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Blood vessels

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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, 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.

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:

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

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.

Frequently Asked Questions about Blood vessels

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

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

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

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

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

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.

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

Final Blood vessels Quiz

Question

The pressure in the atria is greater than in the ventricles. What is the direction of blood flow in this case?

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Answer

Blood flows from the atria to the ventricles as the direction of blood flow in the heart is from a region of high pressure to a region of low pressure.

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Question

Choose the correct options

When the heart muscle (contracts/ relaxes), this results in a region of (high/ low) pressure. Whereas when the heart muscle (contracts/ relaxes), this creates a region of (high/ low) pressure. (Systole/ diastole) is the term to describe the contraction of the heart muscle, whereas (systole/ diastole) is when the heart muscle relaxes.

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Answer

When the heart muscle contracts, this results in a region of high pressure. Whereas when the heart muscle relaxes, this creates a region of low pressure. Systole is the term to describe the heart muscle contraction, whereas diastole is when the heart muscle relaxes.


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Question

Valves ensure blood flows in one direction (True/ False)



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Answer

True

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Question

Valves contract and relax, whereas the heart muscle opens and closes. (True/ False)


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Answer

False - valves open and close

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Question

Choose the correct options.

Pressure changes affect the function of valves. Valves (open/close) when the pressure difference is against the direction of blood flow and (open/ close) when the pressure difference follows the direction of blood flow.


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Answer

Pressure changes affect the function of valves. Valves close when the pressure difference is against the direction of blood flow and open when the pressure difference follows the direction of blood flow.


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Question

Name the three stages of the cardiac cycle.


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Answer

Atrial systole, ventricular systole, and ventricular diastole.

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Question

Describe the events that occur during atrial systole.


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Answer

Muscles in both atria contract

  • Atrial pressure > ventricular pressure

  • Blood flows from atria to ventricles

  • Atrioventricular valves open, semilunar valves shut

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Question

Describe the events that occur during ventricular systole.


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Answer

Muscles in both ventricles contract

  • Pressure in ventricles > atria

  • Atrioventricular valves shut

  • Pressure in ventricles > arteries

  • Blood flows from atria to arteries, semilunar valves open

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Question

Describe the events that occur during ventricular diastole.


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Answer

The whole of the heart muscle relaxes

  • Pressure in veins > atria and ventricle

  • Blood flows in

  • Pressure in arteries > ventricles

  • Semilunar valves close

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Question

Blood flow in ventricular systole is higher in pressure than in atrial systole. (True/ False)


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Answer

True

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Question

Atrioventricular valves shut during atrial systole to prevent blood from flowing back into the vena cava or pulmonary vein. (True/ False)


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Answer

False - semilunar valves shut during atrial systole


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Question

The closing of valves generates heart sounds. (True/ False)

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Answer

True


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Question

The ‘lub’ sound is from the (atrioventricular/ semilunar) valves whereas the (atrioventricular/semilunar) valves make the ‘dub’ sound.


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Answer

The ‘lub’ sound is from the (atrioventricular) valves whereas the (semilunar) valves make the ‘dub’ sound.


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Question

A woman whose cardiac output is 8000cm3min-1 completes one cardiac cycle in 0.5s during that timeframe. Calculate her stroke volume.


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Answer

66.67 cm3 

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Question

Name the five blood vessels.


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Answer

arteries, arterioles, capillaries, venules and veins

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Question

Arterioles have thinner muscle walls and thicker elastic layers. (True/ False)


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Answer

False - Arterioles have thicker muscle walls and thinner elastic layers.


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Question

Fill in the blanks

Arteries have ____ walls composed of _____ layers. The outer layer, tunica externa, is made of________. It is  ____ and _____. The second layer, tunica ______, is a thick muscle layer. This is followed by a thick _____ layer, tunica intima and the ________, tunica ______. Arteries have a _____ lumen than veins.

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Answer

Arteries have thick walls composed of four layers. The outer layer, tunica externa, is made of collagen. It is tough and fibrous. The second layer, tunica media, is a thick muscle layer. This is followed by a thick elastic layer, tunica intima and the endothelium, tunica interna. Arteries have a narrower lumen than veins. 


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Question

Fill in the blanks

The capillary wall is thin (____-____ _____) and capillaries have a _____ lumen. The diameter of the lumen of capillaries is only ______ ______ than that of a red blood cell.


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Answer

The capillary wall is thin (one-cell thick) and capillaries have a narrow lumen. The diameter of the lumen of capillaries is only slightly larger than that of a red blood cell.


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Question

Fill in the blanks

Veins have ____ walls composed of _____ layers. The outer layer, tunica externa, is made of________. It is  ____ and _____. The second layer, tunica ______, is a thin muscle layer. This is followed by a thin _____ layer, tunica intima and the ________, tunica ______. Veins have a _____ lumen than arteries. 

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Answer

Veins have thin walls composed of four layers. The outer layer, tunica externa, is made of collagen. It is tough and fibrous. The second layer, tunica media, is a thin muscle layer. This is followed by a thin elastic layer, tunica intima and the endothelium, tunica interna. Veins have a wider lumen than arteries. 


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Question

Pulmonary and umbilical arteries carry deoxygenated blood whereas pulmonary and umbilical veins carry oxygenated blood. (True/ False)

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Answer

True

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Question

Explain why arteries have thick muscle and elastic layers.

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Answer

Arteries have thick muscle layers to constrict and dilate to regulate the volume of blood passing through, and thick elastic layers to stretch at systole and recoil at diastole to maintain high pressure of blood flow by preventing fluctuations in blood pressure.

Show question

Question

Arteries have valves because the high-pressure blood flow increases the chance of backflow. (True/ False)


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Answer

False - arteries do not have valves because the high-pressure blood flow only causes the blood to move in its intended direction.


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Question

Explain why arterioles have thicker muscle layers but thinner elastic layers.


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Answer

Arterioles have thicker muscle layers to restrict blood flow further to channel blood into arteries, and thinner elastic layers to lower blood pressure as arterioles are further away from the heart.

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Question

Outline how capillaries adapt to have a large surface area.


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Answer

Capillaries form a large network near surrounding tissues, termed capillary beds, to increase the surface area for efficient substance exchange.


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Question

Blood flow in the capillaries is rapid to maximise the efficiency of substance exchange (True/ False)


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Answer

False - Blood flow in the capillaries is slow to maximise the efficiency of substance exchange.


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Question

Suggest why veins have thinner walls than arteries.


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Answer

Thick vessels walls are to prevent the high-pressure blood flow from causing blood vessels to burst. Veins have thinner walls than arteries because the pressure of blood flow is too low to create any risks of veins bursting.


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Question

Outline another way how veins prevent backflow besides having valves.


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Answer

Veins prevent backflow by being located in muscular regions of the body (eg: legs and chest). The contraction of these muscles helps push blood to the heart. 


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