Regulation of the Cardiovascular system 2 Flashcards
What is perfusion?
- the flow of blood through a tissue
adequate tissue perfusion is essential to maintain its nutritive demands - what does too much perfusion or too little perfusion do to a tissue?
- Too little = hypoxia (doesn’t receive oxygen), impaired metabolism leading to necrosis (irreversible death of tissue)
- Too much = capillary/endothelial damage, fluid exudation (fluid leaking)
What is flow through a tube driven by?
- flow through a tube is driven by pressure, and opposed resistance
If the tube is an artery pressure and resistance would be described as what?
- pressure = systemic arterial blood pressure
- resistance = resistance of a vessel
What is systemic arterial pressure a major determinant of?
- major determinant of perfusion pressure, driving tissue perfusion
What would you have to do to maintain tissue perfusion?
- control arterial pressure
Perfusion pressure and arterial control can be described as either of two things - what are these?
- hypertension (high blood pressure)
- hypotension (low blood pressure)
Normal arterial pressure is continuously variable - what factors contribute to this?
- age
- physiological status
- species and breed
- systole and diastole
What is systolic arterial pressure (SAP)?
- peak pressure in the arteries when the left ventricle is ejecting blood during systole
What is diastolic arterial pressure (DAP)?
- residual pressure in the arteries when the left ventricle is filling during diastole
What does MAP stand for?
- Mean arterial pressure
Why is pulse pressure never in the middle of a graph?
- not quite in the middle as the heart spends more time relaxed more than it does contracting
What determines mean arterial pressure?
- cardiac output
- systemic vascular resistance
What is the equation for mean arterial pressure (MAP)?
- MAP = cardiac output x systemic vascular resistance
What can we do in order to maintain adequate perfusion?
- change cardiac output and or change vascular resistance
If cardiac output remains constant what happens to arterial pressure?
- arterial pressure can rise or fall as resistance increases or decreases
What is arterial blood pressure controlled by?
- controlled by negative feedback
There are both short-term and long-term feedback mechanisms for arterial pressure - what is the short term mechanism?
- short term is rapid via baroreceptor regulation
- ANS
What is the long term mechanism for control of arterial pressure?
- long term is slower and via circulating fluid volume
- anti-diuretic hormone AKA vasopressin
- renin-angiotensin-aldosterone-system
Cardiovascular reflexes are ultimately directed at regulation of arterial pressure - how do we regulate this?
- heart rate and force (contractility)
- tone of resistance vessels (arterioles)
- volume of capacitance vessels (e.g., veins)
Short term control of arterial pressure is a function of what?
- of the cardiovascular system
What does long term control of arterial pressure include?
- fluid balance
- renal system
What do cardiovascular reflexes require in terms of pressure?
- sensors (ability to monitor pressure)
- integrating centre (assess correctness of pressure)
- effectors (mechanisms to return pressure to correct level
What are baroreceptors?
- they are sensors
- non-encapsulated nerve endings
Where are baroceptors found?
- in the adventitial layer of arteries
- aortic arch and carotid sinus
What do baroreceptors serve as?
- serve as mechanoreceptors
- increasing firing rate in response to pressure
Central axons terminate where?
- in the medulla oblongata (cardiovascular centres)
How does the autonomic output get to effector organs?
- sympathetic to heart and vessels
- parasympathetic to heart
What does increased blood pressure do to baroreceptor firing rate?
- increases firing rate
What does decreases blood pressure do to baroreceptor firing rate?
- decreases firing rate
What happens when there is an increased rise in blood pressure?
- acute rise in blood pressure
- increased baroreceptor discharge
- goes to brainstem
- this increases vagal output which decreases heart rate
and at the same time - this decreases sympathetic output meaning there is less calcium coming into cells which decreases, contractility (SV), heart rate and arteriolar tone
- all of these actions decrease blood pressure
How fast do baroreceptors act?
- Act extremely fast, within seconds providing pressure buffering for shot/acute changes
What happens to baroreceptors after longer altered pressure?
- there is evidence for resetting of baroreceptors
What type of information do baroreceptors relay?
- information on blood pressure that is relative to the individual set point (not an index of absolute pressure)
Principle long-term mechanism of blood pressure regulation are via what?
- via kidneys through plasma volume
Describe how the kidneys and plasma volume increases arterial pressure:
- Increase plasma circulating volume
- increased preload (Starlings Law)
- increased cardiac output
- increases mean arterial pressure
What does RAAS stand for?
- The Renin-Aldosterone System
What is the RAAS system?
- A hormone system regulating fluid balance and blood pressure
What is the first step in RAAS?
- the release of the enzyme renin
What do triggers for renin release include?
- reduced perfusion of kidney
- sympathetic stimulation
As an enzyme what does renin do?
- renin cleaves a precursor protein (angiotensinogen) into angiotensin 1
What happens after angiotensinogen is cleaved into angiotensin 1?
- Angiotensin 1 is then converted to angiotensin 2 by an enzyme angiotensin converting enzyme and this mainly happens in lungs
Angiotensin 2 binds to receptors around the body and has many effects what are these?
- Vasoconstriction (increases SVR)
- Na+ reabsorption in kidney (increases CO/preload)
- increases sympathetic activity (increases HR and SV)
- releases anti-diuretic hormone (increases CO/preload)
- release aldosterone (more Na reabsorption)
What does the renin-angiotensin-aldosterone axis provide?
- immediate, potent control of MAP via angiotensin 2
- slower, prolonged regulation of MAP via volume control (aldosterone + ADH)
Other long term control mechanism include volume receptors - What are atrial receptors?
- they are found in the heart and are low-pressure baroreceptors that act as volume receptors
When volume receptors detect high volume detected as stretch it leads to what?
- inhibition of sympathetic pathways leading to renal vasodilation (diuresis)
- inhibition of ADH/vasopressin
- other effects via atrial natriuretic peptide (ANP)
Increase in blood volume leads to stimulation of atrial volume receptors
What are the two responses to this?
- inhibit sympathetic vasoconstrictor pathways to kidneys leading to diuresis
or - reflex inhibition of ADH/vasopressin from posterior pituitary gland
What do the two responses to increased blood volume lead to?
- decreased volume
- decreased central venous pressure
- decreased stroke volume
- decreased cardiac output
- decreased arterial pressure
What is acute haemorrhage?
- blood loss
What are the two acute cardiovascular reflexes?
- Baroreceptor reflexes
- atrial volume receptors
What controls longer term restoration of volume?
- RAAS
