Lecture 5-6 -- Micturition and Fluid Regulation

What are the three muscle that are responsible for urination?

Detrusor muscle, external sphincter muscle and internal sphincter muscle.

How does the sympathetic nervous system affect bladder filling?

→ Signals are sent via pelvic nerve through alpha 1 receptor via noradrenaline as neurotransmitter to internal sphincter muscle for contraction
→ Signals are sent via hypogastric nerve through beta 2 receptor via noradrenaline as neurotransmitter to detrusor muscle for relaxation

What effects do parasympathetic nerves have during bladder emptying?

→ Signals are sent via pelvic nerve to internal sphincter muscle for relaxation
→ Signals are sent via hypogastric nerve through muscarinic receptors via ACh to detrusor muscle for relaxation

What nervous system controls the external urethral sphincter?

Somatic nervous system
→ During bladder filling, the external urethral sphincter is continuously being excited via ACh, acting on nicotinic receptors through pudendal nerve → Contraction external urethral sphincter allow bladder filling

→ During bladder emptying, the excitation of external urethral sphincter is inhibited → Inhibit the closure of external urethral sphincter

Describe the whole process of bladder emptying

Step 1: When the filling is sufficient to stretch the detrusor, there is a myogenic reflex = Reflex contraction of smooth muscle not controlled by nerves
Step 2: High urinary pressure stimulate micturition centre of the brain to send signal
Step 3: Opening of internal sphincter → Dilation of neck of bladder
Step 4: Contraction detrusor + Inhibition of external sphincter muscle
Step 5: Urination

What type of muscle is the internal urethral sphincter?

It is smooth muscle controlled by ANS

What type of muscle is the detrusor muscle?

It is smooth muscle controlled by ANS

What type of muscle is the external urethral sphincter?

It is striated muscle controlled by the somatic nervous system.

What is the level at which the pelvic and pudendal nerve originates?

S1-S2

What is the level at which the hypogastric nerve originates?

L1-L2

What hormone promotes calcium absorption in the gastrointestinal tract?

Calcitriol (Active form of vitamin D)

What triggers the production of erythropoietin (EPO)?

Hypoxia or low oxygen levels in tissues
!! Not regulated by the number of RBC !!

Where is erythropoietin produced?

By interstitial cells of the kidney.

Describe the procedure of RBC production in Kidney

Hypoxia leads to increased release of erythropoietin from the interstitial cells of the kidney
→ Erythropoietin travels along the blood
→ Bind to the specific receptor in bone
→ Increase erythrocyte formation

What is the normal fluid intake?

2mls/kg/hr or 50mls/kg/day

What is the normal fluid loss from sensible loss?

1-2ml/kg/hour

What are the sensible and insensible fluid loss?

Sensible loss: Urine output
Insensible fluid loss: Evaporation from the skin/ Exhalation from the lungs/ faces

What percentage of body weight is made up of intracellular fluid in adult animals?

Approximately 40%.

What percentage of body weight is made up of interstitial fluid in adult animals?

15% of body weight

What percentage of body weight is made up of plasma in adult animals?

5% of body weight

What percentage of body weight is made up of extracellular fluid in adult animals?

20% of body weight

What monitoring mechanisms exist for water levels?

Osmoreceptors in the hypothalamus monitor the osmolarity of the blood

What monitoring mechanisms exist for sodium levels?

Blood volume is monitored by stretch-sensitive nerve endings

Blood pressure is monitored by baroreceptors

How do natriuretic peptides affect sodium and chloride excretion?

Natriuretic peptides promote the excretion of sodium and chloride, leading to water excretion.