Ch. 24 Fluids Acid PDF

Chapter 24 Fluids, Electrolytes & Acid-Base Body Water Content Infants: 73% or more water (low body fat, low bone mass) Adult males: ~60% water Adult females: ~50% water (higher fat content, less skeletal muscle mass) Water content declines to ~45% in old age Body Water Content Fluid Compartments To...

Chapter 24 Fluids, Electrolytes & Acid-Base Body Water Content Infants: 73% or more water (low body fat, low bone mass) Adult males: ~60% water Adult females: ~50% water (higher fat content, less skeletal muscle mass) Water content declines to ~45% in old age Body Water Content Fluid Compartments Total body water = 40 L (60% of body weight) 1. Intracellular fluid (ICF) compartment: 2/3 or 25 L in cell 2. Extracellular fluid (ECF) compartment: 1/3 or 15 L Plasma: 3 L Interstitial fluid (IF): 12 L in spaces between cells Other ECF: lymph, CSF, humors of the eye, synovial fluid, serous fluid, and gastrointestinal secretions Fluid Compartments Fluid Compartments Composition of Body Fluids Water: the universal solvent Solutes: nonelectrolytes and electrolytes – Nonelectrolytes: most are organic Do not dissociate in water: e.g., glucose, lipids, creatinine, and urea – Electrolytes Dissociate into ions in water; e.g., inorganic salts, all acids and bases, and some proteins The most abundant (most numerous) solutes Have greater osmotic power than nonelectrolytes, so may contribute to fluid shifts Determine the chemical and physical reactions of fluids Extracellular and Intracellular Fluids Each fluid compartment has a distinctive pattern of electrolytes ECF – All similar, except higher protein content of plasma Major cation: Na+ Major anion: Cl– ICF: – Low Na+ and Cl– – Major cation: K+ – Major anion HPO42– Electrolyte Concentration Fluid Movement Among Compartments Regulated by osmotic and hydrostatic pressures Water moves freely by osmosis Two-way osmotic flow is substantial Ion fluxes require active transport or channels Change in solute concentration of any compartment leads to net water flow Movement Through Body Fluids Water Balance and ECF Osmolality Water intake = water output = 2500 ml/day Water intake: beverages, food, and metabolic water – Beverages 60% – Foods 30% – Metabolism 10% Water output: urine, insensible water loss (skin and lungs), perspiration, and feces – – – – Urine 60% Skin and lungs 28% Sweat 8% Feces 4% Regulation of Water Intake Thirst mechanism is the driving force for water intake The hypothalamic thirst center osmoreceptors are stimulated by – – – – INCREASE in Plasma osmolality of 2–3% Angiotensin II or baroreceptor input Dry mouth Substantial decrease in blood volume or pressure Plasma Osmolality Regulation of Water Output: Influence of ADH Water reabsorption in collecting ducts due to ADH release –  ADH → dilute urine and  volume of body fluids –  ADH → concentrated urine Factors that may trigger ADH release – large changes in blood volume or pressure – Fever, sweating, vomiting, or diarrhea; blood loss, and traumatic burns Regulation of Water Output: Influence of ADH Regulation of Water Output: Influence of ADH Disorders of Water Balance: Dehydration Negative fluid balance – ECF water loss due to: hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water deprivation, diuretic abuse – Signs and symptoms: thirst, dry flushed skin, oliguria – May lead to weight loss, fever, mental confusion, hypovolemic shock, and loss of electrolytes Disorders of Water Balance: Hypotonic Hydration Cellular over hydration, or water intoxication Occurs with renal insufficiency or rapid excess water ingestion ECF is diluted → low ECF Na⁺ concentration → net osmosis into tissue cells → swelling of cells → severe metabolic disturbances (nausea, vomiting, muscular cramping, cerebral edema) → possible death Disorders of Water Balance: Edema Atypical accumulation of IF fluid → tissue swelling Due to anything that increases flow of fluid out of the blood or hinders its return –  Blood pressure,  Capillary permeability , Incompetent