Lecture 18: Bodily Fluid Compartments PDF

Lecture 18: Bodily Fluid Compartments Adapted From: Textbook Of Medical Physiology, 13th Ed. Arthur C. Guyton, John E. Hall Chapter 25 John P. Fisher © Copyright 2024, John P. Fisher, All Rights Reserved ...

Lecture 18: Bodily Fluid Compartments Adapted From: Textbook Of Medical Physiology, 13th Ed. Arthur C. Guyton, John E. Hall Chapter 25 John P. Fisher © Copyright 2024, John P. Fisher, All Rights Reserved  Overview of The Kidney and Body Fluids Introduction The maintenance of volume and a stable composition of body fluids is essential for homeostasis The kidneys are key players that control many functions Overall regulation of body fluid volume Regulation of the constituents of extracellular fluid Regulation of acid-base balance Control of fluid exchange between extracellular and intracellular compartments © Copyright 2024, John P. Fisher, All Rights Reserved  Balance of Fluid Intake and Fluid Output Introduction Water intake comes from two sources Ingested in the form of liquids or within food Normal Exercise Synthesized as a result of the Intake oxidation of carbohydrates Ingested 2100 ? Metabolism 200 200 Water loss occurs in many forms Total Intake 2300 ? Insensible water loss Evaporation through skin Output Humidification of inspired air Insensible - Skin 350 350 Sweat Insensible - Lungs 350 650 Feces Sweat 100 5000 Urine Feces 100 100 The kidneys are a critical Urine 1400 500 mechanism of controlling water Total Output 2300 6600 loss © Copyright 2024, John P. Fisher, All Rights Reserved  Balance of Fluid Intake and Fluid Output Body Fluid Compartments Total body fluid is divided between extracellular fluid, transcellular fluid, and intracellular fluid Extracellular fluid is again divided between blood plasma and interstitial fluid Transcellular fluid includes fluid in the synovial, peritoneal, pericardial, and intraocular space A 70 kg person contains approximately 42 liters of water (60%) and varies with significant physical parameters Age Sex Obesity Guyton & Hall. Textbook of Medical Physiology, 11th Edition © Copyright 2024, John P. Fisher, All Rights Reserved  Balance of Fluid Intake and Fluid Output Body Fluid Compartments About 28 liters are inside the 75 trillion cells of the body and thus are part of the intracellular fluid Despite the vast differences in cell functions, the composition of most cells is relatively similar About 14 liters are extracellular Three quarters of this water is interstitial One quarter of this water is plasma About 5 liters are blood, 3 liters of which are plasma Plasma is 60%, RBCs is 40% Guyton & Hall. Textbook of Medical Physiology, 11th Edition © Copyright 2024, John P. Fisher, All Rights Reserved  Balance of Fluid Intake and Fluid Output Composition of Plasma and Interstitial Fluid Since the plasma and interstitial fluids are separated by highly permeable capillary membranes, their compositions are quite similar However, due to low protein permeability, the interstitial spaces of most tissues have low protein concentrations Donnan Ion Effect Proteins are mostly impermeable through the capillary wall Plasma proteins are negatively charged and therefore bind cations (Na+ and K+ ) as well as repel anions Thus, the concentration of cations is slightly greater in the plasma than in the interstitial fluid © Copyright 2024, John P. Fisher, All Rights Reserved Balance of Fluid Intake and Fluid Output Plasma Interstitial Intracellular (mOsm/l H20) (mOsm/l H20) (mOsm/l H20) Na+ 142.0 139.0 14.0 K+ 4.2 4.0 140.0 Ca++ 1.3 1.2 0.0 Mg+ 0.8 0.7 20.0 Cl- 108.0 108.0 4.0 HCO 3- 24 28.3 10.0 HPO 4-, H 2PO 4- 2.0 2.0 11.0 SO 4- 0.5 0.5 1.0 Phosphocreatine 45.0 Carnosine 14.0 Amino Acids 2.0 2.0 8.0 Creatine 0.2 0.2 9.0 Lactate 1.2 1.2 1.5 Adenosine Triphosphate 5.0 Hexose Monophosphate 3.7 Glucose 5.6 5.6 Protein 1.2 0.2 4.0 Urea 4.0 4.0 4.0 Others 4.8 3.9 10.0 Total mOsm/l 301.8 300.8 301.2 Corrected Osmolar 282.0 281.0 281.0 © Copyright 2024, John P. Fisher, All Rights Reserved  Balance of Fluid Intake and Fluid Output Measurement of Fluid Volumes Total Body Water Add a bolus of radioactive water and measure concentration after a few hours Extracellular Fluid Volume Add a bolus of radioactive sodium and measure concentration after a few hours Intracellular Fluid Volume Equals