Physiology of Micturition (Urination) - Dr. Byron

Micturition

act of passing urine

Four lower urinary tract structures:

1.) kidneys

2.) ureters

3.) bladder

4.) urethra

kidneys

produce urine

ureters

transport urine from the kidneys to the bladder

bladder

stores urine until urination occurs

urethra

allows urine to exit the body

Two urethral sphincters

1.) internal urethral sphincter

2.) external urethral sphincter

internal urethral sphincter

smooth muscle; located proximally

external urethral sphincter

skeletal muscle; located distally

Both urethral sphincters normally maintain _________ tone to keep the urethra closed

basal

Three additional lower urinary tract structures in the male:

1.) prostate

2.) testes

3.) colliculus seminalis

colliculus seminalis

where reproductive tract connects to urethra

Urination involves the coordination of three structures:

1.) bladder

2.) urethra

3.) nervous system

Flow path of urine, starting in the kidneys:

kidney --> renal pelvis --> ureters --> bladder --> urethra --> external environment

The urination system must control two functions:

1.) urine storage

2.) urine voiding

Storage phase of micturition

occurs 99% of the time; allows the bladder to fill with urine while preventing urine leakage

Two requirements for the storage phase of micturition:

1.) detrusor muscle relaxes

2.) urethral sphincters contract

detrusor muscle

layer of smooth muscle in the bladder wall; relaxes to store urine and contracts during urination

Voiding phase of micturition

occurs 1% of the time; goal is to empty the bladder completely

Two requirements of the voiding phase of micturition:

1.) detrusor muscle completely contracts

2.) both urethral sphincters relax

What happens to the detrusor muscle as the bladder fills with urine?

it actively relaxes

As the bladder continues to fill with urine ________ gradually increases

pressure

If the pressure inside the bladder becomes too high...

the detrusor muscle spontaneously contracts to empty the bladder

Usually, the animal will __________ urinate before the detrusor muscle spontaneously contracts

voluntarily

For an animal to have urinary continence, ________ pressure must remain greater than ________ pressure

Urethral; bladder

If urethral pressure falls below bladder pressure....

urine leakage occurs

If sphincters fail to relax during urination....

Urine flow becomes difficult; only small drops or a thin stream is produced and bladder retains urine

Urination requires coordination of three nervous systems:

1.) somatic nervous system

2.) sympathetic nervous system

3.) parasympathetic nervous system

The somatic nervous system controls....

the external urethral sphincter (skeletal muscle)

Nerve of the somatic nervous system involved in micturition

pudendal nerve

Neurotransmitter for the somatic nervous system

Acetylcholine

Receptor type for the somatic nervous system

Nicotinic receptors

Functions of the somatic nervous system during micturition (2):

1.) Maintains baseline contraction of the urethra

2.) Allows urine storage

During urination, the somatic nervous system signal is _________ and the external urethral sphincter _________

inhibited; relaxes

Two things the sympathetic nervous system controls:

1.) detrusor relaxation

2.) internal urethral sphincter contraction

Nerve of the sympathetic nervous system involved in micturition

Hypogastric nerve

The detrusor muscle is relaxed via _______ receptors of the sympathetic nervous system

β receptors

The internal urethral sphincter is contracted via _______ receptors of the sympathetic nervous system

α receptors

α receptors are important clinically because they are...

targets for drugs used to treat urinary incontinence

Function of the sympathetic nervous system during micturition:

urine storage

Parasympathetic nervous system controls...

detrusor contraction

Parasympathetic nervous system nerve

pelvic nerve

Parasympathetic nervous system neurotransmitter

Acetylcholine

Parasympathetic nervous system receptor type

Muscarinic receptors

Two functions of the parasympathetic nervous system:

1.) causes detrusor contraction

2.) initiates voiding

Which nervous system(s) promote urine storage?

somatic and sympathetic

Which nervous system(s) promote urine voiding?

parasympathetic

Brain control of micturition

the bladder contains stretch and pain receptors that transmit signals to the brain to empty the bladder

Two nerves through which stretch/pain receptor signals travel:

1.) pelvic nerve

2.) hypogastric

Once the stretch/pain signal reaches the brain, it goes to three areas:

1.) pontine micturition center

2.) thalamus

3.) cerebral cortex

Once the signal reaches the brain, the brain decides if...

this is an appropriate area to urinate

Six components of continence:

1.) urethral smooth muscle

2.) urethral mucosa

3.) vasculature

4.) connective tissue

5.) pelvic floor

6.) guarding reflex

urethral smooth muscle and continence

Maintains closure

urethral mucosa and continence

Creates a watertight seal

vasculature and continence

Provides structural support; may contribute up to 30% of urethral closure pressure

connective tissue and continence

Holds urinary structures in correct position

pelvic floor and continence

Maintains proper bladder position

guarding reflex and continence

External sphincter contraction in response to increased abdominal pressure

guarding reflex

prevents involuntary emptying of the bladder during coughing or barking

Normally, the bladder is positioned partly in the __________

abdomen

The bladder's position in the abdomen is important because...

it allows pressure to be applied to the bladder and proximal urethra to maintain continence

If the bladder shifts completely into the pelvis...

Pressure is applied mainly to bladder which creates a pressure gradient; leads to urinary leakage

Two major categories of urination disorders:

1.) storage disorders

2.) voiding disorders

storage disorders

animal cannot hold urine, such as urinary incontinence

voiding disorders

animal cannot empty bladder, such as urethral obstruction

Polyuria (PU)

Production of excess urine volume

*usually occurs with Polydipsia

Polydipsia (PD)

Drinking more than normal; excessive water uptake

*usually occurs with Polyuria

Pollakiuria

Frequent urination of small volumes

Pollakiuria occurs with what diseases?

lower urinary tract inflammation (UTIs, bladder stones)

Stranguria

Straining to urinate

Dysuria

Painful or difficult urination

Periuria

Cats only; urinating outside of the litterbox

Examples of questions to ask while taking a urinary history

-Duration?

-Timing of onset?

-Is the storage or voiding phase the problem?

-Is the animal Pu/Pd?

-Other disease processes?

-Orthopedic or neurologic problems?

Physical exam of urinary system includes:

1.) external genital exam

2.) rectal exam

3.) abdominal palpation

4.) orthopedic assessment

5.) neurologic assessment

three things to evaluate during rectal exam:

1.) Anal tone

2.) Urethra

3.) Prostate

Two things to evaluate during abdominal palpation:

1.) bladder size

2.) pain

What should be evaluated during orthopedic assessment?

if animal can posture normally to urinate

Three things evaluated during neurologic assessment:

1.) tail function

2.) rear limb function

3.) perineal reflex