• 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.