Physiology of Body Fluid & Water Balance Lecture 1 PDF

Faculty of Medicine Academic Year: 2024-2025 Year: 1 Semester: 1 Module: BLOOD and body fluids (blf) 103  Body fluid and water balance By: Prof. Dr. Mohamed Hassan Abdelsattar Professor – Faculty of Medicine – Al-Azhar University Department:...

Faculty of Medicine Academic Year: 2024-2025 Year: 1 Semester: 1 Module: BLOOD and body fluids (blf) 103  Body fluid and water balance By: Prof. Dr. Mohamed Hassan Abdelsattar Professor – Faculty of Medicine – Al-Azhar University Department: Medical Physiology 29/12/2024 10/11/2023 HBF - 102 22  Objectives By the end of this lecture the student should be able to: 1. Identify the distribution of water in the body. 2. Compare and contrast between the ECF and ICF. 3. Identify the fluid exchange between ECF and ICF 4. Determine of Volumes of Specific Body Fluid Compartments. 29/12/2024 BLF - 103 33  Objectives By the end of this lecture the student should be able to: 5. List the water gain & water loss 6. Describe the control of water balance 7. Describe the forces involved in the formation & drainage of the tissue fluid. 29/12/2024 BLF - 103 44  Introduction In young adult male: body water constitutes 60 % of body weight (42 L in a 70 kg) This percentage can change, depending on: age, sex & obesity: 1. In old age: due to ↑ fat → ↓ % of water in the body. 2. Sex: Because women normally have more body fat than men, they contain slightly less water (51%). 3. In obese persons: body water constitutes 45%. The body fat is relatively free of water 29/12/2024 BLF - 103 55  INTRODUCTION body fluids 60 % of body wt (42 L in a 70 kg) 2/3 Intracellular 1/3 Extracellular 28 L (40 % of body wt) 14 L (20 %) Extravascular fluid Intravascular (plasma) 11 L (15 %) 3 L (5 %) Interstitial fluid (tissue fluid) Transcellular fluids 29/12/2024 BLF - 103 6  Introduction The body fluids are distributed as follows: 1) Intracellular (ICF): Constitutes about 2/3 of total body water (TBW). About 28 L (40 % of body wt) 2) Extracellular (ECF): Constitutes about 1/3 of TBW. About 14 L (20 % of body wt) a) Intravascular (plasma): 3 L (about 5 % of body wt) 29/12/2024 BLF - 103 77  Introduction b) Extravascular fluid: 11 L (15 % of body wt). It is includes: i. Interstitial fluid (ISF, tissue fluid). ii. Transcellular fluids: e.g. cerebrospinal fluid, intraocular fluid & fluids in the joints, pleura, peritoneum, etc  Then, the blood contains both ECF (plasma) & ICF (the fluid in the RBCs).  The blood volume is 5 L in adult (about 8 % of body wt) 29/12/2024 BLF - 103 88  Introduction (5 %) (20 %) (15 %) TOTAL BODY WATER 60 % of body wt (42 L in a 70 kg) (40 %) 29/12/2024 BLF - 103 99  Composition of the ECF and ICF The main cation the main anion 142 106 24 4 14 10 The main anions 140 40-45 The main cation 29/12/2024 BLF - 103 10  COMPOSITION OF THE ECF and ICF A. ECF  The composition of the ECF is almost the same elsewhere, EXCEPT for the protein concentration which is much higher in the plasma (17 mEq/L) than in the interstitial fluid (5 mEq/L).  The main cation is Na+ (142 mEq/L) while the main anion is Cl- (106 mEq/L). 29/12/2024 BLF - 103 11  COMPOSITION OF THE ECF and ICF  Other cations K+ (4 mEq/L) and small amount of Ca++ and Mg++.  Other anions: HCO3- (24 mEq/L), proteins and small amounts of HPO4-- and SO4--.  ECF also contains non-electrolytes such as glucose, cholesterol, urea, uric acid, creatinine, bile pigments and phospholipids.  