CVS 15 - Cardiovascular 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

How does standing up affect blood pressure and why?

Answer 2

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 3

How does the change in posture affect fluid movement?

Answer 3

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 4

Explain how standing up affect venous return to the heart?

Answer 4

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 5

Which mechanisms are in place to compensate the stress that is standing up?

Answer 5

The increased SYMPATHETIC FIRING compensates for the transient hypotension via:
INCREASED HEART RATE (20 BPM)
INCREASED HEART CONTRACTILITY
INCREASED SPLANCHNIC/RENAL VASOCONSTRICTION
VENOCONSTRICTION
Sensed by baroreceptors which are extremely sensitive to changes around normal BP.

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 (to increase TPR)

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. because osmotic pressure becomes dominant.
NB - erythrocytes can’t be replaced but at least fluid is maintained

Question 9

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

Answer 9

Angiotensin II
Vasopressin (ADH)
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

30% blood loss, after it is shock.

Question 15

What is the problem exercise creates on the body?

Answer 15

While exercising, we need increased blood flow to certain part of the body (heart, lungs, skeletal muscle)
However, increasing blood flow involves vasodilation which will lead to a decrease in TPR and thus, blood pressure (BP = CO X TPR)

Question 16

What mechanisms happen for exercise?

Answer 16

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.

Our response to exercise is controlled by the MEDULLARY CARDIOVASCULAR CENTER
It gets input from:
- a ‘preprogrammed pattern’ which happens automatically in anticipation of exercise
- Muscle chemoreceptors which sense changes due to exercise
The CVS Center activates and inhibits the SNS and PNS

Question 17

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

Answer 17

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.
Other thing - skeletal muscle pump helps with increasing venous return, ie. SV so CO.

Question 18

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

Answer 18

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