NTA Level 4 • Semester 1 • PST04103

Body Fluids and Electrolytes Imbalance

Human Anatomy and Physiology • Source Session/Topic 20

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Session 20: Body Fluids and Electrolytes Imbalance

Total Session Time: 120 minutes

Prerequisites

• None

Learning Tasks

By the end of this session students are expected to be able to:

• Identify Major Body Fluid Compartment

• Differentiate Between Intracellular and Extracellular Fluids

• Explain Mechanism that Maintain Homeostasis of Body Fluid

• Describe Body Fluid Electrolyte Levels and their Functions

• Explain Electrolyte Imbalance

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board, chalk and whiteboard markers

• Overhead Projector and Transparencies or Slide Projector/LCD and

Computer

SESSION OVERVIEW

|2 | 15 |Presentation |Major Body Fluid Compartment |

| | |Buzzing |of |

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning tasks and clarify

ASK students if they have any questions before continuing

STEP 2: Major Body Fluid Compartment (15 minutes)

• In the human body there are several major fluid compartments each of

which is subject to homeostatic regulation.

• Fluid content of the human body ranges from 40% to 60% of bodyweight.

• The following are the major fluid compartments found in human body.

o Intracellular Fluid (ICF) water inside the cell comprises 2/3 of the

body's water.

o Its main function is to facilitate intracellular chemical reactions that

maintain life

▪ Is the largest compartment.

▪ ICF is about 40% of the body weight.

▪ The ICF is primarily a solution of potassium and organic anions,

proteins etc.

▪ The cell membranes and cellular metabolism control the constituents

of this ICF.

o Extracellular Fluid (ECF) is the remaining 1/3 of the body's water, any

fluid not contained inside a cell therefore comprises the extracellular

compartment.

▪ ECF is about 20% of the body weight, it consists mainly of plasma

and lymph fluid (circulating compartment) and interstitial fluid (the

spaces between the cells).

• The ECF is further subdivided into three sub-compartments.

o Interstitial Fluid (ISF)

▪ surrounds the cells, but does not circulate

▪ It comprises about 3/4 of the ECF.

o Plasma circulates as the extracellular component of blood.

▪ It makes up about 1/4 of the ECF.

o Transcellular fluid

▪ It is a set of fluids that are outside of the normal compartments.

▪ These 1-2 litres of fluid make up the Cerebrospinal Fluids (CSF),

Digestive Juices, Mucus, specialized joint fluids aqueous; humour and

vitreous humour the fluids in the eyeball

• The ECF provide a relatively constant environment for cells and

transport substances to and from the cells.

• Example; A 70 kg man contains about 40-45 litres of water divided into

the different compartments as follow

• Out of 45 total litres of body fluid

o Intracellular fluid 27litres

o Extracellular 18 litres, which are further, subdivided as follows;

interstitial 13 litres, plasma 3.5litres and lymph 1.5litres.

STEP 3: Mechanism that Maintain Homeostasis of Body Fluid (35 minutes)

|Activity: Buzzing (5 minutes) |

| |

|ASK students to pair up and buzz on the following question for 2 |

|minutes |

| |

|What are the factors that cause fluid and electrolyte imbalance? |

| |

|ALLOW few pairs to respond and let other pairs to add on points not |

|mentioned |

| |

|WRITE their response on the flip chart/board |

| |

|CLARIFY and SUMMARIZE by using the content below |

• Under normal conditions homeostasis of the total volume of the water

in the body is maintained or restored primarily by adjusting output

urine volume and secondarily by fluid intake

• Regulation of fluid intake; When dehydration begins to develop,

salivary secretion decreases, producing the sensation of thirst;

individual increased fluid intake to offset increased output tends to

restore fluid balance

• Regulation of urine volume; two factors determine urine volume

o Glomerular filtration rate, except under abnormal conditions remain

fairly constant

o Rate of tubular reabsorption of water fluctuates considerably; normally

adjusts urine volume to fluid intake influenced by hormonal mechanisms

• Some of the factors that alter fluid loss under abnormal conditions

are increased rate of respiration and volume of sweat secreted,

certain abnormal conditions such as vomiting, diarrhoea, or intestinal

drainage can produce fluid and electrolyte imbalance

• The volume of extracellular fluid can increase or decrease, even if

the osmolality of the extracellular fluid is maintained within a

narrow range of values

• The following are the major mechanisms that regulate extracellular

fluid volume

o Neural mechanism

▪ Neural mechanism changes the frequency of action potentials carried

by sympathetic neurons to the afferent arterioles of the kidney in

response to change in blood pressure

▪ When baroreceptors detect an increase in arterial and venous

pressure, the frequency of action potential carried by sympathetic

neuron to the afferent arteriole decreases.

