Responses to CVS Stress

Question 1

Why is gravity a problem for the cardiovascular system?

Answer 1

Gravity pushes a column of blood down from the head to the toes causing distention in the veins lower down in the body and increasing the volume of blood in the venous reservoir.

Question 2

Why is blood pressure measurement taken from the arm?

Answer 2

Because it is level with the heart. When standing up, the blood pressure will decrease above the heart and increase below the heart so at the level of the heart, gravity would cause less of a change in pressure.

Question 3

How does standing up affect blood pressure and why?

Answer 3

The blood pressure increases below the heart and decreases above the heart because gravity pushes a column of blood down from the head to the toes.

Question 4

How does the change in posture affect fluid movement?

Answer 4

As gravity pushes blood down the body, there is an increase in capillary hydrostatic pressure meaning that more fluid leaves the capillaries and enters the interstitial compartment. This means that there is reduction in effective circulating blood volume so there is decreased venous return to the heart.

Question 5

Explain how standing up affect venous return to the heart?

Answer 5

Blood pools in the distensible veins and fluid leaves the circulatory system due to the increased hydrostatic pressure so there is a decrease in the venous return to the heart and hence less ventricular filling and less stretching of the muscle fibres (preload) so the force of contraction is decreased (Starling’s Law).

Question 6

What is haemorrhage?

Answer 6

Reduction in actual circulating blood volume.

Question 7

What are the three main responses to haemorrhage?

Answer 7

Increased heart rate
Increased heart contractility
Organ specific vasoconstriction

Question 8

What is the compensatory mechanism that takes place in the capillaries to preserve fluid volume?

Answer 8

Autotransfusion - more fluid is retained in the capillaries (because the hydrostatic pressure is decreased due to the decrease in fluid volume but the colloid osmotic pressure is the same) - this means that less fluid leaves the capillaries and more fluid is reabsorbed.

Question 9

State three hormones that are important in the response to haemorrhage?

Answer 9

Angiotensin II
Vasopressin
Aldosterone

Question 10

What is the action of angiotensin II?

Answer 10

Angiotensin II is a powerful vasoconstrictor - it will decrease the amount of blood flowing to the kidneys and hence it will decrease urine output

Question 11

Where do aldosterone and vasopressin act?

Answer 11

Kidney Collecting Duct

Question 12

What are the effects of aldosterone?

Answer 12

Aldosterone causes the loss of K+ into the urine and the retention of Na+ and water

Question 13

What are the overall effects of these three hormones?

Answer 13

Reduced blood flow to the kidneys causing reduced urinary output
Increased fluid retention to preserve blood volume and pressure

Question 14

What percentage fluid loss can these compensatory mechanisms cope with?

Answer 14

Around 30%

Question 15

Describe a mechanism that causes local vasodilation.

Answer 15

Active Hyperemia - if there is a lot of metabolism going on in a tissue, it will use up a lot of oxygen and glucose. The uptake of oxygen and glucose is detected by the tissue and it causes an increase in vessel radius.

Question 16

What are the two afferent inputs to the medullary cardiovascular centre?

Answer 16

Preprogrammed pattern - autonomic response in anticipation of exercise
Chemoreceptors in the muscle - detect a changing environment

Question 17

How does exercise affect TPR? What is the overall change?

Answer 17

Exercise causes a lot of vasodilation (e.g. to the skeletal muscle and the skin (to radiate heat away)) but there is some organ-specific vasoconstriction (spleen, GI tract). The overall effect is a DECREASE in TPR.

Question 18

How does exercise affect CO? What is the overall change?

Answer 18

Increased sympathetic activity increases heart rate and contractility and the muscle activity causes increased venous return to the heart and so increased preload. However, there is an increase in hydrostatic pressure so more fluid is lost to the interstitium and fluid is lost as sweat. Overall there is an INCREASE in CO.

Question 19

State the equation relating TPR, BP and CO. What is the overall effect of exercise on blood pressure?

Answer 19

BP = CO x TPR

The increase in cardiac output is GREATER than the decrease in TPR and so there is an INCREASE IN BLOOD PRESSURE