Renal And Acid Base Flashcards
What is the most important method of secreting H+ out of tubular cells?
Na+/H+ counter transport
What is the maximal H+ gradient to pump acid into the lumen?
4.5 I.e. a conc of H+ 1000x that of plasma and this is normally reached in the collecting ducts
Why do you need buffers in the urine?
To keep the gradient and enable H+ to be secreted into the lumen
What is the most important buffer system in the body?
The bicarbonate buffer system
Where does H+ mainly bind with HPO4?
Distal tubule and collecting ducts because phosphate is maximally concentrated here.
What is the titratable acidity of urine?
The amount of alkali that is added to urine to return the pH to 7.4, the pH of glomerular filtrate
Why does the phosphate system play a major role in H+ buffering in the tubules?
Because the phosphate buffer system has a pK suited to the pH of the tubular fluid
What else contributes to phosphate being an effective buffer in the tubules?
- Composed of HPO and HPO4 and are poorly reabsorbed and become conc in a/w water reabsorption
- Their pK of 6.8 also makes these active buffers as the urine becomes concentrated through the tubules and the pH falls to about 6
Where are hydrogen ions actively secreted?
In the proximal, distal tubules and the collecting ducts
Can H+ be secreted against a concentration gradient?
Yes until the urine has a pH of 4.5. Therefore a urine pH of <5 is normal.
Is the secretion of H+ in the nephron anything to do with K+?
No, it is exchanged with Na+
What happens to H+ secretion in the presence of aldosterone?
Na reabsorption is the aim with aldosterone, therefore H+ is secreted in increased amounts in the distal tubule
Can H+ ions directly stimulate the respiratory centre?
Yes
How is H+ secreted in the urine?
By combining with urinary buffers
In the PCT- HCO3-
When heaps to be excreted mostly with ammonium salts
Urine pH of <5 contains more H2PO4 than HPO4
Does H2CO3 appear in the urine in acidosis?
No, only very small amounts because most of it will rapidly dissociate into H+ and HCO3-
What are the characteristics of a urine with a pH of less than 5? (4)
- It is within normal limits
- Contains more H2PO4 than HPO4
- Contains more than normal NH4+
- Does not contain HCO3- in greater than normal concentration
What is elevated serum bicarbonate associated with?
- Chronic emphysema
- Duodenal ulcer with obstruction
- Milk-alkali syndrome
- Hyperaldosteronism
Does total parenteral nutrition lead to elevated serum bicarbonate?
NO
Where is K+ reabsorbed?
PCT, predominantly secreted at other sites
What are the 3 most important buffer systems in the body?
Bicarbonate (most important), phosphate, proteins
Why is phosphate a good buffer?
PK of 6.8 which is near plasma pH, but it has low concentration compared with bicarbonate
Why are proteins a good buffer?
PK close to physiological pH but most are within the cells
Why is bicarbonate the most important buffer?
Has a pK of 6.1 which is far from ideal, but it is abundant and can be easily controlled: CO2 at lungs, HCO3 at kidneys. Thus most important buffer in ECF
How does urine get from renal pelvis to bladder?
Regular peristaltic contractions 1-5x per minute
What is the main muscle responsible for micturition?
Detrusor muscle
At what volume is the urge to void first felt?
150mL
When is fullness of th ebladder felt?
400mL
Which nerves innervate the bladder?
Inferior hypogastric nerves, inferiro mesenteric ganglion, pelvic nerves from sacral plexus
Which nerve innervates the external urethral sphincter?
Pudendal nerve
How is continence maintained via the internal urethral sphincter?
The urethral pressure being higher than the bladder pressure
What type of reflex is the micturition reflex?
Autonomic spinal cord reflex
What is the role of the hypogastric nerves on the bladder?
Acts via noradrenaline on B3 and A1 receptors to relax the bladder and excites the urethra and fills the bladder (sympathetic, S= storage)
What is the role of the pelvic nerves from the sacral plexus on the bladder?
Act via Acth on M3 receptors and cause contraction of detrusor and relaxation of the internal sphincter