venous valves, Congestive heart failure, hypertension,  blood volume Blocked (or surgically removed) lymph vessels – Cause leaked proteins to accumulate in IF –  Colloid osmotic pressure of IF draws fluid from the blood Electrolyte Balance Electrolytes are salts, acids, and bases Electrolyte balance usually refers only to salt balance Salts enter the body by ingestion and are lost via perspiration, feces, and urine Importance of salts – ECF concentration normally remains stable – Water follows salts – Controlling fluid movements (H₂O) into and out of ICF Central Role of Sodium Most abundant cation in the ECF Na+ leaks into cells and is pumped out against its electrochemical gradient Na+ content may change but ECF Na+ concentration remains stable due to osmosis Central Role of Sodium Changes in plasma sodium levels affect – Plasma volume, blood pressure Renal acid-base control mechanisms are coupled to sodium ion transport Regulation of Sodium Balance – Na+-water balance is linked to blood pressure and blood volume control mechanisms Regulation of Sodium Balance: Aldosterone Na+ reabsorption – 65% is reabsorbed in the proximal convoluted tubules – 25% is reclaimed in the Loop of Henle Aldosterone → active reabsorption of remaining Na+ at the distal convoluted tubules Water follows Na+ Regulation of Sodium Balance: Aldosterone Renin-angiotensin-aldosterone mechanism is the main trigger for aldosterone release – Granular cells of JGA secrete renin in response to decreased blood pressure – Process is increase blood pressure Regulation of Sodium Balance: Aldosterone Renin catalyzes the production of angiotensin II, which prompts aldosterone release from the adrenal cortex Aldosterone release is also triggered by elevated K+ levels in the ECF Aldosterone brings about its effects slowly (hours to days) Na⁺and K⁺ Maintenance Regulation of Sodium Balance: Atrial Natriuretic Peptide (ANP) Released by atrial cells of heart in response to increased blood pressure Decreases blood pressure and blood volume by: – Decreased ADH release – Decreases renin and aldosterone production – Increases excretion of Na+ and water Cardiovascular System Baroreceptors Baroreceptors alert the brain of increases in blood volume and pressure – Sympathetic nervous system impulses to the kidneys decline – Afferent arterioles dilate – GFR increases – Na+ and water output increase Regulation of Potassium Balance Importance of potassium: – Affects resting membrane potential (RMP) in neurons and muscle cells (especially cardiac muscle) Increased extracellular K+ concentration can cause heart arrhythmias Decreased extracellular K+ concentration can cause lethal effects on the heart, muscular weakness, and eventual paralysis H+ shift in and out of cells – Leads to corresponding shifts in K+ in the opposite direction to maintain cation balance – Interferes with activity of excitable cells Regulation of Potassium Balance Influence of aldosterone – Stimulates Na+ reabsorption at the distal convoluted tubules of nephron – Stimulates K+ secretion into collecting ducts – Increased K+ in the adrenal cortex causes Release of aldosterone Potassium secretion Regulation of Calcium Ca2+ in ECF is important for – Nerve excitability – Muscle contraction – Blood clotting Hypocalcemia →  excitability and muscle tetany Hypercalcemia → Inhibits neurons and muscle cells, may cause heart arrhythmias Calcium balance is controlled by parathyroid hormone (PTH) and calcitonin Regulation of Anions Cl– is the major anion in the ECF – Helps maintain the osmotic pressure of the blood – 99% of Cl– is reabsorbed under normal pH conditions When acidosis occurs, fewer chloride ions are reabsorbed Other anions have transport maximums and excesses are excreted in urine Acid-Base Balance pH affects all functional proteins and biochemical reactions Normal pH of body fluids – Arterial blood: pH 7.4 – Venous blood and IF fluid: pH 7.35 – ICF: pH 7.0 Alkalosis or alkalemia: arterial blood pH >7.45 Acidosis or acidemia: arterial pH < 