Co-ordinated Cardiovascular Responses - Gravity & Exercise

Question 1

Define Orthostasis

Answer 1

Standing up - the cardiovascular system according to the effect of gravity

Question 2

What 2 things happen when we first stand up?

Answer 2

Blood pressure falls at first
Postural hypotension, lack of blood flow to the brain – faint

Quickly recovers
Due to homeostatic mechanisms such as baroreflex.

Baroreflex integrates three smaller changes

Increases:
heart rate
heart contractility
total peripheral resistance

Question 3

What is the arterial pressure in the head, heart, and feet when lying down?

Answer 3

95, 100 and 95mmHg

Question 4

What is the venous pressure when lying down?

Answer 4

10, 3-5 and 10mmHg

Question 5

What is the arterial pressure in the heart, and feet when standing up?

Answer 5

60, 95 and 180

Question 6

What is the venous pressure when standing up?

Answer 6

35mmHg, 0-5mmHg and 90mmHg

Question 7

Describe gravity-induced high venous blood pressures

Answer 7

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

Pressure (P) is higher at the bottom of the tube - Magnitude of pressure depends on the height of the fluid column, the density of the fluid, and gravity

Pressure = phg

We get distension of the veins, where blood volume is high and can be liberated in order to increase cardiac output according to Starlings Law

Question 8

What happens when we stand up (venous pressure)

Answer 8

1. Fall in central venous pressure
2. Decreased end-diastolic pressure
3. Decreased diastolic pressure
4. Decreased stroke cardiac output
5. Poor perfusion of brain - dizziness and fainting

Blood pooling of 500ml in legs reduces blood return to the heart

Increased transmural pressure of 90mmHg

Question 9

What happens when we lie down (fainting)?

Answer 9

1. Increased central venous pressure
2. Increased end-diastolic pressure
   3, Increased stroke volume
3. Increased Cardiac output

Question 10

What is the reflex response to orthostasis?

Answer 10

1. Less stimulation (unloading of baroreceptors)
2. Lower afferent fibre activity
3. Switches off inhibitory nerves that go from Caudal ventrolateral medulla (CVLM) to Rostral ventrolateral medulla (RVLM).
4. Results in RVLM being more active sending efferent signals to heart and arterioles.
5. Increased sympathetic drive to SA node and increased HR.
   Myocardium increased contractility
   Vasoconstriction (arterioles, veins) increases TPR.
   Less vagal parasympathetic activity to SA node – overall increase in blood pressure.

Question 11

What makes postural hypotension worse?

Answer 11

α-adrenergic blockade, generalized sympathetic blockade or other drugs that reduce vascular tone - eg. Side effect with voltage gated calcium channel blockers used to treat hypertension, angina.

Varicose veins - Impairs venous return.

Lack of skeletal muscle activity - Due to paralysis or forced inactivity eg. Long term bed rest, soldiers on guard.

Reduced circulating blood volume - eg. Haemorrhage.

Increased core temperature - Peripheral vasodilatation, less blood volume available eg. standing up after bath.

Question 12

Does microgravity matter whether you are standing up?

Answer 12

doesn’t matter whether you are standing or lying down in microgravity…

Question 13

What initially happens in microgravity

Answer 13

Initially: Blood not pooling in feet and returning to the heart easily, increases atria/ventricle volume and so preload and cardiac output. Sensed 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 reduction in blood volume (BV) by 20%.

Question 14

What are the long term effects of microgravity?

Answer 14

Long-term: Less BV, reduced stress on heart, heart reduces in muscle mass, general drop in BP.

Question 15

What happens when we return to gravity?

Answer 15

On return to gravity: Severe postural hypotension, due to much lower blood volume and smaller heart. Baroreceptor reflex can not compensate.

Question 16

What is a dynamic exercise

Answer 16

Constantly shortening and relaxing with lots of different muscle groups involved – lower BP, lower sympathetic tone

Question 17

What is static exercise

Answer 17

One specific muscle group is being worked without constant movement – higher BP, local metabolic hyperaemia

Question 18

Describe the 4 general points about how the cardiovascular system responds to exercise

Answer 18

Integrated by central command in the brain.
Just anticipation of exercise will cause some of the changes to be initiated.
Once exercise commences there is feedback from the muscles via mechanoreceptors & metaboreceptors.
They are all going to increase sympathetic activity and reduce vagus inhibition.

