16 Cardiovascular Stress

Question 1

Describe the effect of changing posture from supine to standing

Answer 1

Transient hypotension due to increase in the hydrostatic pressure from 25 mmHg to 105 mmHg because of gravity. As a result, there will be reduced venous return and hence stroke volume

Question 2

Why do we not suffer syncope from supine to standing

Answer 2

Change in BP is sensed through baroreceptors on aortic arch and carotid sinus. The most sensitive pressure range is from 90 mmHg to 100 mmHg which is effective at correcting body BP. This is done through reduction in the parasympathetic and increase in sympathetic activity to elicit venoconstriction (increase venous return), renal/splanchnic vasoconstriction, increased contractility and HR

Question 3

What is the physiological response to haemorrhage

Answer 3

Reduction in blood volume decreases baroreceptor firing rate to increase sympathetic activity and so increase SV and HR. There is another mechanism which involves autotransfusion due to the drop in hydrostatic pressure.

Useful hormones include AVP, aldosterone and Ang II

Question 4

What is the cutoff of shock and what is significant about it

Answer 4

Blood loss > 1500 mL. BP is not restored by compensated mechanism. Respiratory frequency increases and decrease mental state

Question 5

What are the input (afferent) during exercise

Answer 5

Preprogrammed pattern
Muscle chemoreceptor
Both these send impulses to the medulla to increase sympathetic activity

Question 6

What effect does exercise have physiologically

Answer 6

Increase cardiac output by increasing HR and contractility
Overall TPR will reduce:
- Vasodilation to skin
- Vasoconstriction to GIT and kidneys
- Local control to reduce resistance to skeletal muscles, lungs and heart (active hyperemia)

Question 7

Why is there vasoconstriction in GIT and kidneys

Answer 7

• To prevent hypotension as vasodilation in skeletal muscle reduces TPR significantly
• This is also to ensure greatest blood flow to active muscles to supply oxygen and nutrients and remove waste products

Question 8

What are the positive and negative effects on stroke volume of the heart during exercise

Answer 8

Positive: skeletal pump
Negative: sweating and increased capillary pressure (across muscle walls) causing loss of fluid