Control of sodium and water excretion

 

Short term BP control

Detectors

(see Baroreceptors)

1. High pressure baroreceptors
* Send signal to vasomotor centre via sensory neural pathway

2. Cardiopulmonary baroreceptors

Effectors

Sympathetic nerve system innervation of vessels

--> Vasoconstriction and venoconstriction

NB:

• Value of the BP setpoint in the long term is determined by kidney, not vasomotor center

 

Intermediate-term BP regulation

Detector

1. Baroreceptors

BP changes in carotid and aortic artery

--> Signal input to vasomotor centre

--> Renal sympathetic nerve stimulation

--> beta1-adrenergic receptors on JG cells activated

--> cAMP increased (in JG cells)

--> Protein kinase A activated

--> Release of renin (from JG cells)

NB:

Alpha1-adrenergic receptors on JG cells are also stimulated
--> Afferent arteriole constriction
--> Decreased GFR and RBF
* However, most of this reduction is blunted by tubuloglomerular feedback

2. Intrarenal baroreceptor (deformation of JG cells)

BP changes in renal afferent arterioles

--> Changes in JG cell deformation

--> Release renin (from JG cells)

NB:

• JG cells act both as a detector and effector
• JG cells are not nerve cells
• No vasomotor centre involvement

3. Macula densa cells of JGA

Monitor the amount of NaCl in the filtrate entering DCT

Which depends on:

• Flow
  * i.e. Rate of filtration
• Concentration
  * i.e. Rate of Na+ reabsorption

When amount of NaCl in the filtrate increases

--> Osmotic swelling of macula densa cells

--> Release of transmitter agent locally
* [WG21:p462] Possibly nitric oxide (NO)

--> Renin production from JG cells decrease

NB:

• No vasomotor centre involvement either
• Also see Tubuloglomerular feedback

 

Effector

Renin release from JG cells

--> Renin converts angiotensinogen to angiotensin I

--> Angiotensin converting enzyme (ACE) converts angiotensin I to angiotensin II

--> Angiotensin II causes vasoconstriction

--> Reinforce vasoconstriction by sympathetic stimulation
* Slower onset (minutes)

Angiotensin II acts directly on JG cells to inhibit renin release

• Negative feedback
• Acts on AT1 receptors on JG cells
  --> Increase intracellular [Ca2+]
  --> Inhibition of renin release

NB:

• ACE is in the endothelial cells of capillaries
  * Especially in lungs, but also in kidney

Long-term BP control

Kidney determines ECF volume (by controlling sodium and water)

--> Blood volume

--> Blood pressure

Thus, vasomotor centre is not involved in long term control