Body Fluids

Water Content in the Body

A significant percentage of the human body is water, which includes intracellular and extracellular fluids.

Learning Objectives

Describe the characteristics of water content in the body

Key Takeaways

Key Points

  • On average, body water can account for 50% of the total human body weight and it is significantly higher in newborns. Obesity decreases the percentage of water in the body.
  • Body water is regulated by hormones, including anti-diuretic hormone (ADH), aldosterone, and atrial natriuretic peptide.
  • Water content in the body can be evaluated using bioelectrical impedance and mass spectrometry.
  • Important functions of water in the body include supporting the cellular metabolism, molecular transport, biochemical reactions, and the physical properties of water, such as surface tension.

Key Terms

  • hydrolysis: A biochemical reaction in which water molecules are used to break down a molecule into smaller molecules.
  • bioelectrical impedance analysis: A commonly used method for estimating body composition, by measuring resistance to the flow of electricity in the body, which is associated with hydration levels.

Water Content

In physiology, body water is the water content of the human body. It makes up a significant percentage of the total composition of a body. Water is a necessary component to support life for many reasons. All cells in the human body are made mostly of water content in their cytoplasm.

This is a 3-dimensional model of hydrogen bonds (labeled 1 on the model) between molecules of water.

Water molecule: A 3-dimensional model of hydrogen bonds (labeled 1) between molecules of water.

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.

Overall Water Content

The total amount of water in a human of average weight (70 kilograms) is approximately 40 liters, averaging 57 percent of his total body weight. In a newborn infant, this may be as high as 79 percent of the body weight, but it progressively decreases from birth to old age, with most of the decrease occurring during the first 10 years of life.

Also, obesity decreases the percentage of water in the body, sometimes to as low as 45 percent. The water in the body is distributed among various fluid compartments that are interspersed in the various cavities of the body through different tissue types. In diseased states where body water is affected, the fluid compartments that have changed can give clues to the nature of the problem.

Water Content Regulation and Measurement

Body water is regulated largely by the renal and neuro-endocrine systems. Water content regulation is one of the most important parts of homeostasis due to its influence on blood pressure and cardiac output. Much of this regulation is mediated by hormones, including anti-diuretic hormone (ADH), renin, angiotensin II, aldosterone, and atrial natriuretic peptide (ANP).

These hormones act as messengers between the kidneys and the hypothalamus; however, the lungs and heart are also involved in the secretion of some of these hormones, such as angiotensin converting enzyme (ACE) and ANP respectively.

There are many clinical methods to determine body water. One way to get an uncertain estimate is by calculation based on body weight and urine output. Another way to measure body water is through dilution and equilibration using mass spectrometry, which measures the abundance of water in breath samples from an individual.

In bioelectrical impedance analysis, a person’s hydration level is calculated from high-precision measurements of the opposition to the flow of an electric current through body tissues. Since water conducts electricity, a lower hydration level will cause a greater amount of resistance to electrical flow through the body.

Fluid Compartments

The major body-fluid compartments includ: intracellular fluid and extracellular fluid (plasma, interstitial fluid, and transcellular fluid).

Learning Objectives

Distinguish between intracellular and extracellular fluids

Key Takeaways

Key Points

  • The intracellular fluid of the cytosol or intracellular fluid (or cytoplasmic matrix) is the liquid found inside cells.
  • The cytosol is a complex mixture of substances that include proteins, ions, and organelles dissolved in water.
  • Extracellular fluid (ECF) or extracellular fluid volume (ECFV) usually denotes all body fluid outside of cells, and consists of plasma, interstitial, and transcellular fluid.
  • An extracellular matrix is an extracellular fluid space containing cell-excreted molecules, and they vary in their type and function.
  • Plasma also serves as an extracellular matrices (ECM) for the cells and molecules of the blood.
  • Interstitial fluid (or tissue fluid) is a solution that bathes and surrounds the cells of multicellular animals.
  • Transcellular fluid is the portion of total body water contained within epithelial -lined spaces.

Key Terms

  • intracellular fluid: The liquid found inside cells, between the endomembrane and the membrane-bound organelles.
  • interstitial fluid: A solution that bathes and surrounds the cells of multicellular animals; also called tissue fluid.
  • plasma: The straw-colored/pale-yellow, liquid component of blood that normally holds the blood cells of whole blood in suspension.

