Human beings are mostly water, ranging from about 75 percent of body mass in infants to about 50–60 percent in adult men and women, to as low as 45 percent in old age. An appropriate balance of solutes inside and outside of cells must be maintained to ensure normal function. As a result, water will move into and out of cells and tissues, depending on the relative concentrations of the water and solutes found there. Osmosis is basically the diffusion of water from regions of higher concentration of water to regions of lower concentration of water, along an osmotic gradient across a semi-permeable membrane. In the body, water moves through semi-permeable membranes of cells and from one compartment of the body to another by a process called osmosis. For instance, sodium ions (Na +) and chloride ions (Cl -) are often referred to as electrolytes. Often in medicine, a mineral dissociated from a salt that carries an electrical charge (an ion) is called an electrolyte. In the human body, solutes vary in different parts of the body, but may include proteins-including those that transport lipids, carbohydrates, and, very importantly, electrolytes. The dissolved substances in a solution are called solutes. The chemical reactions of life take place in aqueous solutions. Identify the causes and symptoms of edema.Explain the importance of protein channels in the movement of solutes.Contrast the composition of the intracellular fluid with that of the extracellular fluid.Explain the importance of water in the body. The physical barrier between the intracellular fluid and interstitial fluid is the cell plasma membrane, and the physical barrier between the interstitial fluid and plasma is the capillary endothelium.By the end of this section, you will be able to: These fluid compartments are emphasized by the gray background color. If we examine this figure carefully, and if we consider the functional interconnections of the fluid compartments, it becomes clear that there are three major fluid compartments: (1) intracellular fluid, (2) interstitial fluid, and (3) plasma. Transcellular fluid generally comprises up to about 2.5% of the total body water (or up to ~0.75 L). Cerebrospinal fluid (the fluid bathing the brain and the spinal cord), intraocular fluids (aqueous and vitreous humors), inner ear fluids (endolymph and perilymph), pericardial fluid, peritoneal fluid, synovial fluids (in joints), as well as some other fluids are components of the transcellular fluid in the body. This fluid is generally separated from the plasma by an additional epithelial layer in addition to the capillary endothelium. Transcellular fluid is found in small amounts in different body regions and, in total, comprises a very small portion (though not unimportant) of the total body water. It is generally about 75% of the volume of ECF (or ~8.25 L). Interstitial fluid is the fluid that directly bathes the cells and tissues in the body. The extravascular compartment is further subdivided into two compartments: interstitial fluid and transcellular fluid. The volume of plasma is about 2.35 L and is generally about 25% of the volume of ECF. Plasma is the fluid portion of blood (i.e., whole blood minus the so-called formed elements red cells, white cells, and platelets). The fluid in the intravascular compartment is the fluid that is present in all of body blood vessels. These two compartments are separated by the capillary endothelium (lining of the walls of the capillaries). The extracellular fluid (ECF) compartment is further subdivided into the intravascular compartment and the extravascular compartment. Approximately 33% of the total body fluid (or ~20% of total body weight) is in the extracellular fluid (ECF) compartment. The intracellular fluid is separated from the extracellular fluid by the cell plasma membrane. Therefore, the total volume of ICF is ~22 L (cytoplasmic volume of all cells combined). Approximately 67% of total body water (or ~40% of total body weight) is in the intracellular fluid (ICF) compartment, which is the fluid that is present in the cytoplasm of all cells of the body. If we use 55 kg (121 lb) to represent the average adult female, the total volume of water in the body can be calculated to be about 30.25 L. In a typical adult female human, approximately 55% of the total body weight is composed of water.
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