4 - micturition

What is micturition

The process of voiding urine from the urinary bladder through the urethra

What are the two main phases of micturition

Filling phase and voiding phase

What happens during the filling phase of micturition

Bladder fills slowly, detrusor muscle relaxes, internal urethral sphincter contracts, external sphincter under voluntary control

Which autonomic system controls bladder storage during the filling phase

Sympathetic nervous system

Which receptors mediate detrusor relaxation during bladder filling

β3-adrenoceptors

Which receptors control contraction of the internal urethral sphincter during filling

α1-adrenoceptors

Which nervous system controls the external urethral sphincter

Somatic nervous system via the pudendal nerve

What triggers the voiding phase of micturition

Stretch receptors in the bladder wall activating the micturition reflex

Which autonomic system mediates detrusor contraction during voiding

Parasympathetic nervous system via muscarinic receptors

What happens to the internal urethral sphincter during voiding

It relaxes due to inhibition of sympathetic nerves

What controls voluntary relaxation of the external urethral sphincter

Cerebral cortex and somatic nervous system

Which spinal cord segments mediate the micturition reflex

Sacral spinal cord segments

What are common clinical consequences of micturition dysfunction

Urinary incontinence, urinary retention, dysuria

What is the normal urine volume in dogs

10–50 mL/kg/day depending on hydration and diet

What gives urine its pale yellow to amber color

Urochrome pigment

What is the usual odor of normal urine

Mild, ammonia smell in concentrated urine

What does turbid urine indicate

Presence of cells, crystals, or infection

What is the normal specific gravity range of urine

1.001–1.080 depending on species and hydration

What are the main chemical components of urine

Water (95–98%), solutes including urea, creatinine, electrolytes, and metabolites

What is the typical pH range of urine in dogs

6–7.5, slightly acidic

Which processes form urine in the kidneys

Glomerular filtration, tubular reabsorption, tubular secretion

Which species produce more concentrated urine

Carnivores

Which species produce more dilute urine

Herbivores

What is renal clearance

The volume of plasma completely cleared of a substance per unit time

Which substance is the gold standard for measuring GFR

Inulin

Which practical clinical marker is used to estimate GFR in veterinary medicine

Creatinine

Which substance is used to measure renal plasma flow

Para-aminohippuric acid (PAH)

How is renal clearance calculated

C_x = (U_x × V) / P_x where U_x is urine concentration, V is urine flow rate, P_x is plasma concentration

What does it mean if clearance is less than GFR

The substance is reabsorbed (e.g., glucose)

What does it mean if clearance equals GFR

The substance is freely filtered and neither secreted nor reabsorbed (e.g., inulin)

What does it mean if clearance is greater than GFR

The substance is secreted into the tubules (e.g., PAH)

Why is renal clearance clinically important

It assesses kidney function, detects early renal disease, monitors progression, and evaluates drug excretion