Body Water Content
Infants have low body fat, low bone mass, and are 73% or more water
Total water content declines throughout life
Healthy males are about 60% water; healthy females are around 50%
This difference reflects females’: Higher body fat Smaller amount of skeletal muscle
In old age, only about 45% of body weight is water
Fluid Compartments
Water occupies two main fluid compartments
1- Intracellular fluid (ICF) – about two thirds by volume, contained in cells ( about 2/3 of fluid)
2 - Extracellular fluid (ECF) – consists of two major subdivisions (about 1/3 0f fluid)
Plasma – the fluid portion of the blood
 Interstitial fluid (IF) – fluid in microscopic spaces between cells
 --also found in lymph, cerebrospinal fluid, eye humors, synovial fluid (joints), serous fluid (includes saliva),
and gastrointestinal secretions
Composition of Body Fluids
Solutes are broadly classified into:
 Electrolytes – inorganic salts, all acids and bases, and some proteins
Electrolytes are chemical compounds that dissociate into ions in water.
They can conduct an electrical current & they carry a charge
Solutes are broadly classified into:
Nonelectrolytes – examples include glucose, lipids, creatinine, and urea
Nonelectrolytes have bonds that prevent them from dissociating in solution therefore, they carry no electrical
 Electrolytes have greater osmotic power than nonelectrolytes
 Water moves according to osmotic gradients
 Electrolytes have the greatest ability to cause fluid shifts
Electrolyte Composition of Body Fluids

Extracellular and Intracellular Fluids
 Each fluid compartment of the body has a distinctive pattern of electrolytes
 Extracellular fluids are similar (except for the high protein content of plasma) (ECF)
Sodium is the chief cation (positive charge)
Chlorine is the major anion (negative charge)
 Intracellular fluids have low sodium and chloride
Potassium is the chief cation (positive)
Phosphate is the chief anion (negative)
 Sodium and potassium concentrations in extra- and intracellular fluids are nearly opposites
 This reflects the activity of cellular ATP-dependent sodium-potassium pumps
 Electrolytes are the most abundant solutes in body fluids & determine the chemical and physical reactions of
        fluids BUT they do NOT constitute the bulk of dissolved solutes in these fluids
 Larger molecules including proteins, phospholipids, cholesterol, and neutral fats (nonelectrolytes) account for:
90% of the mass of solutes in plasma
60% of the mass of solutes in interstitial fluid
97% of the mass of solutes in the intracellular compartment
Fluid Movement Among Compartments
 Anything that changes solute concentration in any compartment leads to net water flows
 This is regulated by osmotic and hydrostatic pressures
 Lymphatic vessels collect “leaked” fluids & return them to bloodstream
 Cell membranes are selectively permeable, restricting some ions from moving through
 Movement of nutrients, respiratory gases, & wastes usually occurs in one direction
 Water moves freely both ways

Extracellular and Intracellular Fluids
 Plasma is the only fluid that circulates throughout the body and links external and internal environments
 Osmolalities of all body fluids are equal; changes in solute concentrations are quickly followed by osmotic
 (osmolality = # of solute particles dissolved in 1 kg of H2O)

Continuous Mixing of Body Fluids
Water Intake and Output
Water Balance and ECF Osmolality
It is typical for an adult to intake 2500 ml of water per day
To remain properly hydrated…
 water intake = water output
Water intake sources
Ingested fluid (60%) and solid food (30%)
Metabolic water or water of oxidation (10%)
Water Balance and ECF Osmolality
Water output
Urine (60%) and feces (4%)
Insensible losses(28%), sweat (8%)
--vaporizes out of lungs in expired air or diffuses through skin
Regulation of Water Intake
increases in plasma osmolality of only 2-3% excites the hypothalamic THIRST CENTER
causes thirst & release of antidiuretic hormone (ADH)
is stimulated:
By a decline in plasma volume of 10%–15%
By increases in plasma osmolality of 1–2%
Via baroreceptor input, angiotensin II, and other stimuli
Thirst is quenched as soon as we begin to drink water
Feedback signals that inhibit the thirst centers include:
Moistening of the mucosa of the mouth and throat
Activation of stomach and intestinal stretch receptors
Regulation of Water Output
Obligatory (unavoidable) water losses include:
Insensible water losses from lungs and skin
Water that accompanies undigested food residues in feces
Obligatory water loss reflects the fact that:
1- Kidneys excrete solutes to maintain blood homeostasis
2 - Urine solutes must be flushed out of the body in water
Minimum of 500 ml of urine excreted per day
Influence and Regulation of ADH
 Water reabsorption in collecting ducts is proportional to ADH release
 **ADH is inhibited within about 30 minutes after hydration & kidneys will begin to eliminate excess water
 Low ADH levels produce dilute urine and reduced volume of body fluids
 High ADH levels produce concentrated urine
 Hypothalamic osmoreceptors trigger or inhibit ADH release
Disorders of Water Balance: Dehydration
 Water loss exceeds water intake and the body is in negative fluid balance
 Causes include: hemorrhage, severe burns, prolonged vomiting or diarrhea, profuse sweating, water
deprivation, and diuretic abuse
 Signs and symptoms: cottonmouth, thirst, dry flushed skin, and oliguria
 Prolonged dehydration may lead to weight loss, fever, and mental confusion
 Other consequences include hypovolemic shock and loss of electrolytes

Disorders of Water Balance:
Hypotonic Hydration
Renal insufficiency or an extraordinary amount of water ingested quickly can lead to cellular overhydration,
                                                                                 or water intoxication
ECF is diluted – sodium content is normal but excess water is present
The resulting hyponatremia promotes net osmosis into tissue cells, causing swelling
These events must be quickly reversed to prevent severe metabolic disturbances, particularly in neurons
Hypotonic Hydration
Disorders of Water Balance: Edema
Atypical accumulation of fluid in the interstitial space, leading to tissue swelling
Caused by anything that increases flow of fluids out of the bloodstream or hinders their return
Factors that accelerate fluid loss include:
Increased blood pressure, inflammation
Congestive heart failure, hypertension, high blood volume
Electrolyte Balance – SALT BALANCE
 Electrolytes are salts, acids, and bases, but electrolyte balance usually refers only to salt balance
 Salts are important for:
 1 - Neuromuscular excitability
 2 - Secretory activity
 3 - Membrane permeability
 4 - Controlling fluid movements
 Salts come IN via eating & drinking
 Salts go OUT via sweat, feces, & urine
Sodium in Fluid and Electrolyte Balance
 **regulating sodium input & output is one of most important renal functions
 Sodium salts:
 Account for 90-95% of all solutes in the ECF
 Sodium is most abundant cation to regulation of fluid and electrolyte balance in body due to its abundance
 It controls water distribution in the body
 “water follows salt”
Sodium in Fluid and Electrolyte Balance
 The role of sodium in controlling ECF volume and water distribution in the body is a result of:
 Sodium being the only cation to exert significant osmotic pressure
 Sodium ions leaking into cells and being pumped out against their electrochemical gradient
 Sodium concentration in the ECF normally remains stable
Sodium in Fluid and Electrolyte Balance
 Since all body fluids are in chemical equilibrium, Changes in plasma sodium levels affect:
 Plasma volume, blood pressure which leads to changes in ICF (intracellular fluid) & insterstitial fluid volumes
 Renal acid-base control mechanisms are (linked) coupled to sodium ion transport
SODIUM BALANCE --is influenced by:
 #1 - Aldosterone
 #2 - ADH (antidiuretic hormone)
 #3 - Cardiovascular baroreceptors
 #4 – ANP (atrial natriurectic peptide)
Regulation of Sodium Balance: #1Aldosterone
“has the most to say” about renal regulation of Na ion concentration in the ECF
Aldosterone is an adrenal cortex hormone
It causes the remaining sodium to be reabsorbed & promotes water retention
When aldosterone levels are high, all remaining Na+ is actively reabsorbed
Water follows sodium if tubule permeability has been increased with ADH
 The renin-angiotensin mechanism triggers the release of aldosterone
Adrenal cortical cells are directly stimulated to release aldosterone by elevated K+ levels in the ECF
Aldosterone brings about its effects (diminished urine output and increased blood volume) slowly
#2 ADH Antidiuretic Hormone
 controls water reabsorption in collecting ducts
 low ADH = dilute urine, reduction of blood volume
 high ADH = high resorption, small volume, concentrated urine
#3 Cardiovascular System Baroreceptors
 Baroreceptors in heart & blood vessels alert brain of increases in blood volume (hence increased blood
 Rise in blood pressure       leading to…… less sympathetic nerve impulses to kidneys leading to….
 Afferent arterioles to dilate leading to….Glomerular filtration rate rises leading to….
Sodium and water output increase

Cardiovascular System Baroreceptors
 Since sodium ion concentration determines fluid volume, baroreceptors can be viewed as “sodium
#4 Atrial Natriuretic Peptide (ANP)
 Reduces blood pressure and blood volume by inhibiting release of ADH, renin, & aldosterone, and
directly causing vasodilation.
 Na+ and water retention
 Is released in the heart atria as a response to stretch (elevated blood pressure)
 Promotes excretion of sodium and water

Influence of Other Hormones on Sodium Balance
 Enhance NaCl reabsorption by renal tubules
 May cause water retention during menstrual cycles
 Are responsible for edema during pregnancy
 Glucocorticoids – enhance reabsorption of sodium & promote edema
 -raise blood pressure

Regulation of Potassium Balance
Relative ICF-ECF potassium ion concentration affects a cell’s resting membrane potential
Excessive ECF potassium decreases membrane potential
Too little K+ causes hyperpolarization and nonresponsiveness
Potassium – chief intracellular cation
 required for normal neuromuscular functioning
 needed for many metabolic activities
 is part of body’s buffer system which resists changes in the pH of body fluids.
 Shifts of H+ ions in & out of cell lead to shifts in K+ in the opposite direction
 this can alter excitable cells’activity
 Hyperkalemia – high levels of potassium
 – reduced excitability of neurons, muscle
 Hypokalemia – low levels of potassium
 - disruption of skeletal/cardiac muscle function
Regulatory Site: Cortical Collecting Ducts
 K+ balance is maintained by renal mechanisms.
 (NOTE: Na+ is NEVER secreted into the filtrate)
Factors Influencing Potassium Concentration
 1 - Blood plasma levels are the most important factor regulating K+ secretion back into the filtrate
 2 – Aldosterone – stimulates cells to reabsorb Na+, simultaneously enhancing K+ secretion
Regulation of Calcium
99% of calcium phosphate salts are found in bones
Ionic calcium in ECF is important for:
1 - Blood clotting  2 - Cell membrane permeability           3 - Secretory behavior
Regulation of Calcium
Increases excitability
Causes muscle tetany
Inhibits neurons and muscle cells
May cause heart arrhythmias
Calcium balance is controlled by:     1- parathyroid hormone (PTH)          2-calcitonin(thyroid)

Regulation of Calcium and Phosphate
 PTH (parathyroid hormone) promotes increase in calcium levels by targeting bones, small intestine, & kidneys
 PTH enhances calcium reabsorption and decreases phosphate reabsorption
 Calcium reabsorption and phosphate excretion go hand in hand
Influence of Calcitonin - released in response to rising blood calcium levels
 Calcitonin - stimulates Ca++ deposit to
Regulation of Anions
 Chloride - is the major anion reabsorbed with sodium in the ECF (extracellular fluid)
 99% of chloride is reabsorbed under normal pH conditions
 chloride helps maintain osmotic pressure of blood
 Other anions have transport maximums and excesses are excreted in urine
Acid-Base Balance
All functional proteins (enzymes, hemoglobin, & others) are influenced by H+ concentration
pH varies from one body fluid to another
Normal pH of body fluids
Arterial blood is 7.4
Venous blood and interstitial fluid is 7.35
Intracellular fluid is 7.0
 Alkalosis or alkalemia – arterial blood pH rises above 7.45
 Acidosis or acidemia – arterial pH drops below 7.35 (physiological acidosis)
Sources of Hydrogen Ions
 Most hydrogen ions originate from cellular metabolism but they can also enter body through ingested foods
Hydrogen Ion Regulation
 Regulation of H+ concentration is by:
 1 - chemical buffer systems – act within secs
 2 – brain stem respiratory center – acts in 1-3 mins
 3 - renal mechanisms – require hrs to days to effect pH changes
Chemical Buffer Systems
One or two molecules that act to resist pH changes when strong acid or base is added
Three major chemical buffer systems
1 - Bicarbonate buffer system
2 - Phosphate buffer system
3 - Protein buffer system
Any drifts in pH are resisted by the entire chemical buffering system
1 - Bicarbonate Buffer System
 A mixture of carbonic acid (H2CO3) and its salt, sodium bicarbonate (NaHCO3) (potassium or magnesium
bicarbonates work as well)
 If strong acid is added:
 Hydrogen ions released combine with the bicarbonate ions and form carbonic acid (a weak acid)
 The pH of the solution decreases only slightly
1 - Bicarbonate Buffer System
 If strong base is added:
 It reacts with the carbonic acid to form sodium bicarbonate (a weak base)
 The pH of the solution rises only slightly
 **This system is the only important ECF buffer
2 - Phosphate Buffer System
 Same as bicarbonate system but with phosphate salts
 This system is an effective buffer in urine and intracellular fluid (**important within cells)
3 - Protein Buffer System
 in plasma and within cells
 is body’s most powerful, widespread buffer system
 Some amino acids of proteins have:
 Free organic acid groups (weak acids)
 Groups that act as weak bases (e.g., amino groups)
Physiological Buffer Systems
 The respiratory system regulation of acid-base balance is a physiological buffering system
Physiological Buffer Systems
 CO2 from cellular metabolism enters red blood cells & is converted to bicarbonate ions for transport in
this causes drop in blood pH, activating deeper, more rapid breathing expelling more CO2

 When blood pH rises, respiratory center is depressed, allowing CO2 to accumulate in the blood, lowering
Renal Mechanisms of Acid-Base Balance
 Chemical buffers can tie up excess acids or bases, but they cannot eliminate them from the body
 The lungs can eliminate carbonic acid by eliminating carbon dioxide
 Only the kidneys can rid the body of metabolic acids (phosphoric, uric, and lactic acids and ketones) and
prevent metabolic acidosis
 ***The ultimate acid-base regulatory organs are the kidneys**
Renal Mechanisms of Acid-Base Balance
 **The most important renal mechanisms for regulating acid-base balance are:
 1 - Conserving (reabsorbing) or generating new bicarbonate ions by secreting & excreting either H+ or
ammonium ions in urine
 2 – Secreting bicarbonate ions
Respiratory Acidosis and Alkalosis
 Result from failure of the respiratory system to balance pH
 partial pressure of CO2 (PCO2)is the single most     important indicator of respiratory inadequacy
Respiratory Acidosis and Alkalosis
 Respiratory acidosis - most common cause of acid-base imbalance
 Occurs when a person breathes shallowly, or gas exchange is hampered by diseases such as pneumonia, cystic
fibrosis, or emphysema
 Respiratory alkalosis - common result of hyperventilation
Metabolic Acidosis
 All pH imbalances except those caused by abnormal blood carbon dioxide levels
 Metabolic acid-base imbalance – bicarbonate ion levels above or below normal
 Metabolic acidosis - second most common cause                               of acid-base imbalance
 Typical causes: ingestion of too much alcohol & excessive loss of bicarbonate ions
 Other causes - accumulation of lactic acid, shock, ketosis in diabetic crisis, starvation, &kidney failure
Metabolic Alkalosis
 Rising blood pH and bicarbonate levels indicate metabolic alkalosis
 Vomiting of acid contents of stomach
 Intake of excess base (e.g., from antacids)
 Constipation, in which excessive bicarbonate is reabsorbed
Respiratory and Renal Compensations
 Acid-base imbalance due to inadequacy of a physiological buffer system is compensated for by the other
 The respiratory system will attempt to correct metabolic acid-base imbalances
 The kidneys will work to correct imbalances caused by respiratory disease
Respiratory Compensation
In metabolic acidosis:
The rate and depth of breathing are elevated
Blood pH is below 7.35 and bicarbonate level is low
As carbon dioxide is eliminated by the respiratory system, PCO2 falls below normal
In respiratory acidosis, the respiratory rate is often depressed and is the immediate cause of the acidosis
In metabolic alkalosis:
Compensation exhibits slow, shallow breathing, allowing carbon dioxide to accumulate in the blood
Correction is revealed by:
High pH (over 7.45) and elevated bicarbonate ion levels
Rising PCO2
Renal Compensation
To correct respiratory acid-base imbalance, renal mechanisms are stepped up
Acidosis has high PCO2 and high bicarbonate levels
The high PCO2 is the cause of acidosis
The high bicarbonate levels indicate the kidneys are retaining bicarbonate to offset the acidosis
Alkalosis has Low PCO2 and high pH
The kidneys eliminate bicarbonate from the body by failing to reclaim it or by actively secreting it
Developmental Aspects
Water content of the body is greatest at birth (70-80%) and declines until adulthood, when it is about 58%
At puberty, sexual differences in body water content arise as males develop greater muscle mass
Homeostatic mechanisms slow down with age
Elders may be unresponsive to thirst clues and are at risk of dehydration
The very young and the very old are the most frequent victims of fluid, acid-base, and electrolyte imbalances
Problems with Fluid, Electrolyte, and Acid-Base Balance
 Occur in the young, reflecting:
 Low residual lung volume
 High rate of fluid intake and output
 High metabolic rate yielding more metabolic wastes
 High rate of insensible water loss
 Inefficiency of kidneys in infants

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