Fluid Compartments

The fluids of the various tissues of the human body are divided into fluid compartments. Fluid compartments are generally used to compare the position and characteristics of fluid in relation to the fluid within other compartments.

While fluid compartments may share some characteristics with the divisions defined by the anatomical compartments of the body, these terms are not one in the same. Fluid compartments are defined by their position relative to the cellular membrane of the cells that make up the body’s tissues.

Intracellular Fluid

The intracellular fluid of the cytosol or intracellular fluid (or cytoplasm ) is the fluid found inside cells. It is separated into compartments by membranes that encircle the various organelles of the cell. For example, the mitochondrial matrix separates the mitochondrion into compartments.

The contents of a eukaryotic cell within the cell membrane, excluding the cell nucleus and other membrane-bound organelles (e.g., mitochondria, plastides, lumen of endoplasmic reticulum, etc.), is referred to as the cytoplasm.

This is an illustration of a cytosol encompassing a variety of molecules in its fluid. The cytosol is the fluid within the plasma membrane of a cell and contains the organelles. The cytosol includes dissolved molecules and water.

The cytosol: The cytosol (11) is the fluid within the plasma membrane of a cell and contains the organelles. The cytosol includes dissolved molecules and water.

The cytosol is a complex mixture of substances dissolved in water. Although water forms the large majority of the cytosol, it mainly functions as a fluid medium for intracellular signaling (signal transduction ) within the cell, and plays a role in determining cell size and shape.

The concentrations of ions, such as sodium and potassium, are generally lower in the cytosol compared to the extracellular fluid; these differences in ion levels are important in processes such as osmoregulation and signal transduction. The cytosol also contains large amounts of macromolecules that can alter how molecules behave, through macromolecular crowding.

Extracellular Fluid

Extracellular fluid (ECF) or extracellular fluid volume (ECFV) usually denotes all the body fluid that is outside of the cells. The extracellular fluid can be divided into two major subcompartments: interstitial fluid and blood plasma.

The extracellular fluid also includes the transcellular fluid; this makes up only about 2.5% of the ECF. In humans, the normal glucose concentration of extracellular fluid that is regulated by homeostasis is approximately 5 mm. The pH of extracellular fluid is tightly regulated by buffers and maintained around 7.4.

The volume of ECF is typically 15L (of which 12L is interstitial fluid and 3L is plasma). The ECF contains extracellular matrices (ECMs) that act as fluids of suspension for cells and molecules inside the ECF.

This is a diagram of the extracellular matrix. It shows the spatial relationship between the blood vessels, basement membranes, and interstitial space between structures.

Extracellular matrix: Spatial relationship between the blood vessels, basement membranes, and interstitial space between structures.

Blood Plasma

Blood plasma is the straw-colored/pale-yellow, liquid component of blood that normally holds the blood cells in whole blood in suspension, making it a type of ECM for blood cells and a diverse group of molecules. It makes up about 55% of total blood volume.

It is the intravascular fluid part of the extracellular fluid. It is mostly water (93% by volume) and contains dissolved proteins (the major proteins are fibrinogens, globulins, and albumins), glucose, clotting factors, mineral ions (Na+, Ca++, Mg++, HCO3- Cl-, etc.), hormones, and carbon dioxide (plasma is the main medium for excretory product transportation). It plays a vital role in intravascular osmotic effects that keep electrolyte levels balanced and protects the body from infection and other blood disorders.

Interstitial Fluid

Interstitial fluid (or tissue fluid) is a solution that bathes and surrounds the cells of multicellular animals. The interstitial fluid is found in the interstitial spaces, also known as the tissue spaces.

On average, a person has about 11 liters (2.4 imperial gallons or about 2.9 U.S. gal) of interstitial fluid that provide the cells of the body with nutrients and a means of waste removal. The majority of the interstitial space functions as an ECM, a fluid space consisting of cell-excreted molecules that lies between the basement membranes of the interstitial spaces. The interstitial ECM contains a great deal of connective tissue and proteins (such as collagen) that are involved in blood clotting and wound healing.

Transcellular Fluid

Transcellular fluid is the portion of total body water contained within the epithelial-lined spaces. It is the smallest component of extracellular fluid, which also includes interstitial fluid and plasma. It is often not calculated as a fraction of the extracellular fluid, but it is about 2.5% of the total body water.