total body water minus extracellular fluid volume Plasma Volume Add a bolus of radioactive serum albumin and measure concentration after a few hours Interstitial Fluid Volume Equals extracellular fluid volume minus plasma volume Total Blood Volume Equals plasma volume / (1 - hematocrit*) *Here hematocrit is total blood cell volume © Copyright 2024, John P. Fisher, All Rights Reserved  Balance of Fluid Intake and Fluid Output Osmosis and Osmotic Pressure Osmosis is the diffusion of water, through a selectively permeable membrane, from a region of low solute concentration to high solute concentration Water moves to try to equilibrate solute concentration on either side of the membrane An osmole is the total number of particles in solution that are establishing an osmotic gradient Osmoles are identical to moles, if the particle does not dissociate However, if a particle does dissociate, then an osmole equals the number of moles multiplied by the number of dissociating particles Osmolality = osmoles / kg solution Osmolarity = osmoles / liter solution © Copyright 2024, John P. Fisher, All Rights Reserved  Balance of Fluid Intake and Fluid Output Osmosis and Osmotic Pressure The pressure applied to one side of a selectively permeable pressure which stops osmosis is called osmotic pressure Osmotic pressure is proportional to its osmolarity Molecular size is not relevant here, rather the number of osmotically active molecules is the critical parameter Osmotic Pressure, Π = CRT C = Concentration of solutes in osmoles per liter (osmolarity) R = Ideal gas constant T = absolute temperature (K) Example: C = 1.0 mOsm/l and T = 310 K, Π = 19.3 mmHg Thus, for each mOsm concentration gradient of an impermeable solute molecule, about 19.3 mmHg osmotic pressure is exerted across the separating membrane © Copyright 2024, John P. Fisher, All Rights Reserved  Balance of Fluid Intake and Fluid Output Osmosis and Osmotic Pressure Basic osmotic pressure calculation: a 0.9% sodium chloride solution 0.9% = 0.9 gm/100 mL or 9.0 gm/l Molecular weight of sodium chloride = 58.5 gm/mol Molarity of the solution = 9 gm/l / 58.5 gm/mol = 0.154 mol/l Osmolarity of the solution = 2 x 0.154 mol/l = 0.308 Osm/l = 308 mOsm/l Osmotic pressure = 308 mOsm/l x 19.3 mmHg/mOsm/l = 5944 mmHg Note that sodium chloride does not completely dissociate, thus the true osmotic pressure is less An osmotic coefficient is thus used as a correction factor Osmotic coefficient for sodium chloride is 0.93 Thus, osmolarity = 0.93 x 308 mOsm/l = 286 mOsm/l © Copyright 2024, John P. Fisher, All Rights Reserved  Balance of Fluid Intake and Fluid Output Osmosis Equilibrium Relatively small changes in concentration of impermeable solutes in the extracellular fluid can cause tremendous changes in cell volume If a cell is placed in a solution with normal osmolarity (280 mOsm/l), the cell will neither shrink or swell - the solution is isotonic Isotonic solutions include 0.9% sodium chloride and 5% glucose If a cell is placed in a solution with less osmolarity (hypotonic), the cell will swell as it tries to dilute its solutes If a cell is placed in a solution with more osmolarity (hypertonic), the cell will shrink Guyton & Hall. Textbook of Medical Physiology, 11th Edition as it tries to concentrate its solutes © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Introduction Abnormalities of the composition and volumes of body fluids are among the most common and important of clinical problems and are of concern for almost all seriously ill Factors that augment extracellular and intracellular volumes include Ingestion of water Dehydration Intravenous infusions Loss of fluid from the gastrointestinal tract Loss of fluid from sweating Loss of fluid through the kidneys Two basic principles guide our discussion Water moves rapidly across the cell membrane, thus the osmolarities of intracellular and extracellular fluids are almost exactly equal Cell membrane is almost completely impermeable to many solutes, thus the number of osmoles in the extracellular and intracellular fluid remains constant unless directly added / removed © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Example: Patient in Water Deficit A patient has an osmolarity of 320 mOsm/l, so how much water should be administered to reduce this to 280 