Passive or active transport classifies the movement of molecules across the membrane. Passive transport aligns with the electrochemical gradient of the molecule, while active transport moves against the electrochemical gradient. Active transport is further divided into primary active transport, which is directly coupled to ATP hydrolysis, and secondary active transport, which occurs with coupled solute carriers. Secondary active transport involves the passive movement of one or more molecules driving the active transport of other molecules. Examples of secondary active transport include the Na+-glucose symporter and Na+-H+ antiporter.

Upon completion of this chapter, the student should be able to answer the following questions:

1. Define steady-state balance and explain the elements required to achieve it using water balance as an example.
2. Discuss the volumes of the body fluid compartments and their changes under different conditions.
3. Compare the composition of different body fluid compartments.
4. Determine the factors that determine the resting membrane potential of cells.
5. Explain how cells regulate their volume in isotonic, hypotonic, and hypertonic solutions.
6. Describe the structural features of epithelial cells, their role in vectorial transport, and the general mechanisms by which transport is regulated.

Normal cellular function relies on maintaining the intracellular composition within a narrow range. This is achieved through the transport of substances and water across the cell membrane via membrane transport proteins. Ingestion of food and water, as well as excretion of waste products, occur daily without significant changes in the volume or composition of body fluid compartments in a healthy individual. Homeostasis refers to the maintenance of constant volume and composition of body fluid compartments, as well as the body's temperature in warm-blooded animals and humans. The human body has multiple systems that contribute to achieving homeostasis, which are explained in detail throughout this book. This chapter outlines the basic principles underlying the maintenance of homeostasis and defines the volume and composition of various body fluid compartments.

The human body functions as an open system, with substances being added to and lost from the body each day. The amount of substances added or lost can vary greatly depending on factors such as the environment, access to food and water, disease processes, and cultural norms. In such an open system, homeostasis is achieved through steady-state balance.

To illustrate steady-state balance, consider a river with a dam that forms a man-made lake. Each day, water enters the lake from different streams and rivers, as well as from underground springs, rain, and snow. At the same time, water is lost through the spillways of the dam and through evaporation. To maintain a constant level in the lake (steady-state balance), the rate of water addition, regardless of source, must match the amount of water lost, regardless of the route. Since water addition and evaporation cannot be easily controlled, the only way to keep the lake level constant is by regulating the amount lost through the spillways.
• Transport across the membrane is classified as passive or active. Passive transport is the movement of molecules as expected from the electrochemical gradient for that molecule. Active transport represents transport against the electrochemical gradient. Active transport is further divided into primary active and secondary active transport. Primary active transport is directly coupled to the hydrolysis of ATP (e.g., ATP-dependent transporters). Secondary active transport occurs with coupled solute carriers, for which passive movement of one or more molecules drives the active transport of other molecules (e.g., Na+-glucose symporter, Na+-H+ antiporter).
Pawlina W. Histology: A Text and Atlas with Correlated Cell and Molecular Biology. 7th ed. Alphen aan den Rijn, The Netherlands: Wolters Kluwer; 2016.
Rojeck A, et al. A current view of mammalian aquaglyceroporins. Annu Rev Physiol. 2008;70:301-327.
Ruess L, Altenberg GA. Mechanisms of ion transport across cell membranes. In: Alpern R, Moe O, Kaplan M, eds. Seldin and Giebisch’s The Kidney—Physiology and Pathophysiology. 5th ed. New York: Academic Press; 2013.
Upon completion of this chapter, the student should be able to answer the following questions: 1.
What is steady-state balance, and, with water balance as an example, what are the elements needed to achieve steady-state balance?
2.
What are the volumes of the body fluid compartments, and how do they change under various conditions?
3.
How do the body fluid compartments differ with regard to their composition?
4.
What determines the resting membrane potential of cells?
5.
How do cells regulate their volume in isotonic, hypotonic, and hypertonic solutions?
6.
What are the structural features of epithelial cells, how do they carry out vectorial transport, and what are the general mechanisms by which transport is regulated?
ormal cellular function requires that the intracellular composition—with regard to ions, small molecules, water, pH, and a host of other substances—be maintained within a narrow range. This is accomplished by the transport of many substances and water into and out of the cell via membrane transport proteins, as described in . In addition, each day, food and water are ingested, and waste products are excreted from the body. In a healthy individual, these processes occur without significant changes in either the volume of the body fluid compartments or their composition. The maintenance of constant volume and composition of the body fluid compartments (and their temperature in warm-blooded animals and humans) is termed homeostasis. The human body has multiple systems designed to achieve homeostasis, the details of which are explained in the various chapters of this book. In this chapter, the basic principles that underlie the maintenance of homeostasis are outlined. In addition, the volume and composition of the various body fluid compartments are defined.
Concept of Steady-State Balance
The human body is an “open system,” which means that substances are added to the body each day and, similarly, substances are lost from the body each day. The amounts added to or lost from the body can vary widely, depending on the environment, access to food and water, disease processes, and even cultural norms. In such an open system, homeostasis occurs through the process of steady-state balance.
To illustrate the concept of steady-state balance, consider a river on which a dam is built to create a synthetic lake. Each day, water enters the lake from the various streams and rivers that feed it. In addition, water is added by underground springs, rain, and snow. At the same time, water is lost through the spillways of the dam and by the process of evaporation. For the level of the lake to remain constant (i.e., steady-state balance), the rate at which water is added, regardless of source, must be exactly matched by the amount of water lost, again regardless of route. Because the addition of water is not easily controlled and the loss by evaporation cannot be controlled, the only way to maintain a constant level of the lake is to regulate the amount that is lost through the spillways.