Body Fluids

Body fluids

Body fluids, the vital liquids coursing through the veins of life, are the unsung heroes of our physiological functioning. These biofluids, ranging from the blood that fuels our cells to the tears that express our deepest emotions, play a pivotal role in maintaining our health and vitality. They serve as the body’s internal transportation system, carrying nutrients to cells, ferrying waste products away, and playing a crucial role in maintaining homeostasis. This article aims to shed light on these essential components of our bodies, delving into their diverse types, unique compositions, and the indispensable roles they play in our survival and well-being. In this article we will see various body fluids.

Intracellular Fluid

Definition: Intracellular fluid is the fluid that exists within the cells of multi-celled organisms.

Composition: It is made up primarily of water and molecules such as dissolved ions and is a major component of the cytoplasm and cytosol.

Volume: In adult humans, it accounts for about 40 percent of total body weight.

pH: The pH of intracellular fluid is between roughly 6.8 and 7.4, depending on the tissue and cell type.

Ion Concentration: Intracellular fluid contains only a small amount of sodium and chloride ions and very few calcium ions but contains relatively large amounts of potassium and phosphate ions.

Function: A variety of chemical reactions within cells take place in the cytoplasm and therefore the intracellular fluid.

Extracellular Fluid

Definition: Extracellular fluid denotes all body fluid outside the cells of any multicellular organism.

Composition: It is found in blood, in lymph, in body cavities lined with serous (moisture-exuding) membrane, in the cavities and channels of the brain and spinal cord, and in muscular and other body tissues.

Volume: The volume of extracellular fluid in a young adult male of 70 kg is 20% of body weight – about fourteen liters.

pH: Extracellular fluid pH is between about 7.3 and 7.52.

Ion Concentration: Relative to intracellular fluid, extracellular fluid generally has a high concentration of sodium and low concentration of potassium.

Function: The extracellular fluid constitutes the body’s internal environment that bathes all of the cells in the body. The ECF composition is therefore crucial for their normal functions, and is maintained by a number of homeostatic mechanisms involving negative feedback.

Amniotic Fluid

Definition: Amniotic fluid is a clear, slightly yellowish liquid that surrounds the unborn baby (fetus) during pregnancy.

Composition: It is primarily composed of water, but also contains nutrients, hormones, antibodies, and other fluids to help keep the fetus healthy and protected.

Function: It serves several essential purposes during pregnancy. It cushions and protects the developing fetus from external shocks and injuries, helps regulate the fetus’s temperature, provides buoyancy, allowing for fetal movement and musculoskeletal development1. The fetus practices breathing by inhaling and swallowing the fluid.

Volume: The amount of amniotic fluid is greatest at about 34 weeks into the pregnancy, when it averages 800 mL.

Aqueous Humor

Definition: Aqueous humor is a transparent water-like fluid similar to blood plasma, but containing low protein concentrations. It is secreted from the ciliary body, a structure supporting the lens of the eyeball.

Composition: It is composed of 98% water, electrolytes, amino acids, ascorbic acid, glutathione, and immunoglobulins.

Function: Aqueous humor maintains the intraocular pressure and inflates the globe of the eye. It provides nutrition for the avascular ocular tissues; posterior cornea, trabecular meshwork, lens, and anterior vitreous.

Production and Drainage: Aqueous humor is secreted into the posterior chamber by the ciliary body. It is continually produced and this rate of production must be balanced by an equal rate of aqueous humor drainage.


Definition: Bile, or gall, is a yellow-green fluid produced by the liver of most vertebrates that aids the digestion of lipids in the small intestine.

Composition: In the human liver, bile is composed of 97–98% water, 0.7% bile salts, 0.2% bilirubin, 0.51% fats (cholesterol, fatty acids, and lecithin), and 200 meq/L inorganic salts.

Function: Bile acts to some extent as a surfactant, helping to emulsify the lipids in food. Bile salt anions are hydrophilic on one side and hydrophobic on the other side; consequently, they tend to aggregate around droplets of lipids to form micelles.

Production: About 400 to 800 milliliters of bile is produced per day in adult human beings.

Blood Plasma

Definition: Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but which contains proteins and other constituents of whole blood in suspension.

Composition: It is mostly water (up to 95% by volume), and contains important dissolved proteins (6–8%; e.g., serum albumins, globulins, and fibrinogen), glucose, clotting factors, electrolytes (Na+, Ca2+, Mg2+, HCO3−, Cl−, etc.), hormones, carbon dioxide (plasma being the main medium for excretory product transportation), and oxygen.

Function: It plays a vital role in an intravascular osmotic effect that keeps electrolyte concentration balanced and protects the body from infection and other blood-related disorders.

