Chapter 3: Water Balance, Osmolality, Electrolytes, pH, and Blood Gases PDF

# Chapter 3: Water Balance, Osmolality, Electrolytes, pH, and Blood Gases ## Definitions - **Electrolyte**: Any of the four ions in plasma (Na+, K+, Cl-, and HCO3-) that exert the greatest influence upon water balance and acid-base relationships. - **Internal environment**: The concentration of io...

# Chapter 3: Water Balance, Osmolality, Electrolytes, pH, and Blood Gases ## Definitions - **Electrolyte**: Any of the four ions in plasma (Na+, K+, Cl-, and HCO3-) that exert the greatest influence upon water balance and acid-base relationships. - **Internal environment**: The concentration of ions and other constituents in body fluids, including H+ and pH. - **Homeostasis**: The maintenance of a steady state in the body, with a relatively constant concentration of ions, pH, and osmotic pressure in the various body fluids. - **Osmolality**: A measure of the number of dissolved particles (ions and undissociated molecules) per unit weight of water. - **Osmotic pressure**: The hydrostatic pressure that would have to be applied to the compartment of higher osmolality to prevent water passage into it from the lower compartment. - **Active transport**: The transfer of ions or molecules across a cell membrane against a concentration gradient, requiring energy in the form of ATP. ## Body Water - The protoplasm of multicellular organisms contains a high percentage of water. - Water is found both inside and outside cells, and can be categorized as: - **Intracellular water**: Within cells. - **Extracellular water**: Outside of cells. - **Plasma**: Fluid contained in blood vessels. - **Interstitial fluid**: Fluid between cells. - Body water, with its dissolved salts and other substances, constitutes the internal environment. ## Water Compartments - The content of body water is about 60% of total body weight. - Approximately 67% of the water is intracellular and 33% is extracellular. ## Water Balance - An adult drinks and takes in with his food about 2500 ml of water daily. - The same amount is lost daily in urine, feces, sweat, and expired air. - Individuals remain in water balance by regulating the intake of fluids to compensate for daily losses. ## Electrolyte Distribution - The concentration of electrolytes in extracellular and intracellular water differs significantly, as shown in Table 3.3 - **Extracellular Fluid:** - Na+ is the principal cation, comprising over 90% of the total. - Cl- and HCO3- are the predominant anions. - **Intracellular Fluid:** - K+ is the principal cation, comprising 77% of the total. - Phosphate is the predominant anion. - Differences in ionic concentration are maintained by active transport mechanisms. - The osmotic pressure of extracellular fluids is determined by the concentration of Na+. ## Plasma Volume - The blood plasma is a subdivision of the extracellular fluid compartment. - Its volume is determined by the concentration of Na+. ## Correction of Disturbed Osmolality - The body responds to disturbances in osmolality by changing water intake and excretion. - **Dehydration(high osmolality)**: - Promotes the feeling of thirst, stimulating the posterior pituitary secretion of ADH, leading to increased water reabsorption and reduced urine output. - **Overhydration(low osmolality)**: - Inhibits the thirst mechanism and ADH secretion, leading to increased urine output and a large volume of dilute urine. ## Sodium - The body of the average-sized adult contains about 80 g of sodium. - 35 g of sodium are present in the extracellular fluids. - The body content of sodium is relatively constant, despite variation in intake. ## Potassium - Potassium is the cation having the highest concentration within cells. - Approximately 30 times higher than in the extracellular fluids. - Potassium salts in the diet are absorbed rapidly from the intestinal lumen but have little effect upon the plasma concentration. ## Chloride - Chloride is the extracellular anion in the highest concentration in serum. - Plays an important role in maintaining electrolyte balance, hydration, and osmotic pressure. ## Acid-Base Homeostasis and Blood Buffer Systems - **Acid-base homeostasis** refers to the maintenance of a stable pH in the body fluids. - **Buffer systems** are essential in maintaining a stable pH. - The principal body buffer systems are: - **Bicarbonate/Carbonic