Chapter 26: Fluid Electrolyte Balance

Around 66% of total body water found here

• K+, proteins, hydrogen phosphate
• Proteins are common anions found here

Intracellular Fluid (ICF)

Around 33% of remaining body water found here

• Plasma and Interstitial fluid (IF) are components of this
• IF = lymph, cerebropsinal fluid, humors, serous and synovial fluid

Extracellular Fluid (ECF)

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1. Anything that dissociates into ions in water (+/- charge)
2. Most abundant form of solutes
3. More responsible for fluid shifts/movement of water
1. Inorganic salts, acids/bases, some proteins
4. Water flows form high # of solutes to low # (not what the solute is)

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Electrolytes

1. Do not dissociate in water

2. No Charge

3. Make up the bulk of body fluids

   1. Ex: Glucose, urea, lipids, etc

Non-Electrolytes

Optimal body water content depends on age, body mass, sex, body fat %
Age

• Infants have a larger SA to body surface ratio (skin) than adults --> infants have a higher water ratio

Sex

• More testosterone = more water

Body Fat

• Increased adipose tissue means lower water content since adipose tissue is vert dehydrated

Body Water Content

1. Igested food and liquid, most water intake comes from diet

2. Metabolic water, water that comes from reactions

Sources of Water Intake

1. Insensible water loss (lungs/skin)
2. Sensible water loss (sweat, urine, feces)

Sources of Water Ouput

Hypothalamic thirst center controls the thirst mechanism is activated by

• Osmoreceptors --> detect changing ECF osmolality
• Dry mouth --> salivary glands cannot draw water from blood to produce saliva
• Decreased blood volume/pressure --> around 5-10% drop initiates thirst mechanism

Regulating Intake

Obligatory water loss --> the body will always lose water, even if we never drink water

• Why? Insensible water loss, kidneys never stop functioning

Urine output depends on fluid intake, other sources of water loss

• Excess water is eliminated in urine

Regulating Water Output

ADH causes aquaporins to be inserted in collecting ducts

• ADH only hormone that regulates water

Release of ADH dependent on, osmoreceptors measuring concentration of ECF, and baroreceptora monitoring BP

Water Balance and Role of ADH

Decrease in ADH produced by hypothalamus or released by posterior pituitary
Symptoms include

• Polyuria (peeing very frequently) and then polydipsia (excessive thirst), very dilute urine fatigue, eventual dehydration

Central Diabetes Insipidus

ADH is produced and released in normal amounts, but they kidneys are unresponsive to it

Nephrogenic Diabetes Insipidus

Influences water movement in body, excitability of neurons, membrane permeability etc, etc

Salt intake comes mostly from diet, with small amount coming from metabolic processes

• Salt loss: urine and feces, sweat, and vomit
• Renal processes help body retain what is needed

Electrolye Balance

NaHCO₃ and NaCl account for around 280mOsm of total ECF solute 

• Most important in estabolishing osmotic gradient since water moves with Na+
  
• Plasma membranes are impermeable to Na+  --> almost always lept out of cells and in the ECF which prevents unneccessary changes of membrane permability

Sodium Balance

Most Na+ reabsorbed in PCT and nephron loop (around 85%)

• Reabsorbing more Na+ = Reabsorbing more water

Regulation of Na+

Release causes increases reabsorption of Na+ in DCT and collecting ducts

• Side effect of aldosterone release; increase in ECF volume

Aldosterone Regulation of Na+

Release causes decreased reabsorption of Na+

• Is diuretic and natriuretic (increase urine formation and promote sodium excretion)

Atrial Netriuretic Peptide Regulation of Na+

Estrogen exerts similar effect as aslosterone

• More estrogen = more Na+ reabsorption

Progesterone is slightly diuretic

Sex Hormones Regulation of Na+

In high plasma levels, exerts very strong aldosterone-like effects (Strong Na+ retention)

• Can contribute substantially to edema

Glucocorticoids Regulation of Na+

Heavy regulation due to effect of resting membrane potential

• Buffer --> K+ moves in the opposite direction of H+ to balance pH

Potassium Balance

1. Principle cells secrete K+ in the DCT and collecting ducts which can alter how much based on what needs to be excreted

2. Type A intercalated cells can reabsorb K+ when levels are exceptionally low

   1. This means hypokalemic to release type A, kidneys are not good at potassium reabsorption

Primary Mechanism of Potassium Balance: Renal

High ECF K+ concentrations drive excess K+ into principal cells --> increased secretition and excretition of K+

• Low ECF K+ concentrations promote reabsorption

K+ Secretion Depends on: Plasma Concentration

Stimulates K+ secretion

• Adrenal cortex secretes aldosterone when K+ ECF concentrations are high overall
• Renal mechanisms will preserve desirable K+ concentration

Aldosterone effect on K+ Regulation

Optimal pH of arterial blood is 7.35-7.45

• Less is alkalosis, more is physiological acidosis

We get H+ from ingested food, and metabolic processes such as lactic acid, loading of CO2, phosphoric acid, etc

pH Balance

Chemical Buffer Systems is one of more compounds that resist changes in pH when strong acids or based are introduced

• Release H+ when pH rises, bind H+ when pH drops

Regulating H+ Concentration

Important for ECF, mixture of carbonic acid (weak acid) and bicarbonate salt (weak base)

• Bicarbonate salt ties up free H+ from a strong acid --> converted to carbonic acid
• Lowers the pH only slightly
• Carbonic acid ties up free OH- from a strong base --> converted to a bicarbonate salt
• Raises the pH only slightly

Bicarbonate Buffer Systems

Strong acid --> weak acid (bicarbonate salt)

• HCl + NaHCO₃ --> H₂CO₃ + NaCl

Strong base --> weak base (carbonic acid)

• NaOH + H₂CO₃ --> NaHCO₃ + H₂O

Bicarbonate Buffer System Equations

Important for ICF and Urine

• Similar to bicarbonate buffer system, but utilizes different weak acids and bases
• Salts of dihydrogen phosphate (weak acid) and monohydrogen phosphate (weak base)

Phosphate Buffer System

Strong acid --> Weak Acid

• HCl + Na₂HPO₄ --> NaH₂PO₄ + NaCl

Strong Base --> Weak Base

• NaOH + NaH₂PO₄ --> Na₂HPO₄ + H₂O

Phosphate Buffer System Equations

Important in ICF and blood plasma

• Carboxyl group (-COOH) can release H+ when pH rises
• Amine group (NH₂) group can bind free H+ when pH decreases

Amphoteric Molecule

• A single protein can react as either an acid or base (anflatiric)
• Depends on the pH of the environment

Protein Buffer System

Proteins acting as an acid

• R-COOH --> R-COO^- + H⁺ 

Proteins acting as a base

• R-NH₂ + H⁺ --> R-NH₃

Protein Buffer System Equations

Rising P_CO2 activates respiratory centers

• Respiratory rate + depth increases

• pH rises as more CO₂ is blown off

Decreasing P_CO2 depresses respiratory centers

• Respiratory rate + depth decreasses

• pH decreases as CO₂ accumulates

Respiratory Regulation of H+

PCT and Type A intercalated cells of collecting ducts can generate new bicarbonate ions to be reabsorbed

• H+ mst be secreted into filtrate at the same time

Generating New Bicarbonate

Type B intercalated cells in collecting ducts can reabsorbed H+ while secreting bicarbonate ions from filtrate

• Secretion of bicarbonate is not efficient --> even in alkalosis, more bicarbonate will be reabsorbed than secreted

Secretion of Bicarbonate