fluid and electrolyte balance

Fluid and Electrolyte Balance

MuseLecture #96/28/12

Fluid, Electrolyte, and Acid–Base Balance

Fluid Balance

Is a daily balance between

Amount of water gained

Amount of water lost to environment

Involves regulating content and distribution of

body water in ECF and ICF


The Digestive System

Is the primary source of water gains

Plus a small amount from metabolic activity

The Urinary System

Is the primary route of water loss


Electrolytes

Are ions released through dissociation of inorganic

compounds (Na+ , K+, Cl-, CO3- )

Can conduct electrical current in solution

Electrolyte balance

When the gains and losses of all electrolytes are equal

Primarily involves balancing rates of absorption across

digestive tract with rates of loss at kidneys and sweat glands


Acid–Base Balance

Precisely balances production and loss of

hydrogen ions (pH)

The body generates acids during normal

metabolism

Tends to reduce pH


The Kidneys

Secrete hydrogen ions into urine

Generate buffers that enter bloodstream

In distal segments of distal convoluted tubule (DCT)

and collecting system

The Lungs

Affect pH balance through elimination of carbon

dioxide

Fluid Compartments

Water Exchange

Water exchange between ICF and ECF

occurs across plasma membranes by

Osmosis

Diffusion

Carrier-mediated transport

Body Fluid Compartments In lean adults, body fluids constitute 55% of female and 60%

of male total body mass

Intracellular fluid (ICF) inside cells

About 2/3 of body fluid

Extracellular fluid (ECF) outside cells

Interstitial fluid between cell is 80% of ECF

Plasma in blood is 20% of ECF

Also includes lymph, cerebrospinal fluid, synovial fluid, aqueous humor,

vitreous body, endolymph, perilymph, and pleural, pericardial, and peritoneal

fluids

Body Fluid Compartments

Daily Water Gain and Loss

Fluid Compartments

Major Subdivisions of ECF

Interstitial fluid of peripheral tissues

Plasma of circulating blood

Fluid Compartments

Minor Subdivisions of ECF

Lymph, perilymph, and endolymph

Cerebrospinal fluid (CSF)

Synovial fluid

Serous fluids (pleural, pericardial, and peritoneal)

Aqueous humor

Fluid Compartments

Figure 27–1a The Composition of the Human Body.

Fluid Movement

Edema

The movement of abnormal amounts of water

from plasma into interstitial fluid

Regulation of body water gain

Mainly by volume of water intake/ how much you drink

Dehydration – when water loss is greater than gain Decrease in volume,

increase in osmolarity of body fluids

Stimulates thirst center in hypothalamus

Regulation of water and solute loss

Elimination of excess body water through urine

Extent of urinary salt (NaCl) loss is the main factor that determines

body fluid volume

Main factor that determines body fluid osmolarity is extent of urinary

water loss

3 hormones regulate renal Na+ and Cl- reabsorption (or not)

Angiotensin II and aldosterone promote urinary Na+ and Cl- reabsorption

of (and water by osmosis) when dehydrated

Atrial natriuretic peptide (ANP) promotes excretion of Na+ and Cl- followed

by water excretion to decrease blood volume

Fluid Compartments

An Overview of the Primary Regulatory

Hormones

Affecting fluid and electrolyte balance:

– Antidiuretic hormone

– Aldosterone

– Natriuretic peptides

Fluid Compartments

ADH Production

Osmoreceptors in hypothalamus

Monitor osmotic concentration of ECF

Change in osmotic concentration

Alters osmoreceptor activity

Osmoreceptor neurons secrete ADH

Major hormone regulating water loss is antidiuretic hormone (ADH)

Also known as vasopressin

Produced by hypothalamus, released from

posterior pituitary

Promotes insertion of aquaporin-2 into principal

cells of collecting duct

Permeability to water increases

Produces concentrated urine

Hormonal Regulation of Na+ and Cl-

Fluid Compartments

Aldosterone

Is secreted by suprarenal cortex in response

to

Rising K+ or falling Na+ levels in blood

Activation of renin–angiotensin system

Determines rate of Na+ absorption and K+ loss

along DCT and collecting system

Fluid Compartments

“Water Follows Salt” salt pumps in PCT and ALOH

High aldosterone plasma concentration

Causes kidneys to conserve salt

Conservation of Na+ by aldosterone

Also stimulates water retention

Concentrations in body fluids

Concentration of ions typically expressed in

milliequivalents per liter (mEq/liter)

