Medical brain notes 27

  WATER COMPARTMENTS   Most of the water of the body, about two-thirds of the total water volume, is found within individual cells and is called  intracellular fluid  (ICF). The remaining third is called  extracellular fluid  (ECF) and includes blood plasma, lymph, tissue fluid, and the specialized fluids such as cerebrospinal fluid, synovial fluid, aque-ous humor, and serous fluid. Water constantly moves from one fluid site in the body to another by the processes of filtration and osmosis. These fluid sites are called  water compart-ments  (Fig. 19–1). The chambers of the heart and all   of the blood vessels form one compartment, and the water within is called plasma. By the process of filtra-tion in capillaries, some plasma is forced out into tis-sue spaces (another compartment) and is then called tissue fluid. When tissue fluid enters cells by the process of osmosis, it has moved to still another com-partment and is called intracellular fl...

  WATER INTAKE AND OUTPUT   Most of the water the body requires comes from the ingestion of liquids; this amount averages 1600 mL per day. The food we eat also contains water. Even foods we think of as somewhat dry, such as bread, contain significant amounts of water. The daily water total from food averages 700 mL. The last source of water, about 200 mL per day, is the metabolic water that is a product of cell respiration. The total intake of water per day, therefore, is about 2500 mL, or 2.5 liters.   Most of the water lost from the body is in the form of urine produced by the kidneys; this averages 1500 mL per day. About 500 mL per day is lost in the form of sweat, another 300 mL per day is in the form of water vapor in exhaled air, and another 200 mL per day is lost in feces. The total output of water is thus about 2500 mL per day.   Naturally, any increase in water output must be compensated for by an increase in intake. Someone who exercises strenuously, for ex...

  ELECTROLYTES   Electrolytes  are chemicals that dissolve in water and   dissociate into their positive and negative ions. Most electrolytes are the inorganic salts, acids, and bases found in all body fluids.   Most organic compounds are non-electrolytes; that is, they do not ionize when in solution. Glucose, for example, dissolves in water but does not ionize; it remains as intact glucose molecules. Some proteins, however, do form ionic bonds and when in solution dissociate into ions.   Positive ions are called  cations . Examples are Na + , K + , Ca 1 2 , Mg 1 2 , Fe 1 2 , and H 1 . Negative ions are called  anions , and examples are Cl 2 , HCO 3 2 , SO 4 2 2  (sulfate), HPO 4 2 2  (phosphate), and protein anions. Electrolytes help create the osmolarity of body flu- ids and, therefore, help regulate the osmosis of water between water compartments. Some electrolytes are involved in acid–base regulatory mechanisms, or they are part of s...

  ACID-BASE BALANCE   You have already learned quite a bit about the regula-tion of the pH of body fluids in the chapters on chemistry, the respiratory system, and the urinary system. In this section, we will put all of that information together.   The normal pH range of blood is 7.35 to 7.45. The pH of tissue fluid is similar but can vary slightly above or below this range. The intracellular fluid has a pH range of 6.8 to 7.0. Notice that these ranges of pH are quite narrow; they must be maintained in order for enzymatic reactions and other processes to proceed normally. Maintenance of acid–base homeostasis is accom-plished by the buffer systems in body fluids, the rate and depth of respiration, and the kidneys. BUFFER SYSTEMS   The purpose of a  buffer system  is to prevent drastic changes in the pH of body fluids by chemically react-ing with strong acids or bases that would otherwise greatly change the pH. A buffer system consists of a weak acid and a we...