B.5.2. 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 from the heart (= the cardiac output) to the organs.

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 decreases 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 / diastolic oscillations).

B. Blood Pressure along the Systemic Circulation:

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Blood Pressures Along The Systemic Circulation
1.
This diagram shows the blood pressure from the left ventricle all the way 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 120 mmHg (systole) and 80 mmHg (diastole). This variation in pressure is called pulsatile.
4.
In the large arteries, the blood pressure also varies between about 120 mmHg (systole) and 80 mmHg (diastole). Not much different from the aorta; only slightly lower.
5.
In the arterioles, the blood pressure drops a lot, to about 20-30 mmHg, 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, 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. Blood Pressure along the Pulmonary Circulation:

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Blood Pressures Along The Pulmonary Circulation
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 25 and 0 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, like in the systemic circulation, 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:

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1.
The most important function of the arterial system is to distribute the blood through the body. This is called the cardiac distribution and the big 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 a meal) 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 needs.

6.
So, for example, after a meal, more blood is needed in the intestines and therefore the intestinal arterioles will dilate, diverting more blood towards the intestines. The same would apply with exercise; then the skeletal arterioles will 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 determines the Blood Pressure?

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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 (this occurs for example during exercise).



Determining Factors Of the Systolic Pressure
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 previously, 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 (What is Peripheral Resistance? See section F).

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 or b), the decrease in blood pressure will be very high and the diastolic pressure very low.




Determining Factors Of the Diastolic Pressure
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 (less lower!).

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 (see red arrow).


Effects of the Heart Frequency on the Diastolic Pressure
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 we are saved every time that the heart beats!




Diagram to show the blood pressure in sudden cardiac arrest

F. What is “Peripheral Resistance”?

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Diagram to show the effects of an increase in 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, especially in the arterioles. 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!

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