Pressure and flow in the systemic circulation Flashcards

1
Q

Where do you measure total peripheral resistance?

A

In the aorta

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What determines mean arterial pressure?

A

Flow generated by the heart (cardic output)

Resistance generated by the arterioles (systemic vascular resistance aka total peripheral resistance)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is pulse pressure?

A

The difference between the systolic and diastolic blood pressure

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the clinical significance of pulse pressure?

A

Diastolic pressure is affected by the resistance in arterioles.

In conditions which cause vasodilation (e.g. sepsis) results in a drop in diastolic pressure, so the pulse pressure widens.

In haemorrhage, diastolic pressure rises so the pulse pressure narrows

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What proportion of the cardiac cycle is spent in systole and diastole

A

Systole: 1/3 (fixed)

Diastole: 2/3 (decreases as heart rate increases)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Describe the structure of the veins

A

Thin walled

Highly compliant

Pressure determined by volume of blood contained

Contains 66% of blood volume

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Factors controlling blood pressure

A

Heart rate

Stroke volume

Changes in compliance

Arteriolar resistance

Mean arterial pressure = CO x TPR

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

How is cardiac output affected by changes in heart rate?

A

CO - HR x SV

If heart rate is doubled and the stroke volume is unchanged, then cardiac output.

However if the heart rate increases, eventually the stroke volume will increase, meaning that cardiac output can increase by 4x.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

How does distensibility affect pulsatile flow?

A

In blood vessels, 1/3 of the stroke volume leaves the arteries during systole. The remaining 2/3 stretches the artereis and they become distended. During diastole, the elastic arteries recoil which drives blood into the arterioles.

Pulsatile flow produces a square pulse pressure (on-off) as the valve is opened and closed repeatedly. In a rigid tube, the output flow mimics this pulsatile flow.

In distensile (elastic) tubes, some of the force from the pulsation is absorbed, which means that the elastic stretches to accommodate changes in volume. This converts the pulsatile flow into more of a constant sigmoid flow. There is always a low level out output.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Explain the changes between in pressure, flow and velocity between the left ventricle and the right atrium

A

Flow of blood is dependent on a pressure gradient. If there is no pressure gradient, there is no flow.

In blood leaving the left ventricle via the aorta, blood leaves at a high pressure. The aorta is a large muscular vessel and has a low cross sectional area so velocity is high. As the blood passes from large arteries to small arteries the vessel diameter is reduced and pressure begins to decrease, which reduces the velocity.

As blood passes from small arteries to arterioles, there is a large drop in pressure due to the large incerase in cross-sectional area. Velocity is reduced.

Lowest blood flow velocity is through the capillaries, which allows more time for exchange of material.

More blood ls found in the veins because they are more compliant. Venous valves produce a flow velocity which transports blood back to the heart.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

Describe the changes in pressure in the systemic circulation

A

Pressure = flow x resistance

The total blood flow through the systemic blood vessels is equal to the cardiac output. Changes in pressure in the systemic circulation is proportional to the resistance of the vessel.

The pressure in the aorta and large arteries is high and pulsatile, and there is only a small drop in pressure as blood passes between them, as blood reaches the small arteries the pulse pressure decreases slightly.

The largest drop in pressure is within the arterioles. These have the largest contribution to peripheral resistance.

Pressure continues to decrease in the capillaries and veins.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What is vessel compliance?

A

Describes the flexibility of the vessels in response to pressure.

The greateer the compliance, the more flexible the vessel is. Vessels which are rigid are less compliant.

Compliance decreases with age

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What is vasomotor tone?

A

The degree of tension of the smooth muscle within the walls of blood vessels, particularly in the arterial tree, which is controlled by the sympathetic nervous system.

(Resting level of contraction)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Name 4 metabolites which cause vasodilation

A

K+

H+

adenosine

CO2

hypoxia in the systemic circulation

Cause vascular smooth muscle relaxaion - vasodilation

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

How does metabolism affect local blood flow?

A

Increased metabolism increases the concentration of vasodilator metabolites. These cause smooth muscle relaxation and vasodilation, overriding vasomotor tone from the sympathetic nervous system.

Vasodilation causes increased supply pressure - thus increasing blood flow. This removes the vasodilator metabolites (reduces concentration). Reduced metabolite concentration restores vasomotor tone causing vasoconstriction.

If there is less metabolism, there are less metabolites, and therefore less flow.