Volume: Blood plasma constitutes about 55% of the body’s total blood volume.

Production and Drainage: Blood plasma is separated from the blood by blood fractionation containing an anticoagulant in a centrifuge until the blood cells fall to the bottom of the tube.

Breast Milk

Definition: Breast milk or mother’s milk is milk produced by the mammary glands in the breast of human females. It is the primary source of nutrition for newborn infants.

Composition: It comprises fats, proteins, carbohydrates, and a varying composition of minerals and vitamins.

Function: Breast milk also contains substances that help protect an infant against infection and inflammation, such as symbiotic bacteria and other microorganisms and immunoglobulin A, whilst also contributing to the healthy development of the infant’s immune system and gut microbiome.

Production: The fluid is made by a group of cells, called the choroid plexus, that are deep inside your brain.

Volume: Your body contains between 5 to 6 quarts (5 liters) of blood.

Cerebrospinal Fluid

Definition: Cerebrospinal fluid (CSF) is a clear, colorless body fluid found within the tissue that surrounds the brain and spinal cord of all vertebrates.

Composition: CSF is produced by specialized ependymal cells in the choroid plexus of the ventricles of the brain, and absorbed in the arachnoid granulations.

Function: CSF acts as a shock absorber, cushion or buffer, providing basic mechanical and immunological protection to the brain inside the skull. CSF also serves a vital function in the cerebral autoregulation of cerebral blood flow.

Volume: In humans, there is about 125 mL of CSF at any one time, and about 500 mL is generated every day.

Production and Drainage: CSF is produced in your brain’s ventricles (fluid-filled spaces). Special cells that line the ventricles also make a smaller amount of cerebrospinal fluid.

Cerumen (Earwax)

Definition: Cerumen, commonly known as earwax, is a natural secretion produced by and found within the external auditory canal.

Composition: It is made by the glands of the ear canal skin and is often a yellow or brownish color. It is composed of long chain fatty acids, both saturated and unsaturated, alcohols, squalene, and cholesterol.

Function: Cerumen plays an important role in ear health. It cleans your ears and protects them from dust, dirt, and infection.

Production: All people make cerumen, and it is healthy for the ear because it helps keep the ear clean and keep infection away.


Definition: Chyle is a milky bodily fluid consisting of lymph and emulsified fats, or free fatty acids (FFAs). It is formed in the small intestine during digestion of fatty foods, and taken up by lymph vessels specifically known as lacteals.

Composition: The lipids in the chyle are colloidally suspended in chylomicrons.

Function: Chyle has important roles as part of the first line of defense of the ear from micro-organisms and optimizing function of the tympanic membrane and EAC.

Clinical Significance: A chyle fistula occurs when defect(s) of lymphatic vessel(s) result in leakage of lymphatic fluid, typically accumulating in the thoracic (pleural) or abdominal (peritoneal) cavities.


Definition: An exudate is any fluid that filters from the circulatory system into lesions or areas of inflammation.

Composition: The fluid is composed of serum, fibrin, and leukocytes.

Function: Exudate may ooze from cuts or from areas of infection or inflammation. It is also called pus.

Production: When an injury occurs, leaving skin exposed, it leaks out of the blood vessels and into nearby tissues.

Gastric Juice

Definition: Gastric juice is a mixture of secretions produced by the different types of glands found in the stomach.

Composition: It is made up of water, electrolytes, hydrochloric acid, enzymes, mucus, and intrinsic factor. Hydrochloric acid is a strong acid secreted by the parietal cells, and it lowers your stomach’s pH to around. Pepsinogen is secreted by chief cells, and when it’s in the presence of hydrochloric acid, it’s converted to pepsin.

Function: Gastric juice helps dissolve and break down food in the stomach1. Hydrochloric acid converts pepsinogen into pepsin and breaks various nutrients apart from the food you eat. It also kills bacteria that comes along with your food.

Production: Gastric juice is produced by special glands found in the lining of your stomach.


Definition: Lymph is the fluid that flows through the lymphatic system, a system composed of lymph vessels (channels) and intervening lymph nodes whose function, like the venous system, is to return fluid from the tissues to be recirculated.

Composition: Lymph contains many different substances, including proteins, minerals, fats, damaged cells, cancer cells, and germs.

Function: Lymph helps remove bacteria that might otherwise cause infection. It also transports fats from the digestive system (beginning in the lacteals) to the blood via chylomicrons.

Production: Lymph is formed from the extracellular fluids surrounding cells.


Definition: Mucus is a natural fluid that protects and lubricates various organs in your body.

Composition: Mucus is produced by cells in your mouth, throat, nose, and sinuses. Its slippery consistency helps protect and moisturize, and traps potential irritants.

