Short term control of blood pressure

Question 1

Why is regulating MAP so important?

Answer 1

Because the MAP must be kept within the set range to avoid fainting (syncope) or hypertentsion and its long term effects (causes damage to fine blood vessels throughout the body and therefore causing the symptoms of things like stroke, heart disease, impotence, blindness. So so you need to keep your mean arterial pressure in the right ballpark.

Question 2

Give the componants of the arterial baroreceptor reflex and explain their function

Answer 2

MAP monitored by baroreceptors (effectively stretch reflexes) Aortic arch and Internal carotid archeries.

These send signals up to the medullary cortex in the brain (brainstem). Carotid sinuses via glossopharyngeal nerve and aortic via vagus nerve, they fire more often the more it is stretched.

Brain (medullary cardiovascular center) decides what to do and sends signals down either by :
Sympathetic nerves - can stimulate Beta 1 receptors in SA node, throughout heart, in Arterery and Venous smooth muscle.
Parasympathetic (vagus) nerve - via ACh on muscarinic receptors on SA node.

Question 3

What are the inputs to the medullary cardiovascular centre?

Answer 3

Negative feedback systems:

Cardiopulmonary baroreceptors
Sensing central blood volume (stretch reflexes again) - eg in arteries, atrium and blood vessels in lung

Central chemoreceptors (in medulla) - 
Sensing arterial pCO2 and pO2 

Chemoreceptors in muscle
Sensing metabolite concentrations eg increasing CO2 release

Joint receptors
Sensing joint movement

Feed forward systems:

Higher centres
Hypothalamus and cerebral cortex - eg try to preempt increase in HR when get onto a bike

Question 4

What is the relationship between CO, TPR and MAP?

Answer 4

MAP = CO x TPR

Question 5

Why is it important to have coordinated control of the cardiovascular system (use the example of exercise)

Answer 5

So the MAP is maintained and nutrient and waste delivery is optimised.

Question 6

What effect does changing posture have on the CVS?

Answer 6

veins = capacitance vessles, blood “shlumps”, Reduced Vr - edv - sv - co - map - detected by baroreflexes which send less signals over set time, initiates sympathetic and inhibits parasympathetic.

Question 7

What is the Valsalva manouvre and what affect does it have on the CVS?

Answer 7

Increased thoracic pressure by forced expiration against a closed glottis. (pushing out a poo).

Basically increases thoracic pressure above venous pressure and so reduces/stops venous return, so decreased vr, edv, preload, sv, co, map detected and compensated for by increasing hr and constricting art/venules etc, then when released all blood from veins floods back to heart, significantly incresaing map, so hr is reduced to comensate, and eventually returns to normal.

In other words:

1. Increased thoracic pressure is transmitted through to aorta – hence the immediate jump
2. Increased thoracic pressure reduces the filling pressure from the veins, which therefore decreases all  VR,  EDV,  SV,  CO,  MAP
3. .The reduced MAP is detected by baroreceptors which initiate a reflex increase in CO and TPR
4. At end of the manoeuvre, the decrease in thoracic pressure is transmitted through to the aorta – hence the drop
5. VR is restored so SV increases, but reflex effects have not worn off
6. Eventually, back to normal

Question 8

What responce is initiated by a reduction in MAP (eg standing up)

Answer 8

Elements of the reflex response that is initiated will include:
A parasympathetic mediated dis-inhibition of HR causing an increase in CO
A sympathetic mediated increase in HR causing an increase in CO
A sympathetic mediated increase in contractility causing an increase in SV and CO
A sympathetic mediated venoconstriction causing an increase in preload, stroke volume and CO
A sympathetic mediated arteriolar constriction causing an increase in TPR