▪ Consequently the afferent arteriole dilates

▪ This increases glomerular capillary pressure, resulting in an

increase in the glomerular filtration rate, an increase in filtrate

volume and an increase in urine volume

▪ When the baroreceptors detect a decrease in arterial and venous

pressure the action potential carried by sympathetic neuron causes

constriction of afferent arteriole and the opposite occurs

o Renin-angiotensin-aldosterone mechanism

▪ The Renin angiotensin-aldosterone mechanism responds to small

changes in the blood volume

▪ Increased blood pressure results from increased blood volume

▪ Juxtaglomerular cells detect increase in blood pressure in the

afferent arteriole and decrease the rate of renin secretion

▪ The decrease in renin secretion results in a decreased conversion of

angiotensinogen to angiotensin II

▪ Reduced angiotensin II causes a decrease in the rate of aldosterone

secretion from the adrenal cortex

▪ Decreased aldosterone level reduces the rate of sodium (Na+)

reabsorption from the distal renal tubules and collecting ducts

▪ Consequently more Na+ remains in the filtrate and fewer are

reabsorbed

▪ The effect is to increase the osmolality of the filtrate, which

reduces the ability of the kidney to reabsorb water

▪ The water remains with the excess Na in the filtrate

▪ Thus the volume of urine produced increases and extracellular fluid

volume decreases

▪ The opposite occur in case of decreased in blood volume

o Atrial natriuretic hormone (ANH) mechanism

▪ The ANH mechanism is most important in responding to increases in

extracellular fluid volume

▪ An increase in pressure in the atria of the heart, which usually

results from an increase in blood volume, stimulates the secretion of

ANH, which decreases Na+ reabsorption in the distal and collecting

ducts

▪ This increase the rate of Na+ and water loss in the urine

▪ Thus increased ANH secretion decreases extracellular fluid volume

▪ ANH does not appear to respond strongly to decreases in blood volume

however a decrease in pressure in the atria of the heart inhibit the

secretion of ANH

o Antidiuretic hormone (ADH) mechanism

▪ The ADH mechanism plays an important role in regulating

extracellular fluid volume in response to large changes in the blood

pressure

▪ An increase in blood pressure results in a decrease in ADH secretion

▪ As a result, the reabsorption of water from the lumen of the distal

tubule and collecting ducts decreases, resulting in a large volume of

dilute urine

▪ This response helps to decrease extracellular fluid volume and blood

pressure

▪ A decrease in blood pressure results in an increase in ADH secretion

and the opposite occurs.

|Activity: Brainstorming (5 minutes) |

| |

|Ask students to brainstorm on the following question: |

| |

|How water enters and leaves the body? |

| |

|ALLOW few students to respond? |

| |

|WRITE their responses on the flip chart/ board |

| |

|CLARIFY and SUMMARISE by using the content below |

• Water enters the body via the digestive tract water is also added to

the total fluid volume from each cell as it catabolizes food and the

resulting water enters the bloodstream

• Water leaves the body via four exits

o As urine through the kidney

o As water in expired air through the lungs

o As sweat through the skin

o As faeces from the intestine

STEP 4: Body Electrolytes and their Functions (30 minutes)

|Activity: Brainstorming (5 minutes) |

| |

|Ask students to brainstorm on the following question: |

| |

|What are the body electrolytes? |

| |

|ALLOW few students to respond |

| |

|WRITE their responses on the flip chart/ board |

| |

|CLARIFY and SUMMARISE by using the content below |

•

Electrolytes are ionized molecules found throughout the blood, tissues,

and cells of the body.

• An electrolyte balance exists when the quantities of electrolytes

(molecules that release ions in water) the body gains equal those

lost.

• The electrolytes of greatest importance to cellular functions are

sodium, potassium, calcium, magnesium, chloride, sulphate, phosphate,

bicarbonate and hydrogen ions.

• Sodium ions are predominant extracellular cations.

• Because of their abundance in the extracellular fluid, they exert

substantial osmotic pressure.

• The kidneys are the major route by which Na+ is excreted.

• The predominant anion in extracellular fluid is chloride ions.

• The extracellular concentration of Potassium ions K+ must be

maintained within narrow range.

• The concentration gradient of K across the plasma membrane has a major

influence on the resting membrane potential, and cells that are

electrically excitable are highly sensitive to slight changes in that

concentration gradient.

• Therefore Potassium is important in maintaining the nerve impulse

conduction and cardiac muscles fibre contractions.

• Abnormal potassium (K+) levels may cause these cells to function

abnormally. Kidneys are the primary site for potassium regulation.

• The extracellular concentration of calcium ions like that of K+ is

regulated within a narrow range they are important for electrical

properties of excitable tissue.

• Parathyroid hormone increases extracellular Ca+ level and reduces

extracellular phosphate levels.

