Blood vessels

Blood vessels are tube-like structures that facilitate blood movement to specific tissues in the body. One can compare blood vessels to transport networks in a city.

What are the types of Blood Vessels?

There are five types of blood vessels:arteries,arterioles,capillaries,venules,andveins．We will look in more detail at the characteristics of each type later on in the explanation.

You also need to know the names of blood vessels that supply blood to theheart,lungs, andkidneys, as summarised in the table:

Structure of Blood Vessels

The structures of arteries, veins, and capillaries differ from each other. Arterioles branch off from arteries and venules branch off from veins, similar to how bronchioles branch off from bronchi (you can find out more about this in the Human Gas Exchange explanation).

Thus, arterioles and venules have similar structures to their ‘parent vessels’ – arteries and veins. However, arterioles have a thicker muscle wall and a thinner elastic layer.

To look at the structure of these blood vessels in a bit more detail, we can do a cross-section of arteries, capillaries and veins to see the layers making up these blood vessels, as shown in the table below:

In an exam, you may be asked to draw out the structures of blood vessels based on microscope slides. The diagram below demonstrates how to draw these blood vessels out. Do note the relative proportions of each layer comprising the vessel wall.

Fig. 1 -Drawing of an artery, vein, and capillary

How do Blood Vessels function?

Given how the structure of these blood vessels differs from each other, you may have guessed that each type of blood vessel functions differently. Below is a summary of the role of these blood vessels:

• Arteries:move blood away fromthe heartinto arterioles.
• Arterioles:‘smaller arteries’ that move blood from arteries into capillaries.
• Capillaries:tiny vessels that move blood from arterioles into veins.
• Venules:‘smaller veins’ that move blood from capillaries to veins.
• Veins:move blood away from the venules intothe heart．

Arteries and arterioles carry oxygenated blood, whereas veins and venules carry deoxygenated blood.

However, there are exceptions to this rule:pulmonaryvessels andumbilicalvessels(which supply blood from the mother to the fetus during pregnancy) donotcarry oxygenated blood. Pulmonary and umbilical arteries carry deoxygenated blood whereas pulmonary and umbilical veins carry oxygenated blood.

Relationship between the structure and function of Blood vessels

We will explore in detail how the structure of blood vessels affects their function.

First off, we will look at how the structure ofarteriesaffects their function. Arteries have:

• Thick muscle layersthat constrict and dilate to regulate the volume of blood passing through
• Thick elastic layersthat stretch at systole and recoil at diastole to maintain high blood flow pressure by preventing fluctuations in blood pressure. Refer to the article on theCardiac Cyclefor more information about the concepts of systole and diastole.
• Thick walls防止动脉破裂在前高ssure, just like river dams are thick to prevent structural damage.
• No valves– blood in the arteries does not flow backwards as it flows under high pressure

Considering arterioles have thicker muscle layers and thinner elastic layers than arteries, how does the structure of arterioles affect their function? The thicker muscle layers of arterioles allows arterioles to either increase or decrease blood into capillaries. This plays an important role intemperature control．Arterioles have thinner elastic layers because blood travels in arterioles at lower pressures, reducing the need for an elastin layer. This lower pressure is due to the fact that arterioles are further away from the heart when compared to arteries, which have more elastin.

Now, let's take a look at the relationship between the structure of capillaries and their function. Capillaries are sites ofsubstance exchange, for instance,gas exchangeoccurs in the alveoli capillaries. As such, they have:

• A thin (one cell thick) wallto shorten the diffusion distance between the capillaries and the surrounding tissues

• Large surface areaas capillaries form an extensive network near surrounding tissues (capillary beds) to increase surface area for efficientsubstance exchange．

• Slow blood flowto give sufficient time to maximise the efficiency of substance exchange

• Gaps between endothelialcells(except for the nervous system)to facilitatesubstance exchangebetween the capillaries and tissue

As for veins, they have:

• A thin muscle layeras blood moves away from the tissues whereas constriction and dilation of muscles play a more significant role in moving blood to tissues.

• A thin elastic layeras the pressure of blood flow is too low to cause a recoil action.

• Thin wallsas the blood flow pressure is too low to create any risks of veins bursting.

• A wide lumenthat holds a large reservoir of blood and allows blood to flow easily, which is why blood samples are obtained from the veins

• Valvesto prevent blood from flowing away fromthe heart, as blood in veins moves under low pressure due to the lack of a pulse; hence gravity may affect the movement of blood to the heart.

Furthermore, veins have adapted to prevent a backflow of blood by being located in muscular regions of the body (e.g., the legs and chest). The contraction of these muscles helps push blood tothe heart．

How do these blood vessels compare with each other?

Is all the detail behind all the blood vessels overwhelming? The table below compares the blood vessels to help you grasp the addressed concepts: