Renal Regulation of H+ Flashcards
Body Fluid pH’s (normal range and why is it important keep within these ranges?)
the ECF and ICF normally have a pH of between 7.35 to 7.45, any deviation from this can disrupt metabolism since it would bring the pH of the body fluids away from the pH for optimal enzyme activity
- metabolism itself produces many acidic by-products, there it’s essential that there are mechanisms in place to resist changes in body fluid pH
What is a chemical buffer?
- can bind to H+ and remove it from solution as its concentration increases (solution becomes more acidic) or release H+ as its concentration decreases (solution becomes more alkaline)
- this type of buffer involves a weak acid and a weak base, they’re present in all body fluids and can restore normal pH within a fraction of a second
- often the first line of defence against changes in pH
Example of Chemical Buffer
the bicarbonate buffer system
- present in both ICF and ECF
- In this system Carbonic acid (H2CO3) is formed from a reaction between CO2 and H2O, which occurs in the presence of the enzyme carbonic anhydrase (CA). H2CO3 can then dissociate to form H+ and HCO3-
CA CO2+H2O <-> H2CO3 <-> H+ + HCO3
If there is an excess of H+ ions they are buffered, since HCO3- combines with them to form H2CO3, pushing the reaction to the left, in other words HCO3- acts as a base. If the conditions are basic, a relative deficiency of H+ H2CO3 dissociates to form H+ and HCO3-, pushing the reaction to the right
What is a physiological buffer?
- stabilises pH by controlling the output of acids or bases (renal system), or volatile gases like CO2 (respiratory system)
- since they’re actually getting rid of excess ions they have a much greater buffering capacity than the chemical buffers which merely bind to them
- Of the two physiological buffers the renal system has the greatest buffering capacity, it is able to excrete large amounts of H+ ions, it does however take several hours to a day for it to have a significant effect on body fluid pH.
- The respiratory system, in contrast, has less buffering capacity (it can only excrete volatile acids), but has a significant effect in just a few minutes
- physiological can utilise chemical buffers for part of their mechanism for pH control
H+ secretion in renal system
- it’s the Na+H+ exchange pump which secretes H+ ions into the tubular fluid
- the H+ is produced within the tubular cells by the combination of CO2 and H2O to from H2CO3, which then dissociates to release H+ (bicarbonate buffer system)
- once the H+ ions have been secreted they can combine with filtered HCO3- to form CO2 and H2O, some of which will be excreted in the urine, some reabsorbed
HCO3- filtration and reabsorption
- additional HCO3- is made within the tubular cell from H2O and CO2 retrieved from the tubular fluid and renal interstitial fluid
- these ions diffuse across the tubular cell membrane into the interstitial fluid and ultimately into the peritubular capillaries
- combines with H+ ions to form H2CO3
Correction of acidosis
- an increase in H+ ion secretion and more base (HCO3-) is added to the ECF.
- results from an increase in CO2 diffusing from the peritubular capillaries
CA CO2+H2O <-> H2CO3 <-> H+ + HCO3-
An increase in H+ ions in ECF shifts equation to the left towards CO2 formation increasing its concentration gradient between the renal interstitial fluid and tubular cell interior
- increase in CO2 diffusion into tubular cell and therefore H2CO3 production
- Since this molecule dissociates to form H+ ions, there will obviously be an increase in intracellular [H+], these ions can then be secreted by the Na+, H+ exchanger.
- Since, for every H+ ion secreted there is a HCO3- produced the amount of this ion which is reabsorbed into the plasma also increases.
Correction of alkalosis
- more HCO3- needs to be excreted
- Since the bicarbonate buffer equation will be shifted to the right in alkalosis, there is a relative reduction in [CO2], and therefore a shift away from HCO3- production within the tubular epithelial cell. Any HCO3 that is filtered will therefore simply be excreted, with a reduction in HCO3- reabsorption.
Buffering of H+ ions in urine
Tubular secretion of H+ ions will only occur if there is a concentration gradient for H+ ions to favour it (high [H+] in tubular cell and low [H+] in the tubular fluid).
If the pH of the tubular fluid drops below 4.5 H+ secretion will stop, therefore this is known as the limiting pH for tubular secretion of H+ ions.
Some of the H+ secreted combines with HCO3-, however there is still a lot of free H+. If this free H+ was allowed to persist the limiting pH would soon be reached.
What happens to avoid this is that H+ combines with other ions to form a weak acid (one which has little tendency to dissociate). That acid is ammonium chloride (NH4Cl).
- it is formed from H+ ions secreted into the tubular fluid, NH3 which has diffused out of the tubular cells and chloride ions which have been filtered at the glomerulus
Micturition Reflex
- involuntary component
- When the bladder contains about 200mls of urine stretch receptors in the bladder wall send afferent impulses to the sacral region of the spinal cord via parasympathetic fibres, these fibres are conveyed by the pelvic nerves.
- By way of a reflex through the spinal cord, parasympathetic fibres send signals back to the bladder via the same nerves to cause contraction of the bladder detrusor muscle. This reflex also causes relaxation of the internal urethral sphincter.
- This reflex empties the bladders of infants and young children until voluntary control of the external urethral sphincter is achieved with the development of the somatic fibres which innervate it.
Control of Higher brain centres (urination)
- Sensory signals to the sacral region of the spinal cord also stimulate ascending pathways to the micturition centre of the pons.
- This centre integrates the information from the stretch receptors with information from the cerebrum (responsible for higher brain functions like decision making) to establish the appropriateness of urinating at that time.
- If convenient:
The pons facilitates the micturition reflex (detrusor muscle contraction and internal sphincter relaxation), and decreases the frequency of impulses conveyed to the external urethral sphincter via the pudendal nerve. As a result it relaxes allowing urine to flow down the urethra out of the body. - If not convenient:
The pons inhibits the parasympathetic effects of contracting the detrusor muscle and relaxing the internal urethral sphincter. Impulses reaching the external urethral sphincter via the pudendal nerve keeps the external urethral sphincter closed (this sphincter is closed unless urination is required). As the bladder becomes more and more stretched however, the frequency and intensity of the response increases. Eventually, if voluntary urination is not permitted, the external sphincter is forced open by the sheer force of the pressure exerted by the fluid behind it.