Examples of this fluid are cerebrospinal fluid, ocular fluid, joint fluid, and the pleaural cavity that contains fluid that is only found in their respective epithelium-lined spaces.

The function of transcellular fluid is mainly lubrication of these cavities, and sometimes electrolyte transport.

Body Fluid Composition

The composition of tissue fluid depends upon the exchanges between the cells in the biological tissue and the blood.

Learning Objectives

Describe the composition of intracellular and extracellular fluid in the body

Key Takeaways

Key Points

  • The cytosol or intracellular fluid consists mostly of water, dissolved ions, small molecules, and large, water-soluble molecules (such as proteins).
  • Enzymes in the cytosol are important for cellular metabolism.
  • The extracellular fluid is mainly cations and anions.
  • Plasma is mostly water and dissolved proteins, but also contains metabolic blood gasses, hormones, and glucose.
  • The composition of transcellular fluid varies, but some of its main electrolytes include sodium ions, chloride ions, and bicarbonate ions.

Key Terms

  • electrolyte: Any of the various ions (such as sodium or chloride) that regulate the electric charge on cells and the flow of water across their membranes.
  • transcellular fluid: The portion of total body water contained within epithelial-lined spaces, such as the cerebrospinal fluid, and the fluid of the eyes and joints.
  • ion: An atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge.

Body Fluid Composition

The composition of tissue fluid depends upon the exchanges between the cells in the biological tissue and the blood. This means that fluid composition varies between body compartments.

Intracellular Fluid Composition

The cytosol or intracellular fluid consists mostly of water, dissolved ions, small molecules, and large, water-soluble molecules (such as proteins). This mixture of small molecules is extraordinarily complex, as the variety of enzymes that are involved in cellular metabolism is immense.

This is a diagram of ions in a solution.

Ions: Ions in solution.

These enzymes are involved in the biochemical processes that sustain cells and activate or deactivate toxins. Most of the cytosol is water, which makes up about 70% of the total volume of a typical cell. The pH of the intracellular fluid is 7.4. The cell membrane separates cytosol from extracellular fluid, but can pass through the membrane via specialized channels and pumps during passive and active transport.

The concentrations of the other ions in cytosol or intracellular fluid are quite different from those in extracellular fluid. The cytosol also contains much higher amounts of charged macromolecules, such as proteins and nucleic acids, than the outside of the cell.

In contrast to extracellular fluid, cytosol has a high concentration of potassium ions and a low concentration of sodium ions. The reason for these specific sodium and potassium ion concentrations are Na+/K ATPase pumps that facilitate the active transport of these ions. These pumps transport ions against their concentration gradients to maintain the cytosol fluid composition of the ions.

Extracellular Fluid Composition

The extracellular fluid is mainly cations and anions. The cations include: sodium (Na+ = 136-145 mEq/L), potassium (K+ = 3.5-5.5 mEq/L) and calcium (Ca2+ = 8.4-10.5 mEq/L). Anions include: chloride ( mEq/L) and hydrogen carbonate (HCO3- 22-26 mM). These ions are important for water transport throughout the body.

Plasma is mostly water (93% by volume) and contains dissolved proteins (the major proteins are fibrinogens, globulins, and albumins), glucose, clotting factors, mineral ions (Na+, Ca++, Mg++, HCO3- Cl- etc.), hormones and carbon dioxide (plasma being the main medium for excretory product transportation). These dissolved substances are involved in many varied physiological processes, such as gas exchange, immune system function, and drug distribution throughout the body.

Transcellular Fluid Composition

Due to the varying locations of transcellular fluid, the composition changes dramatically. Some of the electrolytes present in the transcellular fluid are sodium ions, chloride ions, and bicarbonate ions.

Cerebrospinal fluid is similar in composition to blood plasma, but lacks most proteins, such as albumins, because they are too large to pass through the blood–brain barrier. Ocular fluid in the eyes contrasts with cerebrospinal fluid by containing high concentrations of proteins, including antibodies.

Movement of Fluid Among Compartments

How fluid moves through compartments depends on several variables described by Starling’s equation.