This resposnse allows blood to be directed to tissues that are more metabolically active.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Describe how transmural pressure is controlled

A

Transmural pressure is the pressure between two sides of the vessel (or organ) wall.

It can be controlled by altering the volume of the vessel (vasoconstriction or vasodilation) - compliance

17
Q

Describe how hormones affect vasomotor tone

A

Adrenaline and noradrenaline released from sympathetic nerves bind to a1-adrenergic receptors on smooth muscle in the arteriole and cause vasoconstriction, binding to b2-receptors causes vasodilation

Angiontensin II produced by ACE enzyme causes vasoconstriction which increases peripheral resistance

ANP released in response to atrial stretch causes vasodilation by regulating Na+ balance and blood volume

18
Q

What is myogenic contraction?

A

Response of blood vessels to changes in blood pressure to keep the blood flow within the blood vessel constant.

Passive stretching of arterioles by a sudden rise in bp stimulates contraction of smooth muscle cells leading to vasoconstriction (increased resistance to flow).

The smooth muscle of the blood vessels reacts to the stretching of the muscle by opening Ca2+ channels, which cause the muscle to depolarize, leading to muscle contraction. This reduces the volume of blood able to pass through the lumen, reducing blood flow through the blood vessel.

Reduction in pressure favours relacation of the smooth muscle causing vasodilation and decreased resistance to flow

19
Q

Describe the effects of chemicals released by endothelial cells on blood vessels

A

Endothelial cells secrete chemicals which both vasoconstrict and vasodilate blood vessels back acting on smooth muscle cells.

Nitric oxide is a vasodilator

Endothelin I is a vasoconstrictor.

20
Q

How do you calculate mean arterial pressue?

Why is it important?

A

diastolic pressure + 1/3 of the pulse pressure

The mean arterial pressure is the average pressure driving the blood into the tissues over the entire cardiac cycle.

21
Q

Describe the neural control of vasomotor tone

A

Vasomotor tone is an autonomic reflex regulated from the medulla oblongata.

The vasomotor centre activates sympathetic nerves which can raise blood pressure by releasing NorA.

Acts on arterioles causing smooth muscle contraction which increases resistance to flow. As a result pressure in the capilaries is reduced so there is more reabsorption of fluid into the circulation

This binds to b-adrenergic receptors in the heart to stimulate the heart rate and contractility, increasing cardiac output

22
Q

Explain the pattern of the systemic ventricular and arterial waveforms

A

The systemic arterial pressure waveform results from ejection of blood from the left ventricle into the aorta during systole

Systolic peak, dicrotic notch as aortic valve closes, diastolic drop

23
Q

What is autoregulation of blood flow?

A

Where blood flow is adjusted to the existing metabolic activity in the tissues.

Metabolic activity releases vasodilator metabolites into the tissue to increase O2 delivery from the blood. An increase in blood flow decreases the the tissue concentration of metabolites.

24
Q

What determines the pressure in veins?

A

Volume of blood contained within them

Veins are thin walled and highly compliant

25
Q

Name four factors that affect regional blood flow

A

Vasomotor tone

Local metabolites

Transmural pressure

Myogenic contraction

Vasoactive agents

Hormones

Autonomic nervous activity

26
Q

Why is autoregulation important?

A

Local blood flow control mechanisms help keep blood flow constant over a wide range of arterial pressure. Autoregulation ensures that critical organs (kidneys, brain) have an adequate blood flow and oxygen delivery.

A change in mean arterial pressure, e.g. hypovolemia or circulatory shock, In athersclerosis an increase in pressure decreases flow, this causes the vessels to dilate.

At extremes of pressure autoregulation breaks down.

27
Q

What is perfusion pressure?

A

The pressure gradient required to perfuse the tissue with blood (equal to the difference between the arterial and venous pressure)

28
Q

Name four factors that affect venous return

A

Muscle contraction. Contraction of limb muscles during activity promotes venous return

Decreased venous compliance. Sympathetic activation of veins decreases venous compliance, increases central venous pressure and promotes venous return

Respiratory activity. venous return increases because of a decrease in right atrial pressure.

Vena cava compression. An increase in the resistance of the vena cava decreases return . e.g. thoracic vena cava becomes compressed during a Valsalva maneuver or during late pregnancy Gravity. decreases venous return

29
Q

What is the value of mean arterial pressure

A

93 mmHg

Diastolic pressure + 1/3 pulse pressure