Acid Buffer System:** - The most important buffer system in plasma. - Can be influenced by pulmonary ventilation. - **Hemoglobin Buffer System:** - Plays a significant role in red blood cells. - **Phosphate Buffer System:** - A minor component of the total blood buffer system, but is important in the elimination of H+ in the urine. ## Blood Buffer Systems - When fats, carbohydrates, and proteins are catabolized for energy purposes, the carbon atoms in the molecules are converted toCO2 if the oxidation is complete. - This CO2 forms a weak acid, H2CO3, when dissolved in water. - The reaction is reversed in the lung alveoli, and the CO2 is rapidly eliminated by the lungs during respiration. - Incomplete oxidation of metabolites, however, can lead to the formation of nonvolatile acids, which requires the kidney to eliminate about 50 mmoles of acid daily. - The kidneys are responsible for the excretion of nonvolatile acids, and the lungs are responsible for the elimination of HCO3. ## Effects of CO2, HCO3, and H2CO3 on pH - The pH of plasma depends upon the ratio [HCO3]/[H2CO3]. - This can be calculated using the Henderson-Hasselbach equation. ## Compensation of Acid-Base Disturbances - Disturbances in blood pH are usually compensated to a greater or lesser extent by appropriate responses of the respiratory and renal systems. - **Metabolic acidosis:** - Can be compensated by increased pulmonary ventilation, which reduces the [H2CO3] and increases the [HCO3]/[H2CO3] ratio, raising the pH. - The kidneys may also compensate by increasing the rate of formation of NH3, and excreting NH3 salts in the urine. - **Metabolic alkalosis:** - Can be compensated by renal excretion of HCO3 and depression of the respiration rate, which decreases the [HCO3]/[H2CO3] ratio and lowers the pH. ## CO2 Content - The CO2 content consists of the sum of the concentrations of dissolved CO2, undissociated H2CO3, and carbamino-bound CO2. - Plasma or serum is usually taken for the determination of CO2 content. - The CO2 content of plasma or serum is measured by automated, continuous-flow colorimetric methods or by ion-selective electrodes. ## Blood Oxygen Content and Oxygen Saturation - The oxygen content and saturation of whole blood may be measured by several different techniques, including gasometrically, spectrophotometrically, or polarographically. ## Blood Gases: pH, pCO2, and pO2 - The plasma CO2 content is intimately involved in the plasma pH. - The lungs play a critical role in the elimination of CO2. - The average person produces and exhales approximately 20,000 mmoles of CO2 daily. ## Blood Oxygen Content and Oxygen Saturation - Although it is not part of the "blood gas package," oxygen content and oxygen saturation values are sometimes requested. - They are often utilized in heart catheterization, where information is acquired regarding the location of the catheter tip. ## Acid-Base Homeostasis and Blood Buffer Systems - A review of some definitions would be of benefit before proceeding with *acid-base homeostasis*: - **Buffer base (BB)**: The sum of the concentrations of buffer anions present in whole blood. - **Base excess (BE)**: The deviation of buffer base from the normal. - **Oxygen tension (pO2)**: The partial pressure of oxygen in blood. - **Oxygen content**: The volume occupied by the oxygen bound to hemoglobin plus that dissolved in 100 ml of whole blood, when completely liberated. - **Oxygen saturation (capacity)**: The actual O2 content of blood, expressed as a percentage of the O2 content of the same blood when fully oxygenated. ## Blood Buffer Systems - The carbon atoms in fats, carbohydrates, and proteins are converted to CO2 if the oxidation is complete. - The reaction is reversed in the lung alveoli, and the CO2 is rapidly eliminated by the lungs during respiration. - Incomplete oxidation of products can lead to the formation of nonvolatile acids. - These acids must be excreted by the kidney. - The kidneys are responsible for the excretion of nonvolatile acids, and the lungs are responsible for the elimination of HCO3. ## Effects of CO2, HCO3, and H2CO3 on pH - CO2 is the ever-present product of oxidative metabolism. - It is also the source of the formation of plasma HCO3. - The pH of plasma depends upon the ratio [HCO3]/[H2CO3], which can be calculated using the Henderson-Hasselbalch equation. ## Compensation of Acid-Base Disturbances - Disturbances in blood pH are compensated to a greater or lesser extent by the respiratory and renal systems. - **Metabolic acidosis:** - Can be compensated by increased pulmonary ventilation and renal excretion of NH3 salts. - **Metabolic alkalosis:** - Can be compensated by renal excretion of HCO3 and depression of the respiration rate. ## CO2 Content - The CO2 content consists of the sum of the concentrations of dissolved CO2, undissociated H2CO3, and carbamino-bound CO2. - Plasma or serum is usually taken for the determination of CO2 content, as it is not affected by factors such as erythrocyte count or degree of oxygen saturation. ## Blood Oxygen Content and Oxygen Saturation - Oxygen content and oxygen saturation values are often acquired in heart catheterization. - Information is gathered regarding the location of the catheter tip. ## Blood Gases: pH, pCO2, and pO2 - The plasma CO2 content is intimately involved in the plasma pH. - The lungs play a critical role in the elimination of CO2. - The average person produces and exhales approximately 20,000 mmoles of CO2 daily. ## Blood Oxygen Content and Oxygen Saturation - Although it is not part of the "blood gas package," oxygen content and oxygen saturation values are sometimes requested. - They are often utilized in heart catheterization, where information is gathered regarding the location of the catheter tip. ## Table 3.1: Distribution of Body Water in the Adult | Compartment | Percent of Body Weight | Volume (L) | Percent of Total Body Water | |---|---|---|---| | Extracellular | 5.0 | 3.5 | 8 | | Plasma | 15.0 | 10.5 | 25 | | Interstitial | 40.0 | 28 | 67 | | Intracellular | 60.0 | 42 | 100 | ## Table 3.2: Daily Water Loss | Site | Vol/Day (ml) | |---|---| | Skin | 500 | | Expired Air | 350 | | Urine | 1500 | | Feces | 150 | | **Total** | **2500** | ## Table 3.3: Concentration of Cations and Anions in Extracellular and Intracellular Water | **Cation** | **Extracellular** | **Intracellular** | **Anion** | **Extracellular** | **Intracellular** | |---|---|---|---|---|---| | Na+ | 154 meq/L | 15 meq/L | HCO3- | 29 meq/L | 10 meq/L | | K+ | 5 meq/L | 150 meq/L | Cl- | 111 meq/L | 1 meq/L | | Ca2+ | 5.4 meq/L | 27 meq/L | HPO4- | 2 meq/L | 100 meq/L | | Mg2+ | 2.6 meq/L | 2 meq/L | SO4- | 1 meq/L | 20 meq/L | | | | | Organic Acid | 7 meq/L | 63 meq/L | | | | | Protein | 17 meq/L | 33 meq/L | | **Total** | **167 meq/L** | **194 meq/L** | **Total** | **167 meq/L** | **194 meq/L** | ## Table 3.4: Changes in Acid-Base Parameters of Whole Arterial Blood in Acid-Base Imbalances | Pathological State | pH (Uncompensated/Compensated) | pCO2 (Uncompensated/Compensated) | [HCO3] (Uncompensated/Compensated) | [H2CO3] (Uncompensated/Compensated) | |---|---|---|---|---| | Metabolic Alkalosis | ↑/↑ | ↑/↑ | ↑/↑ | ↑/↑ | | Respiratory Alkalosis | ↑/↑ | -/↓ | -/↓ | -/↓ | | Metabolic Acidosis | ↓/↓ | ↓/↓ | ↓/↓ | ↓/↓ | | Respiratory Acidosis | ↓/↓ | ↑/↑ | -/↑ | ↑/↑ | ## Table 3.5: Normal Values (38°C) of Acid-Base and Blood Gas Constituents | Constituent | Arterial or Capillary Whole Blood | Venous Plasma (separated at 30°C) | |---|---|---| | pH | 7.37-7.44 | 7.35-7.45 | | pCO2 (men) | 34-45 mm Hg | 36-50 mm Hg | | pCO2 (women) | 31-42 mm Hg | 34-48 mm Hg | | CO2 Content (Plasma) (men) | 24-30 mmol/L | 26-31 mmol/L | | CO2 Content (Plasma) (women) | 21-30 mmol/L | 24-29 mmol/L | | pO2 | 85-95 mm Hg | 30-50 mm Hg | | O2 Saturation | 94-97% | 50-70% | | O2 Content | 20 ml/dl | 15 ml/dl | ## Figure 3.1: Transport of electrons and H+ in the respiratory chain in mitochondria. - Reduced substrates such as lactate and malate are oxidized to pyruvate and oxaloacetate, respectively. ## Figure 3.2: Siggaard-Andersen Alignment Nomogram - By knowing the concentration of any two constituents of the following three - pCO2, pH, HCO3 (or CO2 content) - one can obtain the third by laying a straight edge on the two values and reading the third from the proper column. ## Figure 3.3: Calibration curve for oxygen saturation - Prepared by blowing nitrogen over a thin film of blood, to obtain zero percent oxygen saturation. - 100% saturated blood is prepared by exposing a thin film of blood to atmospheric air. - The absorbances of both bloods are measured at 650 and 805 nm, and the absorbance ratio 650/805 is calculated. - The absorbance ratio is plotted as ordinate against the percentage of O2 saturation as abscissa