We previously learned that an increase in the amount of resistance in the arteries leaving the heart,
called an increased Afterload can make it harder for the ventricles to pump blood out of the heart.
However, this difficulty pushing against pressure is not only seen at the heart; it is a pattern we see throughout
the circulatory system. Any time there is high resistance in a blood vessel, it will decrease blood flow through that
vessel. In this lesson you will learn about factors that regulate blood flow through the circulatory system.
Blood Flow is the continuous movement of blood through the circulatory system. As blood moves through your body
it can encounter resistance to its flow. There are a number of factors that influence blood flow.
These factors include the thickness, or viscosity of the blood and the length of the blood vessel.
In other words, if the blood is thick, more like molasses than water, there will be more resistance to flow.
Likewise, if the blood vessel is long, there is more resistance to blood flow.
Another factor is the difference in pressure at the beginning and end of the vessel.
This pressure difference is important because later we will see that blood flows from high to low pressure.
However one of the most important factors influencing blood flow is the size or
radius of the blood vessel the blood is passing through.
Blood vessels, and in particular the more muscular arteries, are often the source of resistance.
One way an artery can resist blood flow is by contracting the smooth muscle within its wall, causing the artery to
constrict. When an artery constricts, we call it Vasoconstriction. This is an easy term to recall if you
remember that 'vaso' refers to blood vessel. During vasoconstriction, the artery gets smaller and offers
increased resistance. This causes a decreased blood flow through its lumen or hollow center. If, on the other hand,
the smooth muscle in the wall of the artery relaxes, the blood vessel moves into a state of Vasodilation and its
lumen dilates and gets bigger. This offers decreased resistance and causes an increased blood flow.
This is much like the nozzle on the end of a hose. If the nozzle is almost closed, it will allow less water to get
through. If the nozzle is opened wide, it will allow more water to flow through.
We mentioned earlier that one of the most important factors influencing blood flow is the size, or radius,
of the blood vessel. You recall that radius is the math term that measures the distance from the center of a
circle out to the edge. Blood flow is proportional to the 4th power of the vessel's internal radius.
This is known as Poiseuille's Law, which can be pronounced like the words pause and wee put together.
It is an equation that is used to determine the flow of any fluid through a tube, so you might want to
use its pronunciation to help you remember what the law talks about. For example, if the blood vessel has a small
radius, the blood flow will be small or 'pause'. If the blood vessel's radius is big, blood flow will be big;
we say 'wee' when we open the flood gates. In simple terms, what Poiseuille's Law tells us is that if the radius
of a blood vessel doubles, like it can do during vasodilation, then the flow will increase 16 fold,
because 2 to the 4th power is 16. The same would be true of your hose nozzle. If you take your hose nozzle and
open it twice as big, you could expect the water flow to increase 16 fold. The reverse is also true,
if the radius of the vessel is reduced in half, because of vasoconstriction, then the blood flow will reduce by
16 fold. The take-away learning point here is that it only takes small changes in
the size of the artery to give us big changes in blood flow.
If we look closely at the circulatory system, we see that blood flow through the body is considered in different
ways. One way we can look at blood flow is how much is needed by an individual organ.
For example, if blood flow to an organ is too low, then the blood vessels leading to the organ will dilate and
allow more blood to reach the organ. Conversely, if blood flow to an organ is too high, then the blood vessels
leading to the organ will constrict, and reduce flow to an appropriate level.
This is a piece of cardiac physiology that is important for events such as exercise. For example, if you are doing
bicep curls, blood must increase to your bicep muscles so we see that the arteries feeding these muscles dilate.
But during intense exercise you do not need to digest your food,
so arteries to your digestive system will constrict
The other way to look at blood flow is to consider the entire circulatory system as a whole.
Total Peripheral Resistance is defined as the total resistance to flow of blood in the systemic circulation.
So with total peripheral resistance, we are looking at the resistance of all of the blood vessels in the body.
Of all of the blood vessels, the most important ones to consider when we talk about resistance of blood flow are
the arterioles. We previously learned that arterioles have a smaller diameter than arteries.
This smaller size provides greater resistance to blood flow than arteries, just like our smaller hose nozzle.
Arterioles also have relatively more smooth muscle in their walls, which allows them to constrict and dilate
efficiently. Therefore, we see that the small arterioles are able to provide a significant amount of control over
the flow of blood. Blood flow to individual organs or throughout the entire circulatory system can be
regulated by changing the diameter of the arterioles. Vasodilation of the arterioles decreases resistance and
increases blood flow. Vasoconstriction of the arteriole increases resistance and decreases blood flow
Let's review...
Blood Flow is the continuous movement of blood through the circulatory system. Blood flow decreases when there
is increased resistance to its flow. Some factors that decrease blood flow and increase resistance include
increased viscosity of the blood, increased length of the blood vessel and decreased radius of the blood vessel.
Blood vessels, and in particular the more muscular arteries, are often the source of resistance.
When an artery is in a state of Vasoconstriction, the lumen of the artery gets smaller and blood flow
decreases. When the artery is in a state of Vasodilation, the lumen of the artery gets bigger and blood flow
increases. We use a law known as Poiseuille's Law, to calculate blood flow through an artery.
The law states that blood flow is proportional to the 4th power of the vessel's internal radius.
Therefore, if a blood vessel's radius doubles in size due to vasodilation,
the blood flow through the vessel will increase 16-fold.
