In the steady state, the volume of water that diffuses across the membrane in either direction is balanced precisely so that no net diffusion of water occurs and the volume of the cell remains unchanged. In some cells, the number of AQPs, and thus the permeability to water, can be altered in response to hormones. The number of AQPs, also known as water channels, differs in membranes of different tissues. Although water is a polar molecule, it is able to penetrate the nonpolar lipid region of membranes through a group of transmembrane channel proteins called aquaporins (AQPs), which form channels through which water can readily diffuse. Most cell membranes are semipermeable, that is, relatively impermeable to most solutes but highly permeable to water. The process of osmosis also explains the movement of water across cell membranes. This pressure is called the osmotic pressure. As shown in diagram III, application of a pressure can prevent osmotic movement of water across the semipermeable membrane. ![]() Because the volume of compartment B increased, there were no significant changes in hydrostatic pressure in the compartments. In this example, the volume of compartment B increased, whereas the volume of compartment A decreased. The movement of water across the semipermeable membrane leads to a change in the initial volumes at equilibrium. There is net movement of water from A to B until the solutions in the two compartments are iso-osmotic, as shown in diagram II. In diagram I, compartments A and B are shown filled with equal volumes of solution, but the solution in compartment A is hypo-osmotic with reference to the solution in compartment B. This is achieved by removing excess heat from the body by evaporative water loss from the skin.įIGURE 35-2 Osmosis and osmotic pressure can be illustrated by two compartments separated by a semipermeable membrane, permeable to water but not to solutes (circles). Another important physiological function of water is its role in the regulation of body temperature. The intracellular water establishes the physicochemical medium that allows various metabolic processes to take place. The extracellular water bathing the cells serves as a medium for the transport of nutrients and oxygen to the cells and for removing wastes from the cells, which will be eliminated by the liver and kidneys. It is the principal fluid medium in which nutrients, minerals, gases, and enzymes are dissolved. Water performs several functions that are essential to life. ![]() The volume of the intracellular fluid provides turgor to the tissues, which is important for the tissue or organ form and ultimately the body form. It lubricates joints and moistens tissues such as those in the eyes, nose, and mouth. In the body, water is present inside and around the cells and within all blood vessels. The relationship between the intracellular fluid compartment, interstitial fluid, and plasma can be seen in this figure.Water is an essential nutrient vital to the existence of both animals and plants. Note that this figure only demonstrates the relative sizes of body fluid compartments and does not provide any information regarding how these compartments are related to one another. In adult women, the total fluid volume is approximately 55% of the total body weight. In adult men, the total fluid volume is approximately 60% of the total body weight. All percentages shown in this figure refer to the percentage of the total body fluid volume in an adult human being. ![]() Transcellular fluid is found in small amounts in specialized compartments (e.g., cerebrospinal fluid, intraocular fluids, inner ear fluids, pericardial fluid, peritoneal fluid, synovial fluids, etc.). Interstitial fluid is the fluid that directly bathes the cells and tissues in the body. This compartment can be further subdivided into the interstitial fluid (~25% of total body fluid volume), plasma (~7% of total body fluid volume), and transcellular fluid (~1% of total body fluid volume). The extracellular fluid compartment makes up approximately 33% of the total volume of body fluids. This compartment holds approximately 67% of the total volume of body fluids. The intracellular fluid compartment contains the fluid that is present in the cytoplasm of all cells of the body. As shown in other figures for men and women, body fluids are located in two main compartments: intracellular fluid (ICF) compartment and extracellular fluid (ECF) compartment.
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