Fluids in the Body or Biofluids

In physiology, fluids in the body is the liquid content of the human body. It accounts for a significant percentage of the total composition of the body. Water is a necessary part of maintaining life for many reasons. Most of all cells in the human body consist of the water content in the cytoplasm. Water also provides a fluid environment for extracellular communication and molecular transport throughout the body.

Water itself is also a key component of biochemical reactions involved in physiology, such as hydrolysis. Many organ systems depend on the physical properties of water, such as the surface tension of water in the alveoli of the lungs. Body fluids, body fluids or biological fluids are liquids in the human body. In lean and healthy adult men, total body moisture accounts for about 60% (60-67%) of total weight; women are usually slightly lower (52-55%).

The exact percentage of fluid relative to body weight is inversely proportional to the percentage of body fat. Vascular volume is divided into venous volume and arterial volume; arterial volume has a conceptually useful but immeasurable subunit called effective arterial blood volume


The total amount of water for people with an average weight (70 kg) is about 40 liters, accounting for an average of 57% of their total weight. In newborn babies, this may be as high as 79% of body weight, but gradually decreases from birth to old age, most of the reduction occurs in the first 10 years of life. In addition, obesity reduces the percentage of water in the body, sometimes as low as 45%. The water in the body is distributed in various fluid compartments, which are interspersed in various cavities of the body through different tissue types. In the diseased state where the body’s water is affected, the fluid compartment that has changed can provide clues to the nature of the problem. The main humoral chambers include: intracellular fluid and extracellular fluid(plasma, interstitial fluid and cross-cell fluid). Fluids from various tissues of the human body are divided into fluid compartments. Fluid compartments are usually used to compare the position and characteristics of the fluid relative to the fluid in other compartments.

Intracellular fluids in the body

Intracellular fluid or intracellular fluid (or cytoplasm) of the cytoplasm is a liquid found inside the cell. It is separated into compartments by the membranes of various organelles that surround the cell. For example, the mitochondrial matrix separates the mitochondria into compartments. The contents of the cell membrane of eukaryotic cells do not include the nucleus and other membrane-bound organelles (for example, mitochondria, plastids, lumen of the endoplasmic reticulum, etc.), known as cytoplasm. Cytosol is a complex mixture of substances dissolved in water. Although water makes up most of the cytosols, it mainly serves as a fluid medium for intracellular signaling (signal transduction) and plays a role in determining cell size and shape. Compared with extracellular fluid, the concentration of ions (such as sodium and potassium) in the cytosol is usually lower; these differences in ion levels are important in processes such as osmotic pressure regulation and signal transduction. Cytosol also contains a large number of macromolecules, which can change the behavior of molecules through macromolecular crowding.

Extracellular fluid

Extracellular fluid (ECF) or extracellular fluid volume (ECFV) usually represents all body fluids outside the cell. Extracellular fluid can be divided into two main sub-parts: interstitial fluid and plasma. Extracellular fluid also includes extracellular fluid; this only accounts for about 2.5% of ECF. In humans, the normal glucose concentration of extracellular fluid regulated by homeostasis is about 5mm. The pH of the extracellular fluid is strictly regulated by the buffer and maintained near 7.4. The volume of ECF is usually 15L(of which 12l is interstitial fluid and 3L is plasma). ECF contains an extracellular matrix (Ecm) that acts as a suspension of cells and molecules within the ECF.


Plasma is the straw-colored/light yellow liquid component of blood, usually the blood cells in the whole blood are suspended in the suspension, making it an ECM type of blood cells and a variety of molecules. It accounts for about 55% of total blood volume. It is the intravascular fluid part of extracellular fluid. It is mainly water(93% by volume), contains dissolved proteins(the main proteins are fibrinogen, globulin and albumin), glucose, coagulation factors, mineral ions(Na+, Ca++, etc), hormones and carbon dioxide(plasma is the main medium for the transport of excreted products). It plays a vital role in maintaining electrolyte level balance and protecting the body from intravascular penetration of infections and other blood diseases.

Interstitial fluid

Interstitial fluid (or tissue fluid) is a solution that bathes and surrounds multicellular animal cells. Interstitial fluid exists in interstitial space, also known as tissue space. On average, a person has about 11 liters (2.4 imperial gallons or about 2.9 US gallons) of interstitial fluid, which provides nutrients to the body’s cells and means of removing waste. Most of the functions of the interstitial space are as ECM, and the fluid space consists of cell excretory molecules located between the basement membrane of the interstitial space. Interstitial ECM contains a large amount of connective tissue and proteins(such as collagen), which are involved in blood coagulation and wound healing.

Transcellular fluid

Transcellular fluid is part of the total body moisture contained in the epithelial lining space. It is the smallest component of extracellular fluid, which also includes interstitial fluid and plasma. It is usually not counted as a small part of extracellular fluid, but it accounts for about 2.5% of total body water. Examples of such liquids are cerebrospinal fluid, eye fluid, joint fluid, and thoracic cavities that contain liquids found only in their respective epithelial lining spaces. The function of the cell fluid is mainly to lubricate these cavities, and sometimes the transport of electrolytes.


  • Blood
  • Urine
  • Blood plasma
  • Cerebrospinal Fluid
  • Interstitial fluid
  • Lymph
  • Synovial fluid
  • Saliva
  • Serum
  • Mucus
  • Semen
  • Tears
  • Aqueous humour
  • Amniotic fluid
  • Breast Milk
  • Bile
  • Vaginal Lubrication
  • Gastric Acid
  • Vitreous Body
  • Transudale
  • Glycolytic
  • Metabolic Water
  • Sebum
  • Pericardia Effusion
  • Hemoglobin
  • Pus
  • Chyle
  • Ear Wax
  • Pre-ejaculate
  • Rheum
  • Phlegm
  • Arterial Blood
  • Chym
  • Colostrum
  • Human Feceas
  • Substrate
  • Intestinal Juice
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