1. Anatomy and Physiology
Biology 2401
Chapter-27
Fluids, Electrolytes and Acid Base Balance

2. Fluids and Electrolytes
Life would be impossible without water.
In humans water represents as much as 75% of body weight depending on gender, body mass, and age.
Most of that water is found in body fluids: a body fluid is a mixture of water (solvent) and dissolved molecules (solutes) within body fluid compartments.
Body fluids homeostasis is essential to life: there should always be balance between body fluids and tissue cells.
Body fluids imbalances have serious consequences on human health.

3. Body Fluids Compartments
Body fluids are located in two places:
Inside cells: intracellular fluid (ICF)
Outside cells: extracellular fluid (ECF)
ICF represents 2/3 of body fluids.
ECF represents 1/3 of body fluids.
ECF is made of two main fluids:
Interstitial fluid (80%) (found between cells, lymph, CSF, GI tract, synovial cavities, pericardial cavity, pleural cavity, peritoneal cavity, etc…)
Blood plasma (20%) within blood vessels

4. Body Fluids Compartments

5. Fluid Exchange Fluid exchange occurs between
Body cells and the interstitium: body cells have selectively permeable plasma membranes.
Blood capillaries and the interstitium: blood capillaries have thin walls.
Osmosis is responsible for water movement between compartments.
Active and Passive transports are responsible for electrolytes movement.

6. Abnormalities in Fluid Movement
Edema
The movement of abnormal amounts of water from plasma into interstitial fluid
Lymphedema
Edema caused by blockage of lymphatic drainage

7. Water Balance

8. Regulation of Water Loss
The thirst center in the hypothalamus respond to dehydration.
Dehydration is a deficit of water that occurs when water loss is higher than water gain.
The body responds in several ways:
Increased water reabsorption in the kidney to offset water loss:
ADH is released by the posterior pituitary gland to increase reabsorption of water by the collecting ducts of the kidney.
Mobilization of the RAAS mechanism (see Ch26) where angiotensin 2 decreases blood flow to the nephrons, and aldosterone increases NaCl reabsorption from the collecting duct.
Reduced activity of atriopeptin (ANP) from the heart.
Drinking water (fluid intake).

9. Regulation of Water Gain
Excess water is eliminated by mainly by urination, but also via perspiration, respiration and defecation.
Increased urine production occurs when the previous situation is reversed:
Posterior pituitary gland releases less ADH
Macula densa cells in the kidney release less renin
Heart releases more ANP
Regulation of water balance shows that where sodium goes water follows.
Na+ is the key electrolyte in body fluids.

10. Electrolytes
Electrolytes are charged atoms or molecules located within body fluids.
Cations are positively charged: Na+, Ca+2, K+, Mg+2 , H+.
Anions are negatively charged: HCO3- , Cl-, PO4-3
Electrolytes play a crucial role in osmotic balance, electrochemical gradient, acid balance of cells.
They are also important enzyme cofactors.

11. Electrolytes Concentration in Body Fluids
Blood plasma and interstitial fluid are very similar, differing mainly in the concentration of proteins.
However ICF and ECF differ greatly in all categories.

12. Sodium Ions Sodium (Na+) ions are the dominant ECF cations.
They are mainly responsible for osmotic pressure
Hypernatremia is an excess of Na+ in blood plasma
Hyponatremia is deficit of Na+ the in blood plasma
Small quantities of Na+ are lost in sweat
Regulation of Na+ ions:
ANP promotes elimination of Na+ (natriuriesis)
Aldosterone promotes Na+ reabsorption
ADH secretion is sensitive to Na+ concentration

13. Rising ECF volume by fluid gain or fluid and Na gain
Figure The Integration of Fluid Volume Regulation and Sodium Ion Concentrations in Body Fluids
Responses to Natriuretic Peptides
Combined
Effects
Increased Na loss in urine
Reduced
blood
volume
Rising blood
pressure and
volume
Increased water loss in urine
Natriuretic peptides
released by cardiac
muscle cells
Reduced thirst
Reduced
blood
pressure
Inhibition of ADH, aldosterone,
epinephrine, and norepinephrine
release
Increased blood
volume and
atrial distension
HOMEOSTASIS
RESTORED
HOMEOSTASIS
DISTURBED
Falling ECF
volume
Rising ECF volume by fluid
gain or fluid and Na gain
HOMEOSTASIS
Start
Normal ECF
volume 13

