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Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Balance Concept • Internal pool – the quantity of any particular substance in the ECF • If quantity is to remain stable within the body – Input must be balanced with output •Ingestion •Metabolic consumption •Excretion •Metabolic consumption Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Balance Concept • Input must equal output to maintain a stable balance in ECT. – Positive balance exists when input exceeds output – Negative balance exists when output exceeds input – Input • Input of substances into plasma is poorly controlled or not controlled • Eating habits are variable – Output • Compensatory adjustments usually occur on output side by urinary excretion Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Balance Concept Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Fluid Balance • Water – Most abundant substance in body – Amount varies in different kinds of tissues – Content remains fairly constant within an individual • Minor ECF components – Lymph – Transcellular fluid • • • • • Cerebrospinal fluid Intraocular fluid Synovial fluid Pericardial, intrapleural, and peritoneal fluids Digestive juices Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Classification of Body Fluids Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Barriers Separating Body-Fluid Compartments • Barrier between plasma and interstitial fluid – Blood vessel walls • Barrier between ECF and ICF – Cellular plasma membranes – Major differences between ECF and ICF • Presence of cell proteins in ICF that cannot permeate the cell membrane to leave the cells • Unequal distribution of Na+ and K+ and their attendant ions as a result of the action of the membrane-bound Na+ - K+ ATPase pump present in all cells Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Ionic Composition of the Major Body-Fluid Compartments Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Fluid Balance • ECF serves as an intermediary between the cells and external environment • Two factors are regulated to maintain fluid balance in the body – ECF volume must be closely regulated to help maintain blood pressure • Maintaining salt balance is very important in long-term regulation of ECF volume – ECF osmolarity must be closely regulated to prevent swelling or shrinking of cells • Maintaining water balance is very important in regulating ECF osmolarity Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Salt Balance • Very important in regulating ECF volume • Salt input occurs by ingestion – Often not well controlled • Salt balance maintained by outputs in urine – Salt also lost in perspiration and in feces • Kidneys keep salt constant in ECF – Glomerular filtration rate (GFR) – Tubular reabsorption of sodium Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Daily Salt Balance Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Osmolarity • Measure of the concentration of individual solute particles dissolved in a fluid • Circumstances that result in a loss or gain of free H2O lead to changes in ECF osmolarity – Deficit of free water in ECF • Osmolarity becomes hypertonic • Often associated with dehydration – Excess of free water in ECF • Osmolarity becomes hypotonic • Usually associated with overhydration Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Osmolarity • Hypertonicity – Cells tend to shrink – Causes • Insufficient water intake • Excessive water loss • Diabetes insipidus – Symptoms and effects • Shrinking of brain neurons – Confusion, irritability, delirium, convulsions, coma • Circulatory disturbances – Reduction in plasma volume, lowering of blood pressure, circulatory shock • Dry skin, sunken eyeballs, dry tongue Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Osmolarity • Hypotonicity – Cells tend to swell – Causes • Patients with renal failure who cannot excrete a dilute urine become hypotonic when they consume more water than solutes • Can occur in healthy people when water is rapidly ingested and kidney’s do not respond quickly enough • When excess water is retained in body due to inappropriate secretion of vasopressin – Symptoms and effects • Swelling of brain cells – Confusion, irritability, lethargy, headache, dizziness, vomiting, drowsiness, convulsions, coma, death • Weakness (due to swelling of muscle cells) • Circulatory disturbances (hypertension and edema) • Water intoxication Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning H2O Input and Output • In order to maintain stable water balance, water input must equal water output. • Input – Drinking liquids – Eating solid foods – Metabolically produced water • Output – Insensible loss • Lungs • Nonsweating skin – Sensible loss • Sweating • Feces • Urine excretion Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Vasopressin • Produced by hypothalamus • Stored in posterior pituitary gland • Released on command from hypothalamus – Also location of thirst center • Hypothalamic osmoreceptors – Located near vasopressin-secreting cells and thirst center – Osmolarity increase → vasopressin secretion and thirst stimulated – Osmolarity decrease → vasopressin secretion decreased and thirst suppressed Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Vasopressin • Left atrial receptors – Monitor pressure of blood flowing through (reflects ECF volume) – Upon detection of major reduction in arterial pressure, receptors stimulate vasopressin secretion and thirst – Upon detection of elevated arterial pressure, vasopressin and thirst are both inhibited • Angiotensin II – Stimulates vasopressin secretion and thirst when renin-angiotensin-aldosterone mechanism is activated to conserve Na+ Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Nonregulatory Factors Not Linked to Vasopressin and Thirst • Regulatory factors that do not link vasopressin and thirst – Dryness of mouth stimulates thirst but not vasopressin • Oral metering – Some animals will rapidly drink only enough H2O to satisfy its H2O deficit – Mechanism is less effective in humans • Nonphysiologic influences on fluid intake – Fluid intake often influenced by habit and sociological factors Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Acid-Base Balance • Refers to precise regulation of free H+ concentration in body fluids • Acids – Group of H+ containing substances that dissociate in solution to release free H+ and anions • Bases – Substance that can combine with free H+ and remove it from solution • pH – Designation used to express the concentration of H+ – pH 7 – neutral – pH less than 7 → acidic – pH greater than 7 → basic Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning pH Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Comparison of pH Values of Common Substances Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Acid-Base Balance • Arterial pH less than 6.8 or greater than 8.0 is not compatible with life • Acidosis – Exists when blood pH falls below 7.35 • Alkalosis – Occurs when blood pH is above 7.45 Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Acid-Base Balance • Consequences of fluctuations in pH – Changes in excitability of nerve and muscle cells – Marked influence on enzyme activity – Changes influence K+ levels in body • Sources of H+ in the body – Carbonic acid formation – Inorganic nutrients produced during breakdown of nutrients – Organic acids resulting from intermediary metabolism Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Lines of Defense Against pH Changes • Chemical buffer systems • Respiratory system • Kidneys Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Chemical Buffer Systems • Minimize changes in pH by binding with or yielding free H+ • First line of defense • Body has four buffer systems – H2CO3-, HCO3- buffer system • Primary ECF buffer for noncarbonic acids – Protein buffer system • Primary ICF buffer; also buffers ECF – Hemoglobin buffer system • Primary buffer against carbonic acid changes – Phosphate buffer system • Important urinary buffer; also buffers ICF Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory System • Second line of defense again changes in pH • Acts at a moderate speed • Regulates pH by controlling rate of CO2 removal Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Kidneys • Third line of defense against change in hydrogen ion concentration • Kidneys require hours to days to compensate for changes in body-fluid pH • Control pH of body fluids by adjusting – H+ excretion – HCO3- excretion – Ammonia secretion Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Acid-Base Imbalances • Can arise from either respiratory dysfunction or metabolic disturbances • Deviations divided into four general categories – Respiratory acidosis – Respiratory alkalosis – Metabolic acidosis – Metabolic alkalosis Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Acidosis • Result of abnormal CO2 retention arising from hypoventilation • Possible causes – Lung disease – Depression of respiratory center by drugs or disease – Nerve or muscle disorders that reduce respiratory muscle activity – Holding breath • Compensations – Chemical buffers immediately take up additional H+ – Kidneys are most important in compensating for respiratory acidosis Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Alkalosis • Primarily due to excessive loss of CO2 from body as result of hyperventilation • Possible causes – Fever – Anxiety – Aspirin poisoning – Physiologic mechanisms at high altitude • Compensations – Chemical buffer systems liberate H+ – If situation continues a few days, kidneys compensate by conserving H+ and excreting more HCO3- Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Metabolic Acidosis • Includes all types of acidosis other than those caused by excess CO2 in body fluids • Causes – Severe diarrhea – Diabetes mellitus – Strenuous exercise – Uremic acidosis • Compensations – Buffers take up extra H+ – Lungs blow off additional H+ generating CO2 – Kidneys excrete more H+ and conserve more HCO3- Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Metabolic Alkalosis • Reduction in plasma pH caused by relative deficiency of noncarbonic acids • Causes – Vomiting – Ingestion of alkaline drugs • Compensations – Chemical buffer systems immediately liberate H+ – Ventilation is reduced – If condition persists for several days, kidneys conserve H+ and excrete excess HCO3- in the urine Chapter 15 Fluid and Acid-Base Balance Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning