Lesson # 22

1. Fluid, Electrolytes, and Acid-Base Balance. Lesson # 22 Chapter 27 Objectives: 1- Explain what is meant by the terms fluid balance, electrolyte balance, and acid-base balance. 2- Identify the hormones that play important roles in fluid and electrolyte regulation. 3- Discuss the mechanisms by which sodium is regulated. 4- Explain the principal buffer systems.

2. The Importance of pH control + + A salt is an ionic compound containing cations other than H and anions other than OH - Electrolytes + + + + + + _ _ _ _ Na O O H Na H Na H H Na Salt Cl Cl Cl Cl + Acid Hydrogen proton An acid is a substance that yields hydrogen protons when put in aqueous solution, decreasing the pH. - + Base Hydroxide group A base is a substance that accepts hydrogen ions when put in aqueous solution increasing the pH.

3. An Introduction to Fluid, Electrolyte, and Acid–Base Balance Fluid Balance • It is a daily balance between: • Amount of water gained • Amount of water lost to environment • The Digestive System is the primary source of water gains, plus a small amount from metabolic activity. • The Urinary System is the primary route of water loss, plus a small amount in sweat. Electrolyte Balance • Electrolytes are ions released through the dissociation of inorganic compounds.They can conduct electrical current in solution. • Electrolyte balance is when the gains and losses of all electrolytes are equal. • Electrolyte balance primarily involves balancing rates of absorption across digestive tract with rates of loss at kidneys and sweat glands.

4. Acid–Base Balance • Acid–Base Balance is when there is a precisely balances production and loss of hydrogen ions. • The body generates acids during normal metabolism, which tend to reduce the pH. CO2 + H2O H2CO3 HCO3- + H+ The kidneys secrete hydrogen ions into urine and generate buffers (bicarbonate) that enter bloodstream, in the distal segments of distal convoluted tubule (DCT) and collecting ducts. Carbon dioxide Water Carbonic acid Bicarbonate ion Hydrogen ion The lungs affect pH balance through elimination of carbon dioxide:

5. Water is the most important constituent of the body. About two thirds (60%) of the human body is water. Ex: If a person weighs 70 kg, about 38.5 kgof his body is water. 60% • Water is 99% of the fluid outside the cells (extracellular fluid). Water 60% 40% • It is an essential ingredient of the cytosol (intracellular fluid). Solids 40% • All cellular operations rely on water. 20% • It is a diffusion medium for gases, nutrients, and waste products.

6. The body composition (by weight, averaged for both sexes) and major body fluid compartments of a 70-kg individual. WATER (38.5 kg; 84.7 lbs) Kg Liters SOLID COMPONENTS 20 (31.5 kg; 69.3 lbs) Other (1%) 15 33% Plasma 4.5% 15 10 Interstitial fluid 21.5% 10 5 5 0 0 Intracellular fluid Extracellular fluid Minerals Carbohydrates Miscellaneous Proteins Lipids

7. The ECF and the ICF • Major Subdivisions of ECF • -Interstitial fluid of peripheral tissues (21.5%). • -Plasma of circulating blood (4.5%). Liters 20 • Minor Subdivisions of ECF (1%) • - Lymph, perilymph, and endolymph. • - Cerebrospinal fluid (CSF). • - Synovial fluid. • - Serous fluids (pleural, pericardial, and peritoneal). • - Aqueous humor. Other (1%) 33% Plasma 4.5% 15 Interstitial fluid 21.5% 10 Water exchange among subdivisions of ECF Water Exchange • Water exchange between ICF and ECF 5 • Across endothelial lining of capillaries. • From interstitial spaces to plasma through lymphatic vessels that drain into the venous system. • Osmosis • Diffusion • Carrier-mediated transport 0 Intracellular fluid Extracellular fluid

8. Water exchange between ICF and ECF • Osmosis • Diffusion • Carrier-mediated transport Extracellular Fluid (ECF) • Carrier-mediated transport • Diffusion • Osmosis Intracellular Fluid (ICF)

9. Water exchange among subdivisions of ECF • Across endothelial lining of capillaries. • From interstitial spaces to plasma through lymphatic vessels that drain into the venous system. • Fluid continually filters from the blood capillaries into the tissue spaces. But the blood capillaries reabsorb only 85%. 15% (2 – 4 L/day) of the water and about half of the plasma proteins are not absorbed by capillaries. • One function of the lymphatic system is to reabsorb this excess and to return it to the blood. 85% 7 mm Hg 85% 15% 13 mm Hg

10. An Overview of the Primary Regulatory Hormones 1- Antidiuretic Hormone (ADH) • Osmoreceptors in hypothalamus monitor osmotic concentra- tionof ECF. • A change in the osmotic concentration of ECF alters the osmoreceptor activity and neurons secrete ADH It is secreted by posterior lobe of pituitary in response to dehydration and rising blood osmolarity. It makes the collecting duct more permeable to water. Water in the tubular fluid reenters the tissue fluid and bloodstream rather than being lost in urine. Consequently, less volume of more concentrated urine is produced. ADH stimulates the hypothalamic thirst center.

11. 2- Aldosterone It is a steroid secreted by the adrenal cortex. • It stimulates the reabsorption of more Na+ and secretion of K+ along the DCT and collecting duct. Water and Cl- follow the Na+(“water follows salt”). The net effect is that the body retains NaCl and water and blood pressure rises. 3- Natriuretic Peptides (ANP and BNP) They are secreted by the myocardium of the heart in response to high blood pressure (it produces an abnormal stretching of heart walls). They reduce thirst and block the release of Aldosterone and ADH, thus increasing the diuresis and decreasing blood volume and pressure.

12. Fluid Movement within the Extracellular Fluid (ECF) • It is a daily balance between the amount of water gained and the amount of water lost to environment. Fluid Balance Water Losses: 1- Insensible perspiration (skin and respiratory tract): 1150 mL 2-Sensible perspiration (sweat glands): Variable 3- Water lost in feces: 150 mL 4- Water lost in urine: 1000 mL Total water lost each day: 2500 mL Water vapor lost in respiration and evaporation from moist surfaces (1150 mL) ECF Water lost in feces (150 mL) ICF Water secreted by sweat glands (variable) Water lost in urine (1000 mL) Plasma membranes

13. Fluid Movement within the Extracellular Fluid (ECF) • It is a daily balance between the amount of water gained and the amount of water lost to environment. Fluid Balance Water Gains: 1- Water gain through eating: 1000 mL 2- Water gain through drinking: 1200 mL 3- Metabolic generation (oxidative phosphorylation in electron transport system): 300 mL Water absorbed across digestive epithelium (2200 mL) Total water gain each day: 2500 mL Water vapor lost in respiration and evaporation from moist surfaces (1150 mL) Metabolic water (300 mL) ECF Water lost in feces (150 mL) ICF Water secreted by sweat glands (variable) Water lost in urine (1000 mL) Plasma membranes

15. Electrolyte Balance Salts + Electrolytes + + _ _ Na A salt is an ionic compound containing anions other than OH- and cations other than H+ Na Salt Cl Cl Changes in the concentration of electrolytes in body fluids will disturb almost every vital function. Ex: Declining potassium levels will lead to a general muscular paralysis. Rising concentrations of potassium will cause weak and irregular heartbeats.

16. Sodium Balance Sodium is the dominant cation in ECF. • Sodium salts provide 90% of the osmotic concentration in the extracellular fluid. • The principal sodium salts in the ECF are sodium chloride (NaCl) and sodium bicarbonate (NaHCO3). • Normal Sodium Concentrations in ECF: about 140 mEq/L. • Normal Sodium Concentrations In ICF: 10 mEq/L or less. • Body water content rises (overhydration) • ECF Na+ concentration <136 mEq/L Hyponatremia • Body water content declines (dehydration) • ECF Na+ concentration >145 mEq/L Hypernatremia • Total amount of sodium in ECF represents a balance between two factors: • Sodium ion uptake across digestive epithelium. • 2. Sodium ion excretion in urine and perspiration.

17. Recall of Fluids ADH Secretion Increases The secretion of ADH restricts water loss and stimulates thirst, promoting additional water consumption. Because the ECF osmolarity increases, water shifts out of the ICF, increasing ECF volume and lowering Na concentrations. Osmoreceptors in hypothalamus stimulated HOMEOSTASIS DISTURBED HOMEOSTASIS RESTORED Decreased Na levels in ECF Increased Na levels in ECF HOMEOSTASIS Normal Na concentration in ECF Start

18. ADH Secretion Decreases HOMEOSTASIS DISTURBED HOMEOSTASIS RESTORED Decreased Na levels in ECF Increased Na levels in ECF Osmoreceptors in hypothalamus inhibited Water loss reduces ECF volume, concentrates ions As soon as the osmotic concentration of the ECF drops by 2 percent or more, ADH secretion decreases, so thirst is suppressed and water losses at the kidneys increase. HOMEOSTASIS Normal Na concentration in ECF Start

19. Potassium Balance • 98% of potassium in the human body is in the intracellular fluid (ICF). • Cells expend energy (in the sodium-potassium pump) to recover potassium ions diffused from cytoplasm into the extracellular fluid (ECF).

20. Potassium Balance depends on two factors: • 1. Rate of gain across digestive epithelium. • 2. Rate of loss into urine. Aldosterone levels affect K+ loss in urine. • Aldosterone stimulates the ion pumps to reabsorb Na+ from filtrate in exchange for K+ from peritubular fluid. • High K+ plasma concentrations stimulate aldosterone. Na+ K+

21. pH - - - - - - - - - + + + + + + + + + Hydrochloric Acid ACIDIC A substance is acidic when contains more hydrogen ions than hydroxide ions. An acid is a substance that dissociates in solution and releases hydrogen ions when put in aqueous solution (proton donor). + H

22. pH HO 2 - - - - - - - - OH OH OH OH OH OH OH OH Sodium Hydroxide (NaOH) BASIC A substance is basic when contains more hydroxide ions than hydrogen ions. + H - - OH OH + - OH - - OH OH A base is a substance that removes hydrogen ions when put in aqueous solution (proton acceptor). - OH

23. H O 2 SOME BASES ACEPT HYDROGEN PROTONS BECAUSE THEY YIELD HYDROXIDE GROUPS, WHICH COMBINE WITH HYDROGEN PROTONS TO FORM WATER MOLECULES: - OH + OTHER BASES DO NOT YIELD HYDROXIDE GROUPS + + + H H H + 2 - Carbonic acid Bicarbonate ion NH H CO HCO NH 3 4 3 + 3 + Ammonia Ammonium ion

24. Protein Buffer System Aminoacid Proteins are long chains of aminoacids Carboxyl or acid group Amino group If If H+ H+ + H+

25. - - + + H CO H CO H CO H CO H H 2 2 3 3 3 3 The Carbonic Acid – Bicarbonate Buffer System - + H CO H CO 2 3 3 CARBONIC ACID BICARBONATE ION Week base Week acid + pH does not change drastically If H+ CARBONIC ACID CARBONIC ACID BICARBONATE ION BICARBONATE ION + pH does not change drastically If H+

26. H2O CO2 H2CO3 H HCO3 H HCO3 H2CO3 H2O CO2     PCO2 40–45 mm Hg pH 7.35–7.45 HOMEOSTASIS If PCO2 rises If PCO2 falls When carbon dioxide levels rise, more carbonic acid forms, additional hydrogen ions and bicarbonate ions are released, and the pH goes down. When the PCO2 falls, the reaction runs in reverse, and carbonic acid dissociates into carbon dioxide and water. This removes H ions from solution and increases the pH. pH PCO2 PCO2 pH

27. The most important buffer system in human organism are: 1- Protein buffer systems. 2- Carbonic acid – bicarbonate buffer system. 3- Phosphate buffer system.