Download hydrogen ions

Fluid, Electrolyte, and
Acid-Base Balance
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
The ECF and the ICF are two distinct fluid compartment

Fluids in the body are compose of water and solutes

There are 2 distinct fluid compartments


Intracellular fluid (ICF)

The cytosol of cells

Makes up about two-thirds of the total body water
Extracellular fluid (ECF)

Major components include the interstitial fluid, plasma
and lymph

Minor components include all other extracellular fluids
(water in dense CT, bone, fluid between visceral and
parietal membranes etc.)
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Functions of water in human physiology
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All chemical reactions occur in the medium.
It is crucial in regulating chemical and
bioelectrical distributions within cells.
Transports substances such as hormones and
nutrients.
O2 transport from lungs to body cells.
CO2 transport in the opposite direction.
Dilutes toxic substances and waste products and
transports them to the kidneys and the liver.
Distributes heat around the body
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http://www.jdaross.mcmail.com/fluid.html
Composition of fluid compartments - solutes
Water serve as universal solvent

Solutes can be divided into

Nonelectrolytes – do not dissociate in the water – no
electrical charge



Most are organic molecules (ex. – glucose, lipids)
Electrolytes – dissociate into ions – electrical charge
Osmotic power of electrolyte is greater because they
contribute more molecules to the solution by dissociating
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Main electrolyte in body fluid compartments

Extracellular
fluids
have
relatively
high
concentrations of sodium (Na+), chloride (Cl-), and
bicarbonate (HCO3-) ions.

Intracellular fluid has relatively high concentrations
of potassium (K+), magnesium (Mg+2), and phosphate
(PO4-3) ions.
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Cations and Anions in Body Fluids
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Figure 27.2
Definitions

Osmolarity – the number of solutes particles dissolved
in 1 liter of water – reflect the solution ability to cause
osmosis

Determined by the number of particles and not their type
or nature

The osmotic activity of 10 sodium ions is equal to that of
10 glucose or amino acid molecules in the same volume
of solution

Osmol – one mole of substrate in 1 L of water
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Acids and bases definition

Acids: defined as any compound, which forms hydrogen ions
in solution.


For this reason acids are sometimes referred to as "proton
donors".
Bases: a compound that combines with hydrogen ions in
solution.

Therefore, bases can be referred to as "proton acceptors".

Strong Acids - a compound that ionizes completely in solution
to form hydrogen ions and a base.

Weak Acids and Bases - compounds that are only partially
ionized in solution.
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Acid-Base Balance



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
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pH and enzyme function

Hydrogen ion concentration has a widespread effect on the
function of the body's enzyme systems.

The hydrogen ion is highly reactive and will combine with
bases or negatively charged ions at very low concentrations.

Proteins contain many negatively charged and basic groups
within their structure.

Thus, a change in pH will alter the degree ionization of a
protein, which may in turn affect its functioning.

At more extreme hydrogen ion concentrations a protein's
structure may be completely disrupted (the protein is then
said to be denatured).
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Sources of Hydrogen Ions

Most hydrogen
metabolism
ions
originate
from
cellular

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
releases hydrogen ions
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as
bicarbonate
Types of acids in the body

Volatile acid comes from carbohydrate and fat metabolism

Can leave solution and enter the atmosphere (e.g. carbonic
acid – H2CO3)

Breaks in the lungs to carbon dioxide and water

In the tissues CO2 reacts with water to form carbonic acid,
which dissociate to give hydrogen ions and bicarbonate ions

This reaction occurs spontaneously, but happens faster with
the presence of carbonic anhydrase (CA)

PCO2 and pH are inversely related
CO2 + H20 H2CO3 HCO3- + H+
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Types of acids in the body


Fixed acids

Acids that do not leave solution (e.g. sulfuric and
phosphoric acids – produced during catabolism of
amino acids)

Eliminated by the kidneys
Organic acids

by-products of aerobic metabolism such as lactic acid,
ketone bodies
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Buffers

Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Buffers - compound
that limits the change
in hydrogen ion
concentration (and so
pH) when hydrogen
ions are added or
removed from the
solution.
http://www.nda.ox.ac.uk/wfsa/html/u13/u1312f03.htm
Buffer systems

Two types of buffer in the body

Chemical buffers
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Bicarbonate, phosphate and protein systems

