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.

Blood vessels Blood vessels

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Table of contents

    What are the types of Blood Vessels?

    There are five types of blood vessels: arteries, arterioles, capillaries, venules, and veins. 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 the heart, lungs, and kidneys, as summarised in the table:

    OrganVesselThe direction of blood flow
    HeartVena cavaTo the heart
    AortaAway from the heart
    LungsPulmonary veinAway from the lungs
    Pulmonary arteryTo the lungs
    KidneysRenal veinAway from the kidneys
    Renal arteryTo the kidneys

    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:

    ArteryCapillary Vein

    Thick wall

    Wall has four layers:

    • Tough, fibrous collagen outer layer; tunica externa
    • Thick muscle layer; tunica media
    • Thick elastic layer; tunica intima
    • Endothelium; tunica interna

    Narrower lumen than vein

    Wall is a one-cell thick layer.

    Narrow diameter, thus narrow lumen

    Thinner wall than artery

    Wall has four layers:

    • Tough, fibrous collagen outer layer; tunica externa
    • Thin muscle layer; tunica media
    • Thin elastic layer; tunica intima
    • Endothelium; tunica interna

    Wide lumen

    Lumen is the hollow cavity of an organ or blood vessel.

    Study tip: tunica is the Latin name given to the layers of the blood vessel. Furthermore, students may be confused between epithelium and endothelium. Epithelium covers other organs (e.g., airways, small intestine) whereas endothelium is specific to blood vessels.

    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.

    Artery, vein and capillary, Blood Vessels, Substance Exchange, StudySmarterFig. 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.

    Study tip: some students may confuse bronchi and bronchioles as blood vessels. These are not blood vessels. Instead, they belong to the respiratory system and transport air into and out of the lungs.

    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.

    Study tip: make sure you use the keywords that refer specifically to blood flow when writing your answers. For example, the muscle walls constrict and dilate, whereas the elastic walls stretch and recoil.

    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:

    WallThickThin (one cell thick)Thin
    Muscle layerThickAbsentThin
    Elastic layerThickAbsentThin
    Size of lumenSmallVery small (10μm)Large

    Blood Vessels - Key takeaways

    • The five types of blood vessels are arteries, arterioles, capillaries, venules, and veins.
    • Arteries transport blood away from the heart into capillaries. Arterioles branch off from arteries and connect arteries to capillaries. Artery walls consist of four layers with thick muscle walls and elastic layers. Blood flows in arteries under high pressure.
    • Capillaries are the site of substance exchange between the blood and tissues. Capillaries adapt to their role in substance exchange via their thin walls forming a network near tissues, slow blood movement, and gaps between endothelial cells.
    • Veins transport blood away from capillaries into the heart. Venules connect capillaries to veins. The walls of veins consist of four layers through the muscle walls, and elastic layers are thin. Blood flows in veins under low pressure.
    Blood vessels Blood vessels
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    Frequently Asked Questions about Blood vessels

    What are blood vessels?

    Blood vessels are pipe-like structures that transport blood from the heart to tissues in the body and vice versa.

    Which type of blood vessel contains valves?

    Veins and venules contain valves, essentially blood vessels whose blood flow is under low pressure.

    Which blood vessels carry oxygenated blood?

    Arteries and arterioles carry oxygenated blood, except for the pulmonary artery.

    What are the five major blood vessels?

    The five major blood vessels include arteries, arterioles, capillaries, venules, and veins.

    What blood vessels carry blood away from the heart?

    Arteries carry blood away from the heart, including the pulmonary artery.

    What are venules and arterioles?

    Venules and arterioles are blood vessels that branch off veins and arteries. They are similar in structure and function as veins and arteries, though they are smaller in diameter.

    What are the 3 layers of a blood vessel?

    Arteries and veins (including arterioles and venules) have three layers to them. These layers include the external layer, internal layer, and endothelial layer.

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