Secretion + Micturition Reflex

Question 1

About secretion

Answer 1

• Secretion is the reverse of (R)
• Transfer of molecules from the circulation into the nephron tubule

Why have secretion when there is already filtration?

• a back up for filtration
• can further lower plasma concen of waste products or toxins
• Secretion = primary method of excretion for some compounds
  
  • drugs eg. 80% of a dose of penicillin is secreted by the nephrons + eliminated in urine within 3-4 hrs
  • major role in removal of acid (H+)
  • urea is secreted into LoH
• Via epithelial transport.
• Just as (R), secretion occurs at all levels of the nephron tubule, + the primary site for secretion is the PT

Question 2

Examples of secretion

Answer 2

• In PT, H+ secreted in exchange for Na+ (i.e. NHE – sodium - hydrogen ion exchanger)
• In DT, specialised epithelial cells called intercalated cells secrete H+ in exchange for K+ (H+-K+ATPase) + reabsorb HCO3- (Cl-/HCO3-exchanger).
• In the PT organic ions are secreted via a family of organic ion transporters
  organic anion transporters, organic cation transporters = cause secretion of many drugs
• Unlike (R), these have broad specificity
  = makes sense: w/ secretion = want to be able to eliminate substances that the body doesn’t necessarily recognise (eg. foreign toxins), whereas when (R) materials back into the blood stream you want it to be highly specific.
• because multiple ions are moved by same carrier, there can be competition b/w dif anions/cations for secretion
• eg. the rate of secretion of penicillin can be reduced by increasing the conc. of other organic anions. One drug commonly used is probenicid.

Question 3

Intercalated Cell (Type A) = used in acidosis

Answer 3

• Secretion of H+ in the DT/CD (acidosis) = process of getting acid from the blood into the urine to be excreted

1. In acidosis, high [H+] combines w/ HCO3 to produce CO2 which diffuses into epithelial cell
2. The CO2 then combines w/ water again to produce H+ and HCO3
3. The H+ is pumped into the lumen via the H+/K+ ATPase then and K+ is (R)
4. H+ is also secreted via the H+ ATPase
5. The HCO3 diffuses back into the IF + the cycle repeats

Question 4

Intercalated Cell (Type B) = used in alkalosis

Answer 4

• Secretion of HCO3- in the DT/CD (alkalosis) = same as type A except everything has switched sides

Question 5

Urination: Micturition reflex

Answer 5

• Once the TF leaves the CD + enters the calyx it can no longer be modified
• The TF is now urine
• The urine flows from the minor into the major calyces + then into the renal pelvis.
• It is then pushed down the ureter via peristaltic contraction to the bladder.
• Urine is stored in the bladder until micturition (the process of urinating).
• The processes of storage + micturition are reflexes
  
  1. the storage reflex is a spinal reflex
  2. the micturition reflex is a brainstem reflex

Question 6

Spinal + supraspinal regulation of micturition

Answer 6

Lumbar spinal cord:

• contain sympathetic nerves that:
  1. relax detrusor muscle
  2. contract internal sphincter

Sacral spinal cord:

• contain afferent nerves from stretch receptors in detrusor muscle (signals stretch)
• contain parasympathetic nerves that:
  1. contract detrusor muscle
  2. relax internal sphincter
• as well as somatomotor nerve which contract external sphincter

Pons (lower brainstem):
- contains:
1. Pontine micturition centre (PMC)
2. Pontine storage centre (PSC)
\+ descending projections to spinal cord

Question 7

Reflexes maintaining storage (spinal)

Answer 7

FLOW CHART:
gradual filling of bladder –> low-level firing from stretch receptor afferents in detrusor muscle –> 1. reflex stimulation of sympathetic outflow to bladder –> 2. (i) reflex stimulation of somatomotor outflow to external sphincter + (ii) descending stimulation from PSC –> 1. Detrusor muscle relaxation + internal sphincter constriction –> 2. external sphincter contraction –> CONTINENCE (STORAGE)

• high sympathetic activity
• low parasympathetic activity

Question 8

Micturition Reflex (brainstem)

Answer 8

FLOW CHART:
Bladder fills –> firing from bladder stretch receptors increases –> (i) conscious sensation, voluntary decision to void –> (ii) activation of PMC* –> 1. reflex inhibition of sympathetic outflow to bladder –> 2. reflex stimulation of parasympathetic outflow to bladder –> detrusor muscle contraction + internal sphincter relaxation –> MICTURITION

*–> inhibition of motor neuron –> external sphincter relaxation –> micturition

• sympathetic activity inhibited
• parasympathetic activity increases