Control of Arterial Blood Pressure 2

Question 1

explain long term regulation of mean arterial blood pressure

Answer 1

can be controlled by controlling the extracellular fluid volume (~60% of body weight is water)

total body fluid = intracellular fluid (2/3) + extracellular fluid - normally 1/3 of total
extracellular fluid volume = plasma volume + interstitial fluid volume (fluid which bathes the cells and acts as the go-between the blood and body cells)

is plasma volume falls, compensatory mechanisms shifts the fluid from the interstitial compartment to the plasma compartment
plasma volume and hence steady state blood volume and mean arterial blood pressure would be controlled if extracellular fluid volume is controlled

Question 2

describe the role of hormones in regulating blood plasma volume and hence mean arterial blood pressure

Answer 2

act as effectors to regulate the extracellular fluid volume (including plasma volume) by regulating water and salt balance in our bodies
healthy people stay in a stable water and salt balance, where water input = water output

Question 3

describe hormones which act as effectors to regulate plasma volume and mean arterial blood pressure

Answer 3

renin-angiotensin aldosterone system
natriuretic peptides
antidiuretic hormone

Question 4

describe renin-angiotensin aldosterone system

Answer 4

plays an important role in the regulation of plasma volume and systolic ventricular resistance and hence the regulation of MAP
3 important components;
decrease in plasma volume and blood pressure;

renin - released from kidneys and stimulates formation of angiotensin I in the blood from angiotensinogen (produced by liver)

angiotensin I - is converted to angiotensin II by angiotensin converting enzyme - ACE (produced by pulmonary vascular endothelium)
angiotensin II - stimulates release of aldosterone from adrenal cortex. This causes systemic vasoconstriction, increasing systolic ventricular resistance . Also stimulates thirst and ADH release (contributing to increasing plasma volume mainly brought about by aldosterone)

aldosterone (steroid hormone) - acts on kidneys to increase sodium and water retention - increasing plasma volume and blood pressure

Question 5

describe natriuretic peptides

Answer 5

peptide hormones synthesised by heart (also brain and other organs)
released in response to cardiac distension or neurohormonal stimuli
they cause excretion of salt and water in the kidneys, thereby reducing blood volume and blood pressure
decrease renin release - decreasing blood pressure
act as vasodilators - decrease systolic ventricular resistance and blood pressure
provides a counter-regulatory system for RAAS

2 types of NPs released by heart;
atrial natriuretic peptide - 28 amino acid peptide synthesised and stored by atrial muscle cells (atrial myocytes), released in response to atrial distension (hypervolemic states)

brain-type natriuretic peptide - 32 amino acid peptide synthesised by; heart ventricles, brain and other organs. First synthesised as prepro-BNP, then cleaved to pro-BNP and finally BNP
serum BNP and N-terminal piece of pro-BNPP (NT-pro-BNP) can be measured in patients with suspected heart failure

Question 6

describe antidiuretic (vasopressin) hormone

Answer 6

peptide hormones dervied from a prehormone percursor sytnheised by hypothalamus and stored in the posterior pituitary
secretion stimulated by;
reduced extracellular fluid volume
increased extracellular fluid osmolality (main stimulus)

plasma osmolality indicates relative solute-water balance
plasma osmolality is monitored by osmoreceptors mainly in the brain in close proximity to hypothalamus
ADH released is stimulated by increased plasma osmolality
ADH acts in the kidney tubules to increase the reabsorption of water (conserve water) (e.g. concentrate urine (antidiuresis))
this increases extracellular and plasma volume and hence cardiac output and blood pressure
ADH (vasopressin) also acts on blood vessels to cause vasoconstriction - increase systolic ventricular resistance and blood pressure - the effect is small in normal people but becomes important n hypovolaemic shock (haemorrhage)

Question 7

describe 2 main factors that affect extracellular fluid volume

Answer 7

water excess or deficit

Na+ excess or deficit

Question 8

describe renin secretion

Answer 8

rate limiting step for RAAS - released from juxtaglomerular apparatus in the kidneys;
renal artery hypotension caused by systemic hypotension
stimulation of renal sympathetic nerves
decreased [Na+] in renal tubular fluid - sensed by macula densa (specialised cells of kidney tubules)

Question 9

facts **

Answer 9

ADH - regulates extracellular fluid volume (including plasma) and osmolality to deal with fluid loads/deficits
RAAS - regulation of MAP
natriuretic peptides - counter-regulatory to RAAS