Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Chapter 15 Fluid and Acid-Base Balance

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 H 2 O 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 H 2 O 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 H 2 O to satisfy its H 2 O 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 –H 2 CO 3 -, HCO 3 - 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 CO 2 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 –HCO 3 - 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 CO 2 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 CO 2 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 HCO 3 -

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 CO 2 in body fluids Causes –Severe diarrhea –Diabetes mellitus –Strenuous exercise –Uremic acidosis Compensations –Buffers take up extra H + –Lungs blow off additional H + generating CO 2 –Kidneys excrete more H + and conserve more HCO 3 -

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 HCO 3 - in the urine