CHAPTER 27LECTURE OUTLINEI) INTRODUCTIONA) Body fluid refers to body water and its dissolved substances.B) Regulatory mechanisms insure homeostasis of body fluids since their malfunction may seriously endanger nervous system and organ functioning.II) FLUID COMPARTMENS AND FLUID BALANCEA) Introduction1) In lean adults body fluids comprise about 55-60% (Figure 27.1) of total body weight.2) Water is the main component of all body fluids.3) About two-thirds of the body’s fluid is located in cells and is called intracellular fluid (ICF).4) The other third is called extracellular fluid (ECF).(a) About 80% of the ECF is interstitial fluid and 20% is blood plasma.(b) Some of the interstitial fluid is localized in specific places, such as lymph; cerebrospinal fluid; gastrointestinal tract fluids; synovial fluid; fluids of the eyes (aqueous humor and vitreous body) and ears (endolymph and perilymph); pleural, pericardial, and peritoneal fluids between serous membranes; and glomerular filtrate in the kidneys.5) Selectively permeable membranes separate body fluids into distinct compartments. Plasma membranes of individual cells separate intracellular fluid from interstitial fluid. Blood vessel walls divide interstitial fluid from blood plasma. Although fluids are in constant motion from one compartment to another, the volume of fluid in each compartment remains fairly stable – another example of homeostasis.6) Fluid balance means that the various body compartments contain the required amount of water, proportioned according to their needs.(a) Osmosis is the primary way in which water moves in and out of body compartments. Theconcentrations of solutes in the fluids is therefore a major determinant of fluid balance.(b) Most solutes in body fluids are electrolytes, compounds that dissociate into ions.(c) Fluid balance, then, means water balance, but also implies electrolyte balance; the two are inseparable.B) Sources of Body Water Gain and Loss1) Water is the largest single constituent in the body, varying from 45% to 75% of body weight, depending on age and the amount of fat present.2) Fluid intake (gain) normally equals fluid output (loss), so the body maintains a constant volume (Figure 27.2).C) Regulation of Water Gain1) Metabolic water volume depends mostly on the level of aerobic cellular respiration, which reflects the demand for ATP in body cells.2) The main way to regulate body water balance is by adjusting the volume of water intake.3) When water loss is greater than water gain, dehydration occurs (Figure 27.3).4) The stimulus for fluid intake (gain) is dehydration resulting in thirst sensations; one mechanism for stimulating the thirst center in the hypothalamus is the renin-angiotensin II pathway, which responds to decreased blood volume (therefore, decreased blood pressure) (Figure 27.3).D) Regulation of Water and Solute Loss1) Although increased amounts of water and solutes are lost through sweating and exhalation during exercise, loss of body water or excess solutes depends mainly on regulating how muchis lost in the urine (Figure 27.4).2) Under normal conditions, fluid output (loss) is adjusted by antidiuretic hormone (ADH), atrial natriuretic peptide (ANP), Angiotensin II and aldosterone, all of which regulate urine production.3) Table 27.1 summarizes the factors that maintain body water balance.E) Movement of Water Between Body Fluid Compartments1) A fluid imbalance between the intracellular and interstitial fluids can be caused by a change in their osmolarity.2) Most often a change in osmolarity is due to a change in the concentration of Na+.3) When water is consumed faster than the kidneys can excrete it, water intoxication may result (Figure 27.5).4) Repeated use of enemas can increase the risk of fluid and electrolyte imbalances. (Clinical Connection)III) ELECTROLYTES IN BODY FLUIDSA) Electrolytes serve four general functions in the body.1) Because they are more numerous than nonelectrolytes, electrolytes control the osmosis of water between body compartments.2) They help maintain the acid-base balance required for normal cellular activities.3) They carry electrical current, which allows production of action potentials and graded potentials and controls secretion of some hormones and neurotransmitters. Electrical currents are also important during development.4) Several ions are cofactors needed for optimal activity of enzymes.B) Concentrations of Electrolytes in Body Fluids1) To compare the charge carried by ions in different solutions, the concentration is typically expressed in milliequivalents/liter (mEg/Liter), which gives the concentration of cations or anions in a solution.2) The chief difference between plasma and interstitial fluid is that plasma contains quite a few protein anions, where interstitial fluid has hardly any since plasma proteins generally cannotmove out of impermeable blood vessel walls. Plasma also contains slightly more sodium ionsbut fewer chloride ions than the interstitial fluid. In other respects, the two fluids are similar.3) Intracellular fluid (ICF) differs considerably from extracellular fluid (ECF), however.4) Figure 27.6 compares the concentrations of the main electrolytes and protein anions in plasma, interstitial fluid, and intracellular fluid.C) Sodium (Na+) is the most abundant extracellular ion.1) It is involved in impulse transmission, muscle contraction, and participates in fluid and electrolyte balance by creating most of the osmotic pressure of extracellular fluid.2) The average daily intake of sodium far exceeds the body’s normal daily requirements. The kidneys excrete excess sodium and conserve it during periods of sodium restriction.3) Its level in the blood is controlled by aldosterone, antidiuretic hormone, and atrial natriuretic peptide.4) Excess Na+ in the body can result in edema. Excess loss of Na+ causes excessive loss of water, which results in hypovolemia, an abnormally low blood volume. (Clinical Connection)D) Chloride (Cl-) is the major extracellular anion.1) It plays a role in regulating osmotic pressure between compartments and forming HCl in the stomach.2) Regulation of Cl- balance in body fluids is indirectly controlled by aldosterone. Aldosterone regulate sodium reabsorption; the negatively charged chloride follows the positively charged sodium passively by electrical attraction.E) Potassium (K+) is the most abundant cation in intracellular fluid.1) It is involved in maintaining fluid volume, impulse