General principles of physiology

Foundations of homeostasis

Life functions (e.g., growth and development, reproduction, breathing, cognition, movement, self-defense, and responses to environmental stress) require either the input of energy into the body or the transformation of energy from one form to a more useful form by the body. For anything to happen in an organism, there must be a force, an energy, or work in the form of some gradient—chemical, mechanical, or electrical. The human body is not at equilibrium with the environment, although conditions may be fairly constant, in which the body is in or near a steady state.

The important physiologic force-flow relationships relate “intensive” and “extensive” properties of state. These are called conjugate properties.

• Intensive properties have to do with each part of a system at large and are not additive. Chemical potential, electrical potential, temperature, and pressure are intensive properties of a system.

• Extensive properties, such as mass, moles, volume, charge, and entropy are additive.

For example, all the moles (extensive) of a part of a solution add up to the total moles of substance, whereas the concentration (intensive) in each part of a solution is not additive. The concentration of salt in the harbor is the same at high and low tide, but the volume and the moles of (sodium) Na ⁺ are different: There is a greater volume of solution and greater mass of Na ⁺ at high tide. But the stable concentration, the ratio of moles to volume, exemplifies that the quotient of two extensive properties (solution volume and Na ⁺ mass in this example) is the conjugate intensive property (concentration).

The difference of an intensive property at two points of a system is the “driving force” for the conjugate extensive property:

• Electrical gradients (intensive) drive flow of charge (extensive).

• Pressure gradients (intensive) drive flow of volume (extensive).

• Chemical gradients (intensive) drive flow of moles (extensive).

This chapter provides an overview of homeostasis, the tendency of the body’s physiologic systems to maintain a relatively stable state and internal milieu despite changing external and internal environments by keeping physiologic variables within acceptable ranges. Some background information and appendices available on the companion website for this text provide detail on general properties of cell membranes, mechanisms by which solutes and water traverse membranes, membrane transport, mechanisms by which cells receive and respond to signals, and how cells in the body communicate with each other, all of which are means by which homeostasis occurs.

Transport processes in life

Dynamic processes govern the movement of fluids and gases within the body do not necessarily occur across cell membranes. The transport of fluids, such as blood and water like solutes, requires a driving force, such as a pressure gradient. The consequent movement depends on the nature of any barrier and the magnitude and direction of the pressure gradient.

• The heart is a pump that generates a pressure, driving blood through the circulation system, where the force is the hydrostatic, (or in a moving fluid hydraulic) pressure gradient and the flow is blood flow.

• The flow of gases in the respiratory tree follows the same pressure gradient rules, but unlike liquids, gases are compressible fluids.

Fluids (liquids and gases) not only flow through tubes and across cell membranes, but they can leave the circulation by crossing capillary basement membranes into the extravascular space surrounding cells, joining the interstitial fluid.

Cell membrane structure and transport functions

The contents of cells are sealed off from the extracellular environment by a lipid barrier called the plasma membrane that protects the cell’s interior from the changing conditions of the extracellular environment and helps maintain the specific chemical milieu supporting intracellular metabolic processes. Additional background information about the structure and function of the plasma membrane can be found in the online appendix to this chapter.

Types of membrane transport

Because the plasma membrane is made up predominantly of hydrophobic (lipophilic) molecules, other hydrophobic molecules are able to enter or exit cells by dissolving into or through the membrane. Hydrophilic (lipophobic) molecules, however, must cross the plasma membrane through water-filled channels or via specific carrier proteins.

Transport across the plasma membrane

Transport across the plasma membrane occurs by several mechanisms, including:

• Diffusion

• Osmosis

• Endocytosis

• Exocytosis

• Protein-mediated transport

( Fig. 1.1 ). We would like to post appendices on the website for this text.

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