Water, Body Fluids, and Electrolytes

• Water and Body Fluids are key components of the human body
• Fluid balance, body fluid composition, and water balance regulation are vital
• Major electrolytes play a crucial role in body fluids

Body Fluid Compartments

• Body fluids make up 55% of body mass in adult females
• Body fluids make up 60% of body mass in adult males
• Intracellular fluid (ICF) is inside cells and accounts for about 2/3 of body fluid
• Extracellular fluid (ECF) is outside cells
• Interstitial fluid between cells is 80% of ECF
• Plasma in blood is 20% of ECF
• Lymph, cerebrospinal fluid, synovial fluid, aqueous humor, vitreous body, endolymph, perilymph, pleural, and peritoneal fluids are also part of ECF

Composition of Body Fluids

• Extracellular fluid contains large quantities of sodium, chlorides, bicarbonate, glucose, fatty acids, and oxygen
• Intracellular fluid contains large quantities of potassium, magnesium, phosphates, sulphates and proteins
• The pH of ECF is 7.4
• The pH of ICF is 7

Significance of Body Fluids

• Proper quantities of glucose, amino acids, lipids, vitamins, ions, oxygen, etc. needed for Growth and functions of cells
• Water inside the cells creates the medium for various metabolic reactions
• The characteristic form and texture of various tissues is dependant on the water inside the cells
• Body water plays a vital role in maintaining normal body temperature

Fluid Balance

• Two barriers separate ICF, interstitial fluid, and plasma
• The plasma membrane separates ICF from surrounding interstitial fluid
• The blood vessel wall divides interstitial fluid from plasma
• Body is in fluid balance when required amounts of water and solutes are present and correctly proportioned among compartments
• Filtration, reabsorption, diffusion, and osmosis all maintain continual exchange of water and solutes among compartments

Sources of Body Water Gain and Loss

• Fluid balance is related to electrolyte balance
• Intake of water and electrolytes are rarely proportional
• Kidneys excrete excess water through dilute urine or excess electrolytes through concentrated urine
• Water can be gained through ingestion of liquids and moist foods (2300mL/day)
• Metabolic synthesis of water during cellular respiration and dehydration synthesis accounts for (200mL/day)
• Water is lost through Kidneys (1500mL/day)
• Water is lost through evaporation from skin (600mL/day)
• Water is lost through exhalation from lungs (300mL/day)
• Water is lost through feces (100mL/day)
• When intake is more than the output a positive water balance develops
• Physiological positive water balance can be due to growth, convalescence, and pregnancy
• Pathological positive water balance is also known as edema
• Edema is developed in heart failure, renal failure, malnutrition, and inflammation of tissue
• Dehydration indicates excessive body water loss
• Severe vomiting, excessive sweating, fluid loss in burns, Addison's disease, diabetes, and renal disease can lead to dehydration
• Clinical symptoms include low cardiac output, rapid pulse rate, low blood pressure, decreased skin turgor, acidosis, sunken eyeballs, lethargy, confusion and coma
• Intracellular dehydration can lead to death
• Treatment involves the replenishment of body with water and electrolytes
• Electrolytes and pH should be monitored carefully during severe dehydration

Regulation of Body Water Gain

• Volume of water intake determines how much you drink
• Dehydration occurs when water loss exceeds gain
• A decrease in volume and an increase in osmolarity of body fluids stimulates the thirst center in the hypothalamus
• Increased osmolarity in the blood stimulates the secretion of antidiuretic hormone (ADH)
• ADH results in increased water reabsorption and more concentrated urine

Antidiuretic Hormone (ADH)

• ADH is also known as vasopressin
• ADH is produced by the hypothalamus and released from the posterior pituitary
• ADH increases permeability to water and produces concentrated urine
• Kidneys respond to reduced blood flow by releasing the enzyme renin
• Renin initiates the activation of the protein angiotensinogen to angiotensin
• Angiotensin signals the adrenal glands to secrete aldosterone
• Angiotensin causes the blood vessels to constrict, raising pressure
• The hypothalamus responds to high salt concentrations in the blood by stimulating the pituitary gland
• The pituitary gland releases antidiuretic hormone (ADH)

Sodium Regulation

• Aldosterone and ADH signal the kidneys to retain sodium and water to increase blood volume
• Low aldosterone levels favor increased sodium excretion in the urine, whereas aldosterone stimulates decreased sodium excretion
• Cells normally neither shrink nor swell because intracellular and interstitial fluids have the same osmolarity
• Increasing osmolarity of interstitial fluid draws water out of cells and causes cells to shrink
• Decreasing osmolarity of interstitial fluid causes cells to swell
• Changes in osmolarity from changes in Na+ concentration
• Water intoxication occurs when drinking water faster than the kidneys can excrete it, which can lead to convulsions, coma, or death
• A series of events in water intoxication include excessive blood loss, sweating, vomiting, or diarrhea coupled with the intake of plain water
• This results in decreased Na+ concentration of interstitial fluid and plasma (hyponatremia)
• Decreased osmolarity of interstitial fluid and plasma causes osmosis of water from interstitial fluid into intracellular fluid causing water intoxication.
• Water intoxication causes cells to swell
• Water intoxication causes convulsions, coma, and possible death

Electrolytes in Body Fluids

• Ions form when electrolytes dissolve and dissociate
• Electrolytes have four general functions
• Electrolytes Control osmosis of water between body fluid compartments
• Electrolytes Help maintain the acid-base balance
• Electrolytes Carry electrical current
• Electrolytes Serve as cofactors
• ECF most abundant cation is Na+, and the anion is Cl-
• ICF most abundant cation is K+, and the anion is proteins and phosphates (HPO42-)
• Na+/K+ pumps play a major role in keeping K+ high inside cells and Na+ high outside cell
• Sodium Na+ is the most abundant ion in ECF
• It accounts for 90% of extracellular cations
• Sodium plays a pivotal role in fluid and electrolyte balance
• Sodium accounts for almost half of the osmolarity of ECF
• Sodium levels in blood are controlled by aldosterone, which increases renal reabsorption
• If sodium is too low ADH release stops

Acid-Base Balance

• Major homeostatic challenge is keeping H+ concentration (pH) of body fluids at the appropriate level
• 3D shape of proteins are sensitive to pH
• Diets with large amounts of proteins produce more acids than bases, which acids blood
• Several mechanisms help maintain the pH of arterial blood between 7.35 and 7.45
• Buffer systems, exhalation of CO2, and kidney excretion of H+

Review Questions

• Majority of the fluid in the body is in the intracellular compartment
• The major extracellular cation is Na+
• With a hypertonic IV solution, red blood cells will shrink
• Oxytocin does not have a role in the regulation of body water homeostasis