14. Falling ECF volume by fluid loss or fluid and Na loss
Figure The Integration of Fluid Volume Regulation and Sodium Ion Concentrations in Body Fluids
HOMEOSTASIS
Start
Normal ECF
volume
HOMEOSTASIS
DISTURBED
HOMEOSTASIS
RESTORED
Falling ECF volume by fluid
loss or fluid and Na loss
Rising ECF
volume
Decreased blood
volume and
blood pressure
Endocrine Responses
Combined Effects
Increased renin secretion
and angiotensin II
activation
Increased urinary Na
retention
Decreased urinary water
loss
Increased aldosterone
release
Falling blood
pressure and
volume
Increased thirst
Increased ADH release
Increased water intake 14

15. Chloride Ions Chloride ions are the most abundant anions in the ECF.
They form a natural association with Na+ ions and are therefore indirectly regulated by aldosterone.
They balance the negative charge of many anions, particularly that of bicarbonate ions during the chloride shift.
They are used by gastric glands to make hydrochloric acid in the stomach.

16. Potassium Ions K+ ions are the predominant ions in the ICF.
Affect a number of mechanisms such as resting membrane potentials, membrane repolarization and hyperpolarization.
Aldosterone regulates the amount of K+ secreted into urine.
Aldosterone maintains K+ ions in a narrow range in blood plasma.
Hyperkalemia occurs when the level of potassium in the bloodstream is higher than normal.
Hypokalemia occurs when the level of potassium in the bloodstream is lower than normal. It is also known as potassium deficiency.
Both conditions may affect cardiac function, blood pressure, and neuromuscular interaction.

17. Calcium (Ca+2) ions Ca+2 ions are found outside the cell.
It is the most abundant ion in the body
Ca+2 is stored mainly in bones as calcium phosphate (hydroxyapatite).
Besides its stuctural role in bone it plays important roles in blood clotting, release of neurotransmitter , muscle contraction, nerve and muscle function.
Increase in blood calcium levels is regulated by parathyroid hormone (PTH).
PTH increases Ca+2 extracellular levels by stimulating osteoclasts to break down bone matrix.
Vitamin D stimulates Ca+2 uptake in intestines
Calcitonin decreases extracellular Ca+2 levels
Hypocalcemia: low blood calcium level
Hypercalcemia: high blood calcium level

18. Magnesium (Mg+2) Ions Mg+2 ions in principally an intracellular ion
Capacity of kidney to reabsorb Mg+2 is limited
Magnesium is an important metallic cofactor.
It is also involved in cardiac, neuromuscular, and CNS functions.
Hypomagnesimia is magnesium deficiency
Hypermagnesimia is magnesium excess
Both conditions are rare
Excess lost in urine
Decreased extracellular magnesium results in greater degree of reabsorption

19. Acid Base Balance
Acid base balance is achieved by controlling the amount of H+ in body fluids.
H+ homeostasis is absolutely necessary for protein structure, and blood pH (Blood pH range: 7.35 to 7.45)
Acids release H+ into solution
Bases remove H+ from solution

20. Regulatory Mechanisms
The 3 types of pH regulation:
Buffer regulation
Respiratory regulation
Renal regulation

21. Buffer Systems

22. Protein Buffer Systems
Intracellular and Plasma proteins.
Proteins are a large pool of molecules that can act as buffer.
Provide about 75% of the buffer capacity of the body.
Hemoglobin act as a buffer in erythrocytes.
Albumin act as a buffer in blood plasma.
The buffer capacity of protein is provided by their amino acids:
The carboxyl group releases H+
The amino group bunds H+

23. Carbonic acid / bicarbonate ion
Important buffer system of the ECF and the ICF.
Carbonic acid is a weak acid: releases H+
Bicarbonate ion is a weak base: binds H+
Not as powerful as the protein buffer, but important for the maintenance of blood pH