Learning Objectives

Describe the movement of fluid between extracellular compartments

Key Takeaways

Key Points

  • Interstitial fluid is formed when hydrostatic pressure generated by the heart pushes water out of the capillaries. The water passes from a high concentration outside of the vessels to a low concentration inside of the vessels, but equilibrium is never reached because the constant blood flow.
  • Osmotic pressure works opposite to hydrostatic pressure to hold water and substances in the capillaries.
  • Hydrostatic pressure is stronger in the arterial ends of the capillaries, while osmotic pressure is stronger at the venous ends of the capillaries.
  • Interstitial fluid is removed through the surrounding lymph vessels, and eventually ends up rejoining the blood. Sometimes the removal of tissue fluid does not function correctly and there is a buildup, called edema.
  • The Starling equation describes the pressure gradients that drive the movement of water across fluid compartments.

Key Terms

  • Starling equation: An equation that illustrates the role of hydrostatic and oncotic forces in the movement of fluid across capillary membranes.
  • interstitial fluid: A solution that bathes and surrounds the cells of multicellular animals.

Fluid Movement

Extracellular fluid is separated among the various compartments of the body by membranes. These membranes are hydrophobic and repel water; however, there a few ways that fluids can move between body compartments. There are small gaps in membranes, such as the tight junctions, that allow fluids and some of their contents to pass through membranes by way of pressure gradients.

Formation of Interstitial Fluid

Hydrostatic pressure is generated by the contractions of the heart during systole. It pushes water out of the small tight junctions in the capillaries. The water potential is created due to the ability of the small solutes to pass through the walls of capillaries.

This buildup of solutes induces osmosis. The water passes from a high concentration (of water) outside of the vessels to a low concentration inside of the vessels, in an attempt to reach an equilibrium. The osmotic pressure drives water back into the vessels. Because the blood in the capillaries is constantly flowing, equilibrium is never reached.

The balance between the two forces differs at different points on the capillaries. At the arterial end of a vessel, the hydrostatic pressure is greater than the osmotic pressure, so the net movement favors water and other solutes being passed into the tissue fluid.

At the venous end, the osmotic pressure is greater, so the net movement favors substances being passed back into the capillary. This difference is created by the direction of the flow of blood and the imbalance in solutes created by the net movement of water that favors the tissue fluid.

Removal of Interstitial Fluid

The lymphatic system plays a part in the transport of tissue fluid by preventing the buildup of tissue fluid that surrounds the cells in the tissue. Tissue fluid passes into the surrounding lymph vessels and eventually rejoins the blood.

Sometimes the removal of tissue fluid does not function correctly and there is a buildup, which is called edema. Edema is responsible for the swelling that occurs during inflammation, and in certain diseases where the lymphatic drainage pathways are obstructed.

Starling Equation

image

The Starling model: Note the concentration of interstitial solutes (orange) increases proportionally to the distance from the arteriole.

Capillary permeability can be increased by the release of certain cytokines, anaphylatoxins, or other mediators (such as leukotrienes, prostaglandins, histamine, bradykinin, etc.) that are released by cells during inflammation. The Starling equation defines the forces across a semipermeable membrane to calculate the net flux.

The solution to the equation is known as the net filtration or net fluid movement. If positive, fluid will tend to leave the capillary (filtration). If negative, fluid will tend to enter the capillary (absorption). This equation has a number of important physiologic implications, especially when disease processes grossly alter one or more of the variables.

This is a diagram of capillary dynamics. The oncotic pressure exerted by the proteins in blood plasma tends to pull water into the circulatory system. The illustration shows a capillary with blood flow in it. As the blood moves to the venous end of the capillary, hydrostatic pressure removes fluid from the blood, and osmotic pressure puts fluid into the blood flow.

Capillary dynamics: Oncotic pressure exerted by the proteins in blood plasma tends to pull water into the circulatory system.

This is a diagram of the Starling model. Note how the concentration of interstitial solutes increases proportionally to the distance from the arteriole.

According to Starling’s equation, the movement of fluid depends on six variables:

  1. Capillary hydrostatic pressure (Pc)
  2. Interstitial hydrostatic pressure (Pi)
  3. Capillary oncotic pressure (πz)
  4. Interstitial oncotic pressure (πi)
  5. Filtration coefficient (Kf)
  6. Reflection coefficient (σ)

The Starling Equation is mathematically described as Flux=Kf[(Pc-Pi)-σ (πz-πi)].