Function: Mucus serves as a protective barrier, a lubricant, or a substance that helps trap foreign bodies or flush them out of your body. In your lungs and gut, mucus makes it harder for bacteria to stick together, reducing your risk for infections.

Production: Most adults will produce between 1 and 1.5 quarts of mucus a day—most of it in the respiratory tract, which includes the mouth, nose, throat, and lungs.

Pericardial Fluid

Definition: Pericardial fluid is a serous fluid secreted by the serous layer of the pericardium into the pericardial cavity. It is found in the double-layered, sac-like structure around the heart, known as the pericardium.

Composition: The fluid is typically clear and straw-colored, similar to plasma.

Function: It serves as a lubricant, reducing friction between the layers of the pericardium as the heart beats.

Volume: In healthy individuals, the pericardium contains about 15 to 50 milliliters of pericardial fluid.

Clinical Significance: An excess accumulation of pericardial fluid, known as pericardial effusion, can put pressure on the heart, affecting its function. This can lead to conditions such as cardiac tamponade, which is a medical emergency.

Pleural Fluid

Definition: Pleural fluid is a thin, translucent fluid that fills the cavity between the parietal (outer) and visceral (inner) pleural layers surrounding the lungs.

Composition: It is primarily composed of water, but also contains proteins and other substances.

Function: Pleural fluid functions by lubricating the space between the pleura, allowing the pleura to glide smoothly during inhalation and exhalation.

Volume: Under normal conditions, the pleural space contains only a small amount of fluid, roughly 1-10 ml.

Clinical Significance: An excess accumulation of pleural fluid, known as pleural effusion, can cause symptoms such as shortness of breath and chest pain.


Definition: Pus is a thick, opaque, usually yellowish-white, fluid matter that is formed as part of an inflammatory response typically associated with an infection.

Composition: It is composed of exudate chiefly containing dead white blood cells (such as neutrophils), tissue debris, and pathogenic microorganisms (such as bacteria).

Function: Pus is a natural result of the body fighting infection.

Production: Pus is produced when the body detects an infection, it sends neutrophils, a type of white blood cell, to destroy the fungi or bacteria.

Clinical Significance: The presence of pus can be a sign of infection, abscess formation, or an immune response to injury.


Definition: Saliva, commonly referred to as spit, is an extracellular fluid produced and secreted by salivary glands in the mouth.

Composition: In humans, saliva is around 99% water, plus electrolytes, mucus, white blood cells, epithelial cells (from which DNA can be extracted), enzymes (such as lipase and amylase), and antimicrobial agents (such as secretory IgA, and lysozymes).

Function: The enzymes found in saliva are essential in beginning the process of digestion of dietary starches and fats. These enzymes also play a role in breaking down food particles entrapped within dental crevices, thus protecting teeth from bacterial decay. Saliva also performs a lubricating function, wetting food and permitting the initiation of swallowing, and protecting the oral mucosa from drying out.

Production: Saliva is produced by special glands found in the lining of your mouth.


Definition: Sebum is an oily, waxy substance produced by your body’s sebaceous glands. It coats, moisturizes, and protects your skin.

Composition: Sebum is composed of 57.5% triglycerides and fatty acids, 26% wax esters, and 12% squalene, a lipid.

Function: Sebum production is a complex process that scientists don’t fully understand. That said, researchers do know that its primary function is to protect your skin and hair from moisture loss. Some scientists speculate that sebum may also have an antimicrobial or antioxidant role. It may even help release pheromones.

Production: Sebum is produced by the sebaceous glands, which sit in the middle layers of the skin, near hair follicles.

Serous Fluid

Definition: Serous fluid is a clear to pale yellow watery fluid that serves various functions in the body. It acts as a lubricant, reducing friction between internal organs.

Composition: Serous fluid contains small amounts of proteins and cells and is found in body cavities like the pleural, pericardial, and peritoneal spaces.

Function: It’s involved in maintaining tissue moisture and providing a medium for the exchange of nutrients and waste products.

Clinical Significance: When occurring in large quantities, it is indicative of a pathological condition (such as cirrhosis or heart failure) or surgical complication.


Definition: Semen, also known as seminal fluid, is a bodily fluid that contains spermatozoa. Spermatozoa are secreted by the male gonads (sexual glands) and other sexual organs of male or hermaphroditic animals and can fertilize the female ovum.

Composition: Semen is composed of sperm cells (spermatozoa) and a sticky, syrupy liquid called seminal fluid. The seminal fluid itself is composed of fluids from three organs: Cowper’s gland, Prostate gland, and Seminal vesicles.