pH: is 7.4 and its osmolality is about 300 mOsm / L. 29/12/2024 BLF - 103 12  B. ICF  The main cations: K+ (140 mEq /L) and Mg++ (58 mEq/L) together with a small amount of Na+ (14 mEq /L) and very little Ca++.  The main anions: HPO4- and protein (40-45 mEq/L) together with small amounts of Cl-, HCO3- and SO4--.  pH: is less than that ECF (about 7) due to its low HCO3- content, while its total osmolality is equal to that of the EC 29/12/2024 BLF - 103 13  Composition of the ECF and ICF 29/12/2024 BLF - 103 14  FLUID EXCHANGE BETWEEN ECF and ICF Normally, fluid exchanges continuously across the cell membranes → an equal osmolality in both ECF & ICF: 1) If i.v. injection of a hypertonic NaCI solution → ↑ ECF osmolality → water transfer from ICF to ECF till an osmotic equilibrium is established and the osmolality of both fluids become equal again → ↓ ICF volume, so the cells are dehydrated and shrink. 29/12/2024 BLF - 103 15  FLUID EXCHANGE BETWEEN ECF and ICF ↑ ECF ↑ ECF osmolality ↓ ECF osmolality water transfer from ICF water transfer ↑ ICF volume ↓ ICF volume ↑ ECF to ECF ↑ ECF from ECF to ICF Total body water 29/12/2024 BLF - 103 16  FLUID EXCHANGE BETWEEN ECF and ICF 2) If i.v. injection of a hypotonic NaCl solution (or water) → ↓ ECF osmolality → water transfer from the ECF to the ICF till an osmotic equilibrium is established and the osmolality of both fluids become equal again → ↑ ICF volume, so the cells are swollen and may rupture. 29/12/2024 BLF - 103 17  FLUID EXCHANGE BETWEEN ECF and ICF 3) Injection of isotonic solutions mainly ↑ ECF volume (resulting in edema), but they do not affect the osmolality of the body fluids. 29/12/2024 BLF - 103 18  Measurement of Fluid Volumes in the Different Body Fluid Compartments Measurement of Total Body Water Using the indicator dilution principle as follows: 1) A known amount of an indicator substance is i.v. injected. The used indicator should penetrate the cell membranes to be dispersed in both the ECF and ICF 29/12/2024 BLF - 103 19  Measurement of Total Body Water The commonly used indicators are (a) Deuterium oxide (D2O or heavy water) (b) Tritium oxide (3H2O) (c) Aminopyrine. 29/12/2024 BLF - 103 20  Measurement of Total Body Water 2) A sufficient time is allowed for complete distribution of the indicator in the TBW. Then its concentration in the plasma is determined 3) TBW volume is calculated by dividing the injected amount of the indicator by its concentration in the plasma. 29/12/2024 BLF - 103 21  MEASUREMENT OF THE ECF VOLUME  Using the indicator dilution principle.  However, the indicator used should NOT penetrate the cell membranes (to be dispersed in the ECF only).  The common indicators used: are inulin and mannitol. 29/12/2024 BLF - 103 22  MEASUREMENT OF THE ICF VOLUME The ICF volume cannot be measured directly. The TBW and ECF volumes should be measured first. ICF volume = TBW volume - ECF volume. 29/12/2024 BLF - 103 23  MEASUREMENT OF THE INTERSTITIAL FLUID VOLUME The interstitial fluid volume cannot also be measured directly. The ECF and plasma volumes should be measured first. ISF volume = ECF volume - Plasma volume. 29/12/2024 BLF - 103 24  Measurement of Plasma Volume The plasma volume can be measured by 2 methods: 1) DYE METHOD:  A known amount of a dye e.g. Evans blue is i.v. injected, and after 5-10 min (allows uniform dispersion of the dye in the plasma), the concentration of the dye in the plasma is determined. 