Physiology 1021 Chapter Notes - Chapter 12: Carbonic Anhydrase, Respiratory Acidosis, Peripheral Chemoreceptors

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Why is this Important?
H+ can alter the shape of proteins that act as enzymes that speed up chemical reactions
any change in the concentration of H+ will affect the activity of almost every cell
The Hydrogen Atom and Hydrogen Ion
H has a single proton, which is positively charged, and a single electron, which is
negatively charged
results in an electrically neutral element
H+ is a H atom that has lost its electron, leaving only the positively charged proton
What Are Acids and Bases?
acid is any molecule that will release H+ when put in a solution
presence of the free H+ that makes a solution acidicthe more free H+, the more acidic
strong acid will dissociate rapidly and release large amounts of H+ in solution
base is any molecule that will accept a hydrogen ion
bases lower the concentration of free H+ in solution by combining with the H+
less free H+, the acidity of the solution will decrease and become more basic or alkaline
The pH Scale
a way of quantifying the concentration of H+ in any solution
pH is the negative logarithm (to the base 10) of the hydrogen ion concentration
the more free H+, the lower the pH
0 to 14 with neutral pH at 7
below 7 is considered acidic
above 7 is alkaline or basic
normal pH of body fluids varies slightly between 7.35 and 7.45 and has an average of 7.4
Note that this is slightly alkaline. Arterial blood has a pH of 7.45 while venous blood has
a pH of 7.35
acidosis is a term used to describe body fluids when the pH is below 7.4
alkalosis occurs when the pH is above 7.4
pH of body fluids is below 6.8 or above 7.8 for long periods of time, death will occur
The Source of Acid in the Body
when cells in the body make energy (ATP) they will produce CO2 as a byproduct
Respiratory System this CO2can, with the help of the enzyme carbonic anhydrase,
combine with water in red blood cells to produce carbonic acid, H2CO3. The carbonic
acid will dissociate into free H+ and bicarbonate ions, HCO3. In the lungs, the reaction
will then reversecarbonic acid will reform, it will convert to CO2 and H2O and the
CO2 will be removed and exhaled. As a result, there is generally no NET increase in free
H+ in the plasma. Because the carbonic acid reforms into CO2, which is then removed at
the lungs, carbonic acid is known as a volatile acid.
metabolic breakdown of various proteins will produce a number of acids including
sulphuric acid, phosphoric acid, lactic acid, and other organic acids
stomach is a large source of hydrochloric acid
these acids cannot be removed by the lungs and are therefore called nonvolatile acids
these acids are a significant source of free H+ and are constantly being produced
throughout the body
Regulation of H+ ConcentrationBuffers
a buffer is any molecule that can reversibly bind (or release) free H+
buffers bind free H+, and thereby reduce the amount of free H+ in solution
they help to stabilize the pH
general reaction between a buffer and a free H+ is:
This reaction shows that the buffer, called “X,” combines with free H+ to make XH
buffers do not prevent the pH from changing; they only help to minimize any pH change
until the free H+ can be removed from the body by either the lungs or kidneys
buffers we have already seen include bicarbonate ions and hemoglobin (Hb)
free H+ can bind with buffers in both the intracellular and extracellular fluid
intracellular buffers include phosphates (which won’t be covered here) and intracellular
proteins, such as hemoglobin (Hb) inside red blood cells
most powerful extracellular buffer is the bicarbonate ion, HCO3
Hb can reversibly bind with free H+ to help stabilize the acidity inside the RBCs
o can also bind CO2 to reduce the potential acidity should the CO2 combine with
H2O to form carbonic acid, H2CO3
Regulation of H+ ConcentrationRespiratory System
involves detection of this gas by central and peripheral chemoreceptors
when CO2 levels increase, both of these receptors detect the change and cause an increase
in ventilation
increased ventilation causes more CO2 to be removed at the lungs, which will then return
