Renal Physiology 3 Flashcards
Where is renin released from?
Juxtaglomerular cells
What controls renin release?
The macula densa, detecting low sodium flux
What effect does hypotension have on renin release?
Increased renin release
What effect do beta blockers have on renin release?
Inhibits renin release
How is the renin-angiotensin system activated?
In response to hypotension, and thus a fall in renal blood flow:
- low BP detected by juxtaglomerular apparatus
- JGA secretes renin
- renin stimulates production of angiotension
- angiotension vasoconstricts and also causes aldosterone release from the adrenal cortex
- vasocontriction increases BP
- aldosterone causes sodium and water reabsorption also increasing BP
What is the juxtaglomerular apparatus?
It’s the intrarenal baroreceptor.
Composed of the:
- macula dena
- JG cells
What is the macula densa? Where is it?
Specialized epithelial cells. They increase renin release in response to low levels of delivered sodium, due to a fall in GFR or increase in PCT reabsorption.
Situated in the wall of the 1st part of the DCT. It abuts the afferent and efferent arterioles.
What are the juxtaglomerular cells?
They secrete renin in response to hypovolaemia, increased Na concentration in blood and sympathetic stimulation.
Located in the wall of the afferent arteriole (in the media). Just before the arteriole enters the glomerulus.
What is renin?
- A glyocoprotein hormone, synthesised and secreted by the JGA
- formed from pro-renin and pre-pro-renin with a 1/2 life of 80mins
What are the actions of renin?
- acts on circulating angiotensinogen, a large liver protein
- cleaves a 10 amino acid peptide, Angiotensin I from angiotensinogen, in the plasma
- Angiotensin I is converted to Angiotensin II in lungs, by angiotensin converting enzyme (ACE)
- ACE is found in the capillary endothelium
- Angiotensin II has multiple effects
- Angiotensin II is converted to angiotensin III in many tissues by aminopeptidase
What is renin secretion controlled by?
- renal sympathetic nerves
- intrarenal baroreceptors
- antiontension II
What increases renin secretion?
- hypovolaemia
- cardiac failure
- cirrhosis
- renal artery stenosis
- catecholamines acting on beta-1 receptors
What is renin secretion decreased by?
- angiotensin II
- vasopressin
What are the actions of angiontensin II?
- powerful vasoconstrictor, constricts afferent and efferent arterioles in the kidney
- effect on efferent arteriole is greater, increasing GFR
- causes aldosterone release
- potentiates sympathetic activity
- stimulates thirst by direct effect on hypothalamus
- causes release of vasopressin (ADH)
- acts directly on renal tubules, esp the PCT to cause Na+ and water retention, by stimulation of Na+/H+ antiporters
Where is aldosterone produced and secreted by?
It’s a mineralocorticoid. Secreted by the zona glomerulosa of the adrenal cortex in response to:
- reduced renal blood flow (via RAS-angiotensin II)
- stress and trauma via adrenocorticotrophin (ACTH) release
- hyperkalaemia - small changes in plasma K+ are important
- hyponatraemia
What are the actions of aldosterone?
- acts on the DCT and renal cortical collecting ducts
- increases Na reabsorption, thus Na and H2O are retained
- potassium and H+ lost in exchange for Na
- (Also increases production of distal nephron transport mechanisms
- works via protein synthesis so effect takes hours)
What does the adrenal cortex produce?
Aldosterone (glomerulosa)
Cortisol (fasiculata)
Androgens (reticularis)
Catecholamines (medulla - the rest are all cortex)
Where is ADH synthesised?
Hypothalamus
Where is ADH secreted from?
Posterior pituitary
What does ADH do?
- under control of baro and osmoreceptor reflexes
- inactivated in liver and kidney
- half life of 18 minutes
- causes water retention due to an increase in water permeability of collecting duct membrane
- inserts protein channels for water into luminal membrane via cyclic AMP (aquaporins)
- at high concentrations, can act on arteriolar smooth muscle - reducing renal blood flow and GFR
- also stimulates sodium reabsorption and K+ excretion from collecting ducts
What do baroreceptors do?
- can be arterial, venous, cardiac
- detect fall in plasma volume
- afferent firing rate to hypothalamus falls
- ADH release from posterior pituitary increased
What do osmoreceptors do?
- osmoreceptors detect changes in plasma osmotic pressure
- fall in plasma osmotic pressure reduces ADH secretion
- rise in plasma osmotic pressure increases ADH secretion
What is ANP?
Atrial natriuretic peptide - secreted in response to atrial stretch. Increases renal excretion of sodium and water. Potent vasodilator. Improves renal blood flow.
- afferent arteriolar dilatation and efferent arteriolar constriction - increases net filtration pressure (increased filtration coefficient)
- released from atrial myocytes
- inhibits renin secretion
- inhibits aldosterone release
- direct action on collecting ducts to decrease sodium reabsorption
What is the normal GFR of an adult?
125ml/min or 180L/day