29/12/2024 BLF - 103 25  Measurement of Plasma Volume The plasma volume can be determined. e.g. If the amount of the injected dye was 30 mg, and its plasma concentration after mixing was 0.01 mg/ml, Then the plasma volume = 30/0.01 = 3000 ml. 29/12/2024 BLF - 103 26  Measurement of Plasma Volume Characteristics of Evans blue: a) It is non- toxic and is not metabolized in the body. b) It combines with the plasma proteins. so it does not rapidly escape into the tissue spaces (and is also not rapidly excreted by the kidneys). c) It is not adsorbed to the red cells and does not enter or hemolyze them. d) Its plasma level is easily measured by the spectrophotometer 29/12/2024 BLF - 103 27  Measurement of Plasma Volume 2) ISOTOPE METHOD : is more accurate than the dye method. A substance must be used that does NOT readily penetrate capillary membranes but remains in the vascular system after injection.  The most commonly used substances is serum albumin labeled with radioactive iodine (125I-albumin). 29/12/2024 BLF - 103 28  Water balance The body water remains within normal range through a balance between water gain & water loss. Daily Intake of Water (2300 ml/d) 1) Exogenous (by oral route, 2100 ml/d): is the main source of water gain. it includes: a) Water and other fluids: that are drunk (1500 ml/d). b) Water present in the eaten food (600 ml/d). 29/12/2024 BLF - 103 29  Water balance 1) Endogenous (metabolic H2O, 200 ml/d): which is formed inside the body as a result of oxidation of H2 ion during metabolic reactions. However, is highly variable among different people depending on climate, habits, and level of physical activity. 29/12/2024 BLF - 103 30  Daily Loss of Body Water (2300 ml/d) In the comfortable zone of atmospheric temperature (20 °C), normally lose of water as follows : 1) Urine (1400 ml/d): ↑ when H2O intake is ↑ or in winter.  Urine volume can be: a. In a dehydrated person: as low as 0.5 L/day or b. In a person who has been drinking tremendous amounts of water: as high as 20 L/day. 29/12/2024 BLF - 103 31  Daily Loss of Body Water 2- Skin: a) Insensible perspiration (350 ml/d): loss of H2O by diffusion through the skin. it is not a sweat secretion. b) Sweating (100 ml/d): with normal temp. it may reach 2 L/h in very hot weather. 3- Expired air (350 ml/d): are loss through evaporation. 4- Feces (100 ml/d): ↑ with sever diarrhea. 29/12/2024 BLF - 103 32  Daily intake and Loss of Water 29/12/2024 BLF - 103 33  CONTROL OF WATER BALANCE (A)CONTROL OF WATER GAIN: mainly by the thirst sensation. Thirst: is an organic sensation indicating that the body needs H2O and derives the person to drink. Mechanism of thirst: A. Central mechanism: It is produced due to stimulation of thirst center in the hypothalamus. This occurs if: 29/12/2024 BLF - 103 34  CONTROL OF WATER BALANCE a) ↓ ECF or plasma volumes (hypovolemia): in which the thirst center is stimulated by angiotensin II and certain cardiovascular reflexes. b) ↑ Osmolality (tonicity) of these fluids (hypertonicity), in which the thirst centre is stimulated directly. 29/12/2024 BLF - 103 35  CONTROL OF WATER BALANCE B. Peripheral mechanism: Thirst sensation is felt as dryness of the mouth and throat. It is mediated by afferents in the glossopharyngeal and vagus nerves. 29/12/2024 BLF - 103 36  CONTROL OF THIRST Increase thirst Decrease thirst ↑ Osmolality ↓ Osmolality Hypovolemia Hypervolemia ↓ Blood pressure ↑ Blood pressure ↑ Angiotensin II ↓ Angiotensin II Dryness of the mouth 29/12/2024 BLF - 103 37  (B) CONTROL OF WATER LOSS  Mainly by controlling the urine volume, and this is largely determined by the antidiuretic hormone (ADH).  Site of release: posterior pituitary gland  Main action: ↑ water reabsorption from the distal segments of the renal tubules. 29/12/2024 BLF - 103 38  (B) CONTROL OF WATER LOSS  Stimulus of ADH: hypovolemia and hypertonicity → ↑ water retention in the body and decreased water excretion in the urine).  ADH is inhibited in: hypervolemia and hypotonicity →↓ water retention in the body and ↑ water excretion in the urine. 29/12/2024 BLF - 103 39  FORMATION & DRAINAGE OF THE TISSUE (INTERSTITIAL) FLUID The ISF is continuously formed and drained by the capillaries, Composition: it con­tains almost the same constituents of the plasma EXCEPT the plasma proteins (which are minimally filtered because of their large molecular sizes). 29/12/2024 BLF - 103 40 Forces Influencing Fluid Movement across the Capillary Wall 4 Forces involved in the formation and drainage of the tissue fluid: 1. The Capillary Blood Pressure (filtering force). 2. Plasma Osmotic Pressure (absorbing force). 3. Interstitial Fluid Hydrostatic Pressure. 4. Interstitial Fluid Osmotic Pressure. 29/12/2024 BLF - 103 41 Mechanism of tissue fluid formation 29/12/2024 BLF - 103 42 Mechanism of tissue fluid formation A. At the arterial end of the capillary: The Capillary Blood Pressure is 35 mmHg. It is higher than the osmotic pressure (OP) of plasma proteins (PP, 25 mmHg )→ Fluids pass from blood to tissue spaces → filtration of 1000 cc/h, containing O2 & nutrients) 29/12/2024 BLF - 103 43 Mechanism of tissue fluid formation B. At the venous end of the capillary: The Capillary Blood Pressure is 15 mmHg which is less than OP of plasma proteins (25 mmHg) → Fluids pass from tissue spaces to blood → reabsorption of 900 cc/h, containing CO2 & waste products) 100 cc H2O/ h are removed by lymph vessels 29/12/2024 BLF - 103 44 Mechanism of tissue fluid formation  If the plasma volume increases (e.g. in cases of hydration):  ↑ hydrostatic pressure and ↓ osmotic pressure → the excess fluid is filtered in the tissue spaces. 29/12/2024 BLF - 103 45 Mechanism of tissue fluid formation If the plasma volume decreases (e.g. in cases or dehydration): → ↑ Osmotic pressure and ↓ hydrostatic pressure → fluids are withdrawn from the tissue spaces into the bloodstream. Thus. the plasma volume is kept constant in both conditions. 29/12/2024 BLF - 103 46  Interactive Question Which of the diagrams ↑ ECF represents the changes (after osmotic equilibrium) ↑ ICF ↑ ECF ↓ ICF ↑ ECF in ECF and ICF volumes and osmolarities after the ↓ ICF ↓ECF infusion of hypotonic fluid? 29/12/2024 BLF- 103 47 47  Interactive Question Which of the diagrams ↑ ECF represents the changes (after osmotic equilibrium) ↑ ICF ↑ ECF ↓ ICF ↑ ECF in ECF and ICF volumes and osmolarities after the ↓ ICF ↓ECF infusion of 3% NaCl? 29/12/2024 BLF- 103 48 48  Summary The body fluids are distributed as ICF & ECF. Measurement of fluid volumes by Indicator-dilution method The body water remains within normal range through a balance between water gain & water loss. Control of water gain mainly by the thirst sensation and control of water loss mainly by controlling the urine volume. Mechanism of tissue fluid formation regulated by different forces. 29/12/2024 BLF - 103 49 49  References 1) GUYTON AND HALL: TEEXT BOOK OF MEDICAL PHYSIOLOGY: 14th. Edition; Philadelphia: Elsevier, Inc.,2020. 2) Staff Members of Physiology Department: Medical physiology books; Faculty of Medicine – Cairo University, 2016. 3) Barerett KE et al.: Ganong's Review of Medical Physiology: 26th. Edition; New York: The McGraw- Hill companies, 2019. 4) Linda S. Costanzo, : BRS Physiology Seventh Edition. 29/12/2024 BLF - 103 50 50