Function: The role of seminal fluid is to transport sperm and provide them with nutrition and a stable environment so they can survive for up to five days to enable fertilization.

Production: Sperm cells are produced in the testicles and then transported to the epididymis, a coiled tube. There, the sperm mature under the influence of hormones from the testicles and pituitary gland. This process takes around two and a half to three months.


Definition: Sputum, or phlegm, is a type of mucus secreted by cells in the lower airways (bronchi and bronchioles) of the respiratory tract.

Composition: Sputum may contain dead cells, foreign debris inhaled into the lung, bacteria, and white blood cells that protect the airway from infection.

Function: The body produces mucus to keep the thin, delicate tissues of sensitive areas — such as the respiratory tract — moist. Mucus lines and protects sensitive surfaces inside the body, and it helps trap and remove small particles of foreign matter that may pose a threat.

Production: Sputum is not the same as saliva, a substance secreted in the mouth to help with digestion. The terms sputum and phlegm are used interchangeably. Sputum or phlegm is coughed up from the bronchi, bronchioles, and trachea rather than glands in the mouth and throat.

Synovial Fluid

Definition: Synovial fluid, also called synovia, is a viscous, non-Newtonian fluid found in the cavities of synovial joints.

Composition: Synovial fluid is an ultrafiltrate from plasma, and contains proteins derived from the blood plasma and proteins that are produced by cells within the joint tissues.

Function: The principal role of synovial fluid is to reduce friction between the articular cartilage of synovial joints during movement.

Production: The inner membrane of synovial joints is called the synovial membrane and secretes synovial fluid into the joint cavity.


Definition: Sweat, also known as perspiration, is the release of fluid from sweat glands on the skin’s surface. Your body produces sweat to help regulate your body temperature and cool you down during exercise or in hot weather.

Composition: Sweat is a salty liquid that your skin’s glands produce. It is roughly 99% water, and the rest is composed of thousands of compounds — both inorganic and organic — exiting the body.

Function: Sweating (also called perspiring) occurs when your body becomes heated and attempts to cool itself. As such, you might notice that you sweat on hot days, when exercising, when experiencing anxiety, or when you have a fever.

Production: Sweat is produced by special glands found in the lining of your skin.


Definition: Tears are a clear liquid secreted by the lacrimal glands (tear gland) found in the eyes of all land mammals.

Composition: Tears are made up of water, electrolytes, proteins, lipids, and mucins that form layers on the surface of eyes.

Function: The functions of tears include lubricating the eyes (basal tears), removing irritants (reflex tears), and also aiding the immune system.

Production: Lacrimal glands above each eye produce your tears. As you blink, tears spread across the surface of the eye. Then the tears drain into puncta, tiny holes in the corners of your upper and lower eyelids.


Definition: Urine is a liquid by-product of metabolism in humans and in many other animals. In placental mammals, urine flows from the kidneys through the ureters to the urinary bladder and is released from the bladder through the urethra during urination.

Composition: Urine is roughly 95 percent water, and the rest is composed of thousands of compounds — both inorganic and organic — exiting the body.

Function: Urine is produced when blood passes through the kidneys, which filter out excess waste and water. This waste travels through tubes known as ureters and is stored in the bladder until you urinate.

Production: Urine is produced by the kidneys and then transported to the bladder via the ureters.


Definition: Vomiting, also known as emesis and throwing up, is the involuntary, forceful expulsion of the contents of one’s stomach through the mouth and sometimes the nose.

Composition: Vomit is typically composed of partially digested food, stomach acid, and other secretions.

Function: Vomiting is your body’s way of getting rid of something harmful in the stomach. It may also be a response to irritation in the gut.

Production: Vomiting is triggered by the brain after it receives signals from the body about something harmful or irritating in the stomach.

pH of various body fluids

  • Blood: pH 7.35 – 7.45
  • Saliva: pH 6.5 – 7.5
  • Gastric Juice (Stomach): pH 1.5 – 4.0
  • Intestine: pH 7 – 8.5
  • Urine: pH varies widely from 4.5 to 8
  • Sweat: pH 4.5 – 7
  • Tears: pH 7.0 – 7.3
  • Cerebrospinal Fluid: pH 7.3


In conclusion, body fluids are a vital part of our physiological functioning, playing diverse roles in maintaining our health and well-being. From the blood that fuels our cells to the tears that express our deepest emotions, each fluid has a unique composition and serves a specific purpose. They act as transport mediums, lubricants, protective barriers, and even participate in the body’s defense mechanisms. Understanding these fluids and their functions can provide valuable insights into our body’s health and functioning. As we continue to explore and understand these fluids, we can better appreciate the intricate and delicate balance that sustains life.

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