blood CO2 levels to normal
Regulation of H+ ConcentrationThe Kidneys
nonvolatile acids are being produced all the time in the body and must be excreted (or
buffered) to maintain a constant pH of 7.4
perform this function by doing three important things
o excrete H+ that come from non-volatile acids
o attempt to reabsorb all the bicarbonate ions that are filtered at the glomerulus
o create new bicarbonate ions which then get absorbed into the circulation
roughly 90% of the bicarbonate that is filtered at the glomerulus is reabsorbed in the
proximal tubule
H+ are secreted into the filtrate in the proximal tubule by
o Na+/H+ exchanger
o the later section of the distal tubule
o collecting duct by a H+-ATP pump
bicarbonate ions, which are filtered freely at the glomerulus into the filtrate, cannot be
directly reabsorbed by the tubule cells
bicarbonate ions must first be converted to CO2, which is then reabsorbed
once in the tubule cells, this CO2 combines with H2O and, with the help of carbonic
anhydrase, will eventually produce bicarbonate (HCO3) and H+
bicarbonate ions then leave the tubule cells by simple diffusion and are reabsorbed back
into the circulation
remaining H+ in the cell are secreted into the lumen by the Na+/H+ exchanger
for every bicarbonate ion reabsorbed, there must be one H+ secreted
H+ are also secreted in the late distal tubule and the collecting duct by active transport
using an ATP powered hydrogen ion pump
pump is located on the luminal side of the tubule cells and secretes one H+ for every ATP
molecule consumed
responsible for secreting only about 5% of the total H+ in the filtrate, it is important for
creating extremely acidic urine
H+ secreted in these later sections of the nephron come from the reactions we have seen
before, where CO2 combines with water inside the cell to eventually form bicarbonate
and H+
note that the CO2 in this case did NOT come from the filtrate but, instead, came from
either the cell itself or the interstitial fluid
Abnormal pHAlkalosis and Acidosis
when pH drops below 7.4 (blood is acidic), a person is said to be suffering acidosis
when the pH is greater than 7.4 (blood is alkaline or basic), a person is suffering alkalosis
acidosis can occur when there is too much acid (H+) in the body or too little bicarbonate
ion (HCO3), the most common base in the body
alkalosis can occur when there is too much HCO3 in the body or too little acid (H+)
There are two types of acidosisrespiratory acidosis and metabolic acidosis. Similarly,
there are two types of alkalosisrespiratory alkalosis and metabolic alkalosis
Respiratory Acidosis and Alkalosis
respiratory acidosis is caused by decreased ventilation and increased PCO2
o respiratory centers in the brain stem are damaged or from lung damage resulting
in a decreased ability to remove CO2 from the blood
respiratory acidosis will be counteracted by buffers in the blood and by excretion of
excess H+ by the kidney
o increase in ventilation and decreased PCO2
although infrequent, respiratory alkalosis can be caused by stress or emotionally induced
hyperventilation will result in increased removal of CO2 from the blood causing a
decrease in PCO2
high altitudes can also result in a respiratory alkalosis when the low oxygen levels in the
air cause low PO2 levels of the blood which stimulates hyperventilation
respiratory alkalosis will be compensated for by the excretion of bicarbonate from the
Metabolic Acidosis and Alkalosis
metabolic acidosis can be caused by a number of factors that generally result in a
decrease in extracellular bicarbonate ions
o does not involve increased CO2 levels in the blood
o kidney failure resulting in the inability to excrete acids in the urine or the inability
to reabsorb bicarbonate from the filtrate
o formation of excess metabolic acids in the body
o ingestion of acids (the most common of which are aspirin and methyl alcohol)
o loss of bicarbonate in diarrhea (most common)
metabolic alkalosis can be caused by either the buildup of bicarbonate ions or the loss of
H+ from the body (most common due to vomiting)
o can also be caused by the ingestion of alkaline drugs such as sodium bicarbonate
for treating ulcers