Short term control of blood pressure

Question 1

What does mean arterial pressure depend on?

Answer 1

Mean arterial pressure= TPR X C.O.
where TPR is the pressure against which the heart has to combat to eject blood
C.O. Is the blood entering the system

Question 2

What is main system which controls short term BP?

Answer 2

ARTERIAL BARORECEPTORS:
found in the aortic arch and in the carotid sinuses.
Fire action potentials:
Aortic arch up the vagus nerve
carotid sinuses up the glossopharangeal nerve
They increase the number of action potentials fired when MAP is high and reduce the number when MAP is low.
These impulses are sent to the medullary centres and compared with a reference value.
The medullary centres then co-ordinate a response.

Question 3

How can MAP be increased

Answer 3

• decreased vagal tone: reduce the release of Ach from the vagus nerve so increase HR
• increase the release of noradrenaline from the sympathetic nerves/ adrenaline from medulla
• Increase venoconstriction- increases Venous return and therefore stroke volume
• increase arteriole constriction- increases TPR

Question 4

What other centres are involved in short term control?

Answer 4

• central chemoreceptors
• cardiopulmonary baroreceptors: stretch receptors, respond to change in volume more than pressure, located in low pressure zones such as the walls of the atria
• joint receptors
• chemoreceptors in muscles- respond to changes in ph and c02 concentration
• higher centre: - feed forward systems in the hypothalamus

Question 5

When are these systems important?

Answer 5

-valsalva manœuvre: forced expiration against the glottis
leads to an increase in pressure in the thorax which reduces blood return to the heart and therefore MAP.
baroreceptors respond by reducing number of action potentials sent to the medullar centres.
they co-ordinate a response.
Blood flows back to the heart- reflex actions still present so BP increases.
then returns to normal

-posture changes: veins and venues affected by gravity and can cause blood pooling.
Reduced venous return lowers MAP so response co-ordinated

Question 6

Valsalva manouvre in the elderly

Answer 6

• fewer elastic fibres in the aorta so baroreceptors less prone to stretch
• MAP will continue to fall

Question 7

Consequence of MAP being too low

Answer 7

Syncope

Question 8

Consequences of MAP being too high

Answer 8

Hypertension