Notes
Acid Buffering and Regulation
Sections
Acid & Base Regulation
Volatile acid is the product of aerobic cellular respiration, which releases carbon dioxide. As we've learned elsewhere, when carbon dioxide interacts with water in the body fluids, carbonic acid forms.
"Volatile" refers to the fact that carbon dioxide is expired by the lungs.
Non-volatile acids are the products of protein and phospholipid metabolism; they include sulfuric acid and phosphoric acid, and they are excreted in the urine as titratable acids.
Because enzymes and other proteins only function at particular pH values, the body must regulate the acidity of the intra- and extracellular fluids.
Three lines of defense against acid-base imbalance:
In the body, fluids (chemical mixtures), called buffers, minimize pH changes until hydrogen balance can be regained.
The respiratory system expires carbon dioxide, the source of volatile acid.
The urinary system excretes the non-volatile, aka, fixed, acids.
These systems operate interdependently to maintain an arterial blood plasma pH of 7.4.
Acid Production by Body Tissues
Body tissues produce acids, including carbon dioxide, sulfuric acid, and phosphoric acid; each of these acids releases hydrogen ions that must be buffered in the body fluid compartments.
Buffering in Fluid Compartments
Intracellular compartment of body tissues = proteins and organic phosphates.
Intracellular compartment of red blood cells = hemoglobin.
Extracellular fluid = bicarbonate, which is the most important extracellular buffer, and, phosphate and proteins.
Acid Removal
In the Lungs:
When carbonic acid reaches the lungs, it dissociates to form carbon dioxide, which is expired in ventilation.
When pH falls, increased ventilation triggers to release excess carbon dioxide.
Rapidly, the partial pressure of carbon dioxide drops, and the acidity of the blood is reduced.
In the Kidneys:
Nephrons reabsorb bicarbonate from the filtrate, and excrete hydrogen ions as tritratable acids and ammonium in the urine.
Nearly all of the filtered bicarbonate is reabsorbed from the proximal tubule.
Some hydrogen ions attach to ammonia and are secreted into the filtrate as ammonium; some of this ammonium is then reabsorbed from the thick ascending limb (where it participates in countercurrent multiplication in the renal interstitial fluid).
In the alpha-intercalated cells of the distal nephron, ammonium is again secreted into the tubular fluid, along with phosphoric acid. Phosphoric acid forms when hydrogen ions attach to phosphate, and is often referred to as "titratable acid"
Ammonium and phosphoric acid formation and secretion involve bicarbonate synthesis and reabsorption; this new bicarbonate contributes to the extracellular buffers.
Both mechanisms are responsive to aldosterone, which is a hormone secreted by the adrenal cortex secreted in response to low pH.
Clinical Correlations
Aldosterone deficiency inhibits ammonium excretion, and causes type 4 renal tubular acidosis. Because potassium excretion is also inhibited by aldosterone deficiency, this type of acidosis is characterized by hyperkalemia.
Alkalosis is a consequence of excessive removal of hydrogen ions from the body fluids; as a result, the blood becomes more alkaline).
Acidosis is the opposite: excessive hydrogen ions are added to the blood or retained; the blood becomes more acidic. Retention of acids can be a sign of renal failure.
Full-Length Text
- Here we will learn about acid production and regulation by the body.
- To begin, start a table, and denote that:
- Volatile acid is the product of aerobic cellular respiration, which releases carbon dioxide. As we've learned elsewhere, when carbon dioxide interacts with water in the body fluids, carbonic acid forms.
- "Volatile" refers to the fact that carbon dioxide is expired by the lungs.
- Non-volatile acids are the products of protein and phospholipid metabolism;
- They include sulfuric acid and phosphoric acid, and, are excreted in the urine as titratable acids.
- Because enzymes and other proteins only function at particular pH values, the body must regulate the acidity of the intra and extracellular fluids.
- Denote that there are three lines of defense against acid-base imbalance:
- In the body, fluids (chemical mixtures), called buffers, minimize pH changes until hydrogen balance can be regained.
- The respiratory system expires carbon dioxide, the source of volatile acid, and,
- The urinary system excretes the non-volatile, aka, fixed, acids.
- These systems operate interdependently to maintain an arterial blood plasma pH of 7.4.
Let's draw out the process of acid production, buffering, and removal.
First, set up the diagram:
- Indicate representative body tissues, which comprise collections of cells,
- Show the interstitium, and,
- A blood vessel with a red blood cell.
- Then, show the fluid compartments, as follows:
- Intracellular fluid is contained in the the body cells and the red blood cells;
- Extracellular fluid includes the interstitial fluid and blood plasma within the vessel.
- Next, show that the body tissues produce acids, including carbon dioxide, sulfuric acid, and phosphoric acid;
- Indicate that each of these acids releases hydrogen ions that must be buffered in the body fluid compartments.
Now, we'll show the key buffers in each compartment.
- In the body tissues, indicate proteins and organic phosphates;
- In the red blood cells, indicate hemoglobin;
- In the extracellular fluid, indicate bicarbonate, which is the most important extracellular buffer, and, phosphate and proteins.
Remember that these buffers do not remove hydrogen ions from the body fluids, they simply weaken them until they are removed from the body.
- Acid removal takes place in the lungs and kidneys.
- Recall that, when carbonic acid reaches the lungs, it dissociates to form carbon dioxide, which is expired in ventilation.
- So, when pH falls, increased ventilation triggers to release excess carbon dioxide.
- Rapidly, the partial pressure of carbon dioxide drops, and the acidity of the blood is reduced.
The renal mechanism is a slower process;
- Write that the nephrons reabsorb bicarbonate from the filtrate, and excrete hydrogen ions as tritratable acids and ammonium in the urine.
- To show this, first draw a nephron,
- Then, show that nearly all of the filtered bicarbonate is reabsorbed from the proximal tubule.
- Then, show that some hydrogen ions attach to ammonia and are secreted into the filtrate as ammonium;
- Some of this ammonium is then reabsorbed from the thick ascending limb (where it participates in countercurrent multiplication in the renal interstitial fluid).
- In the alpha-intercalated cells of the distal nephron,
- Ammonium is again secreted into the tubular fluid, along with phosphoric acid.
- Phosphoric acid forms when hydrogen ions attach to phosphate, and is often referred to as "titratable acid"
- Indicate that both mechanisms are responsive to aldosterone, which is a hormone secreted by the adrenal cortex secreted in response to low pH.
- As a clinical correlation, write that aldosterone deficiency inhibits ammonium excretion, and causes type 4 renal tubular acidosis.
- Because potassium excretion is also inhibited by aldosterone deficiency, this type of acidosis is characterized by hyperkalemia.
- Finally, show that processes of ammonium and phosphoric acid formation and secretion involve bicarbonate synthesis and reabsorption; this new bicarbonate contributes to the extracellular buffers.
- Be aware that the secretion of hydrogen ions as titratable acid and ammonium, and the ensuing production of bicarbonate, involves multiple steps that we have omitted, here, for simplicity.
Finally, let's consider the clinical consequences of acid-base imbalances, which occur when the buffer systems fail to maintain homeostatic pH.
- Alkalosis is a consequence of excessive removal of hydrogen ions from the body fluids; as a result, the blood becomes more alkaline).
- Acidosis is the opposite: excessive hydrogen ions are added to the blood or retained; the blood becomes more acidic.
- Retention of acids can be a sign of renal failure.