mOsm/l? Assume Extracellular volume is 20% body weight, 14 liters Intracellular volume is 40% body weight, 28 liters Thus Extracellular osmolarity = 320 mOsm/l x 14 liters = 4,480 mOsm Intracellular osmolarity = 320 mOsm/l x 28 liters = 8,960 mOsm Total body fluid osmolarity = 320 mOsm/l x 42 liters = 13,440 mOsm We want an osmolarity of 280 mOsm/l, thus the necessary volume should be Extracellular volume = 4,480 mOsm / 280 mOsm/l = 16 liters Intracellular volume = 8,960 mOsm / 280 mOsm/l = 32 liters Total body fluid volume = 13,440 mOsm / 280 mOsm/l = 48 liters Thus, 6 liters need to be administered © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Example: Patient in Water Deficit If an isotonic saline solution is administered The osmolarity of the extracellular fluid does not change No osmosis through the cell membrane occurs The effect is an increase in extracellular fluid volume If a hypertonic solution is administered The osmolarity of the extracellular fluid increases Osmosis pushes water out of the cell and into the extracellular space The effect is a large increase in extracellular fluid volume and a decrease in intracellular fluid volume If a hypotonic solution is administered The osmolarity of the extracellular fluid decreases Osmosis pushed water into the cell and out of the extracellular space The effect is an increase in extracellular fluid volume and a larger increase in intracellular fluid volume © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Example: Patient in Water Deficit What is the effect on extracellular and intracellular fluid volumes when 2 liters of a hypertonic 2.9% sodium chloride solution is administered to a 70 kg patient with an initial osmolarity of 280 mOsm/liter Using the basic assumptions Extracellular osmolarity = 280 mOsm/l x 14 liters = 3,920 mOsm Intracellular osmolarity = 280 mOsm/l x 28 liters = 7,840 mOsm Total body fluid osmolarity = 280 mOsm/l x 42 liters = 11,760 mOsm Next, 2 liters of a 29 g/l sodium chloride (58 g/mol) solution provides 1 mole or 2 osmoles (2,000 mOsm) of sodium chloride The instantaneous change would be Extracellular concentration = (3,920 + 2,000) mOsm / 16 liters = 370 mOsm/l Intracellular concentration remains constant = 280 mOsm/l The long term change would be Total body fluid concentration = 13,760 mOsm / 44 liters = 312.7 mOsm/l Extracellular volume = 5,920 mOsm / 312.7 mOsm/l = 18.9 liters Intracellular volume = 7,840 mOsm / 312.7 mOsm/l = 25.1 liters © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Hyponatremia and Hypernatremia Rather than measuring plasma osmolarity, plasma sodium concentration is easier and therefore more commonly measured Normal sodium concentration is 142 mEq/liter Hyponatremia: Reduced sodium levels Results from loss of sodium chloride from the extracellular fluid or the addition of excess water to the extracellular fluid May result from diarrhea, vomiting, overuse of diuretics, or excess water retention Hypernatremia: Elevated sodium levels Results from loss of water from the extracellular fluid or an excess of sodium in the extracellular fluid © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Edema Edema is the presence of excess fluid in the body, typically in the extracellular fluid space but sometimes involving the intracellular fluid space Intracellular Edema Two conditions lead to intracellular swelling Depression of metabolic systems of the tissues Inadequate nutrient delivery to the cell Extracellular Edema Much more predominant Two general conditions lead to extracellular edema Abnormal leakage of fluid from the plasma through the capillary wall and into the interstitial space Failure of the lymphatics to return fluid from the interstitium back into the blood © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Edema Capillary filtration may be described by Filtration = Kf x (Pc - Pif - πc + πif) where Kf = capillary filtration coefficient Pc = capillary hydrostatic pressure Pif = interstitial fluid hydrostatic pressure πc = capillary osmotic pressure πif = interstitial osmotic pressure Thus, capillary filtration may be increased by Increased capillary filtration coefficient Increased capillary hydrostatic pressure Decreased capillary osmotic pressure © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Causes of Extracellular Edema Increased capillary pressure Increased capillary permeability Excessive kidney retention of salt and water Immune reactions that cause release of Acute or chronic kidney failure histamine and other immune products Mineralocorticoid excess Toxins High venous pressure and venous constriction Bacterial infections Heart failure Vitamin deficiency, especially vitamin C Venous obstruction Prolonged ischemia Failure of venous pumps Burns Paralysis of muscles Blockage of lymph return Immobilization of parts of the body Cancer Failure of venous valves Infections (e.g., filaria nematodes) Decreased arteriolar resistance Surgery Excessive body heat Congenital absence or abnormality of lymphatic vessels Insufficiency of sympathetic nervous system Vasodilator drugs Decreased plasma proteins Loss of proteins in urine (nephrotic syndrome) Loss of protein from denuded skin areas Burns Wounds Failure to produce proteins Liver disease (e.g., cirrhosis) Serious protein or caloric malnutrition © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Edema Caused by Heart Failure In heart failure, the heart fails to pump blood normally As a result, venous pressure rises and capillary pressure rises - and therefore causing more capillary filtration Renal Effects Additionally, arterial pressure falls, decreasing water and salt excretion by the kidneys, increasing blood volume and further increasing capillary pressure Also, diminished blood flow stimulates renin secretion, causing additional salt and water retention In pulmonary edema, blood cannot easily leave the pulmonary veins due to heart damage, increasing pulmonary capillary pressure and therefore water retention - death can occur rapidly © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Edema Caused by Salt and Water Retention In some kidney failures, salt and water excretion is diminished Salt and water build in the blood and leak into the interstitial space, causing Increased in interstitial fluid volume Hypertension due to increase blood volume Edema Caused by Decreased Plasma Proteins A decrease in plasma proteins increases capillary filtration and extracellular edema Causes include Renal abnormalities which allow proteins to leave through the urine Cirrhosis of the liver where fibrous tissue in the liver produces increased plasma protein levels © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Prevention of Edema Three major safety factors prevent fluid accumulation in the interstitial space Low compliance of the interstitium when interstitial fluid pressure is negative The ability of the lymph flow to increase 10 to 50 fold Washdown of interstitial fluid protein Low Compliance of the Interstitum Interstitial fluid hydrostatic pressure is slightly negative, about -3 mmHg Once interstitial fluid pressure rises above 0 mmHg, compliance increases and fluids rapidly accumulate Safety factor is about 3 mmHg Guyton & Hall. Textbook of Medical Physiology, 11th Edition © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Prevention of Edema Increased Lymph Flow The ability of the lymphatics to dramatically increase their flow allows a release mechanism for fluids retained within the interstitial space Increase lymph flow occurs in response to increased capillary filtration and therefore preventing increased interstitial pressure Safety factor is about 7 mmHg Washdown of Interstitial Fluid Protein As increased fluids are filtered into the interstitium, interstitial fluid pressure increases, and causing increased lymph flow This increased lymph flow, washes out protein concentration Safety factor is about 7 mmHg Total safety factor preventing edema is approximately 17 mmHg Capillary pressure in a peripheral tissue can rise approximately 17 mmHg, or double its normal level, without significant edema © Copyright 2024, John P. Fisher, All Rights Reserved  Fluid Regulation in Abnormal States Effusion Edema can occur in tissues adjacent to other volumes - potential spaces - that can also collect fluid Pleural cavity Pericardial cavity Peritoneal cavity Synovial cavities Potential spaces are “filled” by capillaries and “drained” by lymphatics in a manner similar to other tissues When edema occurs with fluid collection in a potential space, it is called effusion © Copyright 2024, John P. Fisher, All Rights Reserved