Na+ or Cl- number of mEq/liter = mmol/liter

Ca2+ or HPO42- number of mEq/liter = 2 x mmol/liter

Chief difference between 2 ECF compartments

(plasma and interstitial fluid) is plasma contains

many more protein anions

Largely responsible for blood colloid osmotic pressure

Fluid Movement

ICF differs considerably from ECF

ECF most abundant cation is Na+, anion is Cl-

ICF most abundant cation is K+, anion are

proteins and phosphates (HPO42-)

Na+ /K+ pumps play major role in keeping K+

high inside cells and Na+ high outside cell

Electrolyte and protein anion concentrations

Electrolytes in body fluids

Ions form when electrolytes dissolve ad dissociate

4 general functions

Control osmosis of water between body fluid

compartments

Help maintain the acid-base balance

Carry electrical current

Serve as cofactors

Sodium Na+

Most abundant ion in ECF

90% of extracellular cations

Plays pivotal role in fluid and electrolyte balance because

it account for almost half of the osmolarity of ECF

Level in blood controlled by

Aldosternone – increases renal reabsorption

ADH – if sodium too low, ADH release stops

Atrial natriuretic peptide – increases renal excretion

Potassium K+

Most abundant cations in ICF

Key role in establishing resting membrane potential in

neurons and muscle fibers

Also helps maintain normal ICF fluid volume

Helps regulate pH of body fluids when exchanged for

H+

Controlled by aldosterone – stimulates principal cells in

renal collecting ducts to secrete excess K+

Chloride Cl-

Most prevalent anions in ECF

Moves relatively easily between ECF and ICF because most

plasma membranes contain Cl- leakage channels and

antiporters

Can help balance levels of anions in different fluids

Chloride shift in RBCs

Regulated by

ADH – governs extent of water loss in urine

Processes that increase or decrease renal reabsorption of Na+ also

affect reabsorption of Cl-

Bicarbonate HCO3-

Second most prevalent extracellular anion

Concentration increases in blood passing through systemic capillaries

picking up carbon dioxide

Carbon dioxide combines with water to form carbonic acid which

dissociates

Drops in pulmonary capillaries when carbon dioxide exhaled

Chloride shift helps maintain correct balance of anions in ECF and ICF

Kidneys are main regulators of blood HCO3-

Can form and release HCO3- when low or excrete excess

Calcium Ca2+

Most abundant mineral in body 98% of calcium in adults in skeleton and teeth In body fluids mainly an extracellular cation Contributes to hardness of teeth and bones Plays important roles in blood clotting,

neurotransmitter release, muscle tone, and excitability of nervous and muscle tissue

Regulated by parathyroid hormone Stimulates osteoclasts to release calcium from bone – resorption Also enhances reabsorption from glomerular filtrate Increases production of calcitrol to increase absorption for GI tract

Calcitonin lowers blood calcium levels

Phosphate

About 85% in adults present as calcium phosphate salts in bone and teeth

Remaining 15% ionized – H2PO4-, HPO4

2-, and PO43- are important

intracellular anions

HPO42- important buffer of H+ in body fluids and urine

Same hormones governing calcium homeostasis also regulate HPO42- in

blood

Parathyroid hormone – stimulates resorption of bone by osteoclasts

releasing calcium and phosphate but inhibits reabsorption of phosphate

ions in kidneys

Calcitrol promotes absorption of phosphates and calcium from GI tract

Magnesium

In adults, about 54% of total body magnesium is part of bone

as magnesium salts

Remaining 46% as Mg2+ in ICF (45%) or ECF (1%)

Second most common intracellular cation

Cofactor for certain enzymes and sodium-potassium pump

Essential for normal neuromuscular activity, synaptic

transmission, and myocardial function

Secretion of parathyroid hormone depends on Mg2+

Regulated in blood plasma by varying rate excreted in urine

Acid-base balance

Major homeostatic challenge is keeping H+ concentration (pH) of body fluids at appropriate level

3D shape of proteins sensitive to pH Diets with large amounts of proteins produce

more acids than bases which acidifies blood Several mechanisms help maintain pH of

arterial blood between 7.35 and 7.45 Buffer systems, exhalation of CO2, and kidney

excretion of H+

Acid–Base Balance

Figure 27–6 The Basic Relationship between PCO2and Plasma pH.

Acid–Base Balance

Three Major Buffer Systems

Protein buffer systems:

Help regulate pH in ECF and ICF

Interact extensively with other buffer systems

Carbonic acid–bicarbonate buffer system:

Most important in ECF

Phosphate buffer system:

Buffers pH of ICF and urine

Acid–Base Balance

Figure 27–7 Buffer Systems in Body Fluids.

Acid–Base Balance

Figure 27–8 The Role of Amino Acids in Protein Buffer Systems.