7.35 Physiological acidosis: pH between 7.0 – 7.35 pH Scale Acid-Base Balance Most H+ is produced by metabolism – Phosphoric acid from breakdown of phosphoruscontaining proteins in ECF – Lactic acid from anaerobic respiration of glucose – Fatty acids and ketone bodies from fat metabolism – H+ liberated when CO2 is converted to HCO3– in blood Acid-Base Balance Concentration of hydrogen ions is regulated sequentially by 1. Chemical buffer systems: rapid; first line of defense 2. Respiratory centers: in the brain stem and act within 1–3 min 3. Renal mechanisms: most potent, but require hours to days to effect pH changes Chemical Buffer Systems Chemical buffer: system of one or more compounds that act to resist pH changes when strong acid or base is added Three major chemical buffers in the body 1. Bicarbonate buffer system 2. Phosphate buffer system 3. Protein buffer system Bicarbonate Buffer System Mixture of H2CO3 (weak acid) and its salts, HCO3– (e.g., NaHCO3, a weak base) in the same solution Buffers ICF and ECF The only important ECF buffer Phosphate Buffer System Action is nearly identical to the bicarbonate buffer Components are sodium salts of: – Dihydrogen phosphate (H2PO4–), a weak acid – Monohydrogen phosphate (HPO42–), a weak base Effective buffer in urine and ICF, where phosphate concentrations are high Protein Buffer System Intracellular proteins are the most plentiful and powerful buffers; plasma proteins are also important Protein molecules are amphoteric (can function as both a weak acid and a weak base) – When pH rises, organic acid or carboxyl (COOH) groups release H+ – When pH falls, NH2 groups bind H+ Respiratory and Renal Systems Respiratory and renal systems – Act more slowly than chemical buffer systems – Have more capacity than chemical buffer systems Respiratory Regulation of H+ Respiratory system eliminates CO2 H+ concentration is reduced when CO2 levels are reduced Increase blood CO2 levels activates medullary chemoreceptors Rising plasma H+ activates peripheral chemoreceptors – More CO2 is removed from the blood – H+ concentration is reduced Respiratory Regulation of H+ Alkalosis depresses the respiratory center – Respiratory rate and depth decrease – H+ concentration increases Respiratory system impairment causes acidbase imbalances – Hypoventilation → respiratory acidosis – Hyperventilation → respiratory alkalosis Renal Mechanisms of Acid-Base Balance Most important renal mechanisms – Conserving (reabsorbing) or generating new HCO3– – Excreting HCO3– Generating or reabsorbing one HCO3– is the same as losing one H+ Excreting one HCO3– is the same as gaining one H+ Renal regulation of acid-base balance depends on secretion of H+ Abnormalities of Acid-Base Balance Respiratory acidosis and alkalosis Metabolic acidosis and alkalosis Respiratory Acidosis and Alkalosis The most important indicator of adequacy of respiratory function is PCO2 level (normally 35–45 mm Hg) – PCO2 above 45 mm Hg → respiratory acidosis Most common cause of acid-base imbalances Due to decrease in ventilation or gas exchange Characterized by falling blood pH and rising P CO2 – PCO2 below 35 mm Hg → respiratory alkalosis A common result of hyperventilation due to stress or pain Metabolic Acidosis and Alkalosis Any pH imbalance not caused by abnormal blood CO2 levels Indicated by abnormal HCO3– levels Causes of metabolic acidosis – Ingestion of too much alcohol (→ acetic acid) – Excessive loss of HCO3– (e.g., persistent diarrhea) – Accumulation of lactic acid, shock, ketosis in diabetic crisis, starvation, and kidney failure Metabolic Acidosis and Alkalosis Metabolic alkalosis is much less common than metabolic acidosis – Indicated by rising blood pH and HCO3– – Caused by vomiting of the acid contents of the stomach or by intake of excess base (e.g., antacids) Effects of Acidosis and Alkalosis Blood pH below 7 → depression of CNS → coma → death Blood pH above 7.8 → excitation of nervous system → muscle tetany, extreme nervousness, convulsions, respiratory arrest