5.15 Acid Base Balance Flashcards
What is an acid
A proton donor that releases H+ ions in a solution
What is a base
A proton acceptor that accepts H+ ions from a solution
What is the difference between a strong base/acid and a weak base/acid
A strong base/acid will completely dissociate into ions in the solution but a weak base/acid will only partly ionise in the solution
What is a conjugate base and a conjugate acid
A conjugate base is what remains after the acid has donated its proton. A conjugate acid is what remains after the base has accepted its proton.
Describe how pH is calculated and what its scale indicates
pH = -log10 [H+]
[H+] being the concentration of hydrogen ions in mol/L
As the pH scale is logarithmic, a 1 unit change corresponds to a 10 fold change in the hydrogen concentration. Hence even a small change in blood pH would have a big impact on the biological function
Describe blood pH in humans
Blood pH is in a scale from 7.35 at the venous end to 7.45 in the arterial end. An average set point is 7.4.
What is acidosis and when is it classified as severe
Acidosis can be a variety of processes that make plasma pH more accidic. Severe acidosis is when blood [H⁺] > 160 nmol/L (aka pH<6.8).
What is alkalosis and when is it classified as severe
Alkalosis can be a variety of processes that make plasma pH more basic. Severe acidosis is when blood [H⁺] < 16 nmol/L (aka pH>7.8).
Describe how volatile acid is produced by the body
From metabolism, the body produces 12-15 moles of CO2 a day. CO2 itself is not an acid but it reacts with water to form carbonic acid (H₂CO₃). Carbonic acid is considered volatile because it can reverse back into CO2 and water. This reaction is catalysed by carbonic anhydrase. This carbonic acid can dissociate into bicarbonate (HCO3-) and hydrogen ions.
Describe how fixed acid is produced by the body.
The body produces 0.1 moles of fixed acid from metabollic processes such as the breakdown of sulfur containing amino acids to produce suphuric acid, gastric secretions producing hydrochloric acid, anaerobic metabolism producing lactic acid. Fixed acids can not be exhaled and must be excreted from the kidneys.
What is the purpose of acid/base homeostasis
To maintain blood pH between 7.35 and 7.45
What are the 3 regulatory mechanisms of acid/base homeostasis
-buffers (as the immediate response)
-respiratory regulation (response taking minutes)
-renal regulation (response taking hours to days)
What is a pH buffer
It is something that resists change in pH when an extra acid or base is added. They can absorb or release H+ in response to this pH change. They can not prevent the pH change but they can help to minimise it.
In what environment does a buffer work best and what dictates this
The buffer works best at its optimal pH, where acid and base levels are near equal. This allows the buffer to resist pH changes by absorbing or releasing H+ when small amounts of acid or base are added.
The actual pH of the buffer is determined by the ratio of conjugate base to weak acid. However the capacity of the buffer is determined by the absolute concentrations of the conjugate base and weak acid
How is the pH of a buffer solution determined
The Henderson-Hasselbalch equation
pH = pKa + log10 ([B-]/[H-B])
pKa = dissociation constant of the weak acid, indicating its strength.
[B-] = Concentration of Conjugate Base
[H-B] = Concentration of Weak Acid
What is the proportion of carbonic acid directly proportional to
The partial pressure of CO2
What is the key balance in determining the pH of blood
The balance of bicarbonate and carbonic acid. An increase in bicarbonate will increase the pH of the blood, an increase in carbonic acid will decrease the pH of the blood. Hence the pH is directly proportional to the bicarbonate concentration and inversely proportional to the partial pressure of carbon dioxide
What are the 3 major buffer systems in the body
Intracellular buffers
Extracellular buffers
Bone buffering
What are intracellular buffers
The principal intracellular buffer is haemoglobin. Other intracellular buffers include other intracellular proteins and the phosphate buffer system
What are extracellular buffers
The principal extracellular buffer is the bicarbonate buffer. Other extracellular buffers include inorganic phosphates and plasma proteins such as
In what situation is bone buffering used
Bone buffering is a long term mechanism that is only used in cases of prolonged acidosis once the over compensatory buffering systems are already overwhelmed. However if bone buffering is used for too long in times of prolonged acidosis, there is continuous calcium and phosphate loss which can result in osteopenia and osteoporosis.
Describe the mechanism of bone buffering
Calcium phosphate in the bone is mobilised to release phosphate ions and carbonate, this will help buffer excess hydrogen ions.
-H+ is absorbed onto the surface of the bone and is exchanged with Na, K and Ca that are released
-hyrdoxyapatite crystals on the bone are dissolved to release carbonate and calcium which will help to buffer any excess H+
-osteoclast activity is increased, this causes further bone resoprtion and release of ions used for buffering
What is the disadvantage of buffering systems as a tool for acid/base homeostasis
They can only provide a short term solution and have limited capacity for longterm pH disturbances. Once the buffer capacity is exceeded, dynamic regulatory systems such as respiration and renal regulatory systems are required.
Describe respiratory regulation of acid/base homeostasis
The respiratory system regulates the CO2 levels which influences the carbonic acid levels. Increased ventilation removes more CO2 which reduces H+ and raises pH. Decreased ventilation retains more CO2 which increases H+ an lowers pH.
