II. Physiology: Total blody water distribution
- See Total Body Water
- Images
-
Total Body Water (TBW) accounts for 60% of body weight in men (50% in women)
- Example: 70 kg man has ~42 L TBW and a 70 kg woman has ~35 L TBW
- Intracellular fluid volume or ICFV (66% of Total Body Water)
- Extracellular fluid volume or ECFV (33% of Total Body Water)
- Example: 70 kg man has ~14 L ECFV and a 70 kg woman has ~12 L ECFV
- Electrolytes
- Interstitial volume (75% of extracellular fluid)
- Plasma volume (25% of extracellular fluid)
- Plasma is 92% water, and the remainder mostly Protein (albumin, Coagulation Factors, Fibrinolytic Proteins, Immunoglobulins)
- Plasma volume is maintained by plasma Proteins via oncotic pressure
- Plasma volume represents 55% of Blood Volume (3L in a 70 kg male)
- Remaining 45% of Blood Volume is cellular (Red Blood Cells, White Blood Cells, Platelets)
- Total Blood Volume from Ideal Body Weight (kg): 7% in adults (5L in 70 kg male) and 8-9% children (80 ml/kg)
III. Physiology: Sodium and water
-
Total Body Water is primarily maintained via extracellular Sodium concentration
- Extracellular Sodium is maintained at 135-145 mEq/L
- Sodium (Na+) is the primary osmole to maintain extracellular fluid volume
- Total body Sodium is typically estimated based only on extracellular Sodium
- Intracellular Sodium is negligible (<20 mEq/L) compared with extracellular Sodium (>135 mEq/L)
- Water follows higher Sodium concentrations (osmotic pressure gradient)
- Increased Sodium concentrations results in greater extracellular water retention
- Results in Fluid Overload (Edematous State) with increased ECFV
- Decreased Sodium concentration results in extracellular water loss
- Results in volume depletion with decreased ECFV
- Increased Sodium concentrations results in greater extracellular water retention
- Sodium is regulated to maintain appropriate extracellular fluid volume
- Sodium homeostasis balances Sodium intake with Sodium excretion
- Fluid Overload and Hypovolemia are defects in Sodium regulation (even when Serum Sodium is normal)
- Hyponatremia is a water excess state (water regulation abnormality)
- May be associated with low, normal or high ECFV (or total body Sodium)
IV. Physiology: Osmolality and Tonicity
- See Serum Osmolality (and Extracellular Fluid Tonicity)
- Sodium is the chief osmole in maintaining plasma osmolality and tonicity
-
Osmolal Gap
- Osmolality may be calculated based on known solutes (Sodium, Glucose, BUN)
- Osmolar Gap is the difference between expected and measured osmolality
-
Osmolar Gap >10 mOsm/L suggests unmeasured solutes
- Example: Toxic Alcohols such as Polyethylene Glycol
- Cellular hydration and tonicity
- Fluid Shifts into or out of cells are based on tonicity
- Rapid changes in extracellular Sodium concentration can seriously impact cell fluid (esp. brain cells)
- Rapid onset Hyponatremia, results in cell swelling
- Rapid onset Hypernatremia, results in cell shrinkage
V. Physiology: Sodium Regulation Mechanisms (Renally Mediated)
- Images
- Nephron (Glomerulus and Renal Tubules)
- Sodium freely crosses the glomerular basement membrane
- Of filtered Sodium and water (180 L/day in a healthy 70 kg person), 65% is reabsorbed
- Sodium filtered through glomerulus into renal tubule has same initial concentration as blood
- Water follows Sodium through glomerulus
- Renal Sodium excretion is responsible for most Sodium loss
- Other causes of Sodium loss include sweating, Diarrhea, Hemorrhage and Burn Injury
- Glomerular Filtration Rate (GFR)
- Glomerular-Tubular Balance
- GFR is correlated with Sodium reabsorption in proximal tubule
- Decreased GFR results in decreased filtered Sodium and decreased Sodium and water reabsorption
- Increased GFR results in increased filtered Sodium and increased Sodium and water reabsorption
- Tubulo-Glomerular Feedback
- Glomerular Filtration Rate (GFR) is modulated by the Macula densa in the renal tubule
- Increased tubular flow at Macula densa results in afferent arteriole constriction and decreased GFR
- Decreased tubular flow at Macula densa results in afferent arteriole dilation and increased GFR
- Glomerular-Tubular Balance
- Renal Tubules
- Water reabsorption (without Sodium) is in the descending loop of henle
- Sodium reabsorption (without water) is in the ascending loop of henle
- Results in hyperosmolar interstitium in the lower aspect of the loop of henle region
- Collecting Duct
- Collecting duct passes through the hyperosmolar interstitium and allows for further water reabsorption
- Collecting duct is porous, and water follows osmotic gradient into interstitium, concentrating the urine
- Antidiuretic Hormone (ADH) increases the collecting duct permeability to water (and water reabsorption)
- Sodium freely crosses the glomerular basement membrane
-
Renin-Angiotensin System (and Aldosterone)
- See Renin-Angiotensin System
- Afferent (sensory)
- Renal juxtaglomerular cells sense decreased renal perfusion and release renin
- Renin converts Angiotensinogen to Angiotensin I
- Angiotensin Converting Enzyme converts Angiotensin I to Angiotensin II
- Efferent (action)
- Angiotensin II stimulates Aldosterone release from the Adrenal Cortex (zona glomerulosa)
- Aldosterone acts to increase Sodium absorption at the distal nephron (cortical collecting tubule)
- Angiotensin II increases proximal tubule Sodium absorption
- Sodium reabsorption is contingent on normal or acidic proximal tubule pH
- Sodium crosses from renal tubule cell into capillary with bicarbonate
- Bicarbonate crosses from renal tubule into renal tubule cell as CO2 via H+
- In alkalosis, H+ is lacking, bicarbonate (and Sodium) is less reabsorbed
- Sodium reabsorption is contingent on normal or acidic proximal tubule pH
- Angiotensin II stimulates Aldosterone release from the Adrenal Cortex (zona glomerulosa)
- Atrial natriuretic factor
- See Brain Natriuretic Peptide (BNP)
- Afferent (sensory)
- Atria and vena cava respond to increased intravascular volume, filling and stretch
- Release atrial natriuretic factor from atria in response to increased volume
- Efferent (action)
- Overall effect is to increase Sodium (and water) excretion by blocking Sodium reabsorption
- Atrial natriuretic factor increases Glomerular Filtration Rate (GFR)
- Dilates afferent glomerular arteriole
- Constricts efferent glomerular arteriole
- Atrial natriuretic factor opposes Renin-Angiotensin System
- Decreases renal Sodium absorption at distal nephron (opposes Aldosterone)
- Inhibits renin secretion
-
Sympathetic Nervous System
- Afferent (sensory)
- Aorta and carotid sinus receptors respond to decreased pressure (low volume)
- Activates Sympathetic System in response to volume depletion (decreased ECFV)
- Efferent (action)
- See Sympathetic Nervous System for effects
- Sympathetic System activation results in renal Sodium retention
- Mediated via Renin-Angiotensin System
- Afferent (sensory)
VI. Physiology: Water Regulation Mechanisms (affects osmolality and tonicity)
- Background
- Water intake and output mechanisms have greatest ECF Sodium concentration effect
- Thirst
- Potent Sensation that increases water intake and prevents Hypernatremia
- Hypernatremia is rare with intact thirst mechanism and adequate water access
- Inducers of thirst
- Increased extracellular fluid osmolality (responds to even a few mOsm/L difference)
- Angiotensin II
- Extracellular Fluid Volume depletion
- Potent Sensation that increases water intake and prevents Hypernatremia
-
Antidiuretic Hormone (ADH or Arginine Vasopressin)
- Overall effect is to increase renal water reaborption
- ADH is released from the posterior pituitary
- Released in response to osmoreceptors in the hypothothalamus detecting hypertonicity
- Hypertonicity also stimulates thirst Sensation
- Response to increased plasma osmolality (and increased plasma Sodium concentration, Hypernatremia)
- Increased ADH secretion
- Water retention by the Kidneys
- Decreased plasma Sodium concentration (and decreased plasma osmolality)
- Response to decreased plasma osmolality (and decreased plasma Sodium concentration, Hyponatremia)
- Decreased ADH secretion
- Free water diuresis
- Increased plasma Sodium concentration (and increased plasma osmolality)
- Abnormal Antidiuretic Hormone
- Syndrome Inappropriate ADH Secretion (SIADH)
- Inappropriate ADH release, resulting in water retention despite normal Sodium and water status
- Results in Isovolemic Hypoosmolar Hyponatremia
- Diabetes Insipidus
- Excessive constant water diuresis due lack of pituitary ADH release or lack of renal response
- Syndrome Inappropriate ADH Secretion (SIADH)
- Renal mechanisms
- Renal responses
- Hypertonic response (e.g. Hypernatremia)
- Kidney retains water
- Urine is more concentrated than plasma
- Hypotonic response (e.g. Hyponatremia)
- Kidney excretes water
- Urine is more dilute than plasma
- Hypertonic response (e.g. Hypernatremia)
- Renal water regulation dependencies
- Adequate Glomerular Filtration Rate
- Concentrating and diluting mechanisms require a minimum GFR of 20% of normal
- Adequate renal tubule concentrating and diluting functions
- Adequate glomerular filtrate delivery to tubules
- Excessive proximal tubule water reabsorption bypasses the distal tubule
- Excessive proximal tubule water reabsorption may result in Hyponatremia
- Volume depletion (e.g. free water replacement of Diarrhea losses)
- Edematous States (e.g. CHF, Cirrhosis, Nephrosis)
- Adequate urine concentrating function (Ascending Loop of Henle)
- Ascending loop of henle reabsorbs 30% of Sodium into the Medullary interstitium
- Sodium (but not water) is reabsorbed via the Sodium-Potassium-2-chloride pump
- Sodium reabsorption in ascending loop of henle is blocked by loop diurectics
- However, Sodium may still be reabsorbed in distal convoluted tubule
- Therefore, Loop Diuretics cause less Hyponatremia and greater water loss
- Contrast with Thiazide Diuretics which block distal convoluted tubule
- Hypertonic Interstitium (via active Sodium reabsorption) allows for a concentrated urine
- Hypertonic Medullary interstitium attracts water from the collecting tubule
- Collecting tubule water permeability is increased by ADH (greater water reabsorption)
- Mediators decreasing interstitial hypertonicity and osmotic gradient (less water reabsorption)
- Decreased hypertonicity with Loop Diuretics
- Decreased hypertonicity with Protein deficiency (nutritional deficiency)
- Urea (Protein breakdown) also increases hyperosmolar interstitium
- Adequate urine diluting function (Distal Convoluted Tubule)
- Additional 5-10% of Sodium and chloride are reabsorbed at the distal convoluted tubule
- Sodium chloride reabsorption is blocked by Thiazide diurectics at the distal convoluted tubule
- Thiazide diurectics cause a relative retention of water more than Sodium
- Results in a greater risk of Hyponatremia than with Loop Diuretics
- Without later water reabsorption, the urine is dilute
- Adequate glomerular filtrate delivery to tubules
- Adequate Antidiuretic Hormone (ADH) functioning
- ADH is key regulator of urine concentration and dilution (1200 mOsm/L to 50 mOsm/L)
- Appropriate central ADH release
- ADH is released in response to small increases in extracellular Sodium concentration
- Also released with volume depletion
- Inappropriately increased ADH release with SIADH
- Results in Isovolemic Hypoosmolar Hyponatremia
- Deficient ADH release occurs with Central Diabetes Insipidus
- Results in decreased collecting tubule permeability, water loss and Hypernatremia
- ADH is released in response to small increases in extracellular Sodium concentration
- Appropriate renal ADH response
- ADH increases Medullary collecting tubule water permeability
- Water flows from the collecting tubule into the hypertonic Medullary interstitium
- Inappropriately increased ADH responsiveness with ADH-like drugs
- Results in SIADH
- Deficient ADH response occurs in Nephrogenic Diabetes Insipidus
- Results in decreased collecting tubule permeability, water loss and Hypernatremia
- ADH increases Medullary collecting tubule water permeability
- Adequate Glomerular Filtration Rate
- Renal responses
VIII. References
- Marino (2014) ICU Book, p. 653-72
- Preston (2011) Acid-Base Fluids and Electrolytes, p. 3-30
- Rose (1989) Clinical Physiology of Acid-Base and Electrolyte Disorders, p. 3-27
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Definition (MSH) | The balance of fluid in the BODY FLUID COMPARTMENTS; total BODY WATER; BLOOD VOLUME; EXTRACELLULAR SPACE; INTRACELLULAR SPACE, maintained by processes in the body that regulate the intake and excretion of WATER and ELECTROLYTES, particularly SODIUM and POTASSIUM. |
Concepts | Organism Function (T040) |
MSH | D014882 |
SnomedCT | 16379007 |
English | Balance, Water-Electrolyte, Water Electrolyte Balance, Water-electrolyte balance, function, osmoregulation, balance electrolyte water, water-electrolyte balance, balance electrolytes water, Water-Electrolyte Balance, Water-electrolyte balance, Water-electrolyte balance, function (observable entity), Water-electrolyte balance (function) |
Swedish | Vätske- och elektrolytbalans |
Czech | vodní a elektrolytová rovnováha |
Finnish | Neste-elektrolyyttitasapaino |
French | Équilibre hydro-électrolytique, Équilibre hydroélectrolytique |
Russian | VODNO-ELEKTROLITNYI BALANS, OBMEN ZHIDKOSTI, ZHIDKOSTI BALANS, ЖИДКОСТИ БАЛАНС, ВОДНО-ЭЛЕКТРОЛИТНЫЙ БАЛАНС, ОБМЕН ЖИДКОСТИ |
Japanese | 水-電解質平衡(スイ-デンカイシツヘイコウ), 水・電解質平衡, 体液平衡, 水・電解質バランス, 水-電解質平衡, 浸透圧調節 |
Portuguese | Equilíbrio Hidroeletrolítico, Balanço Hidreletrolítico, Balanço Hidroeletrolítico, Equilíbrio Hidreletrolítico |
Croatian | VODA-ELEKTROLITI, RAVNOTEŽA |
Polish | Gospodarka wodno-elektrolitowa |
Norwegian | Not Translated[Water-Electrolyte Balance] |
Spanish | Balance Hidroelectrolítico, balance hidroelectrolítico (entidad observable), balance hidroelectrolítico (función), balance hidroelectrolítico, equilibrio hidroelectrolítico (entidad observable), equilibrio hidroelectrolítico, Equilibrio Hidroelectrolítico |
German | Wasser-Elektrolyt-Haushalt |
Italian | Equilibrio acqua-elettrolita |
Dutch | Water-elektrolytbalans |
Ontology: Fluid Shifts (C0242705)
Definition (MSH) | Translocation of body fluids from one compartment to another, such as from the vascular to the interstitial compartments. Fluid shifts are associated with profound changes in vascular permeability and WATER-ELECTROLYTE IMBALANCE. The shift can also be from the lower body to the upper body as in conditions of weightlessness. |
Concepts | Physiologic Function (T039) |
MSH | D018495 |
English | Fluid Shifts, Shifts, Fluid, fluids shift, fluid shifting, fluid shift, fluid shifts |
Swedish | Kroppsvätskeförskjutningar |
Czech | přesuny tekutin |
Finnish | Nestesiirtymät |
French | Mouvements des liquides corporels, Échanges liquidiens, Déplacements des liquides corporels, Transferts liquidiens, Transfert liquidien |
Russian | ZHIDKOSTEI ORGANIZMA PEREMESHCHENIE, ZHIDKOSTEI TELA PEREMESHCHENIE, ЖИДКОСТЕЙ ОРГАНИЗМА ПЕРЕМЕЩЕНИЕ, ЖИДКОСТЕЙ ТЕЛА ПЕРЕМЕЩЕНИЕ |
Portuguese | Deslocamentos de Líquidos Corporais, Transferências de Fluídos Corporais, Desvios de Fluidos Corporais, Desvios de Líquidos Corporais, Transferências de Fluídos, Deslocamentos de Fluídos, Desvios de Fluídos, Deslocamentos de Líquidos, Desvios de Líquidos, Transferências de Líquidos Corporais |
Polish | Przemieszczanie się płynów |
Spanish | Transferencias de Fluidos Corporales, Transferencias de Fluidos, Transferencias de Fluido, Transferencias de Líquidos, Transferencias de Líquidos Corporales |
German | Flüssigkeitsverschiebungen |
Italian | Scorrimento dei fluidi |
Dutch | Verschuiving, vloeistof-, Vloeistofverschuiving |
Ontology: Sodium disorder (C0549563)
Concepts | Pathologic Function (T046) |
SnomedCT | 154754006, 267506007, 123807007 |
English | Sodium disorders, Sodium disorder (disorder), Sodium disorder |
Spanish | trastorno del sodio (trastorno), trastorno del sodio |
Ontology: Sodium Cation (C0597484)
Definition (NCI_CRCH) | Sodium ion requiring one electron to return to its elemental state. |
Definition (CSP) | chief cation of the extracellular body fluids. |
Concepts | Element, Ion, or Isotope (T196) |
English | sodium ion, Sodium Ion, Na+, Sodium Cation, sodium ions, Na+ element, SODIUM CATION, Sodium 1+, Sodium+, Sodium cation, sodium cation, Sodium Ions |
Ontology: sodium ion homeostasis (C1156281)
Definition (GO) | Any process involved in the maintenance of an internal steady state of sodium ions within an organism or cell. [GOC:ai, GOC:jid, GOC:mah] |
Concepts | Organism Function (T040) |
English | sodium ion homeostasis |
Ontology: Osmoregulation (C1510659)
Definition (GO) | Any process that results in a change in state or activity of a cell or an organism (in terms of movement, secretion, enzyme production, gene expression, etc.) as a result of a stimulus indicating an increase or decrease in the concentration of solutes outside the organism or cell. [GOC:jl] |
Definition (MSH) | The response of cells in sensing a difference in OSMOTIC PRESSURE between the inside and outside of the cell. This response includes signaling from osmotic sensors to activate transcription factors, which in turn regulate the expression of osmocompensatory genes, all functioning to maintain CELL VOLUME and the water concentration inside the cells. |
Concepts | Cell Function (T043) |
MSH | D064587 , D014882 |
English | response to osmotic stress, osmotic stress response, osmotic response, osmoregulation, Osmoregulation, Osmotic Stress Regulating Pathway, Osmotic Stress Responses, Responses, Osmotic Stress, Stress Responses, Osmotic, Response, Osmotic Stress, Stress Response, Osmotic, Osmotic Stress Response |
Czech | osmoregulace |
French | Réponse au stress osmotique, Voie de régulation osmotique, Voie de régulation du stress osmotique, Régulation osmotique, Osmorégulation |
German | Osmoregulation |
Italian | Osmoregolazione |
Norwegian | Not Translated[Osmoregulation] |
Russian | OSMOREGULIATSIIA, OSMOTICHESKAIA REGULIATSIIA, ОСМОТИЧЕСКАЯ РЕГУЛЯЦИЯ, ОСМОРЕГУЛЯЦИЯ, OTVET KLETKI NA OSMOTICHESKII STRESS, ОТВЕТ КЛЕТКИ НА ОСМОТИЧЕСКИЙ СТРЕСС |
Croatian | Not Translated[Osmoregulation] |
Spanish | Not Translated[Osmoregulation] |
Portuguese | Not Translated[Osmoregulation] |
Dutch | Osmoregulatie |