What are the roles of the venous system?

The venous system has a number of roles:

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  Transport of deoxygenated blood from the capillaries to the right side of the heart, which is the main role of the venous system. There are a few exceptions to this rule:

  
  
  
  
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     – The pulmonary veins carry oxygenated blood from the pulmonary capillaries to the left side of the heart (see Chapter 23).

     
     
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     – The umbilical vein carries oxygenated blood from the placenta to the foetus (see Chapter 83).

     
     
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     – A portal vein is a vein that connects two capillary networks. For example:

     
     
     
     
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        ▪ The hepatic portal vein carries deoxygenated blood between two capillary beds: from the gut to the liver (see Chapter 65).

        
        
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        ▪ The long hypophyseal portal veins connect the capillary networks of the lower hypothalamus and the anterior lobe of the pituitary gland (see Chapter 80).

        
        
     3. 
        

        ▪ The short hypophyseal portal veins connect the capillary networks of the posterior and anterior lobes of the pituitary gland.

     
     

  
  
  
  
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  Storage of blood. The venous system contains 65% of the circulating blood volume.

  
  
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  Venous return to the heart. The rate at which blood returns to the right atrium determines the cardiac preload and is therefore a major factor determining the cardiac output (CO).

  
  
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  Thermoregulation. Arteriovenous anastomoses are short channels that connect arterioles to venules, bypassing the capillary networks. Arteriovenous anastomoses are plentiful in the skin. They are opened when body temperature increases, allowing a large amount of blood to flow through the skin, dissipating heat to the environment.

How does the structure of a vein differ from an artery?

Arteries have three layers (see Chapter 34):

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  The tunica externa;

  
  
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  The tunica media;

  
  
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  The tunica intima.

The tunica media is the thickest layer, with a much higher proportion of smooth muscle than elastin.

In contrast, veins:

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  Have thinner walls and larger lumens than equivalent-sized arteries;

  
  
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  Have much less smooth muscle and more elastin in their tunica media;

  
  
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  Have as their thickest layer the tunica externa, containing elastin and collagen;

  
  
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  Are much more distensible than arteries and are often collapsed;

  
  
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  Have valves formed by folds of tunica intima that prevent backflow of blood. Note that there are no valves in the venae cavae, portal veins or cerebral veins.

What is compliance? How does this relate to the venous system?

Compliance is the change in volume caused by a unit change in distending pressure. The venous system is around 30 times more compliant than the arterial system. This means that the veins can accommodate large volumes of blood for only a small increase in intraluminal pressure. In fact, the venous system holds 65% of the circulating blood volume compared with only 15% within the arterial system, which is under considerably higher pressure.

The difference in compliance between arteries and veins is of critical importance to the systemic circulation. As previously discussed, the left ventricle acts as a pressure generator. It does this by moving a unit of blood (the stroke volume) from the highly compliant veins to the less compliant arteries. As compliance (C) is the change in volume per unit change in pressure, the pressure generated is: ΔP = ΔV/C. Therefore, as the veins have a high compliance, the same volume of blood gives a low pressure, whereas when it is moved to the low-compliance artery the pressure is much increased.