Question 19

Describe changes in oxygen uptake, transport, heart rate, force of contraction. blood pressure

Answer 19

Increase lung oxygen uptake, transport around body & supply to exercising muscle. Increased HR and force of contraction.
Control BP – despite huge changes in CO and resistance (protect heart from excessive afterload which will reduce CO).
Selectively target areas where the oxygen is delivered so co-ordinated dilation/constriction of vascular beds.

Question 20

Outline 3 small changes that occur in response to exercise

Answer 20

1. Heart rate 3 x (60bpm to 180bpm)
2. Stroke volume 1.5 x (70ml to 120ml)
3. Arteriovenous O2 difference, (A-V)O2 3 x (gradient + Bohr effect)

3x1.5x3= 13.5

Question 21

Describe the factors around the increased uptake of oxygen in response to exercise

Answer 21

1. Increased blood flow & greater O2 gradient.
   Increased lung uptake. From 5 -15 litres/ min
2. Arterio-venous oxygen difference reaches a plateau at high exercise levels. From 50 - 150 ml O2/ litre

Question 22

Describe the factors around the increased cardiac output during exercise

Heart rate

Stroke

Answer 22

1. Heart rate increase is main factor at high workloads. From 60 - 180 beats/min
2. Increase in SV reaches max value.
   Plateau phase on Starling’s curve & max contractility. From 80 - 120 ml and then decreases

Question 23

Describe the effects of exercise-induced tachycardia

What is the maximum heart rate?

Answer 23

Tachycardia
Brain central command (ready for exercise)
& muscle mechanoreceptors (fast feedback on exercise being carried out).

Maximum HR = 220 – age, approximate increase 65 to 195 (3x).

1. Vagal tone (SA & AV nodes)
2. Sympathetic activity (SA & AV nodes)

Question 24

Describe the effects of exercise-induced stroke volume

Increased end-diastolic volume

Faster ejection

decreased end-systolic volume

Answer 24

Stroke volume
 sympathetic activity 70 ml to 105 ml for 30 year old male = 1.5x

Increased end-diastolic volume
 Venous return/CVP through veno-constriction
 Sympathetic activity & calf muscle pump - activates Starling law increasing preload.

Faster ejection
 Contractility by sympathetic activation of 1 receptors (inotropic increase in Ca2+).

Decreased end-systolic volume ( ejection fraction)
Accounts for  stroke volume
 Contractility by sympathetic activation of 1 receptors & Starling’s law,

Question 25

Describe what is happening to increase the cardiac output

Answer 25

Fall in local resistance due to metabolic hyperaemia vasodilatation.
Local sympathetic response and β2-mediated vasodilatation via circulating adrenaline.
β2 receptor expression high in skeletal muscle and coronary artery.

Question 26

Describe the effect of increased cardiac output on blood pressure

Answer 26

Large increase in CO

Relatively small
increase in mean BP due to dilated skeletal muscle arterioles decreasing TPR

Large decrease in TPR

Question 27

Describe compensatory vasoconstriction of non-essential circulations

Answer 27

Compensatory vasoconstriction of non-essential circulations prevents hypotension during exercise-induced
decreased TPR

Compensatory vasoconstriction in inactive and unrequited tissues, for example, GI tract, kidney, inactive muscle

Question 28

Which raises the BP more: Static or Dynamic

Answer 28

Static

Question 29

Describe the small-diameter sensory fibres and reflex effects (metaboreceptors)

Answer 29

Small diameter sensory fibres in skeletal muscle
Chemosensitive - Stimulated by K+, H+, lactate, which increase in exercising muscle.

Reflex effects
Tachycardia (via increased sympathetic activity)
Increased blood pressure
‘Pressor response’ to exercise

Especially important during isometric exercise (increased muscle load). Static exercise raises BP more than dynamic exercise.

Raised BP maintains blood flow to contracted muscle to try to force blood into the contracted muscle.

Contracted muscle supplied by dilated resistance vessels due to metabolism…selective metabolic hyperaemia.

Question 30

Give a summary of cardiovascular responses in exercise

Answer 30

Increased oxygen lung uptake = Increase HR and SV

Increase oxygen transport around the body = Extraction of oxygen from the Bohr shift

Direct the increased oxygen supply to exercising muscles = Decrease in vascular resistance in exercising muscle: metabolic vasodilation

Stabilisation of BP = vasoconstriction in non-exercising and non-required tissue