Blood flow can be regulated to individual organs, or it can be considered in total. Total Peripheral Resistance is
defined as the total resistance to flow of blood in the systemic circulation. The Arterioles are important
regulators of blood flow because of their smaller size and muscular walls.
我们先前了解到，离开心脏的动脉阻力增加，
所谓的增加后负荷会使心室更难将血液泵出心脏。
但是，这种克服压力的困难不仅体现在内心，而且体现在人们的心中。这是我们贯穿始终的模式
该循环系统。每当血管中有高阻力时，都会减少通过
船只。在本课程中，您将学习调节血液循环系统的因素。
血液流动是血液通过循环系统的连续运动。随着血液在您体内流动
它可能会遇到阻力。有许多因素会影响血液流动。
这些因素包括血液的厚度或粘度以及血管的长度。
换句话说，如果血液浓稠，比起水更像糖蜜，那么流动的阻力就会更大。
同样，如果血管较长，则对血流的阻力更大。
另一个因素是容器开始和结束时的压力差。
这个压力差很重要，因为稍后我们将看到血液从高压流向低压。
但是，影响血流的最重要因素之一是大小或
血液通过的血管半径。
血管，尤其是肌肉发达的动脉，通常是抵抗力的来源。
动脉可以抵抗血液流动的一种方法是收缩其壁内的平滑肌，使动脉
收缩当动脉收缩时，我们称其为血管收缩。这是一个容易记住的术语，如果您
请记住，“血管”是指血管。在血管收缩期间，动脉变小并提供
抵抗力增强。这导致流经其内腔或中空中心的血流量减少。另一方面，如果
动脉壁的平滑肌松弛，血管进入血管舒张状态
流明扩张并变大。这提供了降低的抵抗力并导致增加的血流。
这很像软管末端的喷嘴。如果喷嘴几乎关闭，则将减少水流
通过。如果喷嘴张开，它将允许更多的水流过。
前面我们提到影响血液流动的最重要因素之一是大小或半径，
血管。您还记得，半径是一个数学术语，用于测量距圆心的距离
划到边缘。血流与血管内半径的四次方成正比。
这就是所谓的泊瓦耶定律，它的发音可以像“停顿”和“放荡”这两个词一样。
这是一个方程，用于确定任何流体通过管道的流量，因此您可能需要
用它的发音来帮助您记住法律的内容。例如，如果血管很小
在半径范围内，血流会很小或“暂停”。如果血管的半径大，那么血流就会很大；反之，
当我们打开防洪闸时，我们说“ wee”。简而言之，泊瓦约定律告诉我们，如果半径
血管扩张一倍，就像血管扩张时一样，那么流量会增加16倍，
因为2到4的幂是16。您的软管喷嘴也是如此。如果带上软管喷嘴，
将其打开两倍，可以预期水流量会增加16倍。反之亦然，
如果由于血管收缩使血管半径缩小一半，那么血流将减少
16折。这里的主要学习点是，它只需要对
动脉的大小使我们的血流发生了很大的变化。
如果我们仔细观察循环系统，就会发现通过人体的血液流动被认为是不同的。
方法。我们观察血液流动的一种方法是单个器官需要多少血液。
例如，如果流向器官的血液太少，则通向该器官的血管会扩张并
让更多的血液到达器官。相反，如果流向器官的血液过多，则血管
导致器官收缩，并将血流减少到适当水平。
这是心脏生理的一部分，对诸如运动等事件很重要。例如，如果您正在
二头肌卷曲，血液必须增加到您的二头肌肌肉，因此我们看到喂养这些肌肉的动脉会扩张。
但是在剧烈运动中，您不需要消化食物，
所以消化系统的动脉会收缩
查看血液流动的另一种方法是将整个循环系统视为一个整体。
总外周阻力定义为对全身循环中血液流动的总阻力。
因此，考虑到总的外周阻力，我们正在研究体内所有血管的阻力。
在所有血管中，当我们谈论血流阻力时要考虑的最重要的是
小动脉。我们先前了解到，小动脉的直径比动脉小。
与我们较小的软管喷嘴一样，这种较小的尺寸比动脉具有更大的血流阻力。
小动脉壁上的肌肉也相对较光滑，这使它们可以收缩和扩张
有效率的。因此，我们看到小动脉能够提供对
血液流动。血液可以流向各个器官或整个循环系统
通过改变小动脉的直径来调节。小动脉的血管舒张降低了抵抗力，
增加血液流动。小动脉的血管收缩会增加抵抗力并减少血液流量
我们来复习...
血液流动是血液通过循环系统的连续运动。那里的时候血流量减少
增加了对其流动的阻力。减少血液流动和增加抵抗力的一些因素包括
血液粘度增加，血管长度增加，血管半径减小。
血管，尤其是肌肉发达的动脉，通常是抵抗力的来源。
当动脉处于血管收缩状态时，动脉腔变小，血液流动
减少。当动脉处于血管舒张状态时，动脉腔变大，血液流动
增加。我们使用被称为泊雪尔定律的定律来计算通过动脉的血流量。
法律规定，血流与血管内半径的四次方成正比。
因此，如果由于血管扩张血管的半径增加了一倍，
通过血管的血流量将增加16倍。
可以调节单个器官的血流量，也可以整体考虑。总外围电阻为
定义为对全身循环中血液流动的总阻力。小动脉很重要
调节血流量的调节器，因为其尺寸较小且肌肉发达。