Homeostatic and Fluid Balance

Homeostatic Balance

• Maintaining a stable internal environment is essential for cellular function

Balance

• Cellular function requires a fluid medium with a carefully controlled composition
• There are three types of homeostatic balance -Water -Electrolyte -Acid-Base
• Maintaining balance requires the collective action of several body systems -Urinary, Respiratory, Digestive, Integumentary, Endocrine, Nervous, Cardiovascular, and Lymphatic.

Total Body Water

• Total body fluid is comprised of 2/3 Intracellular fluid (ICF)
• 1/3 Extracellular fluid (ECF) comprised of 80% Interstitial Fluid, 20% Plasma
• Total body mass (female) is 55% Fluids, 45% Solids
• Total body mass (male) is 60% Fluids, 40% Solids

Fluid Balance

• Sensible water loss - measured water loss
• Insensible water loss - unmeasured water loss
• Obligatory water loss - required water loss
• Daily Water Intake is 2,500 mL/day -Metabolic Water: 200 mL -Food: 700 mL -Drink: 1,600 mL
• Daily Water Output is 2,500 mL/day -Feces: 200 mL -Expired air: 300 mL -Cutaneous Transpiration: 400 mL -Sweat: 100 mL -Urine: 1,500 mL

Dehydration

• Dehydration occurs when fluid output exceeds intake over a long period
• This is signaled by increased blood osmolarity, leading to the following: -Stimulation of hypothalamic osmoreceptors -Resulting in reduced salivation and dry mouth -Reduced blood pressure will cause renin to form Angiotensin II -Stimulating hypothalamic osmoreceptors
• This process leads to a sense of thirst, resulting in the ingestion of water
• Ingestion of water cools and moistens the mouth and rehydrates the blood -This is short-term and long-term inhibition of thirst by distending the stomach and intestines

ADH Secretion

• ADH is secreted in response to low H₂0 or high Na+
• Dehydration leads to increased blood osmolarity
• Stimulates hypothalamic osmoreceptors: -Stimulates posterior pituitary to release antidiuretic hormone (ADH) -Stimulates distal convoluted tubule and collecting duct -Increases water reabsorption -Reduces urine volume -Increases the ratio of Na+: H₂O in urine

Disorders of Water Balance

• Abnormality of total volume, concentration, or distribution of fluid among the compartments

Fluid Deficiency

• Fluid output exceeds intake over long period -Leads to Volume depletion (hypovolemia)
• Water and sodium are lost without replacement -Total body water declines, but osmolarity remains normal -Can be caused by hemorrhage, severe burns, chronic vomiting, or diarrhea

Dehydration

• Negative water balance where the body eliminates significantly more water than sodium -Total body water declines, osmolarity rises
• Caused by lack of drinking water, diabetes, ADH hyposecretion, profuse sweating, overuse of diuretics

Fluid Balance in Cold Weather

• The body conserves heat by constricting blood vessels of the skin -Raises BP -Inhibits ADH secretion and increases ANP secretion -Urine output is increased and blood volume is reduced
• Cold air is drier and increases respiratory water loss - reducing blood volume -Cold weather respiratory and urinary loses cause a state of reduced blood volume -Leads to insufficient blood for rest of the body; weakness, fatigue, or fainting -Exercise dilates vessels in skeletal muscles

Dehydration from Excessive Sweating

• Water is lost from sweating
• Sweat is produced by capillary filtration
• Blood volume and pressure drop, osmolarity rises
• Blood absorbs tissue fluid to replace loss
• Tissue fluid is pulled from ICF
• Leads to all 3 compartments losing water
• An example with 300 ml from tissue fluid (700 ml from ICF)

Fluid Excess

• Less common than fluid deficiency -Can be caused by Renal failure
• Includes these types: -Volume excess -Both Na+ and Water retained equally -ECF remains isotonic -Can be caused by Aldosterone hypersecretion or renal failure -Hypotonic hydration (water intoxication) -More water than Na+ retained or ingested -ECF hypotonic -Leads to Cellular swelling -Pulmonary and cerebral edema

Electrolyte Concentrations and Distribution

• Key elements include Na+, K+, Ca+, Mg+, Cl-, HCO3
• Extracellular fluids include: Blood plasma and Interstitial fluid
• There are different concentrations in intracellular fluid vs. Extracellular

Sodium

• Key point is responsible for the RMPs
• Principal cation in ECF -Accounts for 90 - 95% of osmolarity of ECF -Most significant solute
• Substances involved regulate sodium concentration: -SGLT and Na+- K+ pump -NaHCO3 has major role in buffering pH in ECF -ADH – causes reabsorption of water due to high sodium concentration -Aldosterone - "salt retaining hormone" -ANP and BNP inhibit sodium and water reabsorption
• Imbalances include Hyper- and Hyponatremia

Potasium

• Is the Most abundant cation of ICF
• Greatest determinant of intracellular osmolarity and cell volume
• Substances include: -Na+-K+ pump -Essential cofactor -Potassium homeostasis is closely linked to that of sodium (Aldosterone)

Aldosterone

• Hyponatremia and hyperkalemia directly stimulate Aldosterone secretion
• Hypotension activates Renin-Angiotensin system
• Process: Stimulates adrenal cortex to secrete aldosterone Stimulates renal tubules and Increases Na+ reabsorption -Less Na+ and H₂O in urine Supports existing fluid volume and Na+ concentration pending oral intake -Increases K+ secretion More K+ in urine

Chloride

• The most abundant anion in ECF
• HCI
• Chloride shift (CO₂ loading/unloading in RBCs)
• Major role in regulating body pH
• Homeostasis - primary homeostasis achieved as an effect of Na+ -As sodium is retained, chloride ions passively follow
• Imbalances include: Hyper- and Hypochloremia

