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The Arterial System: the Arterioles

 

 

Purpose:

The arterioles are the small arteries that connect the large arteries to the capillaries and that play an important role in distributing the blood (= the cardiac output) to the organs.

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A. Introduction:

1.

All arteries (large and small) have a thick muscular coat with which the vessel can dilate or constrict. This is called vasodilatation and vasoconstriction (vaso = vessel).

2.

As the blood flows through the arterial system to the tissues, the blood pressure will decrease. But the decrease is not uniform.

3.

Throughout the large arteries, the blood pressure hardly changes at all (only a few mmHg).

4.

When the blood flows through the smaller vessels (arterioles), then the resistance increases and the pressure drops a lot.

5.

Arterioles are the most important vasoconstrictor and vasodilatators in the arterial system.

6.

After the arterioles, the blood pressure has dropped to approximately 30-35 mmHg and is no longer pulsatile (= no more systolic and diastolic oscillations).

 

 

B. Pressure along the Systemic Circulation:

Blood pressures in the systemic circulation larger?

1.

This diagram shows the blood pressure from the left ventricle to the right atrium.

2.

In the left ventricle, the blood pressure varies between about 0 mmHg (diastole) and 120 mmHg (systole).

3.

In the aorta, the blood pressure varies between about 80 mmHg (diastole) and 120 mmHg (systole). This variation in pressure is called pulsatile.

4.

In the large arteries, the blood pressure also varies between about 80 mmHg (diastole) and 120 mmHg (systole). Not much different from the aorta; only slightly lower.

5.

In the arterioles, the blood pressure drops a lot, because the vessels are relatively narrow. In addition, the pulsatile flow gradually disappears and the blood flow becomes non-pulsatile.

6.

In the venules, veins, large veins and vena cava superior and inferior, the blood pressure continues to drop. The lowest pressure in the systemic circulation is found when the blood enters the right atrium (close to 0 mmHg).

C. Pressure along the Pulmonary Circulation:

blood pressures in the pulmonary circulation larger?

1.

This diagram shows the blood pressure in the pulmonary system, from the right ventricle all the way to the left atrium.

2.

In the right ventricle, the blood pressure varies between 0 and 25 mmHg (much lower pressures then in the left ventricle).

3.

In the pulmonary artery, the blood pressure is pulsatile between 25 and 8 mmHg.

4.

The blood pressure decreases a lot in the pulmonary arterioles and becomes non-pulsatile.

5.

The pressure continues to decrease gradually along the pulmonary venules and veins.

6.

The lowest pressure in the pulmonary circulation (about 0 mmHg) is found in the left atrium.

D. Cardiac Distribution:

1.

One of the most important function of the arterial system, and of the arterioles, is to distribute the blood through the body. This is called the cardiac distribution and the question is always how much blood goes to which tissues.

 

2.

Some organs need more blood than others; major users are the kidney (25%) and the brain (15%) for example.

(% of the cardiac output; which is, at rest, about 5L/min).

3.

Some organs or tissues need a lot of blood when they are active but not a lot when they are quiet. Examples are the gut (needs a lot of blood after feeding) and the skeletal muscles (during exercise).

4.

The amount of blood that flows to an organ is determined by the activity of that organ. If the organ works hard, then the arterioles feeding that organ will dilate and lots of blood will flow to it. If the organ works less, then the arterioles will vasoconstrict and less blood will flow to it.

5.

So, the distribution of blood is determined by constricting vessels (=arterioles) to some tissues and relaxing (dilating) other vessels leading to other tissues; as determined by their respective need.

6.

So, for example, after a meal, more blood is needed in the intestines and therefore the intestinal arteries will dilate, diverting more blood towards the intestines. The same would apply with exercise; then the skeletal arteries would dilate.

7.

But this could lead to a conflict. If one exercises after a meal then both gut and muscles need blood. There might then not be enough for all the organs in the body; like for the brain. This is one cause of fainting.

8.

That is why your mother did not allow you to swim after lunch; the gut would need more blood, the muscles will need more blood and there might be not enough for your brain; you might then faint which, in a swimming pool, is quite dangerous!

 

E. What is the "Peripheral Resistance"?:

peripheral resistance

1.

Some students have difficulty in understanding the concept or the idea of the “peripheral resistance”.

2.

We all know that as the blood flows through the blood vessels, it is “resisted” by the vessel wall. If the vessel is narrow, the resistance is high; if the diameter is very large, the resistance is very low.

3.

All the arterioles together could be considered as one giant “resistance”. If they all (vaso) constrict, the peripheral resistance will be high, and the blood pressure before the resistance will increase.

4.

If however, all the arterioles vasodilate, then the resistance will be very low and the blood pressure (especially the diastolic pressure) will decrease.

5.

So, why is this resistance called “peripheral”?

6.

Because it is “peripheral” (away) from the heart (which is considered “central”). That’s all!

 

F. What determines the Blood Pressures:

1. What determines the height of the systolic blood pressure?

Mainly the contraction force of the heart. If the ventricle contracts weakly then the contraction strength will be lower, the ejection will be less and the maximum pressure achieved will decrease. If however the contraction is very strong, then the opposite will occur and the systolic pressure will increase (for example during exercise).

 

effects of contraction on systolic pressure

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2. What determines the height of the diastolic pressure?

This is, mainly, determined by the flow of blood into all the organs. As you saw above, this is determined by the arterioles in the body. All these arterioles resist to various degrees the flow of blood. Together, this is called the peripheral resistance (Why is this called "peripheral"? Because "central" is the heart here).

In the diagram, if the resistance is very high (c), the pressure decrease will be very slow during diastole and the diastolic pressure will be high. If however, the peripheral resistance is very low (a and b), the drop will be very high and the diastolic pressure very low

 

 

 

effects of peripheral resistance on diastolic pressure

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3. The frequency of the heart.

Interestingly, the frequency of the heart has also an influence on the diastolic pressure. This is because the frequency of the heart determines the amount of time the blood is allowed to flow away in the periphery during the diastole.

If the heart frequency is high, then there is less time before the next systole and therefore the diastolic pressure will be higher.

If however the heart rate is low, then there will be more time for the pressure to drop (before the next heart beat) and therefore the diastolic pressure will be lower.

effects of cardiac frequency on diastolic pressure

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4. Why don't we die all the time?

This leads us to my favourite subject! Why don't we die all the time? Better said, what would happen if the heart (suddenly) stopped contracting? As shown in the diagram, the blood pressure will continue to decrease and reach zero quite quickly (within a few minutes). Why don't we die? Because the heart will save us every time from this fatal decrease in blood pressure. We are in danger of dying all the time but saved every time the heart beats!

whay don't we die; because the heart keeps beating!

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large arteries
Contents
capillaries

 

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