloride, and bicarbonate), organic acids, and proteins. Interestingly, the ionic composition of plasma is similar to that of interstitial fluids like lymph, with plasma having a slightly higher protein content than that of lymph.

    Saliva and Other Mucosal Secretions

    Saliva is actually a type of mucus. Mucus is the slime that covers mucous membranes and is made of glandular secretions, inorganic salts, leukocytes, and sloughed-off skin (desquamated) cells.

    Saliva is clear, alkaline, and somewhat viscous. It’s secreted by the parotid, sublingual, submaxillary, and sublingual glands as well as some smaller mucous glands. The salivary enzyme α-amylase contributes to the digestion of food. Furthermore, saliva moistens and softens food.

    In addition to α-amylase, which breaks down starch into maltose, saliva also contains globulin, serum albumin, mucin, leukoctyes, potassium thiocynatate and epithelial debris. Additionally, depending on exposure, toxins can also be found in saliva.

    The composition of saliva and other types of mucosal secretion varies based on the requirements of the specific anatomical sites that they wet or moisten. Some functions that these fluids help perform include the following:

    • Nutrition intake
    • Excretion of waste products
    • Gas exchange
    • Protection from chemical and mechanical stresses
    • Protection from microbes (bacteria)

    Saliva and other mucosal secretions share most of the same proteins. These proteins are mixed differently in different mucosal secretions based on their intended function. The only proteins that are specific to saliva are histatins and acidic proline-rich proteins (PRPs).

    Histatins possess antibacterial and antifungicidal properties. They also help form the pellicle, or thin skin or film, that lines the mouth. Furthermore, histatins are anti-inflammatory proteins which inhibit the release of histamine by mast cells. 

    Acidic PRPs in saliva are rich in the amino acids like proline, glycine, and glutamic acid. These proteins may help with calcium and other mineral homeostasis in the mouth. (Calcium is a chief component of teeth and bone.) Acidic PRPs may also neutralize toxic substances found in food. Of note, basic PRPs are found not only in saliva but also in bronchial and nasal secretions and may proffer more general protective functions.

    Proteins more generally found in all mucosal secretions contribute to functions common to all mucosal surfaces like lubrication. These proteins fall into two categories:

    The first category consists of proteins that are produced by identical genes found in all salivary and mucous glands: lisozyme (enzyme) and sIgA (an antibody with immune function).

    The second category consists of proteins that are not identical but rather share genetic and structural similarities, such as mucins, α-amylase (enzyme), kallikreins (enzymes) and cystatins. Mucins give saliva and other types of mucus their viscosity.

    In a 2011 paper published in Proteome Science, Ali and co-authors identified 55 different types of mucins present in the human airway. Importantly, mucins form large (high-molecular-weight) glycosylated complexes with other proteins like sIgA and albumin. These complexes help protect against dehydration, maintain viscoelasticity, protect cells present on mucosal surfaces, and clear bacteria.

    Tears

    Tears are a special type of mucus. They are produced by the lacrimal glands. Tears produce a protective film that lubricates the eye and flushes it of dust and other irritants. They also oxygenate the eyes and help with the refraction of light through the cornea and onto the lens on its way to the retina.

    Tears contain a complex mixture of salts, water, proteins, lipids, and mucins. There are 1526 different types of proteins in tears. Interestingly, compared with serum and plasma, tears are less complex.

    One important protein found in tears is the enzyme lysozyme, which protects the eyes from bacterial infection. Furthermore, secretory Immunoglobulin A (sIgA) is the main immunoglobulin found in tears and works to defend they eye against invading pathogens.

    Urine

    Urine is produced by the kidneys. It is by and large made of water. Additionally, it contains ammonia, cations (sodium, potassium, and so forth) and anions (chloride, bicarbonate, and so forth). Urine also contains traces of heavy metals, such as copper, mercury, nickel, and zinc.

    Semen

    Human semen is a suspension of sperm in nutrient plasma and composed of secretions from the Cowper (bulbourethral) and Littre glands, prostate gland, ampulla and epididymis, and seminal vesicles. The secretions of these different glands are incompletely mixed in whole semen.