• Most of the phosphate ions are found inside the cells forming covalent

bond with organic molecules.

• Most of the magnesium in the body is stored in the bones or in the

intracellular fluid as it is used as cofactor for intracellular

enzymes.

• Sulphur is an important element in protein synthesis; mostly are found

inside the cell.

• These electrolytes primarily obtained from foods, but they may also be

found in drinking water and other beverages.

• Some electrolytes are by-products of metabolic reactions.

• The body loses some electrolytes by perspiring or through kidney

during urine production

• The kidney alters renal electrolyte losses to maintain the proper

composition of body fluids

• Plasma and interstitial fluid are almost identical in chemical make-

up, with intracellular fluid showing striking differences

• The table below shows the approximate values of electrolytes in

different body fluid compartments

Summary Table of electrolytes constituents in different body fluid

compartments

|mmol/L Na+ |142 |Fluid |Fluid |

| | |145 |12 |

|K+ |4 |4.1 |150 |

|Ca2+ |1-2 |1-2 |<10-6 mol/l |

|Mg2+ |0.75-1.5 |1.3 |34 |

|Cl- |104 |117 |4 |

|HCO3- |24 |27 |12 |

|Protein |14 |0.0 |54 |

• Note: The above values are approximate values; each textbook has its

own set of approximate values that may vary from these by one or two

mmol/l.

• The overall osmolality of all three compartments is identical at about

300 mOsmol/l

STEP 5: Electrolyte Imbalance (25 minutes)

Disturbances of Water and Sodium Balance

• Pure water deficiency

o This is direct deprivation or loss of water without a corresponding

depletion of electrolytes.

o The condition is uncommon but may occur in few clinical following water

deprivations especially in people who fail to swallow due to oesophageal

obstruction, patients in coma or starvation.

• Effect of pure water deficiency

o Increased osmolality of the extracellular compartment

▪ When osmolality of the extracellular compartment increases the

intracellular fluid is transferred to the extracellular with

consequent cellular dehydration.

▪ Water excretion in urine is reduced to minimum.

▪ Clinically patient presents with oliguria and very concentrated

urine.

▪ Plasma changes include increase in concentration of sodium, proteins

and urea.

• Combined sodium and water deficiency

o In this situation sodium and chlorides are being lost in equal

proportions.

o But in vomiting due to gastric disease and in the diuresis that follows

the use of some diuretics, the chloride loss exceeds that of sodium.

▪ This does not in itself affect the volume of extracellular fluid

appreciably as chloride is replaced by carbonate.

Effects of Water and Salt Depletion

• Osmotic pressure of extracellular fluid decreases

• Osmolality of intracellular fluid increases compared to that of

extracellular fluid

• Water leaves extracellular space to enter the cells to try to balance

the osmotic pressure of the two compartments

• Therefore cells become over hydrated and they swell

• This results into extracellular dehydration

Causes of Disturbances of Water and Sodium Balance

• Loss from gastrointestinal tract due to excessive vomiting and

diarrhoea

• Excessive sweating

• Excessive urination

• Conditions like diabetes mellitus, thiazide diuretic administration,

Addison’s disease

• Extensive haemorrhage and severe burn

STEP 6: Key Points (5 minutes)

• Fluid content of the human body ranges from 40% to 60% of bodyweight

• Intracellular Fluid (ICF) water inside the cell comprises 2/3 of the

body's water

• Extracellular Fluid (ECF) is the remaining 1/3 of the body's water

• Mechanisms that regulate extracellular fluid volume are neural

mechanism, Renin angiotensin-aldosterone mechanism, atrial natriuretic

hormone (ANH) mechanism and Antidiuretic hormone (ADH) mechanism

• Electrolytes are primarily obtained from foods, but they may also be

found in drinking

water and other beverages

• Water and Salt Depletion leads to extracellular dehydration

STEP 7: Evaluation (5 minutes)

• What are the types of fluid compartment?

• What are the four mechanisms of extracellular fluid regulation?

• What are the possible causes of water and sodium imbalance?

• What is the electrolyte imbalance?

References

Seeley, R. R., Stephens, T. D., &Tate, P. (2003). Anatomy and Physiology.

(5th ed.).Boston:

McGraw-Hill.

Standring, S. (2008). Grays’s Anatomy; the anatomical basis of clinical

practice.(40th ed.).

Elservier: Churchill Livingstone.

Thibodeau, G. A., & Patton, K. T. (1999). Anatomy & Physiology. Mosby:

Hoffman

Press, Inc.

Tortora, G.J & Derrickson, B (2009). Principles of Anatomy and Physiology

12th Edition,

USA, John Wiley & Sons Inc, USA

Waugh, A., & Grant, A. (2006). Ross and Willson Anatomy and physiology in

Health and

illness. China: Churchill Livingstone Elservier

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