Week 6 - Control of the Cardiovascular System

Question 1

Describe how the cardiovascular system responds to a postural change (for example, by standing up).

Answer 1

1. Orthostatic response
2. Heart rate rises 10-25 bpm driven by drop in MABP: the heart rate starts to increase just after getting up.
3. TPR briefly drops, then increases, especially to skin and gut to offset the drop in mean arterial blood pressure.
4. Increased arterial/venous smooth muscle tone
5. Veins limit gravity pooling leg vein

Question 2

Why does fainting sometimes occur after a long period of standing up (without changing posture)?

Answer 2

1. Defeat CVS homeostasis

2. Muscles in the leg become fatigued - inability to maintain central venous pressure

Question 3

Outline the physiological effects on the cardiovascular system of consuming a large meal.

Answer 3

1. Systemically, ANS - Parasympathetic nervous system activity increases
2. Local autacoids and vasodilators: Local GI vasodilatation leads to a fall in TPR
3. With decreased TPR, there is an initial fall in arterial pressure.
4. As flow out via the liver is increased, central venous pressure increases. Increased CVP leads to increased filling of the right atrium, and therefore a rise in cardiac output
5. Baroreceptors decrease a fall in mean arterial blood pressure: they cause a modest increase in cardiac output via heart rate and stroke volume
6. Increased CVP then decreased by increased pumping of the heart
7. Arterial blood pressure also raised back to normal: supplying greater flow to GI
8. GI Demand met: CVS goes back to normal MABP operating range by adjusting cardiac output

Question 4

Outline the physiological changes that occur (in the cardiovascular system) when an individual stands up.

Answer 4

1. When a person stands up quickly, gravity redistributes blood and the volume of blood in the legs increases temporarily.
2. The blood mainly moves from intra-thoracic vessels.
3. Both legs take up an additional 500 ml.
4. The central venous pressure drops directly, and filling of the right atrium is reduced.
5. According to Starlings Law, this will reduce stroke volume by 10 to 25%.
6. Mean arterial blood pressure falls by 20-25 mmHg.
7. Signal via baroreceptors an acute drop in blood pressure

Question 5

What factors determine venous pressure?

Answer 5

1. Rate the blood enters the veins

2. Rate the heart pumps it out

Question 6

State why venous pressure falls and arterial pressure rises when cardiac output increases and total peripheral resistance remains constant.

Answer 6

Fall in venous pressure due to blood being removed from the venous system: this leads to a decreased volume of blood in the venous system
Rise in arterial pressure: Larger volume of blood needs to be pumped through vessels at the same resistance

Question 7

In what type of blood vessels does the greatest pressure drop occur and why?

Answer 7

Arterioles: The most pressure is lost in the vessels where the greatest resistance is found. Arterioles are small vessels that have a lot of smooth muscle in their wall. This creates high resistance that can be varied by vasoconstriction or dilatation. The more the resistance, the greater the pressure loss across the vessel as energy is lost to overcome the resistance. Almost no pressure is lost through elastic and muscular arteries due to their elastic recoil. Blood pressure is low through capillaries to veins as most pressure has already been lost.

Question 8

What is the effect of increasing ventricular end-diastolic volume (EDV)?

Answer 8

1. Increased stroke volume upto an optimum EDV
2. The relationship between EDV and stroke volume is described by the Frank-Starling law of the heart.
3. Increasing EDV increases the number of potential cross-bridges (by reducing myofilament overlap) that can form during contraction. The effect is an increased force of contraction, which increases stroke volume.
4. This occurs up to an optimum EDV, above which stroke volume no longer increases despite further increases in EDV.

Question 9

What is the effect of increased beta-adrenergic stimulation on the heart? (with respect to Starling’s law of the heart)

Answer 9

1. Increased stroke volume for any given end-diastolic volume
2. Beta-adrenoreceptor stimulation increases ventricular contractility, which reduces ESV for any given EDV
3. Vertical shift upwards for the Frank-Starling curve with increasing sympathetic stimulation - increased ejection fraction for any EDV