short term control of arterial blood pressure

Question 1

Why do we need to control arterial blood pressure?

Answer 1

• in order to ensure adequate blood flow to organs

Question 2

What are baroreceptors?

Answer 2

• **baro = pressure **
• baroreceptor is a** type of sensory nerve ending** (receptor) that is packed with mitochondria & connected to a myelinated or non-myelinated afferent axon
• baroreceptors are present in the adventitia of arteries at 2 main locations

Question 3

what are the** 2 locations that arterial baroreceptors** are located?

Answer 3

1. aortic arch
2. carotid sinus

Question 4

Describe the carotid sinus?

Answer 4

• a thin walled dilatation at the origin of the internal carotid artery
• afferent fibres from the carotid sinus baroreceptors **FORM the carotid sinus nerve **
• this then** joins the glossopharyngeal nerve ** to reach the petrous ganglion which continues to the NTS

Question 5

Describe the** aortic baroreceptors**

Answer 5

• aortic baroreceptors are found on the **arch of the aorta **
• the baroreceptors afferent fibres form the aortic or ‘depressor’ nerve and then **ascend into the vagus nerve **
• the vagus nerve then ascends into the **nodose ganglion **and the central axons terminate in the NTS

Question 6

what is the NTS?

Answer 6

• **Nucleus tractus solitarius **
• located in the medulla oblongata

Question 7

What do baroreceptors respond to?

Answer 7

• stretch
• eg a rise in arterial pressure, stretches the arterial wall which excites the receptor terminals

Question 8

What is the baroreflex?

Answer 8

• a reflex / mechanism the body uses in order to maintain a stable blood pressure / homeostasis
• rapid **negative feedback loop **- in which an elevated blood pressure causes heart rate and blood pressure to decrease….. or a decreased blood pressure will cause an increase in HR and BP in order to restore normal levels

Question 9

Describe the mechanism of the baroreflex

Answer 9

• the reflex starts with stretchreceptors (baroreceptors) located in the walls of the carotid sinus & aortic arch
• in response to high blood pressure, the walls of the CS and AA stretch which causes the** baroreceptors to fire AP’s rapidly**
• the AP’s are** sent via the vagus and glossopharyngeal nerves** to the nucleus tractus solitarius in the medulla
• in response to high BP the NTS activates the PSNS (parasymp)and inhibits the SNS
• PSNS activation & SNS inhibition work together to maximise BP reduction

Question 10

How does the SNS increase blood pressure?

Answer 10

• it **increases arterial tone **which increases the TPR & venous tone which increases the CVP
• it** increases cardiac contractility** and heart rate which increases cardiac output

Question 11

In response to high blood pressure, an activation of the PSNS and an inhibiton of the SNS occurs … what **effect **does this have?

Answer 11

• a fall in SNS activity causes peripheral vasodilatation which reduces the total peripheral resistance

Question 12

How are carotid sinus baroreceptors related to pulse pressure?

Answer 12

• carotid baroreceptors signal not only the MAP but also the size of the oscillation (periodic variation) about the mean pressure ie the pulse pressure
• the greater the pulse pressure, the greater the average baroreceptor activity

Question 13

what are 2**triggers **of the baroreflex?

Answer 13

• a change in body positon from supine (lying down) to upright
• **haemorrhage **

Question 14

Describe how **changing body position **impact blood pressure?

Answer 14

• when a person moves quickly from lying down to a standing position, there is a fall in venous return to the heart (as gravity shifts blood from chest to veins in legs)
• this then leads to a reduction in stroke volume, cardiac output and therefore a reduction in blood pressure
• this is known as postural or orthostatic hypotension

Question 15

How do baroreceptors respond to orthostatic hypotension?

Answer 15

• they rapidly restore normal blood pressure
• a reduction in BP causes a reduced rate of AP firing which results in an increase in SNS activity to the heart and blood vessels
• heart rate is increased and there is a rise in total peripheral resistance

Question 16

How does a haemorrhage impact blood pressure?

Answer 16

• a haemorrhage decreases blood volume
• a decrease in blood volume will cause a decrease in blood pressure as blood vol is proportional to bp
• a decrease in blood volume will cause a decrease in CVP, cardiac output and therefore mean arterial pressure

Question 17

How do baroreceptors respond to haemorrhage & what effects does this have on the CNS?

Answer 17

• as the blood pressure is so low, the baroreceptor traffic slows down a lot
• it causes increased cardiac SNS activity and reduced vagal PSNS activity - this causes tachycardia and increased heart contratility
• increased SNS activity also causes vasoconstriction of peripheral resistance vessels

Question 18

What are catecholamines?

Answer 18

• any class of aromatic amines that include the neurotransmitters adrenaline & noradrenaline

Question 19

what 2 adrenoceptors does Noradrenline interact with?

Answer 19

• alpha adrenoceptors
• B1 adrenoceptors

Question 20

What happens when noradrenaline interacts with the alpha adrenoceptor?

Answer 20

vasoconstriction occurs & therefore an increases TPR - total peripheral resistance and therefore increases blood pressure

Question 21

what happens when noradrenaline binds to a** B1 adrenoceptor**?

Answer 21

• increases cardiac output by increasing the stroke volume & heart rate via B1 adrenoceptor

Question 22

How does adrenaline impact blood pressure?

Answer 22

has little impact on blood pressure

Question 23

When a baroreflex is triggered to** lower blood pressure**, why is TPR reduced?

Answer 23

• a diminished stimulation of vascular smooth muscle cell alpha adrenoceptor as the sympathetic nerve activity is reduced (ie less noradrenaline binding to alpha adrenoceptors to increase TPR)