1.
BODY FLUID COMPARTMENTS &
COMPOSITION
Prof. BL Mtinangi MD, MSc PhD
Department of Physiology
Kampala International University (T)
Dar es Salaam

2.
Homeostasis and Body Fluids
 Maintaining the volume and composition of body fluids are
important
 Body fluids are defined as dilute, watery solutions
containing dissolved chemicals inside or outside of the
cell
 Intracellular Fluid (ICF)
Fluid within cells
 Extracellular Fluid (ECF)
Fluid outside cells
Interstitial fluid is ECF between cells and tissues

3.
BODY FLUID COMPARTMENTS
 Total body water (TBW) = 42L in a70kg
male
 Body water content depends on age and sex
 TBW has two major compartments;
ECF (14L) and ICF (28L).
 ECF = 14L (1/3 of TBW)
 Subdivided into ISF and Intravascular fluid
(Plasma)
 Interstitial = 10L Interstitial fluid (ISF)
 Intravascular = 3.5L Intravascular fluid (IVF)
 Tran cellular = < 0.5L

4.
Body Fluid varies depending on body fat:
 Infant: 73-80%
 Male adult: 60%
 Female adult: 40-50%
 Effects of obesity
 Old age 45%
 Climate Level of physical
activity

5.
BODY FLUID COMPARTMENTS (%)

6.
BODY FLUID COMPARTMENTS

7.
COMPOSITION OF ICF ECF
 ECF most abundant cation is
Na+, anion is Cl-
 ICF most abundant cation is
K+, anion are proteins and
phosphates (HPO4
2-)
 Na+ /K+ pumps play major role
in keeping K+ high inside cells
and Na+ high outside cell

8.
Ion Distribution
Na+
K+
Cl-
PO4
-
Protein-
Cations Anions
Extracellular
Intracellular
Ca+ Mg+
HCo3-
3

9.
Electrolyte and protein anion
concentrations

10.
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

11.
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
ie 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-

12.
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

13.
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

14.
Phosphate
 About 85% in adults present as calcium phosphate salts in
bone and teeth
 Remaining 15% ionized – H2PO4
-, HPO4
2-, and PO4
3- are
important intracellular anions
 HPO4
2- important buffer of H+ in body fluids and urine
 Same hormones governing calcium homeostasis also
regulate HPO4
2- 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

15.
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
2+

16.
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+

17.
Alterations in Water Movement
 Edema
 Fluid accumulation in interstitial
spaces
 Due to any condition that leads to:
 Net movement of fluid out of
capillaries into interstitial tissues

18.
Potassium
 Major intracellular cation
 Needed for nerve, cardiac,
skeletal function
 Excess excreted by kidneys
 Imbalance can cause sudden
death

19.
Calcium
Essential for:
Neuromuscular transmission
Cell membrane permeability
Hormone secretion
Bone growth
Muscle contraction

20.
Magnesium
 Activates enzymes
 50% in bone
 Excreted by kidneys
 CNS effect similar to calcium

21.
ACID BASE BALANCE

22.
Regulation of Acid-Base
 Regulation of [H+]
 Dependent upon:
Respiration
Kidneys
Chemical Buffers
 Precise regulation necessary for peak
enzyme activity

23.
pH Effects on Enzyme Activity
pH
Enzyme
Activity
 activity
Peak Activity
 activity

24.
Acid Base
 Acids release H+
example: HCl -> H+ + Cl-
 Bases absorb H+
example: HCO3
- + H+ -> H2CO3

25.
pH is logarithmic
 pH = log 1/[H+]
 = - log [H+]
 = - log 0.00000004 Eq/L
 pH = 7.4
 Think of pH as ‘power of [H+]

26.
Buffers Resist pH Changes
 Weak acid & conjugate base
pair
 H2CO3  HCO3
-
+ H+
 Conjugate Acid  conjugate
base + acid

27.
Acid Base Compensation
 Buffer System
 Respiratory System
 Renal System

28.
Buffer System
 Immediate Buffer System
 CO2 + H20  H2CO3  H+ +
HCO3
-
 Excessive CO2  acidosis
 Excessive HCO3
-
 alkalosis
Simplified:
CO2  H+

29.
Question...
Is the average pH of the blood lower
in:
a) arteries
b) veins
Veins!
Why?
Because veins pick up the
byproducts of cellular metabolism,
including…
CO2!

30.
Buffer: Respiratory System
Works within Minutes
 CO2  H+
 Respiration : CO2 : H+ 
 Respiration : CO2 : H+ 

31.
Renal System
Works within Hours to days
 Recovery of Bicarbonate
 Excretion of H+
 Excretion of ammonium

32.
Disorders of Buffers
 Respiratory Acidosis
 Respiratory Alkalosis
 Metabolic Acidosis
 Metabolic Alkalosis

33.
Respiratory Acidosis
  CO2 + H20   H2CO3   H+ + HCO3
•Simplified:
• CO2   H+

34.
Respiratory Alkalosis
  CO2 + H20   H2CO3   H+ + HCO3
• Simplified:
• CO2   H+

35.
Metabolic Acidosis
  H+ + HCO3   H2CO3  H20 +  CO2
•Simplified:
•Producing too much H+

36.
Metabolic Alkalosis
  H+ + HCO3   H2CO3  H20 +  CO2
•Simplified:
•Too much HCO3

37.
Normal Values
pH: 7.35 - 7.45
PCO2: 35 - 45

38.
Abnormal Values
pH PCO2
Respiratory Acidosis  
Respiratory Alkalosis  
Metabolic Acidosis  Normal
 if compensating
Metabolic Alkalosis  Normal
 if compensating

39.
Thank you