24. Phosphate Buffer Important intracellular buffer.
Phosphate concentration in ECF is low.
Important buffer in urine.

25. Respiratory Regulation
Respiratory regulation of pH is achieved through carbonic acid/bicarbonate buffer system
As carbon dioxide levels increase, pH decreases
As carbon dioxide levels decrease, pH increases
Carbon dioxide levels and pH affect respiratory centers
Hypoventilation increases blood carbon dioxide levels
Hyperventilation decreases blood carbon dioxide levels

26. Figure 27-9 The Basic Relationship between PCO2 and Plasma pH
40–45
mm Hg
HOMEOSTASIS
If PCO2 rises
H2O 
CO2
H2CO3
HCO3 H
When carbon dioxide levels rise, more carbonic acid
forms, additional hydrogen ions and bicarbonate ions
are released, and the pH goes down.
PCO2 pH 26

27. Figure 27-9 The Basic Relationship between PCO2 and Plasma pH
7.35–7.45
HOMEOSTASIS
If PCO2 falls H 
HCO3
H2CO3
H2O
CO2
When the PCO2 falls, the reaction runs in reverse, and
carbonic acid dissociates into carbon dioxide and water.
This removes H ions from solution and increases the
pH. pH
PCO2 27

28. Respiratory Regulation

29. Renal Regulation
H+ is secreted into the filtrate of the PCT and the collecting duct by tubular cells.
This explain the acidity of urine.
In the PCT H+ is antiported with Na+
In the collecting duct H+ actively transported into the filtrate.
HCO3- in filtrate in the collecting duct is reabsorbed via antiport.
Rate of H+ secretion increases as body fluid pH decreases or as aldosterone levels increase.
Secretion of H+ is inhibited when urine pH falls below 4.5

30. Acidosis and Alkalosis
Normal pH range is 7.35 to 7.45
In acidosis body fluids pH falls below 7.35
Respiratory acidosis is caused by inadequate ventilation
Metabolic acidosis is the results of all conditions other than respiratory that decrease pH
Main consequence: depression of the central nervous system
In alkalosis body fluids pH rises above 7.45
Respiratory alkalosis is caused by hyperventilation
Metabolic alkalosis is the results of all conditions other than respiratory that increase pH
Main consequence: overexcitability of the central nervous system
Compensatory mechanisms reverse acidosis and alkalosis.

31. Figure 27-15a Respiratory Acid–Base Regulation
Responses to Acidosis
Respiratory compensation:
Stimulation of arterial and CSF chemo-
receptors results in increased
respiratory rate.
Increased
PCO2
Renal compensation:
Combined Effects
H ions are secreted and HCO3
ions are generated.
Respiratory Acidosis
Decreased PCO2
Elevated PCO2 results
in a fall in plasma pH
Buffer systems other than the carbonic
acid–bicarbonate system accept H ions.
Decreased H and
increased HCO3
HOMEOSTASIS
DISTURBED
HOMEOSTASIS
RESTORED
HOMEOSTASIS
Hypoventilation
causing increased PCO2
Plasma pH
returns to normal
Normal
acid–base
balance
Respiratory acidosis 31

32. Figure 27-15b Respiratory Acid–Base Regulation
HOMEOSTASIS
HOMEOSTASIS
DISTURBED
HOMEOSTASIS
RESTORED
Normal
acid–base
balance
Hyperventilation
causing decreased PCO2
Plasma pH
returns to normal
Respiratory Alkalosis
Combined Effects
Responses to Alkalosis
Lower PCO2 results
in a rise in plasma pH
Increased PCO2
Respiratory compensation:
Inhibition of arterial and CSF
chemoreceptors results in a decreased
respiratory rate.
Increased H and
decreased HCO3
Renal compensation:
Decreased
PCO2
H ions are generated and HCO3 ions
are secreted.
Buffer systems other than the carbonic
acid–bicarbonate system release H
ions.
Respiratory alkalosis 32

33. Clinical Applications:
Prolong vomiting leads to metabolic alkalosis.
A person with emphysema ehibit signs and symptoms of chronic
respiratory acidosis.
A person with prolong diabetes will develop metabolic acidosis.
A person who consume large amount of NaHCO3( baking Soda)
to settle an upset stomach are at risk of metabolic alkalosis.
Severe kidney damage such as glomerulonephritis often leads to
metabolic acidosis.