Coordinated responses of the CVS

Question 1

Orthostatis

Standing up - CVS changes according to effect of gravity.

Answer 1

1. Blood pressure falls at first - postural hypotensions, lack of blood flow to brain - faint.
2. Quickly recovers- due to homeostatic mechanisms such as baroreflex.
3. Baroreflex integrates three smaller changes by increasing:
   * Heart rate
   * Force of contraction
   * Total peripheral resistance

Question 2

Effects of gravity on blood pressure during orthostasis

Answer 2

Question 3

Gravity induced hig venous blood pressures

Answer 3

Te high pressure in the venous system at the feet is due to hydrostatic pressure.

Pressure = ρhg

Pressure is higher at the bottom of the tube - magnitude of pressure depends on height of fluid colmun, density and gravity.

Question 4

How does orthostasis cause hypotension

Answer 4

Question 5

Reflex to orthostasis

Answer 5

Question 6

What makes postural hypotension worse?

Answer 6

• α-adrenergic blockade, generalised sympathetic blockade or other drugs that reduce vascular tone - eg side effects with calcium channel blockers used to treat hypertension, angina.
• Varicose veins - impairs venous return.
• Lack of skeletal muscle activity - paralysis.
• Reduced circulating blood volume - Haemorrhage.
• Increased core temperature - peripheral vasodilatation.

Question 7

Effect of microgravity on the CVS

Answer 7

Redistribution of blood into chest region:

1. Initially: Blood not pooling in feet, it returns to the heart easily, increases atria and ventricle volume and so preload and cardiac output. Sense by cardiac mechanoreceptors leading to a reduction in sympathetic activity. This reduces ADH and increases atrial natriuretic peptide (ANP), there is increased glomerular filtration rate (GFR) and reduced RAAS, overall 20% reduction of blood volume
2. Long - term: less blood volume, reduced stress on heart, heart reduces in msucle mass, general drop in BP.
3. On return to gravity: Severe postural hypotension due to much lower blood volume and smaller heart. Baroreceptor reflex can no compensate.

Question 8

Summary of the CVS effects of microgravity

Answer 8

Question 9

Cardiovascular responses to exercise

Answer 9

1. Increase lung O2 uptake - transport around body and supply to exercising muscle. increased heart rate and force of contraction.
2. Control of Bp - Despite huge changes in cardiac output and resistance - protect heart from excessive afterload which will reduce cardiac output.
3. Co-ordinated dilation/constriction of vascular beds - Selectively targer areas where the oxygen is delivered.

Question 10

Integration of several small adaptations

Answer 10

Integration of several small adaptations create overall large response to exercise

Question 11

Increase in O2 uptake from lungs

Answer 11

Question 12

Increase in cardiac output during exercise

Answer 12

Question 13

Exercise-induced tachycardia and stroke volume.

Answer 13

Heart rate - tachycardia.

Brain central command:
* Ready for exercise and muscle mechanoreceptors, fast feedback to brain on exercise being carried out.
-decreased signal down vagus nerve to SA and AV nodes.
Increased sympathetic activity to SA and AV nodes.
-Maximum HR = 220-age.

• Stroke volume (ejection) - Sympathetic activity increases stroke volume.
• Increased end-diastolic volume (filling pressure) - Increases sympathetic activity - activates starling law increasing preload
• Faster ejection.
• Decreased end-systolic volume (increased ejection.)

Question 14

Cardiac output and selective flow changes

Answer 14

Question 15

Effect of increased cardiac output on blood pressure

Answer 15

Question 16

Compensatory vasoconstriction of non-essential circulations

Answer 16

Question 17

Metaboreceptors

Answer 17

Question 18

Static exercise raises BP more than dynamic excercise

Answer 18

Question 19

Summary of cardiovascular responses in exercise

Answer 19