Substance that binds H+ and remove it from the solution if
its concentration rises or release it if concentration decreases

Fast reaction within seconds
Physiological


respiratory (fast reaction – few minutes) or urinary (slow
reaction – hours to days)
Regulates pH by controlling the body’s output of bases,
acids or CO2
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Chemical Buffer Systems


Three major chemical buffer systems

Bicarbonate buffer system

Phosphate buffer system

Protein buffer system
Any drifts in pH are resisted by the entire chemical
buffering system
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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
CO2 + H2O  H2CO3  H+ + HCO3¯
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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
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
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
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Buffer Systems in Body Fluids
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Figure 27.7
Physiological Buffer Systems
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The respiratory system regulation of acid-base balance
is a physiological buffering system

The respiratory buffering system takes care of volatile
acids – by-products of glucose and fat metabolism

There is a reversible equilibrium between:

Dissolved carbon dioxide and water

Carbonic acid and the hydrogen and bicarbonate
ions
CO2 + H2O  H2CO3  H+ + HCO3¯
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Physiological Buffer Systems

During carbon dioxide unloading, hydrogen ions are
incorporated into water
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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
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Renal Mechanisms of Acid-Base Balance

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 excrete 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
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Renal Mechanisms of Acid-Base Balance
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The kidney takes care of the non-volatile acid products

By-products of protein metabolism and anaerobic
respiration
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The kidneys must prevent the loss of bicarbonate ions (reabsorb) that is being constantly filtered from the blood.

Both tasks are accomplished by secretion of hydrogen ions

The majority of the hydrogen ions that are secreted will be
used to help re-absorb the bicarbonate ions

Only about 10% of the hydrogen ions secreted will be
excreted

As a result of the H+ excretion the urine is usually acidic
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Reabsorption of Bicarbonate

In a person with normal acid-base balance all the HCO3- in
the tubular fluid is consumed by neutralizing H+ - no
HCO3- in the urine

HCO3- molecules are filtered by the glomerulus and than
reabsorbed and appear in the peritubular capillary (most in
the PCT).


The re-absorption is not direct – the luminar surface of the
tubular cells can not absorb HCO3The kidney cells can also generate new HCO3- if needed
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Reabsorption of Bicarbonate

1. Carbon dioxide combines with water in tubule cells, forming
carbonic acid

2. Carbonic acid splits into hydrogen ions and bicarbonate ions

3. For each hydrogen ion secreted, a sodium ion enters the
tubule cell and a bicarbonate ion enters the peritubular capillary

4. Secreted hydrogen ions form carbonic acid; thus, bicarbonate
disappears from filtrate at the same rate that it enters the
peritubular capillary blood

5. Carbonic acid formed in filtrate dissociates to release carbon
dioxide and water

6. Carbon dioxide then diffuses into tubule cells, where it acts to
trigger further hydrogen ion secretion
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Respiratory Acidosis and Alkalosis

Result from failure of the respiratory system to balance pH

PCO2 is the 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
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
Respiratory Acidosis and Alkalosis

Respiratory acidosis is the most common cause of
acid-base imbalance


Occurs when a person breathes shallowly, or gas
exchange is slowed down by diseases such as
pneumonia, cystic fibrosis, or emphysema
Respiratory alkalosis is a common result of
hyperventilation
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Metabolic Acidosis

Metabolic acidosis is the second most common cause of acid-base imbalance

All pH imbalances except those caused by abnormal blood carbon dioxide
levels

Can be a result of:

Failure of the kidney to excrete metabolic acids


Renal acidosis is either the inability of kidney to excrete H+ or to reabsorb bicarbonate ion
Diarrhea – most common reason of metabolic acidosis

Loss of large amounts of sodium bicarbonate in the feces (which is
normal component of the feces)

Diabetes mellitus – results in breakdown of fat that releases acids

Ingestion of acids

Acetylsalicylic acid (aspirin)

Methyl alcohol (forms acid when metabolized)
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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
Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings
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© 2006 Pearson Education, Inc., publishing as Benjamin Cummings
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Copyright
© 2006 Pearson Education, Inc., publishing as Benjamin Cummings
http://www.mhhe.com/biosci/esp/2002_general/Esp/default.htm