Kidney Function and Micturition Reflex Overview

Excretion

Excretion = Filtration - Reabsorption + Secretion

Renal clearance

The volume of blood completely cleared of a substance per minute.

Inulin

The substance used as the standard to measure renal clearance because it is freely filtered but not reabsorbed or secreted.

Inulin clearance

Glomerular Filtration Rate (GFR)

Clearance < GFR

There is net reabsorption (e.g., urea).

Clearance > GFR

There is net secretion (e.g., penicillin).

Micturition

The process of urination (filling and voiding of the bladder).

Micturition reflex initiation

Stretch receptors in the bladder wall fire as it fills.

Micturition reflex process

Stretch receptors fire, Parasympathetic neurons fire, Motor neurons to external sphincter stop firing, Smooth muscle of bladder contracts, Internal sphincter opens passively, External sphincter relaxes.

Voluntary control of micturition reflex

Yes, higher CNS input can facilitate or inhibit the reflex.