5. Control of plasma volume

Question 1

how are changes to BP sensed

Answer 1

sensed by baroRs in the aortic arch and carotid sinus

Question 2

describe the short term response to increased plasma volume/BP mediated by baroRs

Answer 2

1. alter CO:
   - increase parasympathetic output via vagus n… increase ACh released at mAChRs of SAN… decreased HR… decreased CO
   - decreased sympathetic output… decreased NA release… decrease activation of SAN beta1 Rs and ventricular beta1 Rs… decreased HR and decreased inotropy… decreased CO
2. Alter TPR: decrease sympathetic output… decrease NA release… decrease activation of arteriolar SM alpha Rs… vasodilation… decrease TPR

Question 3

what changes occur in response to increased plasma volume/BP in the kidney in the short term

Answer 3

pressure natriuresis and diuresis

In PCT¨: increased renal arterial pressure…
i) increased peritubular capillary hydrostatic pressure
ii) decreased no. of Na/H antiporters
iii) decreased Na/K ATPase activity
… decreased Na reabsorption… decreased water reabsorption

Question 4

describe the role of ANP in medium/long-term regulation of plasma volume/BP

Answer 4

Over-stretching of atria (i.e. increased BP)… cardiomyocytes release ANP:

i) peripheral vasodilation… decreased TPR
ii) renal afferent arteriole vasodilation… increased GFR… natriuresis and diuresis
iii) inhibits Na reabsorption esp. in CDs… natriuresis and diuresis

Question 5

name the 4 main neurohormonal medium/long term responses to decreased plasma volume/BP

Answer 5

1. RAAS activation
2. SNS activation
3. prostaglandin release
4. ADH release

Question 6

which 3 factors contribute to RAAS activation - what do they stimulate?

Answer 6

1. decreased NaCl delivery to DCT… sensed by NKCC2 on macula densa cells
2. decreased renal perfusion pressure… detected by afferent arteriole baroRs
3. increased sympathetic stimulation (acts on JGA beta1 Rs)

Stimulate granular cells of JGA (of afferent arteriole) to release renin

Question 7

what is the effect of renin release?

Answer 7

1. cleaves angiotensinogen released by liver to angiotensin I
2. AngI cleaved by AngII by pulmonary endothelium ACE

Question 8

decribe the effects of angiotensin on BP (5)

Answer 8

acts at AT1 Rs to:

1. increased stimulation of SNS
2. stimulates PCT apical NHE and basolateral Na/K ATPase, and Na/HCO3 co-transporter… increased Na and H2O reabsorption
3. arteriolar vasoconstriction… increased TPR (EA > AA to prevent decrease GFR)
4. stimulates post. pituitary to release ADH… insertion of apical AQP2 in DCT and CD… increased H2O reabsorption
5. stimulates aldosterone synthase production in adrenal cortex… releases aldosterone… increased expression of apical ENaC and basolateral Na/K ATPase in CD prinicpal cells… increased Na and H2O reabsorption

Question 9

describe the role of the SNS in regulating decreased BP

Answer 9

increased SNS stimulation:

1. arteriolar (AA and EA) vasoconstriction… decreased renal blood flow and GFR… decreased filtering capacity and increased Na and H2O reabsorption
2. stimulates renin releases from JGA cells… RAAS activation
3. activates apical NHE3 and basolateral Na/K ATPase in PCT… increased Na and H2O reabsorption

Question 10

describe the release of and role of prostaglandins in regulating decreased BP

Answer 10

• release stimulated by vasoconstrictors (AngII, NA and ADH)
• cause:
  1. vasodilation of afferent arteriole (very local action) protects glomeruli from ischaemic damage due to too much constriction
  2. enhance renin release