Coordinated Responses of the CVS

Question 1

Describe the effect of gravity on the blood in the blood vessels (include how we figure out how blood gets from the feet back to the heart).

Answer 1

• Due to the effect of gravity on the blood, there is a higher blood pressure at the feet than there is at the head.
• Blood is able to pool in the veins due to the compliant walls, leading to an increased hydrostatic pressure, followed by other complications.
• This means that the arterial pressure gradient is 95 to 185 mmHg.
• In this scenario, Darcy’s Law is not enough to explain how the blood will be able to get from the feet back to the heart.
• Therefore, we have to use Bernoulli’s Law, which takes into consideration pressure, kinetic and potential energy.

Question 2

With the effects of gravity, describe how orthostasis affects the cardiovascular system, effectively causing hypotension.

Answer 2

• On standing up (orthostasis), the cardiovascular system changes according to the effects of gravity.
• The central venous pressure falls at first.
• According to Starling’s Law, this causes a decreased stroke volume, which decreases cardiac output, which leads to decreased blood pressure.
• Thus, there is poor perfusion of the brain, leading to postural hypotension, leaving the person feeling very faint.
• However, the person is able to quickly recover due to homeostatic mechanisms such as baroreflex.
• The baroreflex integrates three small changes (vasoconstriction):
  
  • an increased heart rate,
  • increased heart contractility
  • increased total peripheral resistance.

Question 3

Describe the reflex response to orthostasis.

Answer 3

1. There is a decreased stimulation (unloading) of the baroreceptors (because the heart is less stretched).
2. There is decreased afferent fibre activity.
3. The nucleus tractus solitaries (NTS) switches off inhibitory nerves that go from the caudal ventrolateral medulla (CVLM) to the rostral ventro-lateral medulla (RVLM).
4. This results in the RVLM being more active, sending efferent heart signals to the heart and arterioles.
5. There is increased sympathetic drive to the SA node, increasing the heart rate.
6. Myocardium contractility also increases.
7. Vasoconstriction of the arterioles and veins occurs, increasing the TPR; there is decreased vagal parasympathetic acitivity to the SA node.
8. These together increase BP.

Question 4

What makes postural hypotension worse?

Answer 4

• α-ADRENERGIC BLOCKADE OR GENERALISED SYMPATHETIC BLOCKADE: drugs that reduce vascular tone
• VARICOSE VEINS: impairs venous return
• LACK OF SKELETAL MUSCLE ACTIVITY DUE TO PARALYSIS: eg. long term bed rest, soldiers on guard, etc.
• REDUCED CIRCULATING BLOOD VOLUME: eg. haemorrhage
• INCREASED CORE TEMPERATURE: peripheral vasodilation, less blood volume available (eg. standing up after a bath)

Question 5

Describe the effect of microgravity (space) on the cardiovascular system.

Answer 5

• Blood shifts towards the head in microgravity.
• There less need for the ANS, RAAS, ADH and ANP systems to control blood pressure.
• INITIALLY:
  
  • There is an increase in preload, thus an increase in atria/ventricle volume.
  • This is sensed by mechanoreceptors.
  • There will be decreased sympathetic nerve activity, a reduction in RAAS, ADH and increased GFR, ANP and diuresis - all leading to a 20% reduction in blood volume.
• LONG-TERM:
  
  • Now, with less blood volume, there is reduced stress on the heart, so the heart reduces in muscle mass.
  • This leads to a general drop on BP.
• ON THE RETURN TO GRAVITY:
  
  • They develop severe postural hypotension due to the smaller heart.
  • The baroreceptor reflex cannot compensate.

Question 6

What are the cardiovascular responses to exercise?

Answer 6

• The response varies depending on the exercise taking place.
• For example, static exercise (e.g. weightlifting) raises the blood pressure more than dynamic exercise (e.g. playing football).
• This is because static exercise is the constant contraction of a small number of muscles, so there is a higher load. (as your muscles are contracted causing external constriction of vessels even though they want to dilate to increase blood supply. Therefore you have to increase blood pressure in rest of the body to overcome the contraction).
• With dynamic exercise, there is a shortening/lengthening of many muscles, which is a low load.
• However, there are some general CVS responses to exercise.
  
  • increased lung O2 uptake, which is transported around the body and supplied to exercising muscle
  • controlled BP, despite huge changes in CO and TPR (to protect the heart from excessive afterload)
  • increase mechanoreceptor and metaboreceptor stimulation

Question 7

Describe how the integration of several small adaptations create an overall large response to exercise.

Answer 7

• The O2 uptake by the pulmonary circulation can increase 10-15 times during strenuous exercise.
• This very big change brought about by the integration of three smaller changes: -
  
  • increased heart rate (3x) [60bpm to 180bpm]
  • increased stroke volume (1.5x) [70ml to 120ml]
  • increased arteriovenous O2 difference (3x) [gradient + bohr effect]

3 x 1.5 x 3 = 13.5 times

Question 8

Describe what occurs during exercise-induced tachycardia, and how the stroke volume is affected.

Answer 8

• There is an increased stimulation of the brain central command, along with the increased stimulation of the muscle mechanoreceptors.
• There is a decrease in parasympathetic simulation, and an increase in sympathetic stimulation, both affecting the SA and AV nodes.
• There is an increased stroke volume, due to an increase in sympathetic activity.
• Some factors that contribute to the increased stroke volume are: -
  
  • increased end-diastolic volume - as there is an increase in venous return / CVP through venoconstriction. Increase in sympathetic activity and calf muscle pump activates starling law increasing preload.
  • faster ejection - Increased contractility by symathetic activation of beta-1 receptors (inotropic increase in Ca2+)
  • decreased end-systolic volume

Question 9

Describe the changes in cardiac output and selective flow changes during exercise.

Answer 9

• There is a fall in local resistance due to metabolic hyperaemia (an excess of blood) vasodilation (vasodilation in some places happens idk why).
• There is also increased sympathetic activity and β2-mediated vasodilation via the circulating adrenaline. [there is a high β2-receptor expression in skeletal muscle and coronary arteries]
• This increases cardiac output and flow changes.

Question 10

Describe compensatory vasoconstriction of non-essential circulations during exercise.

Answer 10

• The compensatory vasoconstriction of non-essential circulations prevents hypotension due to an exercise-induced decrease in TPR.
• The compensatory vasoconstrictions of inactive or unrequired tissues (such as in the kidneys, GI tract, inactive muscle) prevents the BP from falling. (note: the RVLM controls specific pre-ganglionic sympathetic nerves in the spinal cord which send out post-ganglionic nerves to specific tissues).

Question 11

As a recap, describe metaboreceptors (including their reflexes, and their role in exercise).

Answer 11

• They are small-diameter sensory fibres in skeletal muscle.
• They are chemosensitive, so they are stimulated by metoboreceptors such as K+, H+ and lactate, which increase in exercising muscle.
• Some of the reflex effects: -
  
  • tachycardia (via increased sympathetic activity) - increased blood pressure. This is especially important during isometric exercise (increased muscle load). Static exercise raises BP more than dynamic exercise. The raised BP maintains blood flow to the contracted muscle. The contracted muscle contains dilated vessels due to metabolism. This is selective metabolic hyperaemia.