The Vessels and Circulation

Introduction to the circulatory system

Blood vessels are dynamic structures which form a system of pipes. There are 3 main types of blood vessels: arteries, veins, and capillaries, and the structure of each vessel varies with its function. The system begins at the heart. Arteries carry blood away from the heart, and veins bring the blood back. There are two circulatory systems; systemic and pulmonary:

Systemic circulation

Blood vessels supply the body. Arteries carry oxygenated blood to where it is required, and the veins bring back the deoxygenated blood.

 

Pulmonary circulation

Arteries carry deoxygenated blood from the heart, into the lungs. Once gas exchange has occurred, the veins bring the oxygenated blood back to the heart, where it is pumped out into systemic circulation.



The structure of blood vessels on a cellular level

Blood vessels are made up of a number of different cell types: smooth muscle cells and endothelial cells. In addition to this there are two types of connective tissue fibres; elastin and collagen. How much of each of these components is present, varies with the function of the vessel

 

Smooth muscle cells

These are spindle shaped cells with a centrally located nucleus, and lack the striations of skeletal and cardiac muscle cells. These cells are tightly arranged to form sheets, and contact to produce a squeezing motion, therefore propelling blood.



Endothelial cells

These are simple squamous epithelial cells, and form the inner (“endo”) lining of blood vessels. These cells are flattened, with a disc shape nucleus, and provide a slick surface for blood flow. These cells form a thin, permeable layer, which is perfect for the exchange of nutrients and waste.



Elastic fibres

These are long, thin fibres which form a branching network. They are made from the elastin protein, which allows them to stretch like elastic under tension. When the tension is released, they recoil back to their original shape.



Collagen fibres

These fibres consist of collagen proteins cross-linked to form a fibril, which bundle to form thick collagen fibres. These fibres are very strong, and are able to withstand large amounts of pressure.



The layers of blood vessels explained

There are three distinctive layers, or tunics, to arteries and veins. These are the tunica intima, tunica media, and tunica externa. In arteries, between these tunics there are also layers of elastin fibres. 

 



Tunica Intima

The innermost layer, containing the endothelium. The squamous epithelial cells reduce friction as blood flows through the vessel. In vessels larger than 1mm wide, there is also a subendothelial layer to support the endothelium.

 

Tunica Media

The middle layer, containing smooth muscle cells and elastin fibres. This layer contracts and relaxes to regulate blood flow and is vital in regulating blood pressure. This layer is supplied by sympathetic nerves.

 

Tunica Externa

The outer layer, containing collagen fibres. This layer’s function is to reinforce the vessel, and the collagen fibres help anchor the vessel in place. This layer also contains nerve fibres, lymphatic vessels, microvasculature, and in larger veins, elastin fibres.

 

 



The arterial system

Arteries can be further divided into 3 groups based on their size and function:

 

Elastic arteries

The major vessels leaving the heart and their branches e.g. the aorta.

  • These are the largest arteries (1 - 2.5cm).
  • Due to their size, they are known as conducting vessels, as their large lumens allow large volumes of blood away from the heart into smaller vessels with little resistance.
  • These arteries contain large amounts of elastin, allowing them to expand and recoil and blood is pumped out of the heart at high pressure
  • Whilst there is a thick layer of smooth muscle, this muscle remains mostly inactive. 


Muscular arteries

Elastic arteries branch into smaller muscular arteries e.g. radial artery.

  • Size ranges from 1cm diameter to 0.3mm.
  • Also known as distributing arteries, as these vessels distribute the blood to specific areas of the body.
  • These vessels have the thickest tunica media of all vessels (relative to their size).
  • They contain a large amount of smooth muscle and some elastin, allowing them to contract and control blood flow.

 



Arterioles

  • Muscular arteries branch into a network of arterioles.
  • The smallest arteries (0.3mm - 10µm).
  • Larger arterioles have all three tunics, and the tunica media is mostly smooth muscle cells.
  • Smaller arterioles (<400 µm) are known as resistance arteries and consist only of a singular smooth muscle layer around the endothelium.
  • Lead to capillaries, and contract to control blood flow into capillaries.
  • Their ability to contract and create resistance makes them vital in controlling blood pressure

 



Capillaries

Capillaries are the smallest blood vessels, and supply most tissues. They are so small (8-10µm) that red blood cells can only fit through in single file. Capillaries only consist of a layer of endothelium (sometimes only one cell thick) and the surrounding basement membrane.

 



The basement membrane consists of connective tissues and sometimes smooth-muscle like cells known as pericytes, which anchor the capillaries in place. The thin structure of capillaries makes them perfectly suited to exchange of gas, nutrients, and waste, between the blood and tissues.

There are three different types of capillary; continuous, fenestrated, and sinusoidal. The function of each type of capillary varies, and therefore they each have different structures.

 

Continuous - Skin and muscles

  • Endothelial cells are joined by tight junctions, therefore provide an uninterrupted lining.
  • Sometimes there are small gaps between the junctions to allow fluid transfer.
  • There are no gaps in the brain capillaries, forming the blood brain barrier.

 



Fenestrated

  • Anywhere filtration occurs e.g. Small intestine, kidneys
  • Contain large pores to allow filtration.
  • Pores are covered by a fine membrane but are still very permeable.

 



Sinusoidal

  • Anywhere involving transfer of large molecules e.g. liver, spleen, bone marrow, adrenal medulla.
  • Endothelium is discontinuous and has fewer tight junctions.
  • These capillaries are “leaky” and allow large molecules to pass through.
  • Macrophages (in the liver) or phagocytes (everywhere else) remove and destroy bacteria.

 

 



Capillaries work in groups called capillary beds to achieve microcirculation. Arterioles bring blood to the capillaries, and if precapillary sphincters are open, blood will flow across the capillary bed and into the venous system. If the sphincters are closed, the blood will flow directly from the arterioles into ametarteriole and into a throughfare channel, therefore bypassing the capillary bed and going straight back into the venous system. 

 

The venous system

After leaving the capillary beds, the blood is carried back to the heart via a series of venous vessels. The venous system comprises of venules and veins. These vessels increase in size as they progress towards the heart.

 

Venules

  • Formed when capillaries unite
  • 8-100µm diameter.
  • The smallest consist of only a layer of endothelium surrounded by pericytes to anchor the vessel.
  • Small venules are still very permeable.
  • Larger venules have a layer or two of smooth muscle, and sometimes a very thin tunica externa.

 



Veins

  • Also known as capacitance vessels, and are formed by venules
  • Known as capacitance vessels as their large lumens allow them to carry large volumes of blood (~65% of total blood volume is held in veins).
  • Walls are very thin, but all three tunics are present.
  • Tunica media is very thin with little smooth muscle or elastin.
  • Tunica externa is very thick with large bundles of collagen.
  • Venae cavae are further reinforced by smooth muscle in the tunica externa.
  • Low pressure – large lumens aid blood flow by offering low resistance. Valves resembling the semi-lunar valves of the heart are required to prevent back-flow.

 



References

Boundless. Arteries, Veins, and Capillaries. [accessed 18/02/16] https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-circulatory-system-40/mammalian-heart-and-blood-vessels-226/arteries-veins-and-capillaries-853-12098/

Ivy Rose Holistic. Blood vessels. [accessed 18/02/16] http://www.ivyroses.com/HumanBody/Blood/Blood_Vessels.php

Marieb EN & Hoehn K. 2010. Human Anatomy and Physiology, Chapter 19. 8th Edition, London: Pearson International Edition.

 

 

 

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