How ionic chemical and electrical gradients control communication between anatomic and physiologic compartments

If you are taking courses in anatomy and physiology at the same time, you may be a little puzzled by what your instructor means when talking about compartmentalization. Anatomical compartments, such as the thoracic cavity, are much easier to visualize than the often abstract compartments described in physiology.

representative human anatomy lab model

Model of Human Body Cavities, photo by Pete Danofric at

In physiology the easiest type of compartments to begin with are those of the body fluids. Anatomic compartments are always separated by a membrane barrier of some kind. Body fluid compartments also use membrane barriers for containment. The fluid compartments are easy to overlook, because they are missing in your anatomic models, dead bones, and even your formalin fixed cadavers. Yet, in a living body they are a major anatomic feature.

For a more detailed description of how the body’s fluid compartments are involved in whole body communication you may want to read “Physiology: Custom-Designed Chemistry”. This small book is available in bookstores and at Amazon and Barnes & Noble.

Water, H2O, is the primary component of the fluid compartments. There are two major fluid compartments, “extracellular” and “intracellular”. About 60% of body water is within cells, intracellular. The remainder of the water is found around cells, i.e. the interstitial fluid, and in blood plasma, lymph, cerebrospinal fluid, gastrointestinal secretion, aqueous humor of the eye, sweat, urine, and in peritoneal and pleural cavities. The barriers between fluid compartments are the plasma membranes of various cells types.

depiction of an open channel in a cell membrane

Movement of Molecules Through a Cell Membrane, illustration by Alex Luengo at

Channels of communication are needed for the body’s fluid compartments to interact with each other. Information proceeds in both directions across membranes. For maximum control of information transfer between compartments, it is necessary that such channels are small and that they open and close in a non-random fashion. Ions, atoms carrying a positive or negative charge, are key messengers that travel between fluid compartments when small doors, or channels, in cellular plasma membranes open.

generalized model of an atom with nucleus and orbiting electrons

Structure of an Atom, illustration by Mr. Jafari at

Some atoms have loosely attached electrons that can be stolen by atoms with larger nuclei that contain more protons. The atom that gets the extra electron(s) has a total negative charge and is called an anion. The atom that is missing an electron or two has a total positive charge and is called a cation. Both are collectively referred to as ions. The abundance of particular ions varies in different fluid compartments.

Ions go readily into water solution, because of their positive and negative nature. Positive ions hang out around water’s oxygen and negative ions around water’s hydrogens. This happens because the sharing of electrons between water’s two hydrogens (very small atoms) and water’s one oxygen (a much larger atom) force an asymmetrical shape upon water. Because oxygen is the stronger atom, it pulls hydrogen’s electrons away from the hydrogen nuclei and toward its own nucleus. However, for the most part, both hydrogens stick with the oxygen and very little becomes a free floating positive hydrogen cation, H+.

model shows polar structure of water with unequal sharing of electrons by hydrogen and oxygen

Asymmetry of the Water Molecule; illustration by Lightspring at

The asymmetric sharing of electrons makes the hydrogen end of a water molecule a little more positive and the oxygen end a little more negative. The partial electrical charge on each end of water molecules allow them to come between anions and cations that might otherwise be attracted to each other.

Ions, enclosed in a layer of water molecules, moving from one fluid compartment to another create a flow of electricity. The major ionic players in creating electrical flow through the membrane of nerve and muscle cells are sodium, Na+, potassium, K+, and calcium Ca++. Calcium also plays several additional roles in skeletal muscle, depending upon the intracellular compartment in which it is stored.

When membrane channels open to allow passage of ions, the direction the ions move is determined by the intracellular and extracellular fluid environment and by the diameter of the channel. The size of the channel is important because some ions are much larger than others. For example, chloride is several times the size of sodium. The size difference is important enough that channels are named for the ion that they allow to pass. In physiology, you will study potassium channels, sodium channels, chloride channels, calcium channels, etc.

Two additional factors regulate an ion’s flow through an open membrane channel. The first is the number of its own kind on each side of the membrane. Second is the net electrical charge on each side of the membrane before the channel opens. The physiologic name for this combination of factors is electrochemical gradient.

trace of electrical rhythms of heart muscle

Electrical Rhythms of Healthy Heart Muscle; adaptation of an illustration by Alila Medical Media at

Ions also play a role in a large array of other physiologic processes. Sodium, potassium, and calcium have many responsibilities in body function beyond their role in generating electrical activity of muscle and nerve. Other ions of major importance include chloride, Cl, hydrogen, H+, zinc, Zn++, iron, Fe++/Fe+++, and magnesium, Mg++. Their many roles will be discussed in future articles.

Continue to think of these small charged atoms as chemical and electrical messengers as you study physiology. Doing so will help make the abstract theories of physiologic processes become less mysterious to you.

You may also like to read Water’s Chemistry Governs Physiology and Ion Channels for a deeper understanding of how cell signaling occurs.

Do you have questions?

Please put your questions in the comment box or send them to me by email at I read and reply to all comments and email.

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Further reading:

3 Simple Secrets to Learning Physiology

Ion Channels

Neurons: Where Does Their Electricity Come From?

Brain Synapses


Reece-4-S2S14-001Margaret Thompson Reece PhD, physiologist, former Senior Scientist and Laboratory Director at academic medical centers in California, New York and Massachusetts and CSO at Serometrix LLC is now CEO at Reece Biomedical Consulting LLC.

Dr. Reece is passionate about helping students, online and in person, pursue careers in life sciences. Her books “Physiology: Custom-Designed Chemistry” (2012), “Inside the Closed World of the Brain” (2015) and the workbook (2017) companion to her course “30-Day Challenge: Craft Your Plan for Learning Physiology” are written for those new to life science.More about her books can be found at

Dr. Reece offers a free 30 minute “how-to-get-started” phone conference to students struggling with human anatomy and physiology. Schedule an appointment by email at


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