Calcium

• Lends strength to the skeleton
• Muscle contraction
• Secondary messenger
• Activates exocytosis of neurotransmitters and other cellular secretions
• Essential factor in blood clotting
• Homeostasis is regulated by PTH, calcitriol, and calcitonin
• Calsequestrin - proteins that bind Ca2+ and keep it unreactive in Ca2+ storage cells

Calcium Balance

• Blood calcium levels are regulated by calcitonin and parathyroid hormone
• Calcitonin: -Released when blood calcium concentration is too high -Leads to less bone resorption and more bone deposition -Reduced osteoclast and increased osteoblast activity
• Parathyroid hormone: -Released when blood calcium concentration is too low -Leads to more bone resorption and less bone deposition -Increased osteoclast and reduced osteoblast activity -Promotes more urinary phosphate excretion and less urinary calcium excretion -Prevention of hydroxyapatite formation Conservation of calcium
• PTH increases excretion of phosphate which increases concentration of free calcium in the ECF

Vitamin D Synthesis

• Begins with 7-dehydrocholesterol in the skin
• Ultraviolet light converts it to Vitamin D3 (cholecalciferol)
• The liver then converts it into Calcidiol
• HO can convert it to Calcitriol -Leads to bone resorption, reducing excretion of Ca2+ and absorption of Ca2+ and phosphate

Acid-Base Balance

• pH Measures how acidic or basic a substance
• Normal pH: 7.35-7.45
• Acidosis happens when pH goes below 7.35
• Alkalosis happens when pH goes above 7.45

Buffers

• Any mechanism that resists changes in pH
• Types: -Physiological: -Urinary system buffers greatest quantity of acid or base (slow) -Takes several hours to days to exert its effect -Respiratory system buffers within minutes (rapid), -Cannot alter pH as much as the urinary system -Chemical buffers: -Restore normal pH in fractions of a second Composed of weak acids and weak bases -Major chemical buffers include: Bicarbonate, Phosphate, Protein Systems

Bicarbonate Buffer System

• A solution of carbonic acid and bicarbonate ions -CO2 + H2O → H2CO3 → HCO3- + H+
• Reversible reaction important in ECF Formula: -CO2 + H2O → H2CO3 → HCO3- + H+ -CO2 + H2O ← H₂CO₃ ← HCO3¯ + H+
• To lower pH, kidneys excrete HCO3-
• To raise pH, kidneys excrete H+ and lungs excrete CO₂

Phosphate Buffer System

• Formula: H2PO4 HPO42- + H+ -Monohydrogen phosphate (HPO42-) weak base -Dihydrogen phosphate (H2PO4) weak acid
• Action is nearly identical to the bicarbonate buffer
• More important buffering the ICF and renal tubules -Constant production of metabolic acids creates pH values from 4.5 to 7.4 in the ICF, avg. 7.0

Protein Buffer System

• Proteins are more concentrated than bicarbonate or phosphate systems, especially in the ICF
• Accounts for about 75% of all chemical buffering
• Carboxyl (-COOH) side groups release H+ when pH begins to rise -COOH -COO- + H+
• Amino (-NH2) side groups bind H⁺ when pH gets too low -NH2 + H+ → -NH3+

Respiratory Control of pH

• Neutralizes 2-3X as much acid as chemical buffers
• CO₂ is constantly produced by aerobic metabolism
• Increased CO₂(low pH) stimulates pulmonary ventilation -CO2 (expired) + H₂O ← H₂CO₃ ← HCO3- + H+
• Decreased CO₂ (elevated pH) inhibits pulmonary ventilation -CO2 + H2O → H2CO3 → HCO3- + H+
• Respiratory compensation – changes in pulmonary ventilation to correct changes in pH of body fluids by expelling or retaining CO₂

Renal Control of pH

• Neutralizes more acid or base than either the respiratory system or chemical buffers
• Renal tubules secrete H+ into the tubular fluid -Excreted in the urine -Other buffer systems only reduce its concentration by binding it to other chemicals
• Renal compensation is slow, but better at restoring a fully normal pH -In Acidosis: renal tubules increase rate of H+ secretion, elevating pH -In Alkalosis: renal tubules decrease rate of H⁺ secretion, lowering pH -Kidneys cannot act quickly enough to compensate for short-term pH imbalances

Disorders of Acid-Base Balance

• Acidosis is hypoventilation, apnea, or respiratory arrest; asthma, emphysema, chronic bronchitis
• Alkalosis is hyperventilation due to emotions or oxygen deficiency (as at high altitudes)
• Acidosis is excess production of organic acids, as in diabetes mellitus and long-term anaerobic fermentation; drugs such as aspirin and laxatives; chronic diarrhea
• Alkalosis is rare but can result from chronic vomiting or overuse of bicarbonates (antacids)
• Acidosis – pH below 7.35 -H+ diffuses into cells and drives out K+, elevating K+ conc. in ECF -Hyperpolarization and Cells are less excitable -CNS depression may lead to confusion, disorientation, coma, possibly death
• Alkalosis – pH above 7.45 -H+ diffuses out of cells and K+ diffuses in -Membranes depolarized and nerves overstimulated -Muscles spasms, tetani, convulsions, respiratory paralysis

Fluid Replacement Therapy

• Fluids may be administered to replenish total body water, restore blood volume and pressure, shift water from one fluid compartment to another, restore and maintain electrolyte and acid-base balance
• Drinking water is the simplest method
• Does not replace electrolytes but broths, juices, and sports drinks replace water, carbohydrates, and electrolytes