    The first portion of ejaculate, which makes up about five percent of total volume, comes from the Cowper and Littre glands. The second portion of ejaculate comes from the prostate gland and makes up between 15 percent and 30 percent of the volume. Next, the ampulla and epididymis contribute a little to the ejaculate. Finally, the seminal vesicles contribute the rest of the ejaculate, and these secretions make up most of the volume of semen.

    The prostate contributes the following molecules, proteins, and ions to semen:

    • Citric acid
    • Inositol (vitamin-like alcohol)
    • Zinc
    • Calcium
    • Magnesium
    • Acid phosphatase (enzyme)

    The concentration of calcium, magnesium, and zinc in semen vary among individual men.

    The seminal vesicles contribute the following:

    • Ascorbic acid
    • Fructose
    • Prostaglandins (hormone-like)

    Although most of the fructose in semen, which is a sugar used as fuel for sperm, is derived from the seminal vesicles, a little bit of fructose is secreted by the ampulla of the ductus deferens. The epididymis contributes L-carnitine and neutral alpha-glucosidase to semen.

    The vagina is a highly acidic environment. However, semen has a high buffering capacity, which allows it to maintain a near-neutral pH and penetrate cervical mucus, which also has a neutral pH. It’s unclear exactly why semen has such a high buffering capacity. Experts hypothesize that HCO3/CO2 (bicarbonate/carbon dioxide), protein, and low–molecular weight components, such as citrate, inorganic phosphate, and pyruvate, all contribute to buffering capacity.

    The osmolarity of semen is pretty high due to high concentrations of sugars (fructose) and ionic salts (magnesium, potassium, sodium, and so forth).

    The rheological properties of semen are quite distinct. On ejaculation, semen first coagulates into a gelatinous material. Coagulation factors are secreted by seminal vesicles. This gelatinous material is then converted to a liquid after liquefying factors from the prostate take effect.

    In addition to providing energy for sperm, fructose also helps form protein complexes in sperm. Furthermore, over time, fructose breaks down by a process called fructolysis and produces lactic acid. Older semen is higher in lactic acid.

    The volume of ejaculate is highly variable and varies depending on whether derived from masturbation or during coitus. Interestingly, even condom use can affect semen volume. Some researchers estimate that the average semen volume is 3.4 mL.

    Breast Milk

    Breast milk comprises all the nutrition that a newborn baby needs. It is a complex fluid that’s rich in fat, proteins, carbohydrates, fatty acids, amino acids, minerals, vitamins, and trace elements. It also contains various bioactive components, such as hormones, antimicrobial factors, digestive enzymes, trophic factors, and growth modulators.

    Looking Forward

    Understanding what body fluids are made of and simulation of these body fluids can have therapeutic and diagnostic applications. For instance, in the field of preventive medicine, there is interest in searching tears for biomarkers to diagnose dry eye disease, glaucoma, retinopathies, cancer, multiple sclerosis, and more.

    Sources

    Hagan S, Martin E, and Enriquez-de-Salamanca A. Tear Fluid Biomarkers in Ocular and Systemic Disease: Potential Use for Predictive, Protective and Personalised Medicine. EPMA Journal. 2016; 7: 15.

    Owen DH and Katz DF. A Review of the Physical and Chemical Properties of HumanSemen and the Formulation of a Semen Simulant. Journal of Andrology. 2005; 26: 4.

    Schenkels, LCPM, Veerman, ECI, and Nieuw Amorongen AV. Biochemical Composition of Human Saliva in Relation to Other Mucosal Fluids. Critical Reviews in Oral Biology & Medicine. 1995; 6: 161-175.

    Shires III G. Fluid and Electrolyte Management of the Surgical Patient. In: Brunicardi F, Andersen DK, Billiar TR, Dunn DL, Hunter JG, Matthews JB, Pollock RE. eds. Schwartz's Principles of Surgery, 10e. New York, NY: McGraw-Hill; 2014.

    Spector, R, Snodgrass SR, and Johanson CE. A Balanced View of the Cerebrospinal Fluid Composition and Funcitions: Focus on Adult Humans. Experimental Neurology. 2015; 273: 57-68.

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