3. The structure and function blood vessels

The structure and function of veins, arteries, capillaries, arterioles. venules and shunt vessels.

The circulatory system operates by a network of blood vessels that carry all the substances that the body wants, and doesn't want, around the body to requisite locations.

The blood flows from the heart to arteries, arterioles (smaller arteries), capillaries, venules (smaller veins), veins and then returned to the heart - reminder diagram of the double circulation system below.

There are three types of blood vessel and each is designed ('adapted') for its particular function.

Arteries

Arteries transport (i) oxygenated blood (except for the pulmonary artery) from the heart to the tissues and organs of the body and (ii) deoxygenated blood away from the heart to the lungs.

The arteries carry blood under high pressure with a pulse, so the artery walls need to be thick and strong with muscle tissue-fibre but these blood vessels must also be elastic too.

The thick walls are an adaptation for high pressure flow.

The artery walls are relatively thick compared to the size of the 'hole' the blood flows through.

The thick artery walls are not permeable.

The 'hole' is called the lumen and is small.

The artery walls are made of a combination of thick layers of strong muscle cells AND elastic fibres which allow flexibility i.e. allowing the artery walls to be stretches and spring back when the pressure is relaxed - a biological example of elasticity.

The arteries branch into the narrower arterioles.

Arterioles are very small blood vessels that branches off from your main arteries and carry blood away from your heart to your tissues and organs - they are effectively small arteries that link up to capillaries, which are even narrower.

 

Capillaries

Capillaries exchange materials with tissues.

Arteries branch into arterioles which then branch into numerous much thinner capillaries.

The pressure falls in arteries and the pulse disappears.

There are no valves in arteries.

 In the organs, in fact in all tissues, blood flows through very narrow, thin-walled blood vessels called capillaries which branch out from the arteries-arterioles.

The network of capillaries in tissues art called capillary beds.

The substances needed by the cells in body tissues pass out of the blood capillaries, and substances produced by the cells pass into the blood, through the thin permeable walls of the capillaries - which are only one cell thick - thin wall adapted for efficient exchange of materials.

This ensures a short distance and movement time for particles - fast diffusion in and out - fast exchange between capillaries and cells - sugars, minerals, amino acids, oxygen into surrounding cells and carbon dioxide, urea and other waste products out of cells.

The blood in capillaries will slowly lose its dissolved oxygen.

The capillaries are the smallest blood vessels, and although these fine blood tubes carrying blood are dispersed in all the tissues of every organ, you can't see individual capillaries, but they reach every cell.

The capillaries have adapted to have large surface area to make exchange of substances as fast and efficient as possible.

Capillaries are the main interface for material exchange eg of sugars, amino acids and other nutrients, oxygen and waste carbon dioxide.

Some capillaries are so fine that blood cells cannot pass through them.

The blood flow through capillaries is the slowest of any of the types of blood vessels. This is important because it allows more time for the exchange of substances through the capillary membranes - more efficient diffusion.

Capillaries eventually join up to form wider blood vessels called venules which connect to veins (next section).

 

Veins

From the capillaries, the venules eventually join up to form veins to carry deoxygenated  blood back to the heart.

Blood flows at a lower pressure in the veins compared to the blood pressure in the arteries and contain valves to ensure the blood flows in the right direction (diagram below).

The adaptation of a larger diameter lumen offers the least flow resistance for the returning blood in the veins (to the heart).

Veins return and transport:

(i) deoxygenated blood from the tissues and organs to the heart,

(ii) convey oxygenated blood to the heart from the lungs and exit the heart via the aorta arteries to the rest of the body.

Veins have non-permeable thinner walls (operating at lower pressure) and periodically have valves to prevent back-flow of blood.

The diagram above shows a cross-section of a vein and the valve system to ensure one-way flow.

If the blood attempts to flow 'backwards', the tissue flap valves close preventing 'back-flow' in the wrong direction - another wonderful adaptation of biological engineering!

Since the pressure in the veins is lower, their walls don't have to be as thick and the lumen is bigger in cross-sectional area.

The bigger lumen allows good blood flow despite the lower pressure, but bits of tissue act as one-way valves - best appreciated in the diagram above.

If the blood attempts to go in the 'wrong direction' the 'flaps' of the valve close together and stop any reverse flow happening.

 

Shunt vessels

A shunt vessel is a blood vessel that connects an artery directly to a vein, allowing the blood to bypass the capillaries in certain areas.

Shunt vessels can control blood flow by their constriction or dilation.
 

The relative diameter, cross-section areas and flow rates of blood vessels

The larger the cross-sectional area of a blood vessel the average velocity of the blood decreases.

Therefore, blood flows more slowly through capillaries than arteries or veins.

However, although capillaries are relatively small in average diameter, their total cross-sectional area is very large, so all the large blood flow from the arteries is dispersed through them.

The slow flow through capillaries is important because it allows more time for the exchange of substances through the capillary membranes.

It also means that the mean blood pressure is highest in the arteries because they are directly connected to the heart (closest).

In fact the total cross-sectional area of the capillaries is greater than that of the arteries which actually causes a fall in blood pressure.

 

Summary note on 'connections'

Strictly speaking, arteries and veins do not connect directly with capillaries.

Arteries branch into arterioles which have a much smaller diameter.

Arterioles then branch out into the even smaller capillaries.

The capillaries then connect to venules which join together to form veins.

Initially the blood vessels get narrower conveying fresh blood, nutrients and oxygen and then get larger conveying deoxygenated blood and waste products away from the tissues.

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Summary of learning objectives and key words or phrases

Be able to interpret diagrams explaining the structure and function of blood and blood vessels including veins, arteries, capillaries, arterioles, venules, shunt vessels and the relative diameter, strength, flow rates and cross-section views of veins and arteries.

• Know and understand that arteries have thick walls containing muscle and elastic fibres.

• The arteries carry blood under high pressure, so their walls need to be thick and strong with muscle tissue-fibre but these blood vessels must be elastic too,

• Know that veins have thinner walls (operating at lower pressure) and periodically have valves to prevent back-flow of blood.

• Know that in the organs, blood flows through very narrow, thin-walled blood vessels called capillaries which branch out from the arteries.

• Know that substances needed by the cells in body tissues pass out of the blood, and substances produced by the cells pass into the blood, through the walls of the capillaries.

• The capillaries are the smallest blood vessels, and these fine blood tubes carrying blood are dispersed in all the tissues of every organ.

• Capillaries are the main interface for material exchange eg of sugars, amino acids and other nutrients, oxygen and waste carbon dioxide.

• Some capillaries are so fine that blood cells cannot pass through them.

• The walls of capillaries are thin with permeable walls only one cell thick, allowing rapid diffusion of substances into the cells (sugars, minerals, amino acids, oxygen) or out of surrounding cells (carbon dioxide, urea and other waste products).

• Capillaries eventually join up to form wider blood vessels called veins. The blood flows at a lower pressure in the veins compared to the blood pressure in the arteries and contain valves to ensure the blood flows in the right direction.

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