Fluid, Electrolyte, and Acid-Base Balance: Introduction to Body Fluids

  Fluid Compartments

Water occupies two main fluid compartments
Intracellular fluid (ICF) - about two thirds by volume, contained in cells Extracellular fluid (ECF) - consists of two major subdivisions Plasma - the fluid portion of the blood
Interstitial fluid (IF) - fluid in spaces between cells
Other ECF - lymph, cerebrospinal fluid, eye humors, synovial fluid, serous fluid, and gastrointestinal secretions Extracellular and Intracellular Fluids
Water is the universal solvent
Solutes are broadly classified into:
Electrolytes - inorganic salts, all acids and bases, and some proteins Electrolytes determine the chemical and physical reactions of fluids
Electrolytes have greater osmotic power than nonelectrolytes Water moves according to osmotic gradients

Nonelectrolytes - examples include glucose, lipids, creatinine, and urea Each fluid compartment of the body has a distinctive pattern of electrolytes Extracellular fluids are similar (except for high protein content of plasma) Sodium is the chief cation

Chloride is the major anion
Intracellular fluids have low sodium and chloride
Potassium is the chief cation
Phosphate is the chief anion
Proteins, phospholipids, cholesterol, and neutral fats 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
Compartmental exchange is regulated by osmotic and hydrostatic pressures Net leakage of fluid from the blood is picked up by lymphatic vessels and returned to the bloodstream Exchanges between interstitial and intracellular fluids are complex due to the selective permeability of the cellular membranes Two-way water flow is substantial

Ion fluxes are restricted and move selectively by active transport Nutrients, respiratory gases, and wastes move unidirectionally 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 changes

Water Balance and ECF Osmolality

To remain properly hydrated, water intake must equal water output Water intake sources
Ingested fluid (60%) and solid food (30%)
Metabolic water or water of oxidation (10%)
Water output
Urine (60%) and feces (4%)
Insensible losses (28%), sweat (8%)
Increases in plasma osmolality trigger thirst and release of antidiuretic hormone (ADH) Regulation of Water - Homeostaisis
Intake - Hypothalmic Thirst Center
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

Influence and Regulation of ADH

Water reabsorption in collecting ducts is proportional to ADH release 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
Factors that specifically trigger ADH release include prolonged fever; excessive sweating, vomiting, or diarrhea; severe blood loss; and traumatic burns 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, mental confusion Other consequences include hypovolemic shock and loss of electrolytes 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 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.

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Factors that accelerate fluid loss include: Increased blood pressure, capillary permeability

Incompetent venous valves, localized blood vessel blockage
Congestive heart failure, hypertension, high blood volume
Hindered fluid return usually reflects an imbalance in colloid osmotic pressures Hypoproteinemia - low levels of plasma proteins
Forces fluids out of capillary beds at the arterial ends
Fluids fail to return at the venous ends
Results from protein malnutrition, liver disease, or glomerulonephritis Blocked (or surgically removed) lymph vessels:
Cause leaked proteins to accumulate in interstitial fluid
Exert increasing colloid osmotic pressure, which draws fluid from the blood Interstitial fluid accumulation results in low blood pressure and severely impaired circulation Sodium in Fluid and Electrolyte Balance

Sodium holds a central position in fluid and electrolyte balance Sodium salts:
Account for 90-95% of all solutes in the ECF
Contribute 280 mOsm of the total 300 mOsm ECF solute concentration Sodium is the single most abundant cation in the ECF
Sodium is the only cation exerting significant osmotic pressure 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

Changes in plasma sodium levels affect:
Plasma volume, blood pressure
ICF and interstitial fluid volumes
Renal acid-base control mechanisms are coupled to sodium ion transport Regulation of Sodium Balance:
Aldosterone
The renin-angiotensin mechanism triggers the release of aldosterone This is mediated by juxtaglomerular apparatus, which releases renin in response to: Sympathetic nervous system stimulation
Decreased filtrate osmolality
Decreased stretch due to decreased blood pressure
Renin catalyzes the production of angiotensin II, which prompts aldosterone release 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

Cardiovascular System Baroreceptors

Baroreceptors alert the brain of increases in blood volume (hence increased blood pressure) Sympathetic nervous system impulses to the kidneys decline
Afferent arterioles dilate
Glomerular filtration rate rises
Sodium and water output increase
This phenomenon, called pressure diuresis, decreases blood pressure Drops in systemic blood pressure lead to opposite actions and systemic blood pressure increases Since sodium ion concentration determines fluid volume, baroreceptors can be viewed as "sodium receptors" Atrial Natriuretic Peptide (ANP)

Reduces blood pressure and blood volume by inhibiting:
Events that promote vasoconstriction
Na+ and water retention
Is released in the heart atria as a response to stretch (elevated blood pressure) Has potent diuretic and natriuretic effects
Promotes excretion of sodium and water
Inhibits angiotensin II production
Influence of Other Hormones on Sodium Balance
Estrogens:
Enhance NaCl reabsorption by renal tubules
May cause water retention during menstrual cycles
Are responsible for edema during pregnancy
Progesterone:
Decreases sodium reabsorption
Acts as a diuretic, promoting sodium and water loss
Glucocorticoids - enhance reabsorption of sodium and promote edema 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 Hyperkalemia and hypokalemia can:
Disrupt electrical conduction in the heart
Lead to sudden death
Hydrogen ions shift in and out of cells
Leads to corresponding shifts in potassium in the opposite direction Interferes with activity of excitable cells
Influence of Aldosterone
Aldosterone stimulates potassium ion secretion by principal cells In cortical collecting ducts, for each Na+ reabsorbed, a K+ is secreted Increased K+ in the ECF around the adrenal cortex causes:
Release of aldosterone -->Potassium secretion
Potassium controls its own ECF concentration via feedback regulation of aldosterone release Regulation of Calcium

