Blood vessels

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 from the heart into 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 into the heart.

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

However, there are exceptions to this rule: pulmonary vessels and umbilical vessels (which supply blood from the mother to the fetus during pregnancy) do not carry 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 of arteries affects their function. Arteries have:

• Thick muscle layers that constrict and dilate to regulate the volume of blood passing through
• Thick elastic layers that stretch at systole and recoil at diastole to maintain high blood flow pressure by preventing fluctuations in blood pressure. Refer to the article on the Cardiac Cycle for more information about the concepts of systole and diastole.
• Thick walls that prevent arteries from bursting under high pressure, 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 in temperature 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 of substance exchange, for instance, gas exchange occurs in the alveoli capillaries. As such, they have:

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

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

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

• Gaps between endothelial cells (except for the nervous system) to facilitate substance exchange between the capillaries and tissue

As for veins, they have:

• A thin muscle layer as 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 layer as the pressure of blood flow is too low to cause a recoil action.

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

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

• Valves to prevent blood from flowing away from the 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 to the 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: