Total Body Water (TBW) and Fluid Distribution

• Total body water (TBW) represents about 60% of body weight.
• TBW is inversely proportional to body fat.
• TBW percentage is higher in neonates and adult males, and lower in adult females and adults with large amounts of adipose tissue.

Water Distribution

• Intracellular fluid (ICF) is two-thirds of TBW.

• The main ICF cations are K⁺ and Mg²⁺.

• The main ICF anions are proteins and organic phosphates (adenosine triphosphate [ATP], adenosine diphosphate [ADP], and adenosine monophosphate [AMP]).

• Extracellular fluid (ECF) is one third of TBW.

• ECF consists of interstitial fluid and plasma. The main ECF cation is Na⁺.

• The main ECF anions are Cl⁻ and HCO₃⁻.

• Plasma represents one fourth of ECF, it is one twelfth of TBW (1/4 × 1/3).

• The main plasma proteins are albumin and globulins.

• Interstitial fluid represents three quarters of the ECF; it is one fourth of TBW (3/4 × 1/3).

• Interstitial fluid composition is the same as plasma, except contains few proteins, it is an ultrafiltrate of plasma.

• 60-40-20 rule: TBW is 60% of body weight, ICF is 40% of body weight, and ECF is 20% of body weight.

Quantification of Liquid Compartment Volumes

• Dilution Method

• A known amount of a substance whose distribution volume is the fluid compartment of interest is administered.

• Examples include tritiated water as a TBW marker, mannitol as an ECF marker, and Evans blue as a plasma volume marker.

• The substance is allowed to equilibrate.

• The substance concentration in plasma is measured and the distribution volume is calculated.

  • Volume = Amount/Concentration
  • Volume = distribution compartment volume (L)
  • Amount = amount of substance present (mg)
  • Concentration = plasma concentration (mg/L)

• Substances Used to Measure the Main Liquid Compartments

  • TBW uses tritiated water, D₂O, and antipyrine.
  • ECF uses sulfate, inulin, and mannitol.
  • Plasma uses radioiodinated serum albumin and Evans blue.
  • Interstitial fluid is quantified indirectly (ECF volume-plasma volume).
  • LIC is ​​quantified indirectly (TBW-ECF volume).

Water Displacement Between Compartments

• Basic Principles

• Osmolarity is the solute particle concentration.

• Plasma osmolarity (Posm) is estimated by:

  • Posm = 2 × Na⁺ + Glucose/18 + BUN/2.8,

• Posm= plasma osmolarity (mOsm/L), Na⁺ = plasma Na⁺ (mEq/L), Glucose = plasma glucose (mg/dL), BUN = blood urea nitrogen (mg/dL)

• LEC and LIC osmolarity is equal in the stable state.

• To achieve this equality, water moves between ECF and ICF.

• Solutes like NaCl and mannitol do not cross cell membranes and are confined to ECF.

Examples of Water Displacement Between Compartments

• Isotonic NaCl Infusion: Isotonic liquid addition
  • Also called isosmotic volume expansion.
  • ECF volume increases, but does not change ECF or ICF osmolarity, no water displacement between compartments.
  • Plasma protein concentration and hematocrit decrease because the liquid addition to the ECF protein and red blood cell dilutes them.Since ECF osmolarity does not vary, so red blood cells will not wrinkle or swell.
  • Arterial pressure increases because ECF volume increases.
• Diarrhea: Isotonic liquid loss
  • Also called isosmotic volume contraction.
  • ECF volume decreases, but ECF or ICF osmolarity does not change. Since osmolarity does not change, no water moves between compartments.
  • Plasma protein concentration and the hematocrit increase because the loss of LEC concentrates the proteins and the red blood cells. Since the osmolarity of LEC does not vary, the red blood cells will not wrinkle or swell.
  • Arterial pressure decreases because ECF volume decreases.
• Excessive NaCl Intake: NaCl addition
  • Also called hyperosmotic volume expansion.
  • ECF osmolarity increases because osmoles (NaCl) have been added to ECF.
  • Water moves from ICF to ECF. As a result of this displacement, the osmolarity of the LIC increases to match the LEC.
  • As a result of water leaving the cells, the LEC volume increases (volume expansion) and the volume of the LIC decreases.
  • Plasma protein concentration and the hematocrit decrease due to increased ECF volume.
• Sweating in the Desert: Water loss
  • It is also called hyperosmotic volume contraction.
    • LEC osmolarity increases because sweating is hypoosmotic (relatively more loss of water than salt).
    • LEC volume decreases due to sweating loss. Water leaves the LIC; as a result, osmolarity of the LIC increases to match that of the LEC, and LIC volume decreases.
    • Plasma protein concentration increases due to the decrease in LEC volume. Although it would be expected that hematocrit would increase, it remains unchanged because erythrocytes lose water, which reduces their volume and compensates for concentrated effect of the LEC volume reduction.
• Syndrome of Inappropriate Vasopressin Secretion (SIADH): Water increase
  • It is also called hypoosmotic volume expansion.
  • LEC osmolarity decreases because of excess water retention.
  • LEC volume increases because of water retention. The water enters the cells; because of this, the LIC osmolarity decreases to match the LEC osmolarity, and the LIC volume increases.
  • Plasma protein concentration decreases due to increased LEC volume. Although it would be expected that hematocrit would decrease, it remains the same because water enters into the erythrocytes, which increases their volume and compensates for diluent effect of increasing LEC.
• Corticosuprarrenal Insufficiency: NaCl loss
  • It is also called hypoosmotic volume contraction.
  • LEC osmolarity decreases. As a result of absent aldosterone in corticosuprarrenal insufficiency, NaCl reabsorption is reduced, and the kidneys excrete more NaCl than water.
  • LEC volume decreases. Water enters the cells; as a result, LIC osmolarity decreases to match the LEC osmolarity, and LIC volume increases.
  • Plasma protein concentration increases due to decreased LEC volume. The hematocrit increases due to decreased LEC volume, because red blood cells swell as result of water entering into them.
  • Arterial pressure decreases due to decreased LEC volume.

Renal Clearance, Renal Blood Flow, and Glomerular Filtration Rate

• Clearance Equation
  • Indicates the plasma volume from which a substance is eliminated per unit time.
  • Clearance units are mL/min and mL/24 h.
  • C = UV/P
    • C = clearance (mL/min or mL/24 hr)
    • U = urine concentration (mg/mL)
    • V = urine volume/time (mL/min)
    • P = plasma concentration (mg/mL)
    • Example: If plasma [Na⁺] is 140 mEq/L, the urine [Na⁺] is 700 mEq/L, and the urine flow (diuresis) is 1 mL/min, the Na⁺ clearance is:
    • CNa+ = ([U]Na+ × V)/[P]Na+ = (700 mEq/L × 1 mL/min)/140 mEq/L = 5 mL/min
• Renal Blood Flow (RBF)
  • It is 25% of the cardiac output.
  • It is directly proportional to pressure gradient between the renal artery and renal vein and inversely proportional to the resistance opposed by the renal vasculature.
  • Activation of the sympathetic nervous system and angiotensin II cause vasoconstriction of the renal arterioles, which reduces the renal blood flow (RBF). At low concentrations, angiotensin II preferentially constricts the efferent arterioles, which "protects" (increases) the glomerular filtration rate (GFR). Angiotensin converting enzyme inhibitors (ACEIs) dilate the efferent arterioles and reduce GFR; these drugs reduce hyperfiltration and the onset of diabetic nephropathy in diabetes.
  • Prostaglandins E2 and I2, bradykinin, nitric oxide, and dopamine cause vasodilation of the renal arterioles, leading to increased RBF.
  • Atrial natriuretic peptide (ANP) causes vasodilation of afferent arterioles and, to a lesser extent, vasoconstriction of efferent arterioles; ANP increases the RBF overall.
• Autoregulation of RBF
  • It is carried out by modifying renal vascular resistance.
  • If the arterial pressure varies, there is a proportional change in renal vascular resistance to maintain constant RBF.
  • RBF remains constant in an arterial pressure range between 80 and 200 mm Hg (autoregulation).
  • The mechanisms of autoregulation include:
    • Myogenic mechanism, the afferent renal arterioles constrict in response to stretching. In this way, increased renal arterial pressure causes the arterioles to stretch, which constricts them and increases resistance to maintain a constant blood flow.
    • Tubuloglomerular Autoregulation, greater renal arterial pressure causes an increase in liquid supply to the dense macula. This detects the elevated load and causes afferent arteriole contraction, increasing resistance to maintain constant blood flow.
    • A high protein diet increases the GFR by increasing the reabsorption of sodium and chloride and decreasing the supply of sodium and chloride to the macula densa, thereby increasing the GFR through tubuloglomerular feedback.
• Determining Renal Plasma Flow: Para-aminohippuric Acid (PAH) Clearance
  • Renal tubules filter and secrete PAН.
  • Clearance of PAH is used to measure renal plasma flow (RPF).
  • PAH clearance measures effective RPF and underestimates actual RPF by 10%. PAH clearance does not determine renal plasma flow from kidney regions that do not filter or secrete PAH, such as adipose tissue.
  • RPF = ( [U]PAH × V) / [P]PAH RPF = renal plasma flow (mL/min or mL/24 h), CPAH = РAH clearance (mL/min or mL/24 h), [U] PAH = PAH concentration in urine (mg/mL), V = urine output or diuresis (mL/min or mL/24 h), [P] PAH = plasma concentration of PAH (mg/mL)
• Determining RBF
• RBF = RPF / (1 – Hematocrit). Observe that the denominator of this formula, 1 – hematocrit, is the fraction of blood volume occupied by plasma.

Glomerular Filtration Rate (GFR)

• Determining GFR: Inulin Clearance
  • Renal tubules filter inulin but do not reabsorb or secrete it.
  • Inulin clearance is used to determine GFR:
    • GFR = ([U]inulin × V) / [P]inulin
      • GFR = glomerular filtration rate (mL/min or mL/24 h), [U]inulin = inulin concentration in urine (mg/mL), V = urine output or diuresis (mL/min or mL/24 h), [P] inulin = plasma concentration of inulin (mg/mL).
• Example Calculation of GFR Se infuse inulin into a patient until reaching a steady-state plasma concentration of 1 mg/mL. Collect a urine sample over 1 hour, obtaining a volume of 60 mL and a urine concentration of inulin of 120 mg/mL. What is the GFR of the patient?
• GFR = (120 mg/mL × 60 mL/h) / 1 mg/mL = (120 mg/mL × 1 mL/min) / 1 mg/mL = 120 mL/min

Estimating GFR with Blood Urea Nitrogen (BUN) and Serum Creatinine

• BUN and Serum Creatinine both increase when GFR decreases.
• In prerenal uremia (hypovolemia), BUN increases more than serum creatinine and the BUN/creatinine ratio increases (> 20 to 1). Hypovolemia increases urea reabsorption in the proximal tubule, increasing the BUN/creatinine.
• GFR decreases with age, serum creatinine remains constant due to lower muscle mass.

Filtration Fraction

• Fraction of RPF filtered through the glomerular capillaries.
  • Filtration Fraction = GFR/RPF normally it's 0.20.
• Increased filtration fraction increases protein concentration in peritubular capillary blood, leading to greater reabsorption.
• Decreased filtration fraction decreases protein concentration in the peritubular capillary blood and the reabsorption at the proximal tubule.

Determination of GFR: Starling Forces

• The driving force of glomerular filtration rate is the net ultrafiltration pressure through the glomerular capillaries.
• In glomerular capillaries, filtration is always favored because net ultrafiltration pressure always favors fluid exiting the capillary. GFR can be expressed by Starling's law:
  • GFR = Kf [(PGC - PBS) - (πGC - πBS)] GFR = Filtration across the glomerular capillaries, Kf = filtration coefficient for glomerular capillaries.
  • The glomerular barrier consists of the capillary endothelium, basement membrane, and filtration slits between podocytes. Normally, anionic glycoproteins line the filtration barrier and limit plasmatic protein filtration as they also have negative charge. In the glomerular disease, the anionic charges present in the barrier may be removed, translating in proteinuria.
• PGC Hydrostatic pressure of the glomerular capillary, which is constant along the whole length of the capillary. Increases with afferent arteriole dilation or efferent arteriole constriction. Increasing PGC elevates net ultrafiltration pressure of the GFR.
• PBS Hydrostatic pressure in Bowman's space and analogous to Pi₁ in the capillaries of general circulation. Increases with ureter constriction. Increasing PBS reduces net ultrafiltration pressure and the GFR.
• πGC Oncotic pressure of the glomerular capillary; it normally increases along the glomerular capillary length, because filtration of water elevates protein concentration in glomerular capillary blood. Increases with protein concentration, increasing πGC lowers net ultrafiltration pressure and GFR.
• πBS Oncotic pressure of the Bowman’s space, usually zero (and therefore often set aside) because only a small amount of protein is normally filtered.
• Typical Calculation of Ultrafiltration Pressure with Starling's Equation: At the afferent arteriolar end of a glomerular capillary, PGC is 45 mmHg, PBS is 10 mmHg, and πGC is 27 mmHg. What are the value and direction of net ultrafiltration pressure?
  • Net Pressure = (PGC - PBS) - πGC = (45 mm Hg - 10 mm Hg) - 27 mm Hg = +8 mm Hg (favors filtration)
• Variation of Starling's Forces: Effect on GFR and Filtration Fraction
• Effects of Changes in Starling's Forces on GFR, RPF, and Filtration Fraction.
  • Constriction of the afferent arteriole (sympathetic example) ↓TFG ↓RPF, no change in the filtration fraction.
  • Constriction of the efferent arteriole ↑TFG ↓RPF, ↑filtration fraction.
  • Increased plasma protein ↓TFG with no change in RPF, ↓filtration fraction.
  • Ureteral calculi ↓TFG with no change in RPF, ↓filtration fraction.

Reabsorption and Secretion, Calculation of Reabsorption and Secretion Rates

• Reabsorption or secretion rate is the difference between the amount filtered through the glomerular capillaries and the amount excreted in urine.
• Calculated as follows:
  • Filtered Load = GFR × [plasma]
  • Excretion Rate = V x [urine]
  • Reabsorption Rate = Filtered Load – Excretion Rate
  • Secretion Rate = Excretion Rate - Filtered Load
• If the filtered load is greater than the excretion rate, then net reabsorption of the substance has occurred. If the filtered load is less than the excretion rate, then net secretion of the substance has occurred.
• Example: woman with untreated diabetes has a GFR of 120 mL/min, plasma concentration of glucose of 400 mg/dL, glucose concentration in urine of 2,500 mg/dL, and diuresis of 4 mL/min. What is the glucose reabsorption rate?
  • Filtered Load = GFR × [plasma glucose] = 120 mL/min × 400 mg/dL = 480 mg/min
  • Excretion = V × [glucose] in urine = 4 mL/min × 2500 mg/dL = 100mg/min
  • Reabsorption = 480 mg/min - 100 mg/min = 380 mg/min

Glucose Transport Curve (Tm)

• Reabsorbed Substance
• Filtered Glucose Load
  • Increases directly proportionally to plasma glucose concentration (filtered glucose load = GFR × [P]glucose).
• Glucose Reabsorption
  • Na⁺-glucose cotransport in the proximal tubule reabsorbs glucose from tubular fluid to blood using limited sodium glucose transporters.
  • At plasma glucose concentrations under 250 mg/dL, all filtered glucose can be reabsorbed because many transporters are available; reabsorption line coincides with the filtration line.
  • At plasma glucose concentrations over 350 mg/dL, transporters are saturated. A plasma concentration increase over 350 mg/dL does not translate into an increased reabsorption rate
• Glucose Rate of reabsorption at which carriers are saturated is the transport maximum. Glucose Excretion
  • At plasma glucose concentrations under 250 mg/dL, all filtered glucose is reabsorbed and excretion is zero. The threshold (defined as the plasma concentration at which glucose first appears in urine) is around 250 mg/dL.
  • At plasma glucose concentrations over 350 mg/dL, reabsorption is saturated (Tm), when the plasma concentration increases, filtered glucose cannot be reabsorbed and is excreted in the urine. Splay
  • Section of glucose curves comprised between the threshold and the transport maximum.
  • Found between plasma glucose concentrations of around 250 and 350 mg/dL.
• Represents urine glucose excretion before reaching reabsorption saturation (Tm).
• Explained with nephron heterogeneity and low Na+-glucose transporter affinity.

Para-aminohippuric Acid (PAH) Transport Curve

• Secreted Substance
• Filtered PAH Load: Increases directly proportionally to plasma PAH concentration.
• PAH Secretion
  • PAH secretion goes from peritubular capillary blood to tubular fluid (urine) via the proximal tubule transporters.
  • At low plasma PAH concentrations, secretion rate increases with plasma concentration.
  • When transporters are saturated, the additional increase in PAH plasma concentration causes no increase in secretion rate (Tm).
• PAH Excretion
  • PAH excretion is the sum of filtration through the glomerular capillaries plus the secretion from the capillary blood to the tubule. Curve shape depends on clearance. Excretion curve is more pronounced at low plasma concentrations than transport maximum. Transport exceeds transport maximum in secretion and all transport molecules are saturated. FPR is determined with PAH clearance at plasma PAH concentrations that are lower than transport maximal.

Relative Clearance of Substances

• Substances with highest clearance are those filtered through glomerular capillaries and secreted from peritubular capillaries to the tubule.
• Substances with lowest clearance are those not filtered (e.g., proteins) or filtered and reabsorbed.

Nonionic Diffusion

• Weak acids have HA and A⁻ forms.
• The HA form is uncharged and lipid-soluble so can "back-diffuse" from urine into the blood.
• The A⁻ form is charged and not lipid-soluble, so it cannot diffuse from the urine into the blood.
• At a pH urinary acid it favors HA, increasing back-diffusion into the blood and diminishing weak acid excretion.
• At a pH urinary alkaline, the A⁻ form is favored, decreasing back-diffusion into the blood increasing the weak acid excretion.
  • Salicylic acid example, can be increased with urine alkalinization. Bases
• Bases form is BH+ and a form B
• B form uncharged, lipid-soluble can "back-diffuse" into the blood.
• form BH+ charged, not lipid-soluble cannot back-diffuse into the blood
• At a pH urinary acid, favors BH+ decreases back-diffusion into the blood increases weak base excretion, use in morphine to increase excretion acidifying the urine
• At a pH urinary acid alkaline, favors B more back-diffusion into the blood less weak base excretion.

Regulation of NaCl

• Nephron terminology:Tubular fluid (TF) is urine at any point in the nephron, Plasma (P) is systemic circulation and is constant.
• TF/Px ratio-concentrates a substance into tubular fluid for any nephron position, TF/P=1 Means, no reabsorption or substance proportionally resorbed. TF/Pna+=1 Na concentration in the tubular fluid like the plasma, =1 for Bowman's space freely filters it TF/P<1 greater water reabsorption. tubular Fluid concentration more than plasm