Buffer systems

Act to quickly temporarily bind H+

Raise pH but do not remove H+

Most consist of weak acid and salt of that acid functioning as weak

base

Protein buffer system

Most abundant buffer in ICF and blood plasma

Hemoglobin in RBCs

Albumin in blood plasma

Free carboxyl group acts like an acid by releasing H+

Free amino group acts as a base to combine with H+

Side chain groups on 7 of 20 amino acids also can buffer H+

Buffer Systems Carbonic acid- bicarbonate buffer system

Based on bicarbonate ion (HCO3-) acting as weak base and carbonic acid (H2CO3)

acting as weak acid

HCO3- is a significant anion in both ICF and ECF

Because CO2 and H2O combine to form this buffer system cannot protect against

pH changes due to respiratory problems in which there is an excess or shortage of

CO2

Phosphate buffer system

Dihydrogen phosphate (H2PO4-) and monohydrogen phosphate (HPO4

2-)

Phosphates are major anions in ICF and minor ones in ECF

Important regulator of pH in cytosol

Acid–Base Balance

The Hemoglobin Buffer System

CO2 diffuses across RBC membrane

No transport mechanism required

As carbonic acid dissociates

Bicarbonate ions diffuse into plasma

In exchange for chloride ions (chloride shift)

Hydrogen ions are buffered by hemoglobin

molecules

Exhalation of carbon dioxide

Increase in carbon dioxide in body fluids lowers

pH of body fluids

Because H2CO3 can be eliminated by exhaling

CO2 it is called a volatile acid

Changes in the rate and depth of breathing can

alter pH of body fluids within minutes

Negative feedback loop

Acid–Base Balance

The Hemoglobin Buffer System

Is the only intracellular buffer system with an

immediate effect on ECF pH

Helps prevent major changes in pH when

plasma PCO2 is rising or falling

Acid–Base Balance

Carbonic Acid–Bicarbonate Buffer System

Carbon Dioxide

Most body cells constantly generate carbon dioxide

Most carbon dioxide is converted to carbonic acid, which

dissociates into H+ and a bicarbonate ion

Is formed by carbonic acid and its dissociation

products

Prevents changes in pH caused by organic acids and

fixed acids in ECF

Acid–Base Balance

Figure 27–9 The Carbonic Acid–Bicarbonate Buffer System

Acid–Base Balance

Phosphate Buffer System

Consists of anion H2PO4- (a weak acid)

Works like the carbonic acid–bicarbonate

buffer system

Is important in buffering pH of ICF

Acid–Base Balance

Limitations of Buffer Systems

Provide only temporary solution to acid–base

imbalance

Do not eliminate H+ ions

Supply of buffer molecules is limited

Acid–Base Balance

Renal Responses to Acidosis

1. Secretion of H+

2. Activity of buffers in tubular fluid

3. Removal of CO2

4. Reabsorption of NaHCO3

Kidney excretion of H+

Metabolic reactions produce nonvolatile acids One way to eliminate this huge load is to excrete

H+ in urine In the proximal convoluted tubule, Na+ /H+

antiporters secrete H+ as they reabsorb Na+

Intercalated cells of collecting duct include proton pumps that secrete H+ into tubule fluid

Urine can be up to 1000 times more acidic than blood

2 other buffers can combine with H+ in collecting duct HPO4

2- and NH3

Secretion of H+ by intercalated cells in the collecting duct

Metabolic alkalosis Abnormally high HCO3

- in systemic arterial blood

Nonrespiratory loss of acid - vomiting of acidic stomach

contents, gastric suctioning

Excessive intake of alkaline drugs (antacids)

Use of certain diuretics

Severe dehydration

Hypoventilation can help

Give fluid solutions to correct Cl-, K+ and other electrolyte

deficiencies and correct cause of alkalosis

Respiratory alkalosis

Abnormally low PCO2 in systemic arterial blood

Cause is hyperventilation due to oxygen deficiency

from high altitude or pulmonary disease, stroke or

severe anxiety

Renal compensation can help

One simple treatment to breather into paper bag

for short time

Regulation of blood pH by the respiratory system

Acid–Base Balance

Renal Responses to Alkalosis

1. Rate of secretion of H+ at kidneys declines

2. Tubule cells do not reclaim bicarbonates in

tubular fluid

3. Collecting system transports HCO3- into

tubular fluid while releasing strong acid

(HCl) into peritubular fluid

Acid–Base Balance Disturbances

Figure 27–15 A Diagnostic Chart for Acid–Base Disorders.

fluid and electrolyte balance

Documents