Ionic calcium in ECF is important for:
Blood clotting
Cell membrane permeability
Secretory behavior
Hypocalcemia: Increases excitability, causes muscle tetany
Hypercalcemia: inhibits neurons and muscle cells; cause heart arrhythmias Calcium balance is controlled by parathyroid hormone and calcitonin PTH promotes increase in calcium levels by targeting:
Bones - PTH activates osteoclasts to break down bone matrix
Small intestine - PTH enhances intestinal absorption of calcium 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 is a PTH antagonist, but its contribution to calcium and phosphate homeostasis is minor to negligible Acid Base Balance

Introduction to Acids and Bases
Strong acids - all their H+ is dissociated completely in water Weak acids - dissociate partially in water and are efficient at preventing pH changes Strong bases - dissociate easily in water and quickly tie up H+ Weak bases - accept H+ more slowly (e.g., HCO3¯ and NH3)

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 Breakdown of phosphorus-containing proteins releases phosphoric acid into the ECF Anaerobic respiration of glucose produces lactic acid

Fat metabolism yields organic acids and ketone bodies
Transporting carbon dioxide as bicarbonate releases hydrogen ions Hydrogen Ion Regulation
Concentration of hydrogen ions is regulated sequentially by: Chemical buffer systems - act within seconds
Physiological buffer systems
The respiratory center in the brain stem - acts within 1-3 minutes Renal mechanisms - require hours to days to effect pH changes Chemical Buffer Systems

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
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
Phosphate Buffer System
Nearly identical to the bicarbonate system
Its components are:
Sodium salts of dihydrogen phosphate (H2PO4¯), a weak acid
Monohydrogen phosphate (HPO42¯), a weak base
This system is an effective buffer in urine and intracellular fluid Protein Buffer System

Plasma and intracellular proteins are the body's most plentiful and powerful buffers Some amino acids of proteins have:
Free organic acid groups (weak acids)
Groups that act as weak bases (e.g., amino groups)
Amphoteric molecules are protein molecules that can function as both a weak acid and a weak base Physiological Buffer Systems

Respiratory Buffer System
The respiratory system regulation of acid-base balance is a physiological buffering system There is a reversible equilibrium between:
Dissolved carbon dioxide and water
Carbonic acid and the hydrogen and bicarbonate ions
CO2 + H2O --> H2CO3 --> H+ + HCO3¯
During carbon dioxide unloading, hydrogen ions are incorporated into water When hypercapnia or rising plasma H+ occurs:
Deeper and more rapid breathing expels more carbon dioxide
Hydrogen ion concentration is reduced
Alkalosis causes slower, more shallow breathing, causing H+ to increase Respiratory system impairment causes acid-base imbalance (respiratory acidosis or respiratory alkalosis) Renal Mechanisms of Acid-Base Balance

Introduction
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

The most important renal mechanisms for regulating acid-base balance are: Conserving (reabsorbing) or generating new bicarbonate ions
Excreting bicarbonate ions
Losing a bicarbonate ion is the same as gaining a hydrogen ion; reabsorbing a bicarbonate ion is the same as losing a hydrogen ion Hydrogen ion secretion occurs in the PCT
Hydrogen ions come from the dissociation of carbonic acid
Reabsorption of Bicarbonate
CO2 combines with water in tubule cells, forming H2CO3
H2CO3 splits into H+ and HCO3-
For each H+ secreted, a Na+ and a HCO3- are reabsorbed by the PCT cells Secreted H+ form H2CO3; thus, HCO3- disappears from filtrate at the same rate that it enters the peritubular capillary blood H2CO3 formed in filtrate dissociates to release CO2 + H2

CO2 then diffuses into tubule cells, where it acts to trigger further H+ secretion Hydrogen Ion Excretion
Dietary H+ must be counteracted by generating new HCO3-
The excreted H+ must bind to buffers in the urine (phosphate buffer system) Intercalated cells actively secrete H+ into urine, which is buffered and excreted HCO3- generated is:
Moved into the interstitial space via a cotransport system
Passively moved into the peritubular capillary blood
In response to acidosis:
Kidneys generate HCO3-and add them to the blood
An equal amount of H+ are added to the urine
Ammonium Ion (NH4+) Excretion
This method uses NH4+ produced by the metabolism of glutamine in PCT cells Each glutamine metabolized produces two ammonium ions and two bicarbonate ions HCO3- moves to the blood and ammonium ions are excreted in urine Respiratory Acidosis and Alkalosis

Result from failure of the respiratory system to balance pH
PCO2 is the single most important indicator of respiratory inadequacy PCO2 levels - normal PCO2 fluctuates between 35 and 45 mm Hg Values above 45 mm Hg signal respiratory acidosis
Values below 35 mm Hg indicate respiratory alkalosis
Respiratory acidosis is the 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 is a 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 (22-26 mEq/L) Metabolic acidosis is second most common cause of acid-base imbalance Typical causes are ingestion of too much alcohol and excessive loss of bicarbonate ions Other causes include accumulation of lactic acid, shock, ketosis in diabetic crisis, starvation, and kidney failure Metabolic Alkalosis

Rising blood pH and bicarbonate levels indicate metabolic alkalosis Typical causes are:
Vomiting of the acid contents of the 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 system The respiratory system will attempt to correct metabolic acid-base imbalances The kidneys will work to correct imbalances caused by respiratory disease Respiratory Compenstaion

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 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
RisingPCO2
Renal Compensation
To correct respiratory acid-base imbalance, renal mechanisms are stepped up Acidosis has high PCO2 and high bicarbonate levels
The high PCO2 s 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