Potassium ions (K+) are the major intracellular positive ion (cation), and sodium ions (Na+) are the major extracellular cation. The distribution is created largely by the sodium-potassium pump (Na+/K+ pump), which actively transports sodium ions out of the cell and potassium ions into the cell. It is also in part due to channels in the cell membrane that determine membrane permeability to these ions. These channels, formed by membrane proteins, can be quite selective; that is, a particular channel may allow only one kind of ion to pass through and exclude all other ions of different size and charge. Thus, even though concentration gradients are present for sodium and potassium, the ability of these ions to diffuse across the cell membrane depends on the presence of channels.
O^1 Reconnect to chapter 3, Cell Membrane, page 69
Some channels are always open, whereas others may be either open or closed, somewhat like a gate. Both chemical and electrical factors can affect the opening and closing of these gated channels (fig. 10.11).
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.