The ICF makes up about 60 percent of the total water in the human body, and in an average-size adult male, the ICF accounts for about 25 litres of fluid (Figure 17.11.3). The ICF lies within cells and is the principal component of the cytosol/cytoplasm. Materials travel between cells and the plasma in capillaries through the IF. Blood plasma is the second part of the ECF. The interstitial fluid (IF) is part of the extracellular fluid (ECF) between the cells. The intracellular fluid (ICF) is the fluid within cells. Extracellular fluid has two primary constituents: the fluid component of the blood (called plasma) and the interstitial fluid (IF) that surrounds all cells not in the blood (Figure 17.11.2). Extracellular fluid (ECF) surrounds all cells in the body. The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes. Fluid Compartmentsīody fluids can be discussed in terms of their specific fluid compartment, a location that is largely separate from another compartment by some form of a physical barrier. Water content varies in different body organs and tissues, from as little as 8 percent in the teeth to as much as 85 percent in the brain. Water content of the body’s organs and tissues. In contrast, teeth have the lowest proportion of water, at 8–10 percent. The brain and kidneys have the highest proportions of water, which composes 80–85 percent of their masses. The percent of body water changes with development, because the proportions of the body given over to each organ and to muscles, fat, bone and other tissues change from infancy to adulthood (Figure 17.11.1). 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 to regions of lower concentration, 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.
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