16. Responses to Cardiovascular Stress

Question 1

Why is the pressure lower above the heart and higher below the heart?

Answer 1

• Gravity forces blood down to the heart
• Pressure above the heart will be lower than 120/80
• Below the heart, blood has to move against gravity towards the heart

Question 2

Why is blood pressure taken from the arm?

Answer 2

• Level with the heart
• Gravity isn’t an issue
• 120/80 normally

Question 3

How much pressure is added if the effects of gravity are added on top of the pressure generated by the heart?

Answer 3

80 mmHg

Question 4

What is the blood pressure in the foot capillaries?

Answer 4

• Normally 25 mmHg

* Add 80 mmHg when standing = 105 mmHg

Question 5

What effect does a change in posture have on blood vessels and blood pressure?

Answer 5

• Arteries are muscular - little impact
• Veins are less muscular - easier to cause venous distention
• Veins stretch and blood can pool - less blood in the arterial system
• This lowers the blood pressure - hypotension

Question 6

What effect does a change in posture have on fluid movement?

Answer 6

• Increase in hydrostatic pressure in the legs (+ gravity)
• More fluid leaves the capillary and enters the tissue
• Reduction in effective circulating blood
• Less venous return => lower diastolic volume => lower CO => hypotension (Starling’s Law)

Question 7

What is transient hypotension?

Answer 7

Sudden, short lasting hypotension as a result of standing up too quickly

Question 8

How to baroreceptors react in response to transient hypotension, and how does this restore BP?

Answer 8

• Most sensitive at 100 mmHg - any deviation is detected
• Pressure goes down
• Baroreceptor firing rate decreases
• Less parasympathetic stimulation - increased heart rate
• Less sympathetic inhibition => more sympathetic
• More noradrenaline
• Increased contractility and heart rate
• Increased cardiac output
• Increased contraction of skeletal muscle
• Increased venous return
• Vasoconstriction - renal and splanchnic - increases TPR
• BP increases

Question 9

What is a Haemorrhage?

Answer 9

Reduction in actual circulating blood volume

Question 10

What are the compensatory mechanisms for a haemorrhage?

Answer 10

• Mechanisms to increase BP - like in transient hypotension i.e. increased CO (heart rate and contractility) and increased TPR (vasoconstriction)
• Higher hydrostatic pressure at the arteriolar end - pressure decreases across the capillary
• Colloid osmotic pressure = same
• Retain more fluid at the venous end
• Autotransfusion - significant reabsorption of fluid back into the capillaries to try and preserve the blood pressure (due to osmotic pressure)
(• Although volume is maintained, lost blood cells need to be replaced)

Question 11

What is the endocrine response to a haemorrhage?

Answer 11

• Angiotensin II - vasoconstriction
• Aldosterone - stimulates sodium reabsorption (collecting duct)
• Vasopressin - stimulates water retention (collecting duct)
• They all reduce urine production
• Promotes retention of fluid

Question 12

What happens when you lose 10%, 30% or over 30% of your blood volume?

Answer 12

• 10% - mechanisms can manage it (alert mental state)
• 30% - decrease in blood pressure (anxiety)
• over 30% - shock, tissues don’t receive enough oxygen (drowsy => confused => unconscious)

Question 13

What happens to your blood pressure when you exercise?

Answer 13

• Blood flow increases to certain tissues
• Increased metabolism
• Vasodilation - increase supply of nutrients (active hyperaemia)
• Decreased total peripheral resistance
• BP drops

Question 14

How does the body prepare for exercise?

Answer 14

• Pre-programmed pattern - autonomic activation of the medullary cardiovascular centre in anticipation
• Chemoreceptors in the muscle detect a change in environment and send a signal to the medullary cardiovascular centre too
• Increased sympathetic response - vasoconstriction in abdominal region - counter the fall in TPR
• Decreased sympathetic response to skin - vasodilation - radiation of heat
• Overall fall in TPR - but the decrease is lessened by the increased TPR in certain places
• Cardiac output increases - increased sympathetic and decreased parasympathetic activity - increase in stroke volume and heart rate
• BP increases

Question 15

How else is cardiac output increased apart from the effect of the autonomic system during exercise?

Answer 15

• Skeletal muscle squeezes veins
• Increased venous return
(• Less blood stored in venous system)
• Increased CO

Question 16

How is the venous return negatively opposed during exercise?

Answer 16

• Increased capillary pressure
• More fluid lost to tissues
• Sweating causes loss of fluid too, as well as a loss of salt
• Decrease in plasma